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Current, Potential Difference and Resistance
Current, Potential Difference and Resistance
This lesson covers the fundamental electrical quantities — current, potential difference and resistance — as required by the AQA GCSE Physics specification (4.2.1). Understanding these three quantities and how they relate to one another is the foundation for every electricity topic in GCSE Physics.
Electric Current
Electric current is the rate of flow of electric charge. In metallic conductors (such as copper wires), the charge carriers are electrons. In electrolytes (solutions that conduct), the charge carriers are ions.
Current is measured in amperes (A) using an ammeter, which is always connected in series with the component being measured.
The equation for electric current is:
Q = I x t
Where:
- Q = charge flow in coulombs (C)
- I = current in amperes (A)
- t = time in seconds (s)
Rearranging for current: I = Q / t
| Quantity | Symbol | Unit | Unit Symbol |
|---|---|---|---|
| Charge | Q | Coulombs | C |
| Current | I | Amperes | A |
| Time | t | Seconds | s |
Conventional Current vs Electron Flow
- Conventional current flows from positive to negative (this was defined before electrons were discovered).
- Electron flow is from negative to positive (electrons are negatively charged, so they are attracted toward the positive terminal).
- In your GCSE exam, you should use conventional current direction unless the question specifically asks about electron flow.
Exam Tip: When asked to define electric current, say "the rate of flow of electric charge." Do not say "the flow of electrons" — current can also be carried by ions in solutions. The AQA mark scheme requires the precise definition involving charge.
Potential Difference (Voltage)
Potential difference (p.d.) is the energy transferred per unit of charge that passes between two points in a circuit. It is sometimes called voltage.
Potential difference is measured in volts (V) using a voltmeter, which is always connected in parallel across the component being measured.
The equation for potential difference is:
V = E / Q
Or equivalently: E = Q x V
Where:
- V = potential difference in volts (V)
- E = energy transferred in joules (J)
- Q = charge in coulombs (C)
| Quantity | Symbol | Unit | Unit Symbol |
|---|---|---|---|
| Potential difference | V | Volts | V |
| Energy transferred | E | Joules | J |
| Charge | Q | Coulombs | C |
Exam Tip: A common mistake is confusing potential difference with current. Remember: potential difference is about energy transfer — it tells you how much energy each coulomb of charge transfers as it passes through a component. Current tells you how much charge flows per second.
Resistance
Resistance is a measure of how much a component opposes the flow of current. The greater the resistance, the harder it is for current to flow.
Resistance is measured in ohms (the symbol is the Greek letter omega: a unit often written as "ohms").
The equation linking potential difference, current and resistance is:
V = I x R
Where:
- V = potential difference in volts (V)
- I = current in amperes (A)
- R = resistance in ohms
Rearranging: R = V / I and I = V / R
| Quantity | Symbol | Unit | Unit Symbol |
|---|---|---|---|
| Potential difference | V | Volts | V |
| Current | I | Amperes | A |
| Resistance | R | Ohms | (ohms) |
What Causes Resistance?
Resistance occurs because charge carriers (usually electrons) collide with the ions in the lattice structure of the conductor. These collisions transfer energy from the electrons to the ions, causing the conductor to heat up. The more collisions, the greater the resistance.
Factors affecting resistance of a wire:
- Length — longer wires have more resistance (more collisions possible).
- Cross-sectional area — thinner wires have more resistance (charge carriers are more "squeezed").
- Material — different materials have different resistivities.
- Temperature — for most conductors, higher temperature means more resistance (ions vibrate more, causing more collisions).
Exam Tip: You must be able to rearrange V = I x R confidently. A useful triangle method: put V at the top, I on the bottom-left and R on the bottom-right. Cover the quantity you want to find and the remaining arrangement gives you the formula.
Circuit Symbols
You must be able to recognise and draw the following standard circuit symbols for your GCSE exam:
| Component | Description |
|---|---|
| Cell | A single cell provides the potential difference (one long line and one short line) |
| Battery | Two or more cells joined together (multiple long and short lines) |
| Switch (open) | A break in the circuit — no current flows |
| Switch (closed) | A complete connection — current can flow |
| Ammeter | Circle with an A inside — measures current (connected in series) |
| Voltmeter | Circle with a V inside — measures p.d. (connected in parallel) |
| Resistor | A rectangle — a component with a fixed resistance |
| Variable resistor | A rectangle with an arrow through it — resistance can be changed |
| Lamp (bulb) | A circle with a cross inside |
| Diode | A triangle pointing to a line — allows current in one direction only |
| LED | A diode symbol with two small arrows pointing outward (light emitted) |
| Thermistor | A rectangle with a line through it — resistance changes with temperature |
| LDR | A rectangle with two arrows pointing toward it — resistance changes with light intensity |
Connecting Meters in a Circuit
Ammeters
- Always connected in series with the component.
- An ammeter has a very low resistance so that it does not significantly affect the current in the circuit.
Voltmeters
- Always connected in parallel across the component.
- A voltmeter has a very high resistance so that very little current flows through it (almost all the current flows through the component being measured).
graph LR
A[Battery] --> B[Ammeter]
B --> C[Resistor]
C --> A
D[Voltmeter] -.-> |connected in parallel| C
Exam Tip: If a question asks you to add an ammeter to a circuit diagram, draw it in the main loop of the circuit (in series). If asked to add a voltmeter, draw it as a separate branch across (in parallel with) the component you are measuring.
Worked Examples
Example 1: Calculating Charge
A current of 3 A flows through a lamp for 2 minutes. Calculate the charge that flows.
Step 1: Convert time to seconds: 2 minutes = 2 x 60 = 120 s
Step 2: Use Q = I x t
Q = 3 x 120 = 360 C
Example 2: Calculating Potential Difference
A charge of 50 C passes through a resistor and transfers 150 J of energy. Calculate the potential difference across the resistor.
Step 1: Use V = E / Q
V = 150 / 50 = 3 V
Example 3: Calculating Resistance
A lamp has a current of 0.5 A flowing through it when the potential difference across it is 6 V. Calculate its resistance.
Step 1: Use R = V / I
R = 6 / 0.5 = 12 ohms
Summary
- Electric current is the rate of flow of charge, measured in amperes (A) with an ammeter in series.
- Charge is calculated using Q = I x t.
- Potential difference is the energy transferred per coulomb of charge, measured in volts (V) with a voltmeter in parallel.
- Energy transferred is calculated using E = Q x V.
- Resistance opposes the flow of current and is measured in ohms.
- The key equation linking all three quantities is V = I x R.
- Resistance depends on the length, cross-sectional area, material and temperature of a conductor.
- You must know all the standard circuit symbols and how to connect ammeters and voltmeters correctly.
Exam Tip: In calculation questions, always show your working clearly: write the formula, substitute the values (with units), and give the final answer with the correct unit. Even if you make an arithmetic error, you can still gain marks for correct method.