Mega Deck Flashcards
1
Q
State two conditions for any object to be in equilibrium
A
Resultant force zero
Resultant moment about any point zero
2
Q
State three vector quantities
A
Any 3 of the following:
Velocity
Acceleration
Force
Displacement
Weight
Momentum
3
Q
State three scalar quantities
A
Any 3 of the following:
Speed
Distance
Mass
Energy
Power
Temperature
4
Q
How can force vectors be arranged to show that an object has constant velocity?
A
- Vectors make a closed shape when rearranged (by scale drawing)
- Or resolve into components and show
- Total up forces = Total Down forces
- Total left forces = Total right forces
5
Q
What is the difference between a vector quantity and a scalar quantity?
A
Vector has a direction
Scalar does not
6
Q
What is meant by centre of mass?
A
The point in a body where the weight of the object appears to act
Also the resultant moment about this point = 0
7
Q
Define the moment of a force
A
Product of the force and the perpendicular distance from the line of action of the force to the point
8
Q
State the principle of moments
A
Sum of the clockwise moments about a point is equal to the sum of the anticlockwise moments for a system in equilibrium
9
Q
Resolve F into its vertical and horizontal components…
A
FH = FcosØ
Fv = FsinØ
10
Q
What mistake has been made in rearranging the vectors for a scale drawing?
A
6N vector has been translated (moved) but also rotated
Should be:
11
Q
What are the steps in working out the resultant force using a tip-tail scale drawing?
A
- Set a scale
- Draw the horizontal or vertical vector first (if there is one)
- Move each vector in turn to the end of the previous one (DO NOT ROTATE THE VECTORS)
- Resultant vector goes from the very start to the very end
12
Q
How is the balancing force different from the resultant force?
A
The balancing force brings the object into equilibrium so makes the resultant force = 0
For a scale drawing, it is the vector that closes the shape
13
Q
If vectors are parallel they can be resolved by…
A
Adding or subtracting the values
14
Q
If vectors are perpendicular they can be resolved by…
A
Making a right angled triangle and using trigonometry and pythagoras
15
Q
What is wrong with this?
A
Vectors of different types can’t be combined
(Here, force and velocity cannot be combined)
16
Q
How do you solve this if the object is in equilibrium?
(3 vectors with 2 unknown sizes)
A
- The vectors must form a closed shape
- Start as you would with a scale drawing
- But draw the third vector meeting for where it connects to the start of the first
- Draws vectors as dotted lines
(x=2.54N, y=3.89N)
17
Q
The box is in equilibrium with no external forces applied
Label the forces acting on the box
A
Notice the angle between weight and perpendicular is also Ø
18
Q
In projectile motion when is the vertical component of the velocity 0?
A
At the peak of a parabola
Not at the start or end
19
Q
How do you calculate the resultant moment? (2 ways)
A
- Multiply perpendicular component of force by distance
- Multiply perpendicular component of distance by force
(First method is shown)
20
Q
What’s wrong with this?
A
Weight must form the hypotenuse of the triangle
21
Q
What is a couple?
A
A pair of equal and opposite coplanar forces which do not act along the same line of action
22
Q
What does it mean if an object is uniform?
A
It has an constant density so its centre of mass acts from the physical centre point of the object
(Weight vector starts from middle of object)
23
Q
When should you use moments?
A
Any situation that has two unknown forces acting on an object
Take moments about one of the unknown forces to find the other
Then use total up force = total down force to find the other
24
Q
What is displacement and how is it different to distance?
A
Displacement is a measure of the line connecting the starting point to the finishing point.
Distance is a measure of the total length of the path travelled.
Also distance is a scalar and displacement is a vector.
25
What does a straight line on a distance-time graph represent?
A constant speed.
26
How is acceleration defined?
Acceleration is the rate of change of velocity.
27
How is speed different to velocity?
Speed is the rate of change of distance.
Velocity is the rate of change of displacement.
28
Describe the motion of this ball
Ball is moving to the right and speeding up.
29
Decribe the motion of this ball.
Ball is moving to the left and speeding up.
30
Describe the motion of this ball.
Ball is moving to the right and slowing down.
31
Describe the motion of this ball.
Ball is moving to the left but slowing down.
32
Is the ball moving to the right?
Only if the velocity vector is also acting to the right.
33
What does a straight line on a displacement-time graph represent?
A constant velocity.
34
What does a curve with an increasing gradient represent on a displacement-time graph?
An increasing velocity (acceleration)
35
What does a curve with a decreasing gradient represent on a displacement-time graph?
A decreasing velocity (decceleration)
36
What does a negative gradient on a displacement-time graph represent?
A negative velocity (travelling back to where it started)
37
What does a straight line on a velocity-time graph represent?
A constant acceleration.
38
What does a curve with an increasing gradient represent on a velocity-time graph?
An increasing acceleration.
39
What does a curve with a decreasing gradient represent on a velocity-time graph?
A decreasing acceleration.
40
What does a negative gradient on a velocity-time graph represent?
A negative acceleration.
41
What does this graph show?
A ball bouncing off a surface
(Dotted lines represent the bounce)
(Red lines represent the ball accelerating towards the ground)
42
What does the acceleration time graph of a ball in freefall look like?
Constant acceleration of 9.81ms-2
43
What does the area of a speed-time graph represent?
How about a velocity-time graph?
44
What's wrong with this?
Displacement takes direction into account.
It should be...
45
When can you use this equation?
When the **acceleration = 0 (constant velocity)**
Or to work out an **average speed**
46
When can you use SUVATs?
When **acceleration is constant**
Or if object has stages of constant acceleration
47
Why can't you use SUVAT's when working with this graph?
Because the **acceleration (gradient) is changing**
48
What does it mean if an object is in freefall?
Only **weight is acting on the object**
It has a constant **acceleration of 9.81ms-2** acting downawards (on Earth)
49
If one ball is dropped as another is projected horizontally which hits the ground first?
They both hit the ground at the same time...
Both in freefall so accelerate at 9.81ms-2
**Vertical motion independent of horizontal motion**
50
What's wrong with this labelling?
Initial velocity and final velocity are not 0
51
How do you start a question involving **angled projectile motion?**
Resolve the velocity into vertical and horizontal components and fill out the corresponding SUVATs
52
What is wrong here?
The acceleration is only 9.81ms-2 if the object is in freefall
53
If this box is in **equilibrium** how would you go about calculating the frictional force and the reaction force?
54
What is Newton's 1st Law of Motion?
If no resultant force acts on a body, then it will either remain at rest, or continue moving with constant velocity (no acceleration)
55
What is Newton's 2nd Law of Motion?
The rate of change of momentum (acceleration) of a body is directly proportional to the resultant force acting on it
**Fres ∝ a**
56
What is Newton's 3rd Law of Motion?
When two objects interact, they exert an equal and opposite force on each other and the forces are of the same type
57
If the **forces** acting on an object **are balanced** what can you say about its motion?
There is **no resultant force** so it will continue moving at a constant velocity. It won't accelerate.
58
What's wrong with this?
In F=ma, **F must be the resultant force!!!**
59
How does an object reach terminal velocity?
As it speeds up, air resistance increases, decreasing the resultant force.
Eventually air resistance = driving force, Fres=0 so a=0.
60
What two things are the case for tension?
1. Tension always acts away from the contact points
2. Tension is constant throughout the rope/wire/ cable
61
Why are objects never truly in freefall?
There will always be **air resistance** opposing the weight
(Apart from when v=0)
62
What is the condition for terminal velocity?
The drag force = driving force (or weight) so **Fres=0** and so **a=0**
63
What factors the drag force on an object?
* Fluid density
* Shape of object
* Cross sectional area of object
* Velocity of object
64
Why does air resistance increase with velocity?
The object is colliding with more air molecules **per second**
65
What does the velocity time graph of an object reaching terminal velocity look like?
66
What's wrong with this?
The **acceleration is not constant** so you **cannot use SUVATs**
Instead use area under graph
67
How is momentum calculated?
68
What's wrong with this?
Direction must be taken into account (as momentum is a vector)
69
What two things is impulse equal to?
1. Rate of change of momentum
2. Impact force x impact time
70
What are the units of impulse and momentum?
71
What does the area under a force-time graph represent?
The **change of momentum** or **impulse**
72
What is the conservation of momentum?
For a system of interacting objects, **the total momentum remains constant...**
**…**provided **no external resultant force acts**
73
In any interaction, what is conserved?
Total momentum is always conserved
Total energy is always conserved
**Kinetic energy is only conserved if collision is elastic**
74
What is an elastic collision?
A collision where kinetic energy is conserved
(as well as momentum)
75
What is wrong here?
You have to c**alculate kinetic energies separately** for each object
76
In physics terms what is an explosion?
The total momentum = 0
77
How do you answer flow rate questions? (momentum of a flowing liquid)
Consider the cylinder made by a liquid's flow after 1 second
Where the **length of the cylinder = velocity of the fluid**
And use density equation to get volume of cylinder
78
How do you work out the area of a curved graph?
1. Split into boxes
2. Count the boxes (pairing up incomplete boxes)
3. Multiply number of boxes by area of each box
79
What is the principle of conservation of energy?
Energy cannot be created or destroyed, only transferred from one type to another.
80
How does an object gain energy from a force?
When the **force does work on the object (same direction as movement)**
81
How does an object lose energy?
By **doing work against a force** (usually frictional)
82
What's wrong here?
In W=Fs you **multiply the parallel components**
83
When using **W=Fs** what must be the case?
The **components must be parallel**
84
What is the **work done by the weight** of this block?
0 because the weight is **perpendicular to the movement**
So there is **no parallel component to displacement**
85
If two objects are **dropped from the same height** and air resistance is negligible, which hits the ground first?
Both hit at the same time because they both accelerate at 9.81ms-2
86
How do you calculate the velocity the object hits the ground using GPE and KE (assuming no air resistance)
Energy equivalency (only works when air resistance = 0)
87
What is power?
The rate of transfer of energy
or
The rate at which work is done
(Measured in **Watts [W]**)
88
What is wrong here?
For P=Fv, **F is not the resultant force**
89
How do you calculate efficiency?
90
If the system **is not 100% efficient** would this be correct?
**No,** because some GPE is converted to thermal and kinetic energy of the snow (working against friction)
91
How is density defined?
The mass per unit volume. [kgm-3]
92
How do you convert 2,3,4 etc… units to SI units?
(eg 5cm3 to m3)
Whatever you do to the unit, you **do the same to the prefix**
(eg 5cm3 = 5x(10-2)3m3 = 5x10-6m3)
93
How do you measure the density of an irregular solid?
1. Read off the volume from the **beaker or measuring cylinder** without and with the object submerged in water
2. The difference in volumes is the volume of the solid
3. Measure the mass **using a balance**
Calculate density using ρ=M/V
94
How do you calculate the average density of an alloy?
(Eg 200cm3 5kg rod of 60% copper (8960kgm-3) and 40% aluminum (2700kgm-3) by volume?)
1. Work out the mass of each and the volume of each
2. Add together to get the total mass and volume
3. Then do the density calculation
95
Define Hooke's Law
When a material is stretched, its extension is proportional to the force applied, up **until the limit of proportionality**
F=kx
96
Define the limit of proportionality
The point at which the material stops obeying Hooke's law.
The graph is no longer a straight line.
97
Define the elastic limit
The point at which when stretched further the material no longer returns to its original length (there is a permanent extension)
98
What do the gradient and area of a force extension graph tell you for a spring.
**Gradient** → The **spring constant** (must be taken before limit of proportionality)
**Area under line →** The strain energy stored loading the spring or energy released unloading the spring
99
What equation **calculates** **energy stored** when a material is **stretched**?
E=½Fx
100
What is the difference between the **elastic limit** and the **limit of proportionality**?
Limit of proportionality is the point at which a stretched spring (or wire) stops obeying Hooke's law.
The elastic limit is the point at which it doesn't return to its original length when unloaded.
101
Will this spring return to its original length if it has been stretched to 35mm?
Yes, because it has **not passed the elastic limit**
102
What is a **ductile** material?
A material with a **large plastic region**.
103
What is a **brittle** material?
A material with a small plastic region.
104
What is the **fracture point** of a material?
The point at which a material breaks
105
How do you know the rubber hasn't stretched passed its elastic limit?
It still returns to its original length when unloaded.
106
What is the formula for **Young's Modulus** that you need to remember?
107
What does the **gradient** and **area** under a **stress-strain** graph give?
**Gradient** → **Young's modulus** (before the limit of proportionality)
**Area** → strain **energy per unit volume**
108
What is a progressive wave?
Oscillations that have a **resultant transfer of energy** in one direction
109
How are mechanical and electromagnetic waves different?
**Mechanical waves require a medium** to oscillate through
Electromagnetic waves don't require matter (oscillate through electric and magnetic fields)
110
What makes a wave transverse?
Oscillations are **perpendicular** to the transfer of energy
111
What makes a wave longitudinal?
Oscillations are **parallel** to the transfer of energy
112
What 2 properties do all electromagnetic waves possess?
1. Always **transverse**
2. Propagate with velocity of **3×108ms-1 through vacuum**
113
Name 3 longitudinal waves
* Sound
* P-waves (Earthquakes)
* Water waves (beneath surface)
114
Name 3 transverse waves
* E-M waves (Light, X-rays, UV etc)
* Waves on string
* P-waves (Earthquakes)
* Water waves (surface)
115
List in order all waves on the E-M spectrum
116
How are displacement and amplitude of a wave different?
Displacement → **Current distance** of a point from the equilibrium position
Amplitude → **Maximum distance** a point reaches from equilibrium position
117
Why do all points on a progressive wave have the same amplitude?
All points have the **same maximum displacement** from equilibrium position
118
What is the time period of a wave?
Time taken for each particle to **complete one full oscillation**
(Return to same position)
119
How is frequency of a wave defined?
The number of complete **oscillations per second**
120
What is the wavelength of a wave?
Distance between two adjacent corresponding points on a wave
(Same displacement, no phase difference)
121
What is the phase difference between A and B on this progressive wave?
**360° ∼ 0°**
**2****π****∼ 0****π**
122
What is the phase difference between A and B on this progressive wave?
**180°**
**π** **∼ Antiphase**
123
What is the phase difference between points A and B on this progressive wave?
**540° ∼ 180°**
**3π** **∼** **π** **∼ antiphase**
124
How is phase difference calculated in degrees?
125
How is phase difference calculated in radians?
126
How do you convert from degrees → radians?
127
What is the phase difference between A and B on this progressive wave?
**420° ∼ 60°**
**14π/6** **∼** **π/3**
128
How are frequency and wavelength related?
129
What are the 2 key features of longitudinal waves?
**Compressions** and **rarefactions**
130
Why can't sound waves be polarised?
**Only transverse waves can be polarised**
(Sound is longitudinal)
131
What is the final intensity?
1. Light vertically polarised through first grating
2. Vertically p[olarised light can't pass through horizontal grating
3. **Final intensity = 0**
132
What is the final intensity?
1. Light vertically polarised through first grating (intensity halves)
2. Vertically polarised light passes through second grating
3. **Final intensity = ½**
133
How do sunglasses reduce glare?
1. When **sunlight reflects** off surfaces it is **polarised**
2. Sunglasses have filter to **block polarised light**
3. Only unpolarised light passes through
134
What is the refractive index of a material?
Ratio of speed of light in a vacuum : speed light passes through material
(The greater n \> the more light slows down)
135
How does θ2 compare to θ1?
θ2 \> θ1
(Light speeds up and **bends away from normal**)
136
How does θ2 compare to θ1?
θ2 \< θ1
(Light speeds up and **bends towards normal**)
137
Is the light refracting here?
**Yes**
It hasn't bent towards or away from normal
**But it has slowed down**
138
How does refraction affect the frequency of a wave?
**Frequency does not change**
(But wavespeed and wavelength do)
139
What is dispersion?
Different wavelength refract by different amounts
So light passing through a prism separates into wavelengths
140
What is wrong here?
In Snell's law **θ1 is the angle between normal and incident ray**
141
What are the 2 conditions for total internal reflection?
1. **θ1 \> θc**
2. **n2 \< n1**
142
How is the critical angle calculated?
143
How do you calculate the angle of incidence in the fibre?
Using basic geometry (angles in triangle add to 180°)
144
Why are optical fibres better than copper cables?
1. Information transmission faster
2. More information can be transmitted
3. Less energy loss (copper heats up)
145
In optical fibres what does cladding do?
1. Protects the core from scratches and spills
2. Stops data loss to adjacent fibres
3. Increases critical angle (reducing modal dispersion)
146
What is modal dispersion and how is it combatted?
Different modes (angles) take different amount of time to propagate through an optical fibre
Leads to **pulse broadening**
Combatted by **making core narrow** and using cladding with low n **(increasing** **θc)**
147
What is **spectral (material) dispersion** and how is it combatted?
**Different wavelengths** (colours) of light **refracted by different** amounts
Leads to **pulse broadening**
**Combatted using monochromatic light**
148
How do these two progressive waves interact when they overlap?
Form a **superposition**
Displacements combined (added or subtracted) at each point
149
What happens when these two pulses overlap?
**Constructive interference** (Displacements combine)
150
What happens when these two pulses overlap?
**Destructive interference** (Displacements cancel)
151
How does a stationary wave form?
1. Progressive wave reflects off a fixed point
2. Two progressive waves propagating in opposite directions (with same c,f,λ,A)
3. Waves overlap and interfere forming superposition
152
On a stationary wave how are nodes and antinodes different?
Nodes → Points of 0 amplitude
Antinodes → Points of maximum amplitude
153
How are progressive waves different from stationary waves?
* All points on a progressive wave have same amplitude (Stationary waves have range)
* Progressive waves resultant energy transfer (Stationary waves have 0 resulatant)
154
How is the wavelength of a stationary wave calculated?
**Each loop = ½****λ**
155
How is the frequency of the nth harmonic of a stationary wave calculated?
1. Calculate the frequency of the 1st harmonic
2. Multiply f1 by n
156
On this stationary wave why do points A and B have different amplitudes?
A and B have **different maximum displacements**
157
On this stationary wave what is the phase difference between A,B,C and D
0° → All points on same side of equilibrium are in phase
158
On this stationary wave what is the phase difference between A,B,C and D
A and B → 180° → All points on oppsoite side of equilibrium are in anti-phase
C and D → 180°
A and C → 0° → All points on same side of equilibrium are in phase
B and D → 0°
159
How can the frequency of the first harmonic on this string be decreased?
* Decrease tension (reduce mass)
* Increase distance between end points
* Use string with greater density (greater μ)
160
What 2 conditions are required to produce an interference pattern?
1. Sources must be **coherent** (same frequency, constant phase difference)
2. Sources must be **monochromatic** (one wavelength)
161
When will two sources interfere constructively?
When their **path difference = n****λ**
So **phase difference = 0°**
**Maxima** forms
162
When will two sources interfere destructively?
When their **path difference = (n+½)****λ**
So **phase difference = 180° (∏ rad or antiphase)**
**Minima** forms
163
When does maximum diffraction occur?
When the wavelength is close to the size of the gap the wave passes through
164
What does the interference pattern of the single slit look like?
## Footnote
**Large central maxima**
**Intensity decreases exponentially**
**Each maxima has half width of central**
165
For the single slit how is the central maxima width affected by λ?
W ∝ λ
166
For the single slit how is the central maxima width affected by the gap size?
W ∝ 1/a
167
For the double slit, how can you increase the widths of the maximas?
1. Increase λ
2. Increase slit to screen distance D
3. Decrease slit separation s
168
How does the intensity graph look for the double slit interference pattern?
## Footnote
**Intensity decreases linearly**
**Width of maximas constant**
169
How is the 1st maxima formed for the diffraction grating
between adjacent slits **Path difference = 1****λ**
So **phase difference = 0°**
170
How is the 3rd maxima formed for the diffraction grating?
between adjacent slits **Path difference = 3****λ**
So **phase difference = 0°**
171
How do you calculate the slit separation for a diffraction grating?
172
How do you calculate the maximum number of observed maximas for the diffraction grating?
**nmax = d/****λ**
**Round Down!!!**
173
Define current (I)
The **rate of flow of charge**
174
How do you work out the number of electrons carrying a charge (eg 10C)?
Divide charge by the charge of each electron (1.6x10-19)
175
What is the difference between **conventional current** and **electron flow**?
**Conventional current** flows from the +ve terminal to the -ve terminal
**Electron flow** shows the direction the electrons flow, from -ve to +ve
176
How is the current in a circuit related to potential difference and resistance?
Increasing potential difference increases the current
Increasing resistance decreases the current
177
What is Ohm's law?
The current flowing through a metallic conductor is proportional to the potential difference applied across it **at constant temperature**
178
When does Ohm's law apply?
When the **component has a fixed resistance** (eg a fixed resistor at a constant temperature, or a filament at a low current)
179
Define **potential difference**
The **work done** (energy transferred) **by** **each coulomb of charge** moving between two points
(Eg a 12V battery adds 12J of energy to each coulomb of charge passing through)
180
How does a circuit ‘short circuit’?
If there is an available path with **0 resistance**
**Current → ∞**
And the circuit **heats up**
181
What is the I-V graph for a fixed resistor?
182
What is the I-V graph for a filament bulb?
183
What is the graph for a semiconductor diode?
184
What's wrong with this?
Resistance is **not calculated using the gradient (**of a tangent**) of an I-V graph!!!**
Instead just **use the voltage and current at that point**
185
Explain the shape of the I-V graph for a filament
As current increases, temperature of filament increases
This **increases lattice ion vibrations**.
Which increases the number of **collisions per second with electrons.**
So **resistance increases.**
186
How does the I-V graph for a fixed resistor prove it is ohmic?
The **straight line passing through the origin**
**proves that current ∝ voltage**
187
Explain the shape of the semiconductor diode (in positive bias)
* As the potential difference increases **weakly bound electrons** in the conductor gain energy
* After the threshold pd, some **electrons become free** to carry a current
* The **lattice vibrations** still increase but this is **less significant**
188
What happens if a semiconductor diode is connected in reverse bias?
No current flows until the breakdown voltage is reached (**~**50V)
The diode breaks and all current flows through
189
What is the difference between a **series** and a **parallel** circuit?
**Parallel circuits have junctions** (3 or more wires connect)
190
Why doesn't adding voltmeters in parallel affect the circuit? (it is still series)
Voltmeters have ~ **∞ R so no current flows through**
191
What are the p.d and current rules for a series circuit?
**P.D is shared** across the components (by resistance)
**Current is constant** throughout
192
What are the p.d and current rules for a series circuit?
**P.D is shared** across the components (by resistance)
**Current is constant** throughout
193
What are the p.d and current rules for a **parallel** circuit?
**P.D is same for parallel branches**
**Current separates at junctions** (according to branch resistance)
194
What is Kirchoff's 1st Law?
At any junction in a circuit the **sum of the current flowing into the junction** is **equal** to the **sum of the current flowing away from it.**
195
What is Kirchoff's 2nd Law?
In any complete “loop” of a circuit the sum of p.d’s equals the source p.d.
196
How do you combine **series resistors in the same branch?** (no junction between them)
Add up their resistances
197
How do you combine resistors in parallel branches? (one junction between them)
Use the following equation…
198
What is the advantage of placing resistors in parallel arrangements?
The **total resistance is always less** than the smallest resistance
199
Will the current split equally?
No, because the **resistance of each branch is different**
200
Will each component receive the same voltage?
No, because the **resistance of the components are different**
201
Why would you place **batteries in parallel**?
* The power delivered is the same
* But they take longer to run flatter
202
What is a potential divider circuit?
A circuit with **2 or more resistors connected in series** with a power supply. (usually one is a thermistor or LDR)
203
How does resistance change for an **NTC Thermistor?**
As **temperature increases, resistance decreases**
204
How does resistance change for a **Light Dependent Resistor (LDR)?**
As **light intensity increases, resistance decreases**
205
What is the advantage of setting up a **rheostat** as a **variable resistor?**
* Simpler circuit
* Current constant throughout
* But cannot get 0V across bulb
206
What is the advantage of setting up a **rheostat** as a **potential divider?**
* Bulb can receive full range of voltage 0V → Vsource
* Current through bulb can be reduced to 0A
* But maximum current is lower
207
How does changing the **dimensions** of a piece of metal **affect its resistance?**
* Increased length → increased resistance
* Increases cross sectional area → decreased resistance
* Increased resistivity (using different material) → increased resistance
208
How do you calculate the **cross sectional area of a wire?**
Assume it to be a cylinder (unless told otherwise)
**A=∏r2**
209
Why do metals with a greater cross sectional area have a lower resistance?
There are **more paths for the electrons to propagate**
210
How do you calculate the potential difference across branches?
* Work out the P.D of each component
* Make a loop connecting the branches
* Subtract the PDs of one branch from the other
211
What is a superconductor?
A material with **0 resistance at and below the critical temperature**
212
Why does a material become **superconducting at and below its critical temperature?**
* The **lattice ion vibrations reduce to 0**
* So **electrons can pass** through **without collision**
213
What is the advantage of superconductors and name a use?
* Transmit large currents with 0 resistance
* So negligible thermal energy losses
* Used to create high power magnets → MRI machines
* High processing power circuits → Supercomputers
214
Define **emf** of a power source
The **potential difference across the terminals** when **no current** is flowing through
215
Define **terminal potential difference** of a circuit
The **potential difference** across the terminals **when a current** is flowing through
216
What is the **lost voltage** in a circuit?
The potential difference used up pushing a current through the battery (**vlost = emf - TPD**)
217
How should you work with a circuit **involving internal resistance?**
Treat the internal resistance as another resistor in series with the components
Then solve as a regular circuit (using ohm's law, kirchoff's laws, P=IV etc)
218
What is the **photoelectric effect?**
Light incident on a **metal surface** causes **electrons to be emitted from the surface**
219
Why are electrons emitted from this surface by shining green and blue light on it? (not red)
Blue and green light are **above the threshold frequency of this metal**
So the **photons of light have an energy \> work function (****φ****)**
220
Why are no electrons emitted when red light shines on this metal?
The red light **photons are below the threshold frequency**
So the **energy of each photon \< work function (****φ****)**
221
Why does making the **red light brighter not cause electrons to be emitted? (Photoelectric effect)**
**Electrons** in the metal interact with photons in a **1-1 interaction**
They only absorb photons which have an **energy \> work function (****φ****)**
222
1. Why do both light source cause electrons to be emitted? (from the surface)
2. What is different about the electrons emitted due to the blue light?
1. Both light sources have **frequency above the threshold frequency (f0)** of the metal
2. The electrons emitted due to the blue light have a **greater maximum kinetic energy** (because blue photons have a greater energy from E=hf)
223
What does **threshold frequency (f0)** of a metal mean?
The **minimum frequency of the incident light** needed to cause electrons to be emitted from the surface
224
1. What can you say about the green light incident on this metal?
2. What difference does the brighter lamp make?
1. The green light is above the threshold frequency so the photelectric effect happens
2. The brighter lamp causes **more photons of light to collide with electrons** so **more photons are emitted per second** (But the electrons have the same maximum kinetic energy)
225
You are shining a light (above f0) on a metal. How do you:
1. **Increase** the maximum **kinetic energy** of the emitted electrons?
2. **Increase** the **number** of **emitted** electrons per second?
1. **Increase the frequency** of the light source
2. **Increase the brightness** of the light source
226
This is a graph for the photelectric effect. What information do the 3 features of the graph provide?
1. Y-intercept
2. X-intercept
3. Gradient
1. Y-intercept = - work function
2. X-intercept = threshold frequency
3. Plancks' Constant
227
This is the photoelectric effect graph for a metal
Plot a line on this graph for a metal with a higher threshold frequency
1. Y-intercept (φ) decreases
2. X-intercept (f0) increases
3. **But the gradient (h) is constant**
228
If you shine a really bright light on a metal but the **light is below the threshold frequency** why will electrons never be emitted?
Electrons interact with the photons in a **1-1 interaction**
But only if the photon has an energy \> work function
**No red light photons have an energy \> work function**
So electron **emission will never occur**
229
What is the definition of the work function (φ) of a metal?
The **minimum energy** required to **liberate an electron from the surface of a metal**
230
How is the **work function (φ) related to the threshold frequency (f0)** of a metal?
231
When light (above f0) is incident on a metal surface how is the maximum kinetic energy of emitted electrons calculated?
Difference between the energy of each photon and the work function (φ)
232
For the gold leaf experiment (to show the photoelectric effect):
1. How do you make the gold leaf rise?
2. Why does the gold leaf fall?
1. A charged rod transfers additional electrons to the plate causing repulsion between the stem and gold leaf
2. Electrons are liberated from the metal surface (by light above f0) so the stem and leaf become neutrally charged again
233
Define the **electron volt**
The **kinetic energy gained by 1 electron passing through a potential difference of 1 volt**
234
How do you **convert between electron volts and Joules?**
**eV → J : multiply by 1.6x10-19J**
**J → divide by 1.6x10-19J**
235
**How is the maximum kinetic energy of photoelectrons** (emitted during the photoelectric effect) **measured?**
1. Connect the system to a circuit
2. Place a battery opposing the current produced by the emitted electrons
3. Measure the **stopping potential** when the total current = 0
4. **Ekmax = eVs**
236
During the Photoelectric effect **why are electrons with a range of kinetic energies emitted?**
**Electrons deeper down require more energy to rise to the surface** before being liberated
(Electrons at the very top of the surface are emitted with maximum kinetic energy)
237
What are the 3 types of line spectra and how are they produced?
1. Continuous - Produced by **blackbody**
2. Emission - Produced by **an excited gas**
3. Absorption - Produced by a **continuous spectrum passing through a cold gas**
238
What are the key ideas of the **Bohr model of the atom?**
* Electrons can only travel in **allowed orbitals (energy levels)**
* Electrons can emit or absorb energies to **instantaneously transition between orbitals**
* Electrons **cannot exist between orbitals**
239
How could an electron excite from the n=2 → n=4 energy level?
It must **absorb an energy = the difference between levels** (By photon or electron collision)
240
How could an electron de-excite from n=3 → n=1 energy level?
It must **emit an photon of energy = the difference between levels**
241
How is the energy of a photon calculated?
242
Why do different gases (made of **different elements) have different emission spectra?**
1. Each element has a different set of orbitals (with different energy levels)
2. So each element has a different set of electron de-excitation energies
3. The different de-excitation energies produce photons with different frequencies (E=hf)
243
How would you show the 488nm hydrogen emission line corresponds to a de-excitation from n=4 → n=2?
1. Calculate then energy difference between the energy levels
2. Convert energy difference to Joules
3. Convert to f or λ (E=hf or E=hc/λ)
244
What is the ionisation energy of an atom?
The energy required for an electron to to become liberated from an atom
## Footnote
**Equal to the energy of the ground state**
245
What is wrong with this?
**Never use work function when talking about energy levels**. Ionisation and work function are different.
246
How is excitation by photon different from excitation by an electron?
* Photon energy = Difference between energy levels
* Electron energy ≥ Difference between energy levels
247
How many photons (of different wavelengths) can be emitted from this hydrogen atom?
6 possible transition so 6 different photons
248
Why is this mercury vapor in the fluorescent tubes kept at low pressure?
So a large enough current (of incident electrons) can be sustained
249
How does fluorescence work in a tube light?
1. Mercury atoms excite by absorbing electrons from the current
2. When the Mercury atoms de-excite they emit UV photons
3. UV photons are absorbed by and excite the phosphor coating
4. When the phosphor coating de-excites it emits visible light
250
When do particles exhibit properties of waves? (refraction, diffraction and polarisation)
When their **Debroglie Wavelength is similar to the size of the gap** they are passing through
251
What does this experiment show?
Wave-Particle duality
Electron diffraction through graphite to form maximas (bright rings) and minimas (dark rings)
252
How is the Debroglie wavelength λdb of a particle calculated?
253
Which part of the atom has the **largest specific charge** and why?
**The electron**
(It has the same magnitude of charge as the proton but a much smaller mass)
254
Why do the proton, neutron and electron deflect differently in a magnetic field?
Neutron → 0 specific charge so zero deflection
Electron → Greatest specific charge so greatest deflection
Proton → Smaller deflection in opposite direction as specific charge smaller and opposite
255
How do you calculate the **specific charge of a nucleus?**
Divide the total charge of the protons by the total mass of nucleus
(Protons + Neutrons)
256
How do you calculate the **specific charge of an ion?**
Charge of the ion (Protons - Electrons) divided by total mass of ion
257
What is an isotope?
An atom with the
1. **same number of protons**
2. **Different number of neutrons**
258
When will an isotope undergo radioactive decay?
If the nucleus has:
1. too many of too few protons
2. Too many nucleons
3. Too much vibrational energy
259
What happens in **alpha (****α****) decay?**
A nucleus ejects a helium nucleus (2 protons and 2 neutrons)
Decreasing its nucleon number by 4
And its proton number by 2
260
What happens in **Beta Minus (****β-****) Decay?**
A **neutron turns into a proton**
**Ejecting a fast moving electron (****β-****) and an anti-electron neutrino**
261
What happens in **Beta Plus (****β+****) Decay?**
A **proton turns into a neutron**
**Ejecting a fast moving positron (****β+****) and an electron neutrino**
262
What is wrong about this Beta Decay equation?
The **nucleon number must not change**
263
Why do the α, β-, β+ and γ deflect differently in a magnetic field?
α and β+ → Deflect in same direction but β+ larger (greater specific charge)
β- → Equal and opposite deflection to β+ (Equal and opposite specific charge)
γ → No deflection (no specific charge)
264
What is an antiparticle?
A particle with the:
1. **Same mass**
2. **But equal and opposite charge**
265
What happens during Annihilation?
A **particle collides** and annihilates with its correspond **antiparticle**
And their **mass energy (E=mc2) is converted to radiation energy**
**Producing at least 2 gamma photons**
266
Why do **at least 2 photons need to be created during annihilation?**
To conserve momentum
Before annihilation ptotal = 0
AFter annihilation **ptotal = 0 (can't be achieved with one photon)**
267
What happens during **pair production?**
A gamma photon (with energy ≥ 2 **×** mass energy)
spontaneously creates a particle, anti-particle pair
268
What condition must pair production meet?
The energy of the gamma photon ≥ Mass energy of the particle anti-particle pair
(Any excess energy is used a kinetic energy for the particles produced)
269
How was the anti-electron neutrino discovered?
During Beta decay the emitted β- had less energy than expected so another particle carried the rest of the energy
270
What are the **four fundamental forces** and their approximate ranges?
1. Strong
2. Weak
3. Electromagnetic
4. Gravitational
271
What does the strong force do?
What is the exchange particle of the strong force?
Holds nucleons together in the nucleus
* By opposing the electromagnetic repulsion of the protons
* By attracting nucleons at small distances but repelling the, at very small distances
Gluons (between quarks), or pions (between hadrons)
272
Describe the nature of the strong force
**Very repulsive over short distance (0-0.5fm)**
**Attractive over larger distances (3fm \> d \> 0.5fm)**
**Negligible beyond 3fm**
273
What does the electromagnetic force act between and what is its exchange particle?
Acts between all **particles with charge**
## Footnote
**Exchange particle is the photon**
274
What does the **gravitational force** act between?
Particles or objects **with mass**
275
What particles does the weak force act on and what does it do?
Acts between leptons and hadrons and causes the decay of hadrons (by changing quark structure)
276
What are **fundamental particles that make up the standard model?** (that you need to know)
NOTE: Each of the leptons and and quarks has an corresponding anti-lepton and anti-quark
277
What is the quark structure of a proton?
Up, Up, Down
278
What is the quark structure of a neutron?
Up, down, down
279
How is a muon different from an electron?
Both are leptons, muon is much heavier than the electron, produced in cosmic ray showers
280
What are hadrons?
**Particles that are made up of quarks**
281
How are baryons and mesons different?
Both are hadrons (made up of quarks)
But **Baryons are made up of 3 quarks**
And **Mesons are made up of 1 quark 1 anti-quark**
282
What are the similarities and differences between **W bosons and photons?**
**Both are exchange particles**
But **W bosons mediate the weak force, Photons mediate electromagnetic**
W bosons carry charge of +1 or -1, Photons have no charge
W bosons have mass, Photons are massless
283
What are the similarities and differences between gluons and pions?
**Both mediate the strong force**
**But gluons act between quarks, Pions act between hadrons** (to keep the nucleus together)
Gluons have no mass, Pions have mass
284
What does the **Higgs Boson do?**
It **creates the Higgs field**
Which **gives mass to particles**
285
What quantities are **always conserved in every interaction?**
* **Total momentum**
* **Total energy**
* **Charge**
* **Baryon**
* **Lepton number**
NOTE 1: **Kinetic energy** is conserved in **elastic collisions**
NOTE 2: **Strangeness** is conserved in all interactions **apart from weak**
286
What must you know about k-mesons? (kaons)
They are made of **1 quark and 1 anti-quark** (mesons)
## Footnote
**They have non-zero strangeness**
**Produced by strong interactions, Decay (into pions) by weak interactions**
287
What must you know about π-mesons? (pions)
They are **made up of 1 quark and 1 anti-quark (mesons)**
## Footnote
**They have strangeness = 0**
288
What is the most stable lepton and what is the most stable hadron?
(That other isolated particle will eventually decay into)
The **electron** and the **proton**
289
Why can't this muon decay happen like this?
(What's the mistake with the logic in the table?)
When electron and muon type particles are involve **each lepton number must be considered separately**
290
What is the formula for a muon decaying into an electron?
291
What is the feynman diagram for an electron-electron collision?
292
What is the feynman diagram for **β- Decay?**
293
What is the feynman diagram for **β+ Decay?**
294
What is the quark feynman diagram for **β- Decay?**
295
What is the quark feynman diagram for **β+ Decay?**
296
Identify the unknown particles in this feynman diagram for electron capture
297
Identify the unknown particle in this feynman diagram for the electron proton collision
298
Identify the unknown quark in the feynman diagram for electron capture
299
Identify the unknown exchange particle in the quark feynman diagram of electron proton collision
300
Which particles have a baryon number = +1?
Which have a B = -1?
Which have a B = 0
**Baryons = +1**
**Anti-Baryons = -1**
**All other particles (including mesons) = 0**
301
Which particles have a Lepton number = +1?
Which have a L = -1?
Which have a L = 0
**Leptons = +1**
**Anti-Leptons = -1**
**All other particles = 0**
302
What is the muon lepton number of an electron?
0! Only muons and muon neutrinos have Lmuon = +1
303
What is the electron lepton number of a muon?
0! Only electrons and electron neutrinos have Lelectron = +1
