The current (I) through a component depends on both the resistance (R) of the component and the potential difference (V) across the component.
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GCSE Physics Revision
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GCSE Physics revision
Current, resistance and potential difference
Current, potential difference and resistance
Your specification
AQA student objectives
Learning pathway
All · Most · Some
Apply the specified scientific knowledge of current, resistance and potential difference to a relevant example.
Analyse an unfamiliar example of current, resistance and potential difference using the specified scientific ideas.
Revision summary
Key knowledge
Read on screen, then print for Cornell-style active revision.
What is an Electrical Circuit?
- An electrical circuit is a closed loop containing a power source, such as a cell, and a conductor like a wire for electrons to flow through.
- Circuits are represented using circuit diagrams, which use standard symbols to show the different components and how they are connected.
Key Circuit Component Symbols
- A cell is represented by one long line (positive terminal) and one short line (negative terminal), and is a simple power source.
- A battery is made up of two or more cells, so its symbol is two cell symbols placed together.
- A filament lamp (light bulb) has its own circuit symbol and will light up when current flows through it.
- A switch can be shown as either open (breaking the circuit and stopping current flow) or closed (completing the circuit and allowing current to flow).
Current (I)
- Current, given the symbol I , is a measure of the flow of electrons around a circuit.
- Current is measured in amperes (amps), given the unit symbol A.
- Current can be thought of like water flowing through a pipe — the greater the flow, the higher the current.
Potential Difference (V)
- Potential difference, given the symbol V , is the driving force that pushes electrons around the circuit and is provided by the cell or battery.
- Potential difference is also known as voltage and is measured in volts (V ).
- It can be thought of as the pump in a water pipe system that forces the water (electrons) to move.
Resistance (R)
- Resistance, given the symbol R, is a measure of how much a component opposes or resists the flow of electrons.
- Resistance is measured in ohms (Ω).
- It can be compared to a partial blockage in a pipe, which reduces the flow of water.
Conventional Current vs. Electron Flow
- Electrons are negatively charged, so they actually flow from the negative terminal to the positive terminal of a cell.
- When electricity was first discovered, scientists assumed current flowed from positive to negative, before the role of electrons was understood.
- This original assumption is kept today as conventional current, meaning current is always said to flow from the positive terminal to the negative terminal in circuit diagrams.
- In exams, always use conventional current — current flows from positive (+) to negative (−).
Identifying Terminals on a Cell
- The longer line in a cell or battery symbol always represents the positive terminal.
- The shorter line in a cell or battery symbol always represents the negative terminal.
Key Circuit Components
- A cell is a source of potential difference that drives current around a circuit.
- An ammeter measures current and is connected in series within the circuit.
- A voltmeter measures potential difference (voltage) and must be connected in parallel across a component.
- A variable resistor allows the resistance in a circuit to be changed during an experiment.
Resistance and Current
- The greater the resistance in a circuit, the smaller the current — resistance opposes the flow of current.
- Resistance is measured in ohms (Ω), and typical values might be 10 Ω, 20 Ω, or 30 Ω.
- To observe the relationship between resistance and current, the potential difference must be kept constant.
Potential Difference and Current
- The greater the potential difference supplied to a circuit, the larger the current that flows.
- To observe the relationship between potential difference and current, the resistance must be kept fixed at a constant value.
- Increasing the number of cells in a circuit is one way to increase the potential difference.
Ohm's Law Equation
- The relationship between potential difference, current, and resistance is given by: V = IR Potential difference (V) is measured in volts (V), current (I) is measured in amps (A), and resistance (R) is measured in ohms (Ω).
- This equation must be memorised as it is not provided in the GCSE exam.
- A formula triangle can be used to rearrange the equation: cover the quantity you want to find to see the correct operation.
Rearranging the Equation
- To find resistance, rearrange the equation to R = V / I.
- To find current, rearrange the equation to I = V / R.
- A calculator is permitted in the GCSE Physics exam and should always be brought to save time on calculations.
Worked Example
- To calculate resistance, use R = V / I with the known values of potential difference and current.
- For example, if V = 3 V and I = 0.2 A, then R = 3 / 0.2 = 15 , Ω.
- Always include the correct unit (Ω) in your final answer.
Graphs of Potential Difference vs Current
- A graph of potential difference (y-axis) against current (x-axis) for a fixed resistor produces a straight line through the origin.
- A steeper gradient on the graph indicates a lower resistance, as more current flows for the same potential difference.
- A shallower gradient indicates a higher resistance, as less current flows for the same potential difference.
- Straight-line graphs are only produced when resistance remains constant throughout the experiment.
Effect of Changing Resistance on Graphs
- If resistance changes during an experiment (e.g. in a filament bulb), the graph will be a curve rather than a straight line.
- Multiple straight lines can be plotted on the same axes to compare different fixed values of resistance.
Connecting Measuring Instruments Correctly
- An ammeter is always connected in series so that the same current flows through it as through the component being measured.
- A voltmeter is always connected in parallel (across) the component — connecting it in series would prevent the circuit from working correctly.
Summary of Key Relationships
- Current increases as potential difference increases, provided resistance is kept constant.
- Current decreases as resistance increases, provided potential difference is kept constant.
- The equation V = IR links all three quantities and is essential for GCSE Physics calculations.