What you'll learn
This revision guide covers the fundamental concepts of electric current, charge and potential difference as specified in the AQA GCSE Physics specification. You'll learn how to calculate charge flow, understand the relationship between current and potential difference, and apply these concepts to both series and parallel circuits. These principles form the foundation for understanding all electrical circuits and appear frequently in both Foundation and Higher tier papers.
Key terms and definitions
Electric current — the rate of flow of electrical charge around a circuit, measured in amperes (A)
Charge — a property of matter that causes it to experience a force in an electric field, measured in coulombs (C)
Potential difference (voltage) — the energy transferred per unit charge between two points in a circuit, measured in volts (V)
Resistance — a measure of how difficult it is for current to flow through a component, measured in ohms (Ω)
Coulomb — the SI unit of electrical charge; one coulomb is the charge that flows past a point when a current of 1 ampere flows for 1 second
Volt — the SI unit of potential difference; one volt means one joule of energy is transferred per coulomb of charge
Ammeter — a device connected in series to measure the current flowing through a component
Voltmeter — a device connected in parallel across a component to measure the potential difference across it
Core concepts
Electric current and charge flow
Electric current is fundamentally the movement of charged particles. In metal conductors, these charged particles are electrons that move from the negative terminal towards the positive terminal. However, by convention, we describe current as flowing from positive to negative (conventional current).
The relationship between current, charge and time is expressed by the equation:
Q = I × t
Where:
- Q = charge flow in coulombs (C)
- I = current in amperes (A)
- t = time in seconds (s)
This equation tells us that:
- A larger current means more charge flows per second
- The same current flowing for longer means more total charge flows
- One ampere represents one coulomb of charge flowing per second
Key points about current:
- Current is the same at all points in a series circuit
- Current is measured using an ammeter connected in series
- Current divides at junctions in parallel circuits
- The total current entering a junction equals the total current leaving it (conservation of charge)
Potential difference and energy transfer
Potential difference (often called voltage) represents the energy transferred by each unit of charge as it moves between two points in a circuit. When charge flows through a component, electrical energy is converted to other forms such as heat, light or kinetic energy.
The relationship between energy transferred, charge and potential difference is:
E = Q × V
Where:
- E = energy transferred in joules (J)
- Q = charge in coulombs (C)
- V = potential difference in volts (V)
This can be rearranged to give:
V = E / Q
This shows that one volt means one joule of energy is transferred per coulomb of charge.
Key points about potential difference:
- Potential difference is measured using a voltmeter connected in parallel across a component
- In a series circuit, the total potential difference across the power supply is shared between components
- In a parallel circuit, the potential difference across each branch is the same as the supply
- Components with higher resistance have a greater potential difference across them (when in series)
Current in series and parallel circuits
Series circuits:
- Current is identical at all points in the circuit
- There is only one path for current to flow
- If one component fails, the entire circuit stops working
- The same charge passes through each component
Parallel circuits:
- Current splits at junctions
- Multiple paths exist for current to flow
- If one branch fails, current continues in other branches
- The sum of currents in each branch equals the total current from the supply
For parallel circuits, the junction rule states:
I(total) = I₁ + I₂ + I₃ + ...
This reflects the conservation of charge — charge cannot be created or destroyed at a junction.
Combining current, charge and potential difference equations
You can combine the fundamental equations to solve more complex problems:
Since Q = I × t and E = Q × V, we can substitute to get:
E = I × V × t
This equation shows the total energy transferred when a current I flows through a potential difference V for time t.
Rearranging gives:
- V = E / (I × t)
- I = E / (V × t)
- t = E / (I × V)
These rearrangements are particularly useful for exam questions involving energy transfers in circuits.
Measuring current and potential difference
Using an ammeter:
- Connect in series with the component
- Current must flow through the ammeter
- Modern digital ammeters have very low resistance
- Select an appropriate range before connecting
- Conventional current flows from red (positive) to black (negative) terminal
Using a voltmeter:
- Connect in parallel across the component
- No significant current flows through an ideal voltmeter
- Voltmeters have very high resistance
- Measures the energy difference between two points
- Red terminal connects to the more positive point
Practical considerations:
- Always check meters are set to d.c. (direct current) for battery circuits
- Zero the meter before use if possible
- Read the scale carefully, noting the divisions
- Record whether using mA (milliamps) or A (amps)
Charge carriers and current
In metal conductors, the charge carriers are free electrons. These electrons:
- Are negatively charged
- Move relatively slowly through the metal (drift velocity)
- Collide with metal ions, causing resistance
- Are already present in the metal before the circuit is switched on
When a cell is connected:
- An electric field is established almost instantly throughout the circuit
- All free electrons begin moving together
- This creates the current immediately
- The electrons themselves move quite slowly (typically mm/s)
The number of charge carriers and their speed determine the current. A thicker wire has more charge carriers available and therefore less resistance to current flow.
Worked examples
Example 1: Calculating charge flow (Foundation/Higher)
Question: A current of 3.0 A flows through a light bulb for 5 minutes. Calculate the charge that flows through the bulb.
Answer:
Step 1: Write down the equation Q = I × t
Step 2: Identify known values
- I = 3.0 A
- t = 5 minutes = 5 × 60 = 300 s (must convert to seconds)
Step 3: Substitute and calculate Q = 3.0 × 300 Q = 900 C
The charge that flows through the bulb is 900 coulombs. ✓
Marks: 3 marks (1 for equation, 1 for correct substitution/time conversion, 1 for answer with unit)
Example 2: Energy transfer in a circuit (Higher)
Question: A 12 V car battery transfers 7200 J of energy to start the engine. Calculate: (a) the charge that flows from the battery (b) the average current if the starter motor runs for 2.5 seconds
Answer:
(a) Step 1: Write the equation E = Q × V, so Q = E / V
Step 2: Substitute values Q = 7200 / 12 Q = 600 C ✓
(b) Step 1: Write the equation Q = I × t, so I = Q / t
Step 2: Substitute values I = 600 / 2.5 I = 240 A ✓
Marks: 5 marks total (2 for part a: equation and answer; 3 for part b: equation, substitution, answer with unit)
Example 3: Current in parallel circuits (Foundation/Higher)
Question: In a parallel circuit, the current from the battery is 1.5 A. The current through one branch is 0.6 A and through a second branch is 0.4 A. Calculate the current through the third branch.
Answer:
Step 1: Apply the junction rule Total current = sum of branch currents I(total) = I₁ + I₂ + I₃
Step 2: Rearrange for the unknown current I₃ = I(total) - I₁ - I₂
Step 3: Substitute values I₃ = 1.5 - 0.6 - 0.4 I₃ = 0.5 A ✓
The current through the third branch is 0.5 A.
Marks: 3 marks (1 for method/equation, 1 for calculation, 1 for answer with unit)
Common mistakes and how to avoid them
Not converting time to seconds — the equation Q = I × t requires time in seconds. Always convert minutes or hours to seconds before calculating. Remember: 1 minute = 60 seconds, 1 hour = 3600 seconds.
Confusing current and charge — current is the rate of flow (amount per second), while charge is the total amount that has flowed. Current is like the speed of water flow; charge is like the total volume of water that has passed.
Mixing up ammeter and voltmeter connections — ammeters go in series (current flows through them), voltmeters go in parallel (connected across components). A reversed connection can damage the meter or give meaningless readings.
Forgetting units in final answers — always include the correct unit (A, C, V, J, or s). Exam mark schemes typically award a separate mark for the correct unit, so you lose marks even with correct numbers.
Incorrectly applying series/parallel rules — in series, current is the same everywhere but voltage divides; in parallel, voltage is the same across each branch but current divides. Learn these rules separately and practice identifying which type of circuit you're analyzing.
Rounding too early in multi-step calculations — keep all digits during intermediate steps and only round your final answer to an appropriate number of significant figures (usually 2 or 3 for GCSE). Early rounding introduces errors that accumulate through calculations.
Exam technique for "Current, charge and potential difference"
Command words matter: "Calculate" requires you to show your working and includes the equation, substitution and answer with units (typically 3-4 marks). "State" needs only a direct answer (1 mark). "Explain" requires reasoning about why something happens (2-3 marks depending on mark allocation).
Show all working clearly: Even if your final answer is wrong, you can gain method marks for correct equations and substitution. Write the equation in symbols first, then substitute numbers, then calculate. This structured approach makes your logic clear to examiners.
Watch for two-part calculations: Questions often require you to calculate an intermediate value first (like finding charge) before using it to find the final answer (like energy). Read the entire question before starting to identify what you need to find at each stage.
Draw on circuit diagrams: If a question shows a circuit, annotate it with given values and what you need to find. Mark current directions and label voltages. This visual approach helps avoid confusion in complex series-parallel combinations and ensures you use the correct values.
Quick revision summary
Electric current is the rate of flow of charge (Q = I × t), measured in amperes. Potential difference measures energy transferred per unit charge (V = E / Q), measured in volts. In series circuits, current is constant but voltage divides; in parallel circuits, voltage is constant but current divides. Ammeters measure current in series; voltmeters measure potential difference in parallel. One ampere equals one coulomb per second; one volt equals one joule per coulomb. Always convert time to seconds and include units in answers.