Current, Resistance and Potential Difference

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CognitoOhm's law and resistance

Physics OnlineI-V characteristics

Key facts & equations

Key definitions

Current (I): rate of flow of charge. Units: amperes (A). I = Q/t.
Potential difference (V): work done per unit charge. Units: volts (V). Energy transferred per coulomb.
Resistance (R): opposition to current flow. Units: ohms (Ω).
Ohm's law: V = IR. At constant temperature, current is proportional to voltage.
Rearranged: I = V/R. R = V/I.

I-V characteristics

Ohmic conductor (e.g. resistor at constant T): straight line through origin. R constant.
Filament lamp: curve — resistance increases as temperature increases (metal gets hotter → more resistance).
Diode: only allows current in one direction (forward bias). Reverse bias: no current. Very low R in forward direction above ~0.7 V.
LDR (light-dependent resistor): resistance decreases as light intensity increases.
Thermistor: resistance decreases as temperature increases (used in temperature sensors).

Required practical — resistance

Circuit: power supply, ammeter in series, voltmeter in parallel across component.
Vary voltage (using variable resistor or rheostat).
Record V and I. Calculate R = V/I for each reading.
Plot I vs V — straight line = Ohmic. Gradient = 1/R.
Repeat for filament lamp: note I-V curve shows decreasing gradient (increasing R).

Exam questions — 4 questions · 10 marks · AQA 8463 style

3 marksV=IR calculationAQA 8463 P2 style 🔢 Calculator

A 12 V battery is connected to a 30 Ω resistor. (a) Calculate the current. (b) Calculate the power dissipated. (3 marks)

Hint: (a) Rearrange V=IR to I=V/R. (b) P = IV (use your calculated current and given voltage).

+30 XP

1 markFilament lamp characteristicAQA 8463 style

Why does the resistance of a filament lamp increase as the voltage across it increases?

A The filament gets shorter as it heats up

B Higher voltage causes the filament to melt partially

C Higher current heats the tungsten filament — higher temperature increases resistance

D Resistance is directly proportional to voltage for all components

3 marksRequired practicalAQA 8463 P2 style

Describe how to carry out an experiment to determine the I-V characteristic of a resistor. Include the circuit setup and the measurements taken. (4 marks)

Hint: Circuit: series with ammeter, parallel voltmeter. Method: vary voltage, record V and I pairs. Plot I vs V.

+30 XP

1 markLDR applicationAQA 8463 style

An LDR is used in a circuit to control a streetlight. As it gets dark, the LDR resistance:

A Decreases, causing the voltage across it to fall, turning the light off

B Increases, causing the voltage across it to rise, which can trigger the light to switch on

C Stays constant, regardless of light level

D Decreases, triggering the lamp off

3 marks Charge flow — time in minutes AQA 8463 style ⚡ Unit conversion trap

A battery supplies a current of 130,000 A. Calculate the charge flow from the battery in 5 minutes. Give your answer in coulombs. (3 marks)

Trap: Time is given in MINUTES. Convert to seconds first: 5 minutes = 5 × 60 = 300 seconds. Then Q = I × t. Unit is coulombs (C), not watts or joules.

+30 XP

Current, Resistance and Potential Difference

Key definitions

I-V characteristics

Required practical — resistance

Module complete! 🎉