Mega Deck Flashcards
State two conditions for any object to be in equilibrium
Resultant force zero
Resultant moment about any point zero
State three vector quantities
Any 3 of the following:
Velocity
Acceleration
Force
Displacement
Weight
Momentum
State three scalar quantities
Any 3 of the following:
Speed
Distance
Mass
Energy
Power
Temperature
How can force vectors be arranged to show that an object has constant velocity?
- 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
What is the difference between a vector quantity and a scalar quantity?
Vector has a direction
Scalar does not
What is meant by centre of mass?
The point in a body where the weight of the object appears to act
Also the resultant moment about this point = 0
Define the moment of a force
Product of the force and the perpendicular distance from the line of action of the force to the point
State the principle of moments
Sum of the clockwise moments about a point is equal to the sum of the anticlockwise moments for a system in equilibrium
Resolve F into its vertical and horizontal components…
FH = FcosØ
Fv = FsinØ
What mistake has been made in rearranging the vectors for a scale drawing?
6N vector has been translated (moved) but also rotated
Should be:
What are the steps in working out the resultant force using a tip-tail scale drawing?
- 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
How is the balancing force different from the resultant force?
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
If vectors are parallel they can be resolved by…
Adding or subtracting the values
If vectors are perpendicular they can be resolved by…
Making a right angled triangle and using trigonometry and pythagoras
What is wrong with this?
Vectors of different types can’t be combined
(Here, force and velocity cannot be combined)
How do you solve this if the object is in equilibrium?
(3 vectors with 2 unknown sizes)
- 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)
The box is in equilibrium with no external forces applied
Label the forces acting on the box
Notice the angle between weight and perpendicular is also Ø
In projectile motion when is the vertical component of the velocity 0?
At the peak of a parabola
Not at the start or end
How do you calculate the resultant moment? (2 ways)
- Multiply perpendicular component of force by distance
- Multiply perpendicular component of distance by force
(First method is shown)
What’s wrong with this?
Weight must form the hypotenuse of the triangle
What is a couple?
A pair of equal and opposite coplanar forces which do not act along the same line of action
What does it mean if an object is uniform?
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)
When should you use moments?
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
What is displacement and how is it different to distance?
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.
What does a straight line on a distance-time graph represent?
A constant speed.
How is acceleration defined?
Acceleration is the rate of change of velocity.
How is speed different to velocity?
Speed is the rate of change of distance.
Velocity is the rate of change of displacement.
Describe the motion of this ball
Ball is moving to the right and speeding up.
Decribe the motion of this ball.
Ball is moving to the left and speeding up.
Describe the motion of this ball.
Ball is moving to the right and slowing down.
Describe the motion of this ball.
Ball is moving to the left but slowing down.
Is the ball moving to the right?
Only if the velocity vector is also acting to the right.
What does a straight line on a displacement-time graph represent?
A constant velocity.
What does a curve with an increasing gradient represent on a displacement-time graph?
An increasing velocity (acceleration)
What does a curve with a decreasing gradient represent on a displacement-time graph?
A decreasing velocity (decceleration)
What does a negative gradient on a displacement-time graph represent?
A negative velocity (travelling back to where it started)
What does a straight line on a velocity-time graph represent?
A constant acceleration.
What does a curve with an increasing gradient represent on a velocity-time graph?
An increasing acceleration.
What does a curve with a decreasing gradient represent on a velocity-time graph?
A decreasing acceleration.
What does a negative gradient on a velocity-time graph represent?
A negative acceleration.
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)
What does the acceleration time graph of a ball in freefall look like?
Constant acceleration of 9.81ms-2
What does the area of a speed-time graph represent?
How about a velocity-time graph?
What’s wrong with this?
Displacement takes direction into account.
It should be…
When can you use this equation?
When the acceleration = 0 (constant velocity)
Or to work out an average speed
When can you use SUVATs?
When acceleration is constant
Or if object has stages of constant acceleration
Why can’t you use SUVAT’s when working with this graph?
Because the acceleration (gradient) is changing
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)
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
What’s wrong with this labelling?
Initial velocity and final velocity are not 0
How do you start a question involving angled projectile motion?
Resolve the velocity into vertical and horizontal components and fill out the corresponding SUVATs
What is wrong here?
The acceleration is only 9.81ms-2 if the object is in freefall
If this box is in equilibrium how would you go about calculating the frictional force and the reaction force?
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)
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
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
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.
What’s wrong with this?
In F=ma, F must be the resultant force!!!
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.
What two things are the case for tension?
- Tension always acts away from the contact points
- Tension is constant throughout the rope/wire/ cable
Why are objects never truly in freefall?
There will always be air resistance opposing the weight
(Apart from when v=0)
What is the condition for terminal velocity?
The drag force = driving force (or weight) so Fres=0 and so a=0
What factors the drag force on an object?
- Fluid density
- Shape of object
- Cross sectional area of object
- Velocity of object
Why does air resistance increase with velocity?
The object is colliding with more air molecules per second
What does the velocity time graph of an object reaching terminal velocity look like?
What’s wrong with this?
The acceleration is not constant so you cannot use SUVATs
Instead use area under graph
How is momentum calculated?
What’s wrong with this?
Direction must be taken into account (as momentum is a vector)
What two things is impulse equal to?
- Rate of change of momentum
- Impact force x impact time
What are the units of impulse and momentum?
What does the area under a force-time graph represent?
The change of momentum or impulse
What is the conservation of momentum?
For a system of interacting objects, the total momentum remains constant…
…provided no external resultant force acts
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
What is an elastic collision?
A collision where kinetic energy is conserved
(as well as momentum)
What is wrong here?
You have to calculate kinetic energies separately for each object
In physics terms what is an explosion?
The total momentum = 0
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
How do you work out the area of a curved graph?
- Split into boxes
- Count the boxes (pairing up incomplete boxes)
- Multiply number of boxes by area of each box
What is the principle of conservation of energy?
Energy cannot be created or destroyed, only transferred from one type to another.
How does an object gain energy from a force?
When the force does work on the object (same direction as movement)
How does an object lose energy?
By doing work against a force (usually frictional)
What’s wrong here?
In W=Fs you multiply the parallel components
When using W=Fs what must be the case?
The components must be parallel
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
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
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)
What is power?
The rate of transfer of energy
or
The rate at which work is done
(Measured in Watts [W])
What is wrong here?
For P=Fv, F is not the resultant force
How do you calculate efficiency?
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)
How is density defined?
The mass per unit volume. [kgm-3]
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)
How do you measure the density of an irregular solid?
- Read off the volume from the beaker or measuring cylinder without and with the object submerged in water
- The difference in volumes is the volume of the solid
- Measure the mass using a balance
Calculate density using ρ=M/V
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?)
- Work out the mass of each and the volume of each
- Add together to get the total mass and volume
- Then do the density calculation
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
Define the limit of proportionality
The point at which the material stops obeying Hooke’s law.
The graph is no longer a straight line.
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)
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
What equation calculates energy stored when a material is stretched?
E=½Fx
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.
Will this spring return to its original length if it has been stretched to 35mm?
Yes, because it has not passed the elastic limit
What is a ductile material?
A material with a large plastic region.
What is a brittle material?
A material with a small plastic region.
What is the fracture point of a material?
The point at which a material breaks
How do you know the rubber hasn’t stretched passed its elastic limit?
It still returns to its original length when unloaded.
What is the formula for Young’s Modulus that you need to remember?
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
What is a progressive wave?
Oscillations that have a resultant transfer of energy in one direction
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)
What makes a wave transverse?
Oscillations are perpendicular to the transfer of energy
What makes a wave longitudinal?
Oscillations are parallel to the transfer of energy
What 2 properties do all electromagnetic waves possess?
- Always transverse
- Propagate with velocity of 3×108ms-1 through vacuum
Name 3 longitudinal waves
- Sound
- P-waves (Earthquakes)
- Water waves (beneath surface)
Name 3 transverse waves
- E-M waves (Light, X-rays, UV etc)
- Waves on string
- P-waves (Earthquakes)
- Water waves (surface)
List in order all waves on the E-M spectrum
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
Why do all points on a progressive wave have the same amplitude?
All points have the same maximum displacement from equilibrium position
What is the time period of a wave?
Time taken for each particle to complete one full oscillation
(Return to same position)
How is frequency of a wave defined?
The number of complete oscillations per second
What is the wavelength of a wave?
Distance between two adjacent corresponding points on a wave
(Same displacement, no phase difference)
What is the phase difference between A and B on this progressive wave?
360° ∼ 0°
2π∼ 0π
What is the phase difference between A and B on this progressive wave?
180°
π ∼ Antiphase
What is the phase difference between points A and B on this progressive wave?
540° ∼ 180°
3π ∼ π ∼ antiphase
How is phase difference calculated in degrees?
How is phase difference calculated in radians?
How do you convert from degrees → radians?
What is the phase difference between A and B on this progressive wave?
420° ∼ 60°
14π/6 ∼ π/3
How are frequency and wavelength related?
What are the 2 key features of longitudinal waves?
Compressions and rarefactions
Why can’t sound waves be polarised?
Only transverse waves can be polarised
(Sound is longitudinal)
What is the final intensity?
- Light vertically polarised through first grating
- Vertically p[olarised light can’t pass through horizontal grating
- Final intensity = 0
What is the final intensity?
- Light vertically polarised through first grating (intensity halves)
- Vertically polarised light passes through second grating
- Final intensity = ½
How do sunglasses reduce glare?
- When sunlight reflects off surfaces it is polarised
- Sunglasses have filter to block polarised light
- Only unpolarised light passes through
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)
How does θ2 compare to θ1?
θ2 > θ1
(Light speeds up and bends away from normal)
How does θ2 compare to θ1?
θ2 < θ1
(Light speeds up and bends towards normal)
Is the light refracting here?
Yes
It hasn’t bent towards or away from normal
But it has slowed down
How does refraction affect the frequency of a wave?
Frequency does not change
(But wavespeed and wavelength do)
What is dispersion?
Different wavelength refract by different amounts
So light passing through a prism separates into wavelengths
What is wrong here?
In Snell’s law θ1 is the angle between normal and incident ray
What are the 2 conditions for total internal reflection?
- θ1 > θc
- n2 < n1
How is the critical angle calculated?
How do you calculate the angle of incidence in the fibre?
Using basic geometry (angles in triangle add to 180°)
Why are optical fibres better than copper cables?
- Information transmission faster
- More information can be transmitted
- Less energy loss (copper heats up)
In optical fibres what does cladding do?
- Protects the core from scratches and spills
- Stops data loss to adjacent fibres
- Increases critical angle (reducing modal dispersion)
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)
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
How do these two progressive waves interact when they overlap?
Form a superposition
Displacements combined (added or subtracted) at each point
What happens when these two pulses overlap?
Constructive interference (Displacements combine)
What happens when these two pulses overlap?
Destructive interference (Displacements cancel)
How does a stationary wave form?
- Progressive wave reflects off a fixed point
- Two progressive waves propagating in opposite directions (with same c,f,λ,A)
- Waves overlap and interfere forming superposition
On a stationary wave how are nodes and antinodes different?
Nodes → Points of 0 amplitude
Antinodes → Points of maximum amplitude
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)
How is the wavelength of a stationary wave calculated?
Each loop = ½λ
How is the frequency of the nth harmonic of a stationary wave calculated?
- Calculate the frequency of the 1st harmonic
- Multiply f1 by n
On this stationary wave why do points A and B have different amplitudes?
A and B have different maximum displacements
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
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°
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)
What 2 conditions are required to produce an interference pattern?
- Sources must be coherent (same frequency, constant phase difference)
- Sources must be monochromatic (one wavelength)
When will two sources interfere constructively?
When their path difference = nλ
So phase difference = 0°
Maxima forms
When will two sources interfere destructively?
When their path difference = (n+½)λ
So phase difference = 180° (∏ rad or antiphase)
Minima forms
When does maximum diffraction occur?
When the wavelength is close to the size of the gap the wave passes through
What does the interference pattern of the single slit look like?
Large central maxima
Intensity decreases exponentially
Each maxima has half width of central
For the single slit how is the central maxima width affected by λ?
W ∝ λ
For the single slit how is the central maxima width affected by the gap size?
W ∝ 1/a
For the double slit, how can you increase the widths of the maximas?
- Increase λ
- Increase slit to screen distance D
- Decrease slit separation s
How does the intensity graph look for the double slit interference pattern?
Intensity decreases linearly
Width of maximas constant
How is the 1st maxima formed for the diffraction grating
between adjacent slits Path difference = 1λ
So phase difference = 0°
How is the 3rd maxima formed for the diffraction grating?
between adjacent slits Path difference = 3λ
So phase difference = 0°
How do you calculate the slit separation for a diffraction grating?
How do you calculate the maximum number of observed maximas for the diffraction grating?
nmax = d/λ
Round Down!!!
Define current (I)
The rate of flow of charge
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)
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
How is the current in a circuit related to potential difference and resistance?
Increasing potential difference increases the current
Increasing resistance decreases the current
What is Ohm’s law?
The current flowing through a metallic conductor is proportional to the potential difference applied across it at constant temperature
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)
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)
How does a circuit ‘short circuit’?
If there is an available path with 0 resistance
Current → ∞
And the circuit heats up
What is the I-V graph for a fixed resistor?
What is the I-V graph for a filament bulb?
What is the graph for a semiconductor diode?
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
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.
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
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
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
What is the difference between a series and a parallel circuit?
Parallel circuits have junctions (3 or more wires connect)
Why doesn’t adding voltmeters in parallel affect the circuit? (it is still series)
Voltmeters have ~ ∞ R so no current flows through
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
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
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)
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.
What is Kirchoff’s 2nd Law?
In any complete “loop” of a circuit the sum of p.d’s equals the source p.d.
How do you combine series resistors in the same branch? (no junction between them)
Add up their resistances
How do you combine resistors in parallel branches? (one junction between them)
Use the following equation…
What is the advantage of placing resistors in parallel arrangements?
The total resistance is always less than the smallest resistance
Will the current split equally?
No, because the resistance of each branch is different
Will each component receive the same voltage?
No, because the resistance of the components are different
Why would you place batteries in parallel?
- The power delivered is the same
- But they take longer to run flatter
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)
How does resistance change for an NTC Thermistor?
As temperature increases, resistance decreases
How does resistance change for a Light Dependent Resistor (LDR)?
As light intensity increases, resistance decreases
What is the advantage of setting up a rheostat as a variable resistor?
- Simpler circuit
- Current constant throughout
- But cannot get 0V across bulb
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
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
How do you calculate the cross sectional area of a wire?
Assume it to be a cylinder (unless told otherwise)
A=∏r2
Why do metals with a greater cross sectional area have a lower resistance?
There are more paths for the electrons to propagate
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
What is a superconductor?
A material with 0 resistance at and below the critical temperature
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
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
Define emf of a power source
The potential difference across the terminals when no current is flowing through
Define terminal potential difference of a circuit
The potential difference across the terminals when a current is flowing through
What is the lost voltage in a circuit?
The potential difference used up pushing a current through the battery (vlost = emf - TPD)
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)
What is the photoelectric effect?
Light incident on a metal surface causes electrons to be emitted from the surface
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)
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)
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)
- Why do both light source cause electrons to be emitted? (from the surface)
- What is different about the electrons emitted due to the blue light?
- Both light sources have frequency above the threshold frequency (f0) of the metal
- The electrons emitted due to the blue light have a greater maximum kinetic energy (because blue photons have a greater energy from E=hf)
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
- What can you say about the green light incident on this metal?
- What difference does the brighter lamp make?
- The green light is above the threshold frequency so the photelectric effect happens
- 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)
You are shining a light (above f0) on a metal. How do you:
- Increase the maximum kinetic energy of the emitted electrons?
- Increase the number of emitted electrons per second?
- Increase the frequency of the light source
- Increase the brightness of the light source
This is a graph for the photelectric effect. What information do the 3 features of the graph provide?
- Y-intercept
- X-intercept
- Gradient
- Y-intercept = - work function
- X-intercept = threshold frequency
- Plancks’ Constant
This is the photoelectric effect graph for a metal
Plot a line on this graph for a metal with a higher threshold frequency
- Y-intercept (φ) decreases
- X-intercept (f0) increases
- But the gradient (h) is constant
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
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
How is the work function (φ) related to the threshold frequency (f0) of a metal?
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)
For the gold leaf experiment (to show the photoelectric effect):
- How do you make the gold leaf rise?
- Why does the gold leaf fall?
- A charged rod transfers additional electrons to the plate causing repulsion between the stem and gold leaf
- Electrons are liberated from the metal surface (by light above f0) so the stem and leaf become neutrally charged again
Define the electron volt
The kinetic energy gained by 1 electron passing through a potential difference of 1 volt
How do you convert between electron volts and Joules?
eV → J : multiply by 1.6x10-19J
J → divide by 1.6x10-19J
How is the maximum kinetic energy of photoelectrons (emitted during the photoelectric effect) measured?
- Connect the system to a circuit
- Place a battery opposing the current produced by the emitted electrons
- Measure the stopping potential when the total current = 0
- Ekmax = eVs
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)
What are the 3 types of line spectra and how are they produced?
- Continuous - Produced by blackbody
- Emission - Produced by an excited gas
- Absorption - Produced by a continuous spectrum passing through a cold gas
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
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)
How could an electron de-excite from n=3 → n=1 energy level?
It must emit an photon of energy = the difference between levels
How is the energy of a photon calculated?
Why do different gases (made of different elements) have different emission spectra?
- Each element has a different set of orbitals (with different energy levels)
- So each element has a different set of electron de-excitation energies
- The different de-excitation energies produce photons with different frequencies (E=hf)
How would you show the 488nm hydrogen emission line corresponds to a de-excitation from n=4 → n=2?
- Calculate then energy difference between the energy levels
- Convert energy difference to Joules
- Convert to f or λ (E=hf or E=hc/λ)
What is the ionisation energy of an atom?
The energy required for an electron to to become liberated from an atom
Equal to the energy of the ground state
What is wrong with this?
Never use work function when talking about energy levels. Ionisation and work function are different.
How is excitation by photon different from excitation by an electron?
- Photon energy = Difference between energy levels
- Electron energy ≥ Difference between energy levels
How many photons (of different wavelengths) can be emitted from this hydrogen atom?
6 possible transition so 6 different photons
Why is this mercury vapor in the fluorescent tubes kept at low pressure?
So a large enough current (of incident electrons) can be sustained
How does fluorescence work in a tube light?
- Mercury atoms excite by absorbing electrons from the current
- When the Mercury atoms de-excite they emit UV photons
- UV photons are absorbed by and excite the phosphor coating
- When the phosphor coating de-excites it emits visible light
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
What does this experiment show?
Wave-Particle duality
Electron diffraction through graphite to form maximas (bright rings) and minimas (dark rings)
How is the Debroglie wavelength λdb of a particle calculated?
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)
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
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)
How do you calculate the specific charge of an ion?
Charge of the ion (Protons - Electrons) divided by total mass of ion
What is an isotope?
An atom with the
- same number of protons
- Different number of neutrons
When will an isotope undergo radioactive decay?
If the nucleus has:
- too many of too few protons
- Too many nucleons
- Too much vibrational energy
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
What happens in Beta Minus (β-) Decay?
A neutron turns into a proton
Ejecting a fast moving electron (β-) and an anti-electron neutrino
What happens in Beta Plus (β+) Decay?
A proton turns into a neutron
Ejecting a fast moving positron (β+) and an electron neutrino
What is wrong about this Beta Decay equation?
The nucleon number must not change
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)
What is an antiparticle?
A particle with the:
- Same mass
- But equal and opposite charge
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
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)
What happens during pair production?
A gamma photon (with energy ≥ 2 × mass energy)
spontaneously creates a particle, anti-particle pair
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)
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
What are the four fundamental forces and their approximate ranges?
- Strong
- Weak
- Electromagnetic
- Gravitational
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)
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
What does the electromagnetic force act between and what is its exchange particle?
Acts between all particles with charge
Exchange particle is the photon
What does the gravitational force act between?
Particles or objects with mass
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)
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
What is the quark structure of a proton?
Up, Up, Down
What is the quark structure of a neutron?
Up, down, down
How is a muon different from an electron?
Both are leptons, muon is much heavier than the electron, produced in cosmic ray showers
What are hadrons?
Particles that are made up of quarks
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
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
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
What does the Higgs Boson do?
It creates the Higgs field
Which gives mass to particles
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
What must you know about k-mesons? (kaons)
They are made of 1 quark and 1 anti-quark (mesons)
They have non-zero strangeness
Produced by strong interactions, Decay (into pions) by weak interactions
What must you know about π-mesons? (pions)
They are made up of 1 quark and 1 anti-quark (mesons)
They have strangeness = 0
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
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
What is the formula for a muon decaying into an electron?
What is the feynman diagram for an electron-electron collision?
What is the feynman diagram for β- Decay?
What is the feynman diagram for β+ Decay?
What is the quark feynman diagram for β- Decay?
What is the quark feynman diagram for β+ Decay?
Identify the unknown particles in this feynman diagram for electron capture
Identify the unknown particle in this feynman diagram for the electron proton collision
Identify the unknown quark in the feynman diagram for electron capture
Identify the unknown exchange particle in the quark feynman diagram of electron proton collision
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
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
What is the muon lepton number of an electron?
0! Only muons and muon neutrinos have Lmuon = +1
What is the electron lepton number of a muon?
0! Only electrons and electron neutrinos have Lelectron = +1
