1
Q
What is current?
A
The rate of flow of charge in a circuit.
2
Q
What are the conditions for current to pass through a circuit?
A
- Circuit must be complete
* Source of p.d.
3
Q
What is the unit for current?
A
Ampere (A)
4
Q
What are charge carriers?
A
Charged particles that move around a circuit, allowing current to flow.
5
Q
What are the charge carriers in metals?
A
Conduction electrons.
6
Q
What are the charge carriers in a salt solution?
A
Ions.
7
Q
What is the unit for charge?
A
Coulomb (C)
8
Q
Which way does conventional current flow?
A
From + to - terminals.
9
Q
Which way do electrons flow in a circuit?
A
From - to + terminals.
10
Q
What must be remembered about the direction of current in a circuit?
A
- Although electrons flow from - to + terminals, this is not used.
- Conventional current is in fact considered, which flows from + to - terminals.
11
Q
What is the symbol for current?
A
I
12
Q
What is the symbol for charge?
A
Q
13
Q
What is a coulomb defined as?
A
The amount of charge that passes in 1 second if the current is 1 ampere.
14
Q
What is the equation relating charge, current and time?
A
Q = I x t
Where:
Q - Charge (C)
I - Current (A)
t - Time (s)
15
Q
What device is used to measure current and how must it be connected?
A
- Ammeter
* Connected in series
16
Q
Why can’t current flow in an insulator?
A
- Each electron is attached to an atom and can’t move away from the atom
- Therefore, electrons can’t flow
17
Q
Why can current easily flow in a metallic conductor?
A
- Most electrons are attached to an atom, but some are delocalised
- These delocalised electrons act as charge carriers
18
Q
What is a semiconductor and why does it behave like this?
A
- Number of charge carriers increases with temperature -> Resistance decreases and resistivity decreases.
- This is because as temperature increases, electrons break free and act as charge carriers
19
Q
What is a pure semiconducting material called and why?
A
Intrinsic semiconductor (intrinsic means natural) -> Conduction is due to electrons that break free from atoms of the material
20
Q
What are the uses of semiconductors?
A
Sensors for detecting changes in the environment (e.g. thermistors and diodes).
21
Q
Give two examples of semiconductors.
A
Thermistors and diodes.
22
Q
What must be done in order to make electric charge flow through a conductor?
A
Do work on it.
23
Q
What is potential difference?
A
The work done (or energy transferred) per unit charge.
24
Q
What is the unit for potential difference?
A
Volt (V)
25
What is the equation relating potential difference, work done and charge?
V = W / Q
Where:
V - Potential difference (V)
W - Work done (J)
Q - Charge (C)
26
What is the symbol for potential difference?
V
27
What device is used to measure potential difference and how must it be connected?
* Voltmeter
| * In parallel
28
Describe the energy changes to an electron in a circuit.
* A battery transfers chemical energy to the electrons
* Each electron has to do work to pass through a component and transfers energy to it
* The battery resupplies electrons with energy
29
When a electron passes through a component, the work done by the electron is equal to...
The loss of energy.
30
Describe what potential difference is in terms of a electrons passing through a component.
* Each electrons has to do work to pass through a component (this is equal to the energy it loses)
* The potential difference across a component is the work done per unit charge
31
What is a volt?
* The potential difference across a component is 1 volt when you convert 1 joule of energy moving 1 coulomb of charge through the component.
* 1V = 1J/C
32
Describe and explain the energy changes in an electrical heater.
* Heater has high resistance
* Charge carriers collide repeatedly with atoms in the device and transfer Ek to them
* Atoms vibrate faster
* Resistor heats up -> Transferred as thermal energy
33
Describe and explain the energy changes in an electric motor.
* Work done on motor = Energy transferred to the load and surroundings -> Constant speed
* Electrons need to be forced through the wires of the spinning motor coil against the opposing force exerted by the magnetic field.
34
Describe and explain the energy changes in a loudspeaker.
* Work done is transferred as sound energy.
* Electrons need to be forced through the wires of the speaker coil against the force on them due to the loudspeaker magnet.
35
For a particular potential difference across a component, what determines the current?
The resistance of the component.
36
What is resistance?
A measure of how difficult it is for current to pass through the component.
37
What is the unit for resistance?
Ohms (Ω)
38
What is the symbol for resistance?
R
39
What equation relates potential difference, current and resistance?
V = I x R
Where:
V - Potential difference (V)
I - Current (A)
R - Resistance (Ω)
40
What is an ohm?
A component has a resistance of 1Ω if a potential difference of 1V makes a current of 1A flow through it.
41
What is the resistance of a voltmeter and ammeter be assumed to be?
Voltmeter - Infinite
| Ammeter - Zero
42
What is Ohm's law?
* Provided that the physical conditions (e.g. temperature) remain constant, the current through an ohmic conductor is directly proportional to the potential difference across it.
* V = I x R
43
What causes resistance?
Repeated collisions between the charge carriers in the material with each other and the fixed ions in the material.
44
What is an ohmic conductor?
* A material or component that obeys Ohm's law.
| * Has constant resistance, regardless of the current.
45
Describe the I/V graph for an ohmic conductor.
* Goes through the origin
| * Straight line
46
Name two physical factors that could affect Ohm's law.
* Light level
| * Temperature
47
What conditions must be met for Ohm's law?
* Ohmic conductor
| * Constant physical conditions
48
Why must light levels and temperature be kept constant in experiments on ohmic conductors?
They could affect the resistance of the conductor.
49
Are most components ohmic conductors?
No, most are non-ohmic components and have their own I-V graph.
50
What does the term "I/V characteristic" refer to?
* A graph of I (y-axis) against V (x-axis)
| * Shows how current through a component changes as the potential difference is increased.
51
On an I/V graph, what is on the x-axis?
V
52
On a V/I graph, what is on the x-axis?
I
53
Describe the circuit that can be used to determine the I/V graph for a component.
* Ammeter
* Component
* Voltmeter (across component)
* Variable resistor
* Battery
54
How can you find the resistance at a certain point on a V/I or I/V graph?
How could you change voltage and current?
* Look at the value for V and for I
* R = V/I
Change resistance of variable resistor to change V and I.
55
On an I/V graph for an ohmic conductor, what does a steep gradient signify?
Low resistance
56
On an V/I graph for an ohmic conductor, what does a shallow gradient signify?
High resistance
57
Describe the I/V graph for a filament lamp.
| This is ohmic :
* Curve
* Starts steep but gets shallower with voltage
* Rotated around origin
58
Why isn't a filament bulb an ohmic conductor (in terms of pd and temperature and resistance)?
The wire heats up with current and potential difference, causing the resistance to increase.
59
Explain how and why the resistance of a filament lamp changes as potential difference is increased.
* As potential difference increases, so does current.
* Increasing current means that some electrical energy is transferred into heat energy, increasing the metal's temperature (due to electron-ion collisions).
* Positive ions in metal vibrate more -> More difficult for charge carriers to pass -> Current can't flow as easily -> Resistance increased
60
Explain how and why the resistance of a filament lamp changes as current is increased.
* Increasing current increases the temperature (due to electron-ion collisions)
* Positive ions in metal vibrate more -> More difficult for charge carriers to pass -> Resistance increased.
IV graph levels off at high current for filament lamps.
61
Compare the resistance of semiconductors and metals..
| How does it vary with temperature?
* Metals are better conductors (i.e. lower resistance)
| * Resistance of metals increases with temperature, while resistance of semiconductors decreases with temperature
62
At low temperatures, why are metals better conductors than semiconductors?
There are more charge carriers available.
| semiconductors have electrons stuck to atoms and aren't free moving
63
What is a thermistor?
A resistor with a resistance that changes with temperature.
64
What type of thermistor do you need to know about?
Negative Temperature Coefficient (NTC) - Resistance decreases as temperature increases
65
How does the resistance of a thermistor change with temperature?
As temperature increases, resistance decreases.
66
Describe the graph of resistance (y-axis) against temperature (x-axis) for a thermistor.
* Downwards curve
| * Gradient becomes less steep with temperature
67
Describe the I/V graph for a thermistor.
* Upwards curve
* Curved upwards away from x-axis
* Rotated around origin
68
Explain the I/V graph for a thermistor.
* As p.d. increases, current increases
* This causes temperature to increase
* More electrons have enough energy to escape from their atoms
* More charge carriers -> Resistance decreases
* More current can flow, so graph curves upwards
69
How can you investigate the resistance of a thermistor?
Use water bath:
Put the thermistor into a beaker and pour boiling water into it, covering the thermistor.
Measure the temperature of the water using a digital thermometer.
Record the current through the circuit with the ammeter.
The potential difference needs to be kept constant throughout the experiment.
Keep recording current and temperature for every 5 degree drop.
From the values for current and potential difference, work out resistance for each temperature.
For NTC resistors, as temperature decreases, resistance increases (current will decrease).
70
What is a diode?
A component that allows current to flow in one direction only
Made from semiconductors
71
What is the forward bias of a diode?
The direction in which the current is allowed to flow.
72
What does LED stand for?
Light emitting diode.
73
Describe how the resistance of a diode changes with potential difference.
* With a negative voltage (in reverse bias) the resistance is very high
* Up to a threshold voltage (usually about 0.6V) the resistance remains high
* After this voltage, the resistance falls rapidly
74
What is the threshold voltage of most diodes?
About 0.6V in the forward direction before they conduct.
75
What is an LED?
A diode that emits light when current flows through it.
76
What does the triangle symbol represent on a diode?
triangle symbol indicates the direction the diode will let current flow.
E.g. a large current can flow left to right (forward bias):
77
Describe the I/V graph for a diode.
* At negative voltage -> Very small NEGATIVE current
* At low positive voltage -> Small positive current
* Above threshold voltage (0.6V) -> Current increases linearly (starts of curved)
78
Name a use of a diode.
Protection of d.c. circuits.
79
What is a cell?
A source of electrical energy.
80
What is a LDR?
* Light-depedent resistor
| * Resistance decreases as temperature is increased.
81
What is a positive temperature coefficient?
When resistance increases with increasing temperature.
82
What is a negative temperature coefficient?
When resistance decreases with increasing temperature.
83
Give an example of a material/component with a positive temperature coefficient.
Metals
84
Give an example of a material/component with a negative temperature coefficient.
Thermistor
85
What is the circuit symbol for a thermistor?
Rectangle with diagonal line across it, with a short line at the end.
86
What is the circuit symbol for a diode?
Circle with a triangle and line in it.
87
What is the circuit symbol for an LED?
Circle with a triangle and line in it. Two lines pointing away from it.
88
What does the symbol for a diode in a circuit tell us?
Current can flow in the direction that the triangle points.
89
What is the circuit symbol for a light bulb?
A circle with a cross in it.
90
What is the circuit symbol for a resistor?
A rectangle.
91
What is the circuit symbol for a variable resistor?
A rectangle with a diagonal arrow.
92
What is the circuit symbol for a LDR?
A rectangle with two arrows pointing towards it.
93
What is the circuit symbol for a heater?
A rectangle divided into 4 squares.
94
What is the circuit symbol for an electric motor?
A circle with an "M" with a line under it that points down and up at either end.
95
What is the resistivity of a material?
The resistance of a 1m length of the material of 1m2 cross-sectional area.
96
What 3 factors affect the resistance of a piece of wire?
1) Length
2) Area (Cross-sectional)
3) Resistivity
97
How does length of a wire affect its resistance?
The longer the wire, the higher the resistance (More difficult to make current flow through)
98
How does the cross-sectional area of a wire affect its resistance?
The wider the wire, the lower the resistance. (Easier for electrons to pass through)
99
What does resistivity measure?
How much a particular material resists current flow
100
What factors affect the resistivity of a material (not including resistance, length, area)?
Property of a material:
Structure of material and environmental factors (e.g. temperature and light intensity)
101
What is the unit for resistivity?
Ohm-metre (Ωm)
102
What is the symbol for resistivity?
p (Greek letter "rho")
103
What is the equation for resistivity?
```
p = RA/L
Where:
p - Resistivity (Ωm)
R - Resistance (Ω)
A - Area (m2)
L - Length (m)
```
104
Is the resistivity of a material a set quantity?
* No, it depends on temperature.
| * Resistivity is usually quoted at a set temperature (e.g. 25*)
105
What is the size of a typical value for the resistivity of a conductor?
Is this good for conducting?
Very small - e.g. 1.72 x 10^-8 Ohm meters.
| Lower resistivity = better at conducting electricity
106
Describe how you can calculate the resistivity of a piece of wire.
Risk assessment first.
Calculate area:
1) Measure the diameter at at least 3 points along the wire using a micrometre -> Find average -> Divide by two to get radius
2) Area = πr^2
Calculate R/l:
1) Set up a circuit with an ammeter, wire and voltmeter.
2) Attach a test wire along a ruler -> Attach one end where the ruler reads 0cm
3) Move the crocodile clip at the other end to adjust the length of the wire
4) Record the length of the wire and the resistance (R = V/I), keep V constant by adjusting the power supply.
5) Repeat this to find an average resistance for that length
6) Vary the length from 0.10 to 1.00m
7) Plot a graph of resistance (y) against length (x) + draw a line of best fit (should be straight line through origin)
Find the resistivity:
1) The gradient is R/l, so it can be subbed in to the equation p = RA/l by multiplying by the area.
2) Take note to maintain the temperature of the wire constant at all times (since resistivity depends on temperature) do this by turning off the power supply between readings to avoid the wire heating.
107
When calculating the resistivity of a piece of wire, what is it important to keep constant and how?
* Temperature -> Resistivity depends on it
| * Only have small currents flow through the wire
108
What is the problem with most materials having some resistivity?
When current is passed through them, they heat up and energy is wasted as thermal energy.
109
What is the easiest way to lower the resistivity of most materials?
Cool them down.
110
What is a superconductor?
A wire or device made of a material that has 0 resistivity below a critical temperature, which depends on the material.
111
Describe how the resistance of a superconductor changes with temperature.
* Below critical temperature -> Zero resistance
| * Above critical temperature -> Resistance increases
112
What is the temperature below which a superconductor has no resistance called?
Threshold temperature / Critical temperature
113
Describe how the resistance of these components varies with temperature:
• Metal
• Semiconductor
• Superconductor
* Metal -> Resistance increases as temperature increases
* Semiconductor -> Resistance decreases as temperature increases
* Superconductor -> Non-zero resistance above critical temperature, zero resistance below it
114
What are the positives of a superconductor and why?
* With no resistance, there is no heating effect -> No energy lost
* You could start a current using a magnet and it would flow forever
115
What is the problem that makes the use of superconductors difficult?
* Most have a transition temperature below 10 kelvin (263*C)
| * It is hard and expensive to cool things that much
116
What is a high-temperature superconductor?
A superconductor with a critical temperature above 77K (the boiling point of nitrogen).
117
What is 77K the boiling point of?
The boiling point of nitrogen.
118
Describe the graph of resistivity (y) against temperature (x) for a superconductor.
* Line along the x-axis up to critical temperature
* Vertical spike at the critical temperature
* Straight line with upwards gradient after this
119
What is the next development in terms of superconductors?
Room-temperature superconductors
120
Give some current uses of superconductors.
High-power electromagnets used in:
• MRI scanners
• Particle accelerators
121
Give some potential uses of superconductors.
* Power cables that transmit energy without energy loss
* Strong electromagnets without a constant power source -used in medicine and Maglev trains
* Electronic circuits that work really fast since there's no resistance to slow them down (therefore minimal energy loss)
122
What is power?
The rate of transfer of energy.
123
What is the symbol for power?
P
124
What is the unit for power?
Watt (W)
125
What is a watt?
1 joule transferred per second.
126
What is the formula for power?
P = E/t
Where:
P - Power (W)
E - Energy transferred (J)
t - Time (s)
127
What is the formula for power in terms of current and voltage?
P = I x V
128
What is the formula for power in terms of resistance and voltage?
P = V²/R
129
What is the formula for power in terms of current and resistance?
P = I² x R
130
How can the different equations for power by derived?
Combining:
• P = I x V
• V = I x R
131
What are the 4 equations for power?
* P = E/t
* P = I x V
* P = V² /R
* P = I² x R
132
What is the equation for energy transferred in terms of power and time?
E = P x t
NOTE: Any of the equations for P can be subbed in here (e.g. E = V x I x t).
133
What causes resistance?
Electrons colliding with atoms in the material and losing energy.
134
What is internal resistance and what causes it?
* The resistance of a battery itself.
| * Caused by electrons colliding with the atoms in the battery.
135
What causes cells and batteries to warm up when they are used?
Their internal resistances.
136
What is the symbol for internal resistance?
r
137
What is the name for the resistance of a cell?
Internal resistance
138
What is load resistance?
The total resistance of all the components in a circuit except the battery.
139
What is the symbol for load resistance?
R
140
What is another name for load resistance?
External resistance
141
What are the two types of resistance in a circuit?
* Internal resistance (r)
| * Load resistance (R)
142
What does e.m.f. stand for?
Electromotive force
143
What is e.m.f.?
* The amount of electrical energy the battery produces and transfers for each coulomb of charge
* i.e. It is the battery’s effective output voltage when no current flows through it
144
What is the symbol for e.m.f.?
ε
145
What are the units for e.m.f.?
Volts (V)
146
What is the equation that defines e.m.f.?
ε = E/Q
147
What is terminal p.d.?
* The potential difference across the load resistance (R).
| * i.e. The energy transferred per coulomb of charge flowing through the load resistance.
148
What is the symbol for terminal p.d.?
V
149
When is the terminal p.d. the same as e.m.f.?
| Why is this uncommon?
* If there is no internal resistance or no current flowing
| * Usually energy is lost in overcoming the internal resistance
150
What is the name for the energy wasted per coulomb in overcoming the internal resistance in a cell?
Lost volts
151
What is the symbol for lost volts?
v
152
What conservation of energy equation is used in e.m.f. calculations?
Energy per coulomb supplied by source (ε) = Energy per coulomb transferred in load resistance (V) + Energy per coulomb wasted in internal resistance (v)
ε = V + v
153
Which e.m.f. equation are you given in the exam?
ε = I(R + r)
154
In e.m.f. calculations, what are the different symbols?
```
ε - emf of battery
V - Terminal pd
v - Lost volts
R - Load resistance
r - Internal resistance
I - Current
```
155
Rearrange the “ε = I(R+r)” equation to give 3 more equations.
* ε = I(R+r)
* ε = V + v
* V = ε - v
* V = ε - Ir
156
How can we find energy lost as heat (energy dissipated) when overcoming the internal resistance in a power supply?
P=I^2
157
How can you work out the total e.m.f. of cells in series?
* Add their individual e.m.f.s
| * ε(total) = ε1 + ε2 + ε3 + ...
158
How can you work out the total e.m.f. of IDENTICAL cells in parallel?
* Total e.m.f. is equal to each individual e.m.f.
| * ε(total) = ε1 = ε2 = ε3 = ...
159
For identical cells in parallel, why is the total e.m.f of the combination of cells the same as the e.m.f of the individual cells?
The amount of charge flowing in the circuit doesn't increase by adding cells in parallel.
The number of paths the charges can take does change.
The current will split equally between identical cells.
The charge only gains e.m.f from the cells it travels through - overall e.m.f in circuit doesn't increase.
160
Three identical cells with an e.m.f. of 2.0V and an internal resistance of 0.20Ω are connected in parallel. A current of 0.90A is flowing through the circuit. Calculate the total p.d. across the cells.
(See diagram of 83 of revision guide)
```
Calculate the lost volts for 1 cell:
• I = 0.90 / 3 = 0.30A
• v = Ir = 0.30 x 0.20 = 0.06V
Find terminal pd across 1 cell:
• V = ε - v
• V = 2 - 0.06 = 1.94V
```
161
Describe an experiment to calculate the e.m.f. and internal resistance of a cell.
1) Connect the cell in series with an ammeter and variable resistor + connect a voltmeter across the cell
2) Vary the current using the variable resistor - start at highest resistance (open the switch and close it again to get two more sets of I and V values and find mean)
3) Record the voltage at each current.
4) Plot a graph of voltage (y) against current (x).
5) y-intercept = ε
Gradient = -r
Make sure external factors are kept the constant like temperature.
162
In the experiment to find the e.m.f. and internal resistance of a cell, how should the V and I values be plotted and why?
```
• V = ε - Ir
Rearranges to:
• V = -rI + ε
• y = mx + c
Therefore:
• Plot V on the y and I on the x
• Gradient = -r
• y-intercept = ε
```
163
What is an easy way to measure a cell’s e.m.f.?
* Connect a high-resistance voltmeter across its terminals
| * A small current flows through the voltmeter, so there are some lost volts, but this is negligible
164
When connecting a high-resistance voltmeter across a cell to find its e.m.f., what is the error in the results?
* Small amount of current flows through the voltmeter
* So there are some lost volts
* Measured value is very slightly less than the e.m.f.
* But this is negligible
165
Does charge get used up?
No
166
What happens to charge and current at a junction?
They are conserved.
167
What quantities are conserved at a junction?
* Current
| * Charge
168
Is voltage conserved at a junction?
Not necessarily.
169
What is Kirchhoff’s first law?
Total current entering junction = Total current leaving it
170
Is energy in a circuit conserved?
Yes
171
Energy transferred to a charge is...
e.m.f.
172
Energy transferred from a charge is...
Potential difference.
173
What is Kirchhoff’s second law?
Total e.m.f. around a series circuit = Sum of the p.d.s across each component
174
Explain Kirchhoff’s second law.
* e.m.f. is the energy transferred to a charge
* p.d. is the energy transferred from a charge
* Conservation of energy says that these two must be equal
* Therefore: Total e.m.f. = Sum of p.d.s
175
State both of Kirchhoff’s laws.
1) Total current entering a junction = Total current leaving it
2) Total e.m.f. around a series circuit = Sum of p.d.s across each component
176
Describe current in a series circuit.
It is the same at all points.
177
Describe e.m.f. in a series circuit.
* e.m.f. is split between components
| * ε = V1 + V2 + V3 + ...
178
Describe resistance in a series circuit.
* Sum of the individual resistances gives total resistance
| * R(total) = R1 + R2 + R3 + ...
179
Describe a series circuit in terms of:
• Current
• e.m.f.
• Resistance
```
CURRENT:
• Same at all points
EMF:
• Shared between components
RESISTANCE:
• Total resistance is sum of the individual resistances
```
180
Describe current in a parallel circuit.
* Split between branches (at each junction)
| * I(total) = I1 + I2 + I3 + ...
181
Describe e.m.f. in a parallel circuit.
* Same p.d. on each branch
* Within each branch, the sum of the p.d.s equals the e.m.f.
* e.m.f. = Branch 1 = Branch 2 = Branch 3 = ...
182
Describe resistance in a parallel circuit.
1/R(total) = 1/R1 + 1/R2 + 1/R3 + ...
183
Describe a parallel circuit in terms of:
• Current
• e.m.f.
• Resistance
CURRENT:
• Split between branches (at each junction)
EMF:
• Same p.d. on each branch
• Within each branch, the sum of the p.d.s equals the e.m.f.
RESISTANCE:
• 1/R(total) = 1/R1 + 1/R2 + 1/R3 + ...
184
Remember to practice conservation of energy and charge questions.
See example pg 85 of revision guide.
185
What is a potential divider?
A setup used to obtain an output voltage equal to a fraction of the source voltage.
186
What is the simplest possible potential divider?
A voltage source with 2 resistors in series (with wires around one of the resistors).
187
What range in potential difference can be provided by a potential divider?
From 0 to the source voltage.
188
How is PD split in a potential divider?
Potential difference across the voltage source (e.g. a battery) is split across the resistors in the ratio of the resistances:
i.e. The higher the resistance, the higher the voltage across that resistor
189
With a 2Ω and 3Ω resistor in series, what is the potential difference across each resistor?
* 2Ω -> 2/5 of the source voltage
| * 3Ω -> 3/5 of the source voltage
190
What does a potential divider look like?
191
For a source voltage, Vs, connected in series with resistors R1 and R2 (with output around R2), what equation gives the output voltage around R2?
V(out) = (R2 / (R1 + R2)) x Vs
i.e. Output voltage = (Output resistor voltage / Total load voltage) x Source voltage
192
How do you get: V(out) = (R2 / (R1 + R2)) x Vs
193
In potential divider diagrams, what is:
• Vs
• R1
• R2
* Vs - Source voltage
* R1 - Non-output resistor
* R2 - Output resistor
194
In a potential divider, if the source voltage is 9V and R1 is 100Ω, what must R2 be in order to get an output of 6V?
200Ω
195
What is a simple potential divider used for?
Calibrating voltmeters, which have a high resistance.
196
In a simple potential divider, what happens when you add a component with low resistance across R2 (the output resistor), what happens and why?
* You now have two resistors in parallel, so the total resistance will be lower than R2.
* This means that R2 will be lower than calculated.
197
In a simple potential divider, what is the effect of replacing R1 (non-output resistor) with a variable resistor?
It allows the output voltage across R2 to be varied.
198
In a simple potential divider with a variable resistor as R1, how does output voltage across R2 vary as the resistance R1 is increased?
* When R1 = 0, V(out) = Vs
| * As you increase R1, V(out) gets smaller
199
How does the resistance of a light-dependent resistor vary?
Resistance decreases as light intensity increases:
LDR has very high resistance in the dark, lower resistance in the light.
200
What is the circuit symbol for an LDR?
* Circle with a rectangle inside it.
* Wires reach to the rectangle, but not through it.
* Arrows point to the circle.
201
How does the resistance of a NTC thermistor vary?
Resistance decreases as temperature increases.
| opposite to normal resistor, but much more
202
How can a potential divider output be varied according to temperature or light intensity?
Using a thermistor or LDR as one of the resistors in the potential divider.
203
When an LDR is connected as R1 in a potential divider, with the output across another resistor, R2, how will the output vary as light increasing increases and why?
* Light gives the LDR a lower resistance.
* So it uses less of the PD and more can go to the output.
* Output voltage (V-out) increases as light intensity increases.
204
What happens to V-out when temperature increases (R1 = thermistor)
205
Give a use of LDRs and thermistors.
They can be included in circuits that control switches, e.g. to turn on a light or heating system.
206
What is a potentiometer?
A type of potential divider that uses a single variable resistor instead of two fixed resistors to give a desired voltage output.
207
How does a potentiometer divider work?
* The slider can be moved along the variable resistor to change R1 and R2 as desired.
* So V(out) can be varied from 0 to the source voltage
208
When is a potentiometer useful?
* When you need to change the voltage continuously
| * e.g. The volume control of a stereo
AQA A-Level Physics
flashcards
Decks in class (14)
# Cards
-
Section 1 - Particles (Use "AQA PARTICLES AND RADIATION" deck for more detailed cards)161
-
Section 2 - EM Radiation and Quantum Phenomena103
-
Section 3 - Waves266
-
Section 4 - Mechanics206
-
Section 5 - Materials133
-
Section 6 - Electricity208
-
Section 7 - Further Mechanics173
-
Section 8 - Thermal Energy Transfer135
-
Section 9 - Gravitational and Electric Fields164
-
Section 10 - Capacitors93
-
Section 11 - Magnetic Fields189
-
Section 12 - Nuclear Physics313
-
Option C - Engineering Physics (Rotational Dynamics)100
-
Option C: Engineering Physics (Thermodynamics)191
