Topic 3 - Electric Circuits Flashcards
1
Q
Define electric current
A
‘The rate of flow of charged particles’
(Spec point 31)
2
Q
Give the equation for current
A
I = delta Q/delta t
3
Q
A)What is the charge of elementary charge
B) what is its unit
A
1.6 x 10^-19
Coulombs
4
Q
What is the charge of an electron in terms of elementary charge?
A
-e
5
Q
What is the symbol for elementary charge?
A
e
6
Q
What is the charge of a proton?
A
e
7
Q
What is 1 mole of something?
A
6.02 x 10 ^23 of that thing
8
Q
What is avogadros constant (give the number)
A
6.02 x 10^23
9
Q
Current is the same/different across a series circuit
A
Same
10
Q
Electrons flow -/+ to -/+
A
Negative to positive always
11
Q
Conventional current flows from the -/+ terminal to the -/+ terminal and why?
A
Positive to negative
Current is defined as flow of positive charge (you can see this from the equation) and as electrons carry negative charge, the direction of current flow is the opposite to the direction of the flow of electrons
12
Q
True or false: only electrons can cause current to flow
A
False - protons and other ions can as well
13
Q
What is the equation for potential difference
A
V = W/Q
Potential difference = work done/charge
14
Q
What is EMF?
A
Electromotive force - the work done per unit charge by the power supply or cell converting energy into electrical energy of the charges
15
Q
Define potential difference
A
The ENERGY TRANSFERRED between two points in a circuit PER UNIT CHARGE
(Things in capitals are important marking points)
16
Q
Net electric charge flow is the sum of what
A
Negative and positive charge flow - do not take the magnitude of the negative flow, as the negative cancels out the positive as net charge flow measures the total imbalance in charge, not the movement of electrons
17
Q
Explain why a proton in a space between a positively charged surface and a negatively charged one (where it is further from the negative surface than the positive), accelerates towards the negatively charged surface. Explain the energy transfers in this process.
A
- The proton has a high electrical potential energy at first when it is far from the negative surface and There is a downward force on the proton
a. Because like charges repel and opposite charges attract, so the charge is repelled from the top and attracted to the bottom- The speed of the proton increases as it accelerates and so the kinetic energy of the proton increases
The electrical potential energy decreases and it reaches the negative surface
- The speed of the proton increases as it accelerates and so the kinetic energy of the proton increases
18
Q
A proton has a higher electrical potential energy far/close to a negatively charged surface
A
Far - as there is a large potential for work to be done
19
Q
True or false: potential is the same as electrical potential energy
A
False
20
Q
Positive charges move from a high/low potential to a high/low potential
A
High to low
As think of all positive things as a high potential as they have a high potential to move to the negative thing as their point charge is pointing outwards. They all want to go to the negative (low potential)
21
Q
Potential is defined between two points/at a point
A
At a point
22
Q
What are the units for potential
A
Volts
23
Q
What is the mass of an electron
A
9.1 x 10 ^-31 kg
24
Q
What is the radius and what is the nucleus of an atom?
A
Radius = 0.1nm = 1 x 10^-10
Nucleus = 10fm = 1 x 10^-14
25
[l] = ?
Metres - m
(The question means what is the unit of length)
26
[ T ] = ?
Kelvin - K
27
Symbol for time
t
28
Moles = [?]
n
29
What is the symbol for the amount of a substance?
n
30
How many degrees Kelvin is 100 degrees Celsius?
373
31
How many degrees Kelvin is 0 degrees Celsius?
273
32
What is 0 degrees Kelvin in Celsius?
-273
33
What happens at O degrees Kelvin
All particles stop moving, they have 0 kinetic energy
34
Give the 7 base units and their symbols
Length - l -metres - m
Time - t - seconds - s
Mass - m - kg
Temperature - T - Kelvin - K
Current - I - Amperes - A
Amount of a substance - n - moles - mol
Luminous intensity - Lv - Candela - Cd
35
How many derived units are there?
Anything that isn’t a base unit is a derived one - and you can always write a derived unit in terms of base units
36
What is Joules expressed in base units using the equation for kinetic energy?
Kg (m/s)^2
REMEMBER THE BRACKETS - kgm^2/s^2
37
Describe in words what V = IR means
Potential difference across the resistor = current through the resistor x the resistance of the resistor
38
What is kirchoff’s current law?
Sum of currents into a junction equals sum of currents out
39
Explain the derivation of kirchoff’s current law using the conservation of charge
- net charge in a closed system doesn’t change
- therefore sum of charge into a junction = sum of charge out of a junction
- Divide the charge on each side by time (ensure you still use sigma to denote sum of in this line of working!)
- therefore sum of current in = sum of current out
40
Is current vector or scalar and why?
Scalar - as it can only be positive or negative, so it is one dimensional
It is also defined as change in current over change in time, and both change in current and change in time are scalar quantities, and you can’t get a vector quantity from a scalar one
41
What is kirchoff’s voltage law?
The sum of the potential difference is equal to the sum of the EMFs around a closed loop in the circuit
42
How is kirchoff’s voltage law derived?
The law of the conservation of energy states energy cannot be created or destroyed
Sum of Energy released by EMFs = sum of energy dissipated by components
Sum of VQ = sum of VQ
Therefore sum of potential difference released by EMF = sum of potential difference across component
43
What is the symbol for femto and what order of magnitude is it?
f - 10^-15
44
What is the symbol for femto and what order of magnitude is it?
f - 10^-15
45
If you move a charge across a voltage of 1V then the charge gains how much energy
An energy whcih is equal to its charge
(If you moved it across more than 1V then it gains energy that is its charge x the voltage )
46
Explain why potential difference across branches of a parallel circuit (with the same component on each) is the same
- Kirchhoff’s voltage law says voltage in a closed loop is equal to voltage of EMF in that loop
- therefore applying this law to the two loops of the two branches shows that the voltage on one branch = voltage on other branch = voltage of EMF
47
Explain why potential difference splits across components connected in series
- kirchoff’s voltage law states that the sum of the voltages = the sum of EMF in a closed loop
- therefore the voltage of the EMF is split across the components connected in series, as they only form one closed loop
48
Describe how current splits across branches in parallel
It splits according to the ratio of the resistances, favouring lower resistances (more current flows in the lower resistance branch)
49
How do you find total resistance in series?
The sum of the resistances
50
What are the only 3 laws you need to know about circuits in a level physics?
Kirchoffs voltage law, his current law, and v = IR
51
Derive the equation for total resistance in a series circuit
NOTE DO NOT USE KVL OR KCL IN THIS QUESTION
KVL says total voltage = sum of voltages
As V = IR, IR total = sum of IR for each component
KCL says I total = sum of currents
Therefore we can cancel the total current to get
R total = sum of individual resistances
52
Derive the equation for total resistance in a parallel circuit
Total current = sum of currents due to KCL
As V=IR, total current = V/R = the sum of V/R for each component
Due to KVL, voltage of EMF is the same as voltage of each component, so that can be cancelled
Therefore 1/Rtotal = sum of 1/R for each component
53
What is the key part of KVL which you need to pay attention to to get questions about KVL correct?
Sum of voltage = sum of EMF IN A CLOSED LOOP (a closed loop means a loop with no breaks (just a loop in series which links the EMF to the component(s))
54
Are Resistor 1 and resistor 3 connected in series or parallel and why?
They are not connected in series, and they are also not connected in parallel!!
They can’t be in series as they have different currents, but they can’t be in parallel because they are not
55
Are Resistor 1 and resistor 3 connected in series or parallel and why?
They are not connected in series, and they are also not connected in parallel!!
They can’t be in series as they have different currents, but they can’t be in parallel because they are not
56
What relationship do R2 and R3 have and what relationship do R1 and Requivalent have?
R2 and R3 are in parallel and R1 and R equivalent are in series
57
What is the relationship between R1 and R2 in this circuit?
How about R2 and R3?
What about R2 and R5?
What about Requivalent (R2+R3) and Ra?
Nothing
Series
Nothing
Parallel
58
What is the potential difference across the resistor?
1.5V
59
How do you calculate the total EMF of multiple EMFs connected in series with each other
Sum of individual EMFs
60
Why would you connect two cells in parallel with each other (given this doesn’t increase total voltage supplied)?
It lengthens the lifetime for each cell, as each cell is providing a lower current (and so the rate of loss of charge for each cell is lower)
This can still make a circuit run as the total current in the other branches is the sum of each current provided by the cells
61
Ammeters have a zero/nonzero resistance in real life and what does this mean for the recorded value of current?
Non zero
It makes the recorded value lower than the true value as V=IR and the resistance increases so current decreases
I think this is wrong i think its actually that pd increases and anything else needs to be worked out
62
What is the resistance of an ideal ammeter?
0 ohms
63
With a normal voltmeter (with non infinite resistance), is the measured potential difference higher or lower than the true value or why
Lower, as the equivalent resistance decreases and so the potential difference decreases - not sure this is actually correct pretty sure this is wrong
64
What is the density of water in g/cm cubed an in kg per metre cubed
1g/cm cubed and 1000kg per metre cubed
65
what is the potential difference across A1 and B1 compared to A2 and B2?
A1 and B1 resistance is 10 Volts as think of the height of the full resistance as 15V, which splits proportionally to resistance
A2 and B2 is 7.5 Volts, as the equivalent resistance across A2 and B2 is 5 ohms, and so the potential difference splits half and half across the 5 ohm resistor and the 5 ohms of equivalent reisstance
66
a battery has an internal ____
resistance
67
the EMF Of a battery is the voltage measured across a) the cell part of the battery OR b) the cell and the internal resistor
a)the cell part of the battery (without internal resistor)
68
what is lost volts
EMF - terminal potential difference (on other words the voltage of the internal resistance of the cell)
69
what is terminal potential difference
potential difference across two ends of a battery (this is lower than EMF as some energy is dissipated by heating up the internal resistor)
70
what is EMF? (in terms of a battery)
EMF of an ideal battery with no internal resistance
71
Explain how you would measure the resistance of a resistor of 1600 ohms using a multimeter
Connect the resistor across the port labelled COM and the one labelled ‘VOhmsmA’( this may vary depending on the current in the circuit)
Then switch to. The 2000ohms setting as the resolution is highest.
72
Explain how a standing wave is formed 6 marker:
1. Two waves must be travelling in opposite directions (usually one wave and its reflection) with the same frequency, amplitude and speed
2. The waves meet and undergo superposition
3. Constructive interference occurs when the waves meet in phase
4. This results in antinodes which are positions of maximum amplitude
5. Destructive interference occurs when the waves meet in antiphase
6. This results in nodes which are positions of zero amplitude
73
74
Give an equation linking the current and resistance of an internal resistor, EMF, and terminal pd
Terminal pd = EMF - current through internal resistor x resistance of internal resistor
75
When does the terminal potential difference in a circuit equal the EMF (give 2 circumstances)
when internal resistance is 0, or current is 0
76
Both of these circuits have 0 internal resistance - Is there a difference in brightness between the bulbs in the left circuit and the right one and why/why not?
The brightness would be the same - as there is no internal resistance in the battery, the ost volts in both cases is the same (0), and doesn’t increase when the current increases (when new bulbs are added). Therefore the potential difference across each bulb is constant (due to KVL), and as power = v^2 / R (and R and V are hence constant for each individual bulb) then the power for each bulb is the same.
The only things whcih change are the resistance (which decreases as you add more branches and therefore give more places for current to flow) and current (which increases as resistance has decreased)
77
Will the bulbs in one circuit be brighter than the other - if so which one and why?
The bulb in the leftmost circuit will be brightest as the lost volts is lower than the other circuit, as the lost volts is current x resistance, and as current is higher in the right circuit (as there are more branches which lowers resistance and hence increases current), the lost volts is higher which means the pd for each branch is lower (as the EMF is a finite source of potential difference)
They therefore have a lower power (as power is V^2 / R) and therefore a lower brightness
78
Give the minimum answer (not necessarily a good answer) for why the headlights of a car are really bright at the start when they are turned on and then quickly get dim
- When the engine is started, the motor is drawing a large current.
- The terminal potential difference across the cell decreases
- the potential difference across each light bulb is smaller
- So the light is dimmer
79
In the practical about finding the EMF and internal resistance of an electrical cell, what do you plot on a graph
terminal pd against current (as current is the independent variable) and terminal pd can be directly measured.
The gradient of that graph is therefore the negative of resistance and the y intercept is the EMF of the cell
This can be derived from the equation terminal pd = EMF - (current x resistance)
80
Describe the circuit set up for the practical on determining the EMF and internal resistance of an electrical cell
81
When writing definitions you must write ___as well
equations
82
___is related to the brightness of a bulb
power
83
What is the definition of power
rate of transfer of energy
84
What is V2/V1 for a transformer
N2/N1
85
What is I2/I1 for a transformer
n1/n2
86
What are the charge carriers in a metal
electrons
87
Only the ___electrons in a metal are conducting electricity
outer
88
Define the number density of free charge carriers
number of charge carriers in 1m cubed of a material
89
What order is the number density of free charge carriers typically
10^28
90
What is the unit of number density of free charge carriers
m^-3
91
What is a free electron in an atom
The electron in the outer shell
92
Define drift velocity
The average speed of the free charge carriers in a conductor under the influence of a potential difference
93
Resistance is ___proportional to drift velocity
Inversely
94
As collision frequency increases, drift velocity ___
Decreases
95
Collision frequency define
How many times per second charge carriers and positive lattice ions collide
96
Why does the light turn on instantly when you turn on the switch
The electromagnetic field propagates at the speed of light, even though the drift velocity is much lower than the speed of light
97
What is the equation for I which includes drift velocity and some other things
I = n q A v, n = number density of free charge carriers, A = cross sectional area of conductor, v = drift velocity, q = charge on each charge carrier
98
Explain, in terms of particle behaviour and drift velocity, why the resistance of a metal wire changes as temperature increases 4 marks
- As temperature increases, positive lattice ions vibrate with a larger amplitude
- As a result, the collision frequency between the free electrons in the wire and the positive lattice ions increases
- Therefore, the mean drift velocity of the electrons decreases
- Since the resistance of a material is inversely proportional to the drift velocity of the charge carriers in the material, so the resistance of the wire increases
99
Explain - in terms of particle behaviour, why the resistance of a filament wire increases when the temperature of the wire increases
1. Temperature increases - positive lattice ions oscillate with a larger amplitude
2. More frequent collisions between electrons and positive lattice ions
3. Mean drift velocity decreases
4. I = nqAv (current decreases for constant n, A, q)
5. R = V/I, V is the same, and I decreases so R increases
100
Explain why resistance of a filament increases with current
101
102
Lithium has one delocalised electron but what should we call this electron instead
A free electron
103
What is a free electron
The electron in the outer shell
104
Compare the number density of free charge carriers in an insulator, semi-conductor and conductor
Insulator = negligible
Semi conductor = medium
Conductor = highest charge carriers per unit volume (around 10^28
105
What defines how good a material is at conducting
The number density of free charge carriers in that material
106
Draw the band diagram of an insulator and a conductor
107
Compare the band diagrams of conductors and insulators
Band gap is larger in insulator
There are electrons in the conduction band of the conductor, but not the insulator
108
In order for a material to conduct there must be what in the __band
Electrons
109
Explain why insulators are not good electrical conductors
- the energy/band gap is too big in the insulator
So at room temp, there isn’t enough energy for electrons to be promoted from the conduction band to the valence band
- so the number of charge carriers in the conduction band is low
So insulators are not good electrical conductors
110
Semi conductors have a high/medium/low band gap
Medium
111
The number density of charge carriers in a semi conductor increases with what
Temperature
112
Explain why the number density of free charge carriers increases in semiconductors as temperature increases
When temperature increases the charges have more molecular kinetic energy
Charge carriers are promoted from the valence band to the conduction band
So there is a higher number of free charge carriers in the conduction band
113
What is a potential divider?
(In an ideal circuit with no internal resistance of battery and ideal ammeter/voltmeter) then the total EMF is shared in the ratio of the resistances (R1/R2=V1/V2))
114
Draw graphs of resistance in ohmic versus non ohmic materials
Straight line through the origin and
115
Draw graphs of resistance in ohmic versus non ohmic materials
Straight line through the origin and
116
How do you calculate resistance from a graph of current versus pd of a non ohmic material
Remember it is not the gradient, as that would be the rate of charge of current with respect to pd which is kind of a meaningless concept and isn’t the same as resistance
117
You can draw what component in a circuit with only that component and a resistor (and no battery) - state what this component is and explain why you can do that
Ohmmeter
It is assumed to be part of a multimeter which has its own battery
118
Explain why it is better to use circuit 2 than circuit 1 to determine the resistance of a component
You cant plot values for V and I close to zero for circuit 1 as it would require an infinite resistance
However for circuit 2 you can get values for I and pd around 0 by setting the resistance of the rheostat to 0 (if you put it to the extreme right)
119
What is the graph of V against I for a filament bulb and why
The v against i is the reflection of the other one in the line y = x
120
With a constant temperature, as current increases then what increases
Drift velocity increases
121
Why is current lower than it would be for a constant temperature component on this graph of a filament bulb plotting current against potential difference
I = nqAv
Amplitude of oscillation increases as temperature increases
Collision frequency increases
So the drift velocity of the charge carriers is limited
122
Explain what happens to the number of free charge carriers in the conduction band when the temperature increases
Electrons gain more kinetic energy
Charge carries are promoted from the valence band to the conduction band
123
When temperature of a thermistor increases then resistance does what
Decreases
124
Explain why when temperature increases in a thermistor, the resistance decreases
When temperature increases, number of free charge carriers increases
As teh charge carriers gain kinetic energy
The charge carriers are promoted from the valence band to the conduction band
When temperature increases, teh positive lattice ions oscillate with larger amplitude
Increasing frequency of collisions between lattice ions and charge carriers
This decreases mean drift velocity
As I = nqAv, I is proportional to v
So as v decreases I decreases
In a thermistor the effect of number density of charge carriers increasing should be reader than the effect of v decreasing
125
When temperature of room increases, resistance of thermistor decreases
Equivalent resistance of thermistor and bulb which are connected in parallel decreases
The circuit shown is a potential divider (where resistor 1 is the equivalent resistance of the bulb and thermistor and resistor 2 is the fixed reisstor) so the potential difference across resistor 1 decreases. So the bulb is going to be dimmer
126
Draw the IV curve for a thermistor
127
Explain why the resistance decreases when the current increases in a thermistor:
- When current increases
- The frequency of collisions between positive lattice ions and free charge carriers increases
- So The positive lattice ions oscillate with a larger amplitude
- So the temperature increases
- So the charge carriers have enough energy to be promoted from the valence band to the conduction band
- This increases the number density of charge carriers in the conduction band
- As I = nqAv, as n increases, current increases
- R = V/I and as I increases by a larger factor than V does, then R decreases
128
Explain why the IV graph for a thermistor shows that as current increases resistance deceases
Gradient is increasing
R is 1/gradient
So R is decreasing
129
What is it called when a diode is conducting (allowing a circuit to have current flow)
Forward bias
130
When a diode is preventing conduction - what do we call this position?
Reverse bias
131
What is the word for the minimum pd across a non ideal diode needed for conduction to happen (for resistance to decrease)
Threshold pd
132
If there is a current flowing through diode and it has resistance of 0 what is the pd across it
The threshold pd, as only values of pd less than the threshold value correspond to 0 current on the graph
133
Explain what happens when you increase EMF 0-1.5V of a circuit with a non ideal diode and a bulb
- Then pd across diode starts increasing but pd across bulb stays at 0 as no current
- Then once pd exceeds threshold value then current starts flowing
- Then pd across diode stays at 0.7V
- And the pd across the bulb starts increasing (due to KVL)
134
Connecting the circuit in which region of the graph of a diode damages the diode
135
Draw the pd diagrams for a non ideal diode connected to a 3V AC EMF
136
Student moved a lamp towards an LDR, the resistance of the LDR decreases
With reference to the electrons in the LDR explain this observation (4 marks)
when light is shone on the LDR, each electron absorbs light (a photon) and gain energy
As the student moved towards the lamp towards the LDR, the light falling on the LDR increases
So more electrons have enough energy to be promoted from the valence band to the conduction band
This increases the number of electrons in the conduction band of the LDR
The increase in the number of conduction electrons decreases the resistance of the LDR
The increase in the number of conduction electrons decreases the resistance of the LDR
(Then elaborate on number of free charge carriers and how it affects the current for the same pd and thus affecting the resistance
137
Explain - in terms of particle behaviour, why the resistance of a filament wire increases when the temperature of the wire increases
1. Temperature increases - positive lattice ions oscillate with a larger amplitude
2. More frequent collisions between electrons and positive lattice ions
3. Mean drift velocity decreases
4. I = nqAv (current decreases for constant n, A, q)
R = V/I, V is the same, and I decreases so R increases
138
draw the IV and the VI graphs for a filament bulb
139
explain why as current increases in a filament bulb, resistance also increases
- as current increases, the electrons collide more frequently with lattice resistor ions, transferring more energy to them per second
- this causes the resistor ions to oscillate with greater amplitudes so the temperature of the resistor increases
- this causes more frequent collisions between the electrons and the lattice ions, limiting the drift velocity
- this causes the current (I = nqvA) to increase by a smaller factor than the potential difference (as n, q, A all fixed)
- so, as resistance is the ratio of pd to current, if pd increases by a greater factor than current does, the ratio R = V/I increases
140
Explain what happens to the number of charge carriers in the conduction band when the temperature increases
- Electrons gain more kinetic energy
- Charge carriers are promoted from the valence band to the conduction band
141
in a circuit with a battery, a thermistor and a bulb, when the temperature increases what happens to the bulb?
temperature increases, resistance of thermistor decreases
EMF is shared in ratio of resistances
so pd across bulb increases
P = V^2/R so the bulb is brighter
142
143
explain why the temperature of a thermistor is high with a high current
more current = high collision frequency between positive lattice ions and charge carriers
energy transferred to the ions
so they vibrate with a higher amplitude
leading to an increase in temperature
144
draw the IV graph for a thermistor and explain why it makes sense
- gradient is increasing
- Resistance is 1/gradient (technically not the gradient instead just the ratio of x to y)
so R is decreasing
145
Explain why the resistance decreases when the current increases in a thermistor:
- When current increases
- The frequency of collisions between positive lattice ions and free charge carriers increases
- So The positive lattice ions oscillate with a larger amplitude
- So the temperature increases
- So the charge carriers have enough energy to be promoted from the valence band to the conduction band
- This increases the number density of charge carriers in the conduction band
- As I = nqAv, as n increases, current increases
- R = V/I and as I increases by a larger factor than V does, then R decrease
146
describe what a diode does
- keeps current flowing in one direction, as it has infinite resistance in one direction and 0 resistance in the other direction
147
draw the diagram for an ideal diode
148
is the triangle part or the vertical line part of the circuit symbol for a diode the side which blocks current
- the line is the side where current is blocked
- the triangle is the side it is allowed
149
draw the circuit diagram for a real diode and label it with:
- forward bias, reverse bias, threshold pd and breakdown pd
150
if the pd across a diode is less than the threshold pd and over 0 will the diode conduct?
no - it needs to be larger than the threshold pd
151
what happens to the pd across the diode as you increase the EMF in a circuit from 0 to double the threshold pd of the diode
- Then pd across diode starts increasing but pd across bulb stays at 0 as no current
- Then once pd exceeds threshold value then current starts flowing
- Then pd across diode stays at 0.7V
- And the pd across the bulb starts increasing
152
connecting the diode in what region of the graph damages the diode
- in the bottom left quadrant (after the breakdown pd) (where the current massively increases)
153
if you connected a bulb and a diode to an AC power supply, draw the graph of the pd against time for the bulb
then draw the graph for the diode (Assume it is ideal)
the positive parts of a normal AC graph only (two humps) for a bulb
then only the negative parts for the diode
154
draw the pd against time graph for a non ideal diode connected to a bulb
for a) a bulb
and b) the diode
-same as for an ideal diode but the humps finish sooner and start later (as the threshold pd curtails the pd for small values of pd)
b) (diode)
- it increases in the positive direction until it hits the threshold pd, and then stays at a maximum of the threshold pd (as a straight line) until it decreases back to 0 and then it completes the whole hump in the negative direction and then repeats
155
what is the equation for light intensity
power per unit area
156
an LDR is a ___
semiconductor
157
what is a photon
a packet of energy
158
what is light as a particle
a photon
159
how are electrons promoted from the valence to the conduction band in an LDR
Light hits conduction band
Photon absorbed by electron then electron
uses that energy to be promoted to conduction band
1 PHOTON interacts with 1 ELECTRON
160
explain why resistance decreases as light intensity increases in an LDR
- Higher intensity means a greater number of photons incident on the LDR per second per metre squared
- So as intensity increases more photons incident per second means more electrons gain energy from the incident photons per second
- More electrons are promoted to the conduction band per second so the number. Density of free charge carriers increases (n)
- Current increases
So as resistance is the ratio of pd to current
161
as the cross sectional area of a wire increases what happens to resistance
it decreases
162
as length of wire increases what happens to resistance
increases
163
keeping the material of the wire as a constant, what two properties of the wire affect resistance
length and area of cross section
164
resistance and length of wire have what relationship
directly proportional
165
resistance and cross sectional area of wire have what relationship
inversely proportional
166
give the equation for resistance in terms of resistivity
R = resistivity (rho) x length / Area
167
what are the units for resistivity
(omega m) ohmmeters
168
what two things does resistivity depend on
material type and temperature
169
what metal has the lowest resistivity
silver
170
what equipment do you use to determine the cross sectional area of a wire experimentally
a micrometer/vernier calliper
171
give a method for determining the cross sectional area of a wire using a micrometer
- measure the wire diameter with the micrometer
- divide by 2
- do pi r squared
172
how do you calculate the uncertainty in the diameter of a wire
measure the diameter at different points on the wire
measure diameter in 3 different directions
then take range/averages
173
what does a wire with an arrow on it mean as a circuit symbol
a slider is used in the circuit
174
explain how to use a micrometer
- Read main scale on the sleeve
- Identify how many whole mm there are and if there are any 0.50mm present too
- Then add the number on the thimble - where the horizontal line on the main scale intersects the thimble scale this is the number
- So this reading would be ___+___= ___
175
what is the resolution of a micrometer
0.01mm
176
what 4 things do you need to check for when reading a micrometer
- Zero error
○ Check that when the micrometer is closed that the value reading is 0
- Measure the diameter of the objects from different orientations
○ As the wire is not circular due to practical manufacturing issues
- Measure the diameter at different positions along the wire
- Use the ratchet so as not to over tighten the spindle
○ To reduce random error
177
How can you ensure accuracy when measuring the cross sectional area of the wire in the resistivity practical?
- Measure the diameter of the wire at different orientations and positions and take a mean
- Ensure eyes perpendicular to scale to avoid parallax error
- Avoid pressing too hard on the wire (which could cause it to deform)
178
Draw the 2 possible diagrams for the resistivity practical
Ohmmeter connected to wire
Or ammeter, resistor and voltmeter and wire with the voltmeter measuring the pd across the wire
179
Draw the 2 possible diagrams for the resistivity practical
Ohmmeter connected to wire
Or ammeter, resistor and voltmeter and wire with the voltmeter measuring the pd across the wire
180
As temperature of thermistor increases, do the charge carriers oscillate with a higher amplitude? Do they gain kinetic energy?
No, no
The positive lattice ions oscillate with a higher amplitude
They just gain energy
181
What is efficiency?
Useful power output/ total power input
