Current, potential difference and resistance

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Circuit Diagrams and Components

Circuit diagrams use standard symbols to represent components.
A cell or battery provides a source of potential difference; a battery is two or more cells.
A switch turns the circuit on (closed) or off (open).
A fixed resistor has a resistance that cannot change; a variable resistor can change its resistance, often used in dimmer switches and volume controls.
A thermistor's resistance depends on temperature: as temperature increases, resistance decreases.
An LDR (light-dependent resistor) has a resistance that depends on light intensity: as light intensity increases, resistance decreases.
A diode allows current to flow in one direction only (forward bias); in reverse bias, it has very high resistance.
An LED is a diode that emits light when current passes through it.
A fuse is a safety device that melts and breaks the circuit if the current becomes too large.
An ammeter measures current and is connected in series; a voltmeter measures potential difference and is connected in parallel.

Electrical symbols used in circuit diagrams

Charge and Current

For charge to flow through a closed circuit, there must be a source of potential difference.
Electric current is the rate of flow of electrical charge.
In metals, the charge that flows is carried by electrons.
The unit of charge is the coulomb (C); one coulomb is the charge passing a point when a current of 1 A flows for 1 second.
Charge flow, current and time are related by the equation: Q = I t (charge = current × time).
Current is measured in amperes (A) using an ammeter connected in series.
In a single closed loop, the current has the same value at any point.
Conventional current is the flow of positive charge from the positive terminal to the negative terminal; electrons flow in the opposite direction.

Potential Difference and Resistance

Potential difference (p.d.) is the energy transferred per unit charge passing between two points; it is measured in volts (V).
Potential difference is measured using a voltmeter connected in parallel with the component.
Resistance is the opposition to current; it is measured in ohms (Ω).
The current through a component depends on both its resistance and the potential difference across it.
For a given potential difference, the greater the resistance, the smaller the current.
The relationship between potential difference, current and resistance is given by: V = I R.
An ohm is equivalent to one volt per ampere (1 V/A).

Required Practical: Investigating Resistance

Investigate how the length of a wire at constant temperature affects its resistance.
Independent variable: length of wire; dependent variable: resistance; control variables: potential difference of power supply and temperature of wire.
Set up a circuit with a thin resistance wire, crocodile clips, ammeter, voltmeter and power supply.
Measure current and potential difference for different lengths (e.g., 10 cm intervals up to 1 m).
Calculate resistance using R = V / I for each length.
Plot a graph of resistance against length; it should be a straight line through the origin, showing resistance is directly proportional to length.
To reduce heating effects, switch off the circuit between readings and use small currents.

Resistors in Series and Parallel

In series, the total resistance is the sum of the individual resistances: R_total = R1 + R2 + ...
In series, the current is the same through each component, and the total potential difference is shared.
In parallel, the total resistance is less than the sum of the individual resistances.
In parallel, the potential difference across each branch is the same, and the current splits between branches.
Adding more resistors in series increases total resistance; adding more in parallel decreases total resistance.

Ohmic Conductors and I–V Graphs

Ohm's Law states that the current through a conductor is directly proportional to the potential difference across it, provided temperature is constant.
Ohmic conductors have a constant resistance and their I–V graph is a straight line through the origin.
Examples of ohmic conductors: fixed resistors, wires, heating elements.
Non-ohmic conductors do not obey Ohm's Law; their resistance changes with current.
Examples of non-ohmic conductors: filament lamps, diodes, thermistors, LDRs.
For a filament lamp, as current increases, the filament temperature increases, causing resistance to increase; the I–V graph curves.
For a diode, current flows in one direction only; the I–V graph shows a sharp increase in forward bias and zero current in reverse bias.

Thermistors and LDRs

A thermistor is a temperature-dependent resistor; its resistance decreases as temperature increases.
Thermistors are used in thermostats, ovens, refrigerators, fire alarms, digital thermometers and boilers.
An LDR is a light-dependent resistor; its resistance decreases as light intensity increases.
LDRs are used in automatic lights (e.g., street lights, garden lights), burglar alarms, and light meters.
To investigate a thermistor, measure current and potential difference as temperature changes; calculate resistance using R = V / I.
To investigate an LDR, measure current and potential difference as light intensity changes; calculate resistance using R = V / I.
In both investigations, the ammeter is connected in series and the voltmeter in parallel with the component.

Required Practical: Investigating I–V Characteristics

Aim: investigate the I–V characteristics of a fixed resistor, filament lamp and diode.
Independent variable: potential difference; dependent variable: current; control variables: potential difference of power supply, same equipment.
Set up a circuit with a variable resistor to change the potential difference across the component.
Record current for a range of potential differences (both positive and negative).
Plot a graph of current against potential difference for each component.
The fixed resistor gives a straight line through the origin (ohmic); the filament lamp gives a curve; the diode gives a sharp increase in forward bias and zero in reverse bias.
Switch off between readings to prevent heating, which would affect resistance.

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1.Which of the following is needed in a circuit for charge to flow?
Easy
- AA source of resistance
- BA closed circuit loop
- CA source of potential difference
- DAn ammeter
2.Which of the following is the correct symbol for a battery?
Easy
- ATwo parallel lines, one long and one short
- BTwo parallel lines of equal length
- CA circle with a cross inside
- DA zigzag line
3.Which instrument is used to measure potential difference?
Easy
- AAmmeter
- BJoulemeter
- CNewtonmeter
- DVoltmeter
4.Which of the following diagrams shows the incorrect placement of a voltmeter?
Easy
- AVoltmeter connected in parallel with a resistor
- BVoltmeter connected in series with a resistor
- CVoltmeter connected across a battery
- DVoltmeter connected across a lamp
5.Which of the following statements is not true about current?
Easy
- ACurrent has the same value anywhere in a closed loop
- BCurrent is the rate of charge flow
- CCurrent is measured by a voltmeter
- DThe unit for current is the coulomb
6.The unit for charge is the coulomb. Which of the following is the correct equation for charge?
Easy
- AQ = I / t
- BQ = t / I
- CQ = I t
- DQ = I + t
7.A resistor of 10 Ω has a current of 0.5 A through it. Calculate the potential difference across the resistor.
Medium
- A0.05 V
- B5 V
- C20 V
- D0.5 V
8.Which of the following is the correct equation linking potential difference, current, and resistance?
Medium
- AV = I R
- BV = I / R
- CV = I + R
- DV = I² R

Thêm câu hỏi

9.Which of the following are non-ohmic conductors? (Select all that apply.)
Medium
- AFixed resistor
- BFilament lamp
- CDiode
- DThermistor
- ECopper wire
10.The current in a closed loop is the same at every point.
Easy
True or false?
11.The resistance of a thermistor increases as its temperature increases.
Easy
True or false?
12.A diode allows current to flow in both directions.
Easy
True or false?
13.Which component is used to measure the current in a circuit?
Easy
- AVoltmeter
- BAmmeter
- CThermistor
- DLDR
14.What is the resistance of a component when a potential difference of 6 V causes a current of 2 A?
Medium
- A3 Ω
- B12 Ω
- C0.33 Ω
- D8 Ω
15.Which of the following is a correct unit for resistance?
Medium
- AVolt (V)
- BAmpere (A)
- COhm (Ω)
- DCoulomb (C)
16.Which of the following components has a resistance that decreases as light intensity increases?
Medium
- AThermistor
- BLDR
- CDiode
- DFixed resistor
17.Which of the following graphs represents an ohmic conductor?
Medium
- AA straight line through the origin
- BA curve that gets steeper
- CA curve that gets less steep
- DA horizontal line
18.A charge of 20 C flows through a lamp in 4 s. What is the current?
Medium
- A5 A
- B80 A
- C0.2 A
- D24 A
19.Which of the following statements about conventional current is correct?
Medium
- AIt flows from negative to positive.
- BIt is the flow of positive charge from positive to negative.
- CIt is the same as electron flow.
- DIt only flows in diodes.
20.Which of the following is the correct symbol for a thermistor?
Medium
- AA rectangle with a diagonal line through it
- BA circle with a cross
- CA rectangle with a line and two arrows pointing towards it
- DA rectangle with a line and two arrows pointing away from it
21.A filament lamp is connected to a variable power supply. As the potential difference across the lamp increases, what happens to its resistance?
Hard
- AIt decreases
- BIt stays the same
- CIt increases
- DIt becomes zero
22.A circuit contains a battery and a fixed resistor. If the resistance of the resistor is doubled while the potential difference remains constant, what happens to the current?
Hard
- AIt doubles
- BIt halves
- CIt stays the same
- DIt becomes zero
23.A student sets up a circuit to investigate the I–V characteristics of a filament lamp. Why should the circuit be switched off between readings?
Hard
- ATo prevent the lamp from cooling down
- BTo prevent the lamp from overheating and changing its resistance
- CTo save battery power
- DTo allow the ammeter to reset
24.Which of the following best describes the relationship between current and potential difference for an ohmic conductor at constant temperature?
Hard
- ACurrent is inversely proportional to potential difference.
- BCurrent is directly proportional to potential difference.
- CCurrent is independent of potential difference.
- DCurrent is proportional to the square of potential difference.
25.A thermistor is used in a circuit to monitor temperature. As the temperature of the thermistor increases, what happens to the current through it if the potential difference across it is constant?
Challenge
- AIt increases
- BIt decreases
- CIt stays the same
- DIt becomes zero
26.Which of the following statements about the I–V graph of a diode is correct?
Challenge
- AIt is a straight line through the origin.
- BIt shows current flowing in both directions.
- CIt shows a sharp increase in current in forward bias and almost zero current in reverse bias.
- DIt shows current decreasing as potential difference increases.
27.Match each component to its correct function.
Medium
- Ammeter
- Voltmeter
- Thermistor
- LDR
- Measures current in a circuit
- Measures potential difference across a component
- Resistance decreases as temperature increases
- Resistance decreases as light intensity increases
28.Place the steps of the required practical to investigate the resistance of a wire in the correct order.
Medium
- Connect the wire to the circuit using crocodile clips 10 cm apart
- Record the potential difference and current
- Move the crocodile clips 10 cm further apart
- Repeat the readings at each length
- Calculate the resistance using R = V/I
29.What shape would you expect the I–V graph to be?
Medium
- AA straight line through the origin
- BA curve that gets steeper
- CA curve that gets less steep
- DA horizontal line
30.What does the straight line through the origin show about the resistor?
Medium
- AResistance increases with current
- BResistance decreases with current
- CCurrent is directly proportional to potential difference
- DCurrent is inversely proportional to potential difference
31.A battery is a source of potential difference in a circuit.
Easy
True or false?
32.Which of the following are true about an ammeter? (Select all that apply.)
Medium
- AIt is connected in series in the circuit
- BIt measures potential difference
- CIts resistance is taken to be negligible in exam questions
- DIt is connected in parallel with a component
- EIt measures the current in a circuit
33.Match each circuit component to the correct description of how it behaves.
Medium
- Thermistor
- LDR
- Diode
- Filament lamp
- Resistance decreases as temperature increases
- Resistance decreases as light intensity increases
- Allows current in one direction only
- Resistance increases as the filament gets hotter
34.A component has a potential difference of 5 V across it and a current of 0.25 A through it. What is its resistance?
Medium
- A0.05 Ω
- B1.25 Ω
- C20 Ω
- D20 V
35.Which of the following are examples of ohmic conductors at constant temperature? (Select all that apply.)
Medium
- AA fixed resistor
- BA filament lamp
- CA wire
- DA diode
- EA heating element
36.Place the steps for investigating how the resistance of a filament lamp varies with potential difference in the correct order.
Medium
- Set up the circuit with the lamp, ammeter in series and voltmeter in parallel
- Adjust the variable resistor to change the potential difference across the lamp
- Record the potential difference and current for each setting
- Switch off the circuit between readings to prevent the lamp heating too much
- Reverse the power supply terminals and take readings for negative values
37.A thermistor is connected in series with a cell. As the temperature of the thermistor increases, what happens to its resistance and the current in the circuit?
Hard
- AResistance increases; current decreases
- BResistance decreases; current increases
- CResistance decreases; current decreases
- DResistance increases; current increases
38.Which of the following statements about the I–V graph of a filament lamp are correct? (Select all that apply.)
Medium
- AIt is a straight line through the origin
- BIt is a curve
- CThe resistance increases as the current increases
- DIt shows the current and potential difference are directly proportional
- EThe resistance is constant
39.A charge of 60 C flows through a heater in 30 s. What is the current through the heater?
Medium
- A0.5 A
- B2 A
- C1800 A
- D30 A
40.A 12 V battery is connected across a resistor. The current through the resistor is 0.25 A. What is the resistance of the resistor?
Medium
- A48 Ω
- B3 Ω
- C0.02 Ω
- D12.25 Ω
41.Calculate the charge that flows when a current of 2 A passes through a component for 10 s. Give your answer in coulombs.
Medium
42.Calculate the resistance of a component when a potential difference of 12 V causes a current of 3 A. Give your answer in ohms.
Medium
43.Complete the sentence about current.
Easy
Current is the rate of flow of electrical ____.
44.Complete the sentence about resistance.
Medium
Resistance is defined as the ____ to current.
45.What is the unit of resistance?
Easy
46.What is the unit of charge?
Easy
47.What is the name of the component whose resistance decreases as light intensity increases?
Medium
48.State the name of the component that allows current to flow in one direction only.
Easy
49.A charge of 15 C flows through a component in 5 s. Calculate the current through the component. Give your answer in amperes.
Medium
50.A charge of 90 C flows through a lamp in 45 s. Calculate the current through the lamp. Give your answer in amperes.
Medium
51.Complete the sentence about resistance using the correct words.
Easy
Resistance is the opposition to ____, and it is measured in ____.
52.Which component allows current to flow in one direction only?
Medium
53.Explain how the resistance of a filament lamp changes as the potential difference across it increases.
Medium
54.Explain why the current in a closed circuit is the same at every point.
Medium
55.Describe how you would investigate how the resistance of a thermistor changes with temperature.
Hard
56.Explain why the current is the same at every point in a single closed loop.
Hard
57.Explain why the current through a filament lamp increases more slowly than the potential difference across it as the potential difference increases.
Hard
58.Calculate the charge that flows when a current of 2 A passes through a component for 10 s. Give your answer in coulombs.
Medium
- A18 C
- B22 C
- C20 C
- D24 C
59.Calculate the resistance of a component when a potential difference of 12 V causes a current of 3 A. Give your answer in ohms.
Medium
- A6 Ω
- B4 Ω
- C5 Ω
- D3 Ω
60.A charge of 15 C flows through a component in 5 s. Calculate the current through the component. Give your answer in amperes.
Medium
- A4 A
- B5 A
- C2 A
- D3 A
61.A charge of 90 C flows through a lamp in 45 s. Calculate the current through the lamp. Give your answer in amperes.
Medium
- A3 A
- B1 A
- C4 A
- D2 A

Thẻ ghi nhớ

What is electric current?

Electric current is the rate of flow of electrical charge. It is measured in amperes (A).

What is the equation linking charge flow, current and time?

charge flow (C) = current (A) × time (s), or Q = It.

What is potential difference?

Potential difference is the energy transferred per unit charge flowing from one point to another. It is measured in volts (V).

What is the equation linking potential difference, current and resistance?

potential difference (V) = current (A) × resistance (Ω), or V = IR.

What is resistance?

Resistance is the opposition to current. It is measured in ohms (Ω).

What is an ohmic conductor?

An ohmic conductor is one where the current through it is directly proportional to the potential difference across it, provided the temperature is constant. Its resistance remains constant.

What does the I–V graph of an ohmic conductor look like?

A straight line through the origin. This shows that current is directly proportional to potential difference.

How does the resistance of a filament lamp change as the current increases?

The resistance increases as the temperature of the filament increases. This makes the I–V graph curve.

What does the I–V graph of a filament lamp look like?

A curve that becomes less steep as potential difference increases. The current increases at a slower rate than the potential difference.

How does a diode behave in a circuit?

A diode allows current to flow in one direction only (forward bias). In the reverse direction, it has a very high resistance and no current flows.

What does the I–V graph of a diode look like?

For forward bias, there is a sharp increase in current after a certain potential difference. For reverse bias, the current is zero.

How does the resistance of a thermistor change with temperature?

As temperature increases, the resistance of a thermistor decreases. As temperature decreases, its resistance increases.

Give two applications of thermistors.

Thermostats in ovens or refrigerators, and digital thermometers.

How does the resistance of an LDR change with light intensity?

As light intensity increases, the resistance of an LDR decreases. As light intensity decreases, its resistance increases.

Give two applications of LDRs.

Automatic street lights that switch on when it gets dark, and burglar alarm circuits.

What is the difference between fixed and variable resistors?

A fixed resistor has a resistance that cannot change. A variable resistor has a resistance that can be changed, for example to control current.

How is an ammeter connected in a circuit?

An ammeter is always connected in series with the component or part of the circuit you want to measure the current through.

How is a voltmeter connected in a circuit?

A voltmeter is always connected in parallel with the component you want to measure the potential difference across.

What is conventional current?

Conventional current is the flow of positive charge from the positive terminal to the negative terminal of a power supply. It is opposite to the direction of electron flow.

In a single closed loop, how does the current at one point compare with the current at another point?

The current has the same value at every point in a single closed loop.

What is needed in a circuit for charge to flow?

A source of potential difference and a closed circuit loop.

What is the unit of charge?

The coulomb (C). One coulomb is the charge that passes a point when a current of 1 A flows for 1 second.

What is the relationship between resistance and current for a given potential difference?

The greater the resistance, the smaller the current for a given potential difference. The lower the resistance, the greater the current.

In the required practical to investigate resistance of a wire, what is the independent variable?

The length of the resistance wire.

In the required practical to investigate resistance of a wire, what is the dependent variable?

The resistance of the wire.

What is the relationship between the length of a wire and its resistance?

The resistance of a wire is directly proportional to its length, provided the temperature is constant.

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