E1: Voltage, Current & Resistance
V=IR, charge, electron flow, conventional current and measuring electrical quantities
Voltage, Current & Resistance
V=IR, charge, electron flow, conventional current and measuring electrical quantities
Key Fact: Voltage is the potential difference that drives charge through a circuit, measured in volts (V).
Key Fact: Current is the rate of flow of charge, measured in amperes (A); 1 A = 1 C/s.
Key Fact: Resistance opposes the flow of current, measured in ohms (Ω).
Key Fact: Ohm's Law states V = IR, linking voltage, current and resistance linearly for ohmic conductors.
Key Fact: Conventional current flows from positive to negative; electron flow is opposite, from negative to positive.
Key Fact: Charge is quantised: Q = It, where charge is measured in coulombs (C).
Key Fact: An ammeter is connected in series to measure current; a voltmeter is connected in parallel to measure voltage.
Key Fact: Ohmic conductors have a constant resistance regardless of voltage; non-ohmic conductors do not.
Key Fact: Electron drift velocity is typically very slow, around fractions of a mm/s in conductors.
Key Fact: Kirchhoff's current law: the sum of currents entering a junction equals the sum leaving it.
Key Fact: Resistance depends on material, length, cross-sectional area and temperature.
Key Fact: Multimeters can measure voltage, current and resistance in a single instrument.
📋 Key Vocabulary and Concepts
For Voltage, Current & Resistance, you must know:
- Voltage: The potential difference between two points in a circuit, measured in volts.
- Current: The rate of flow of electric charge, measured in amperes.
- Resistance: The opposition to the flow of current, measured in ohms.
- Charge: A physical property measured in coulombs; carried by electrons in conductors.
- Conventional current: The assumed flow of positive charge from positive to negative terminal.
- Electron flow: The actual movement of electrons from negative to positive terminal.
❓ Practice Questions
Q: A 12 V battery drives 3 A through a resistor. What is its resistance?
Q: What is the difference between conventional current and electron flow?
Q: A current of 2 A flows for 5 seconds. How much charge passes?
Q: How should a voltmeter be connected in a circuit?
Q: State Ohm's Law and name the quantities involved.
✅ Answers
- Using V = IR: R = V/I = 12/3 = 4 Ω.
- Conventional current flows from positive to negative; electron flow is the actual movement of electrons from negative to positive.
- Q = It = 2 × 5 = 10 C.
- A voltmeter is connected in parallel across the component being measured.
- V = IR, where V is voltage (volts), I is current (amperes) and R is resistance (ohms).
🎯 Exam Tips
- Always state Ohm's Law as V = IR before rearranging in calculations.
- Show full working: write the formula, substitute values, then give the answer with units.
- Remember conventional current is positive to negative; exam questions often use this convention.
- Check that your answer is sensible: resistance should not be negative in basic circuits.
- When describing measurements, always specify whether the meter is in series (ammeter) or parallel (voltmeter).
📝 Exam Technique
GCSE Electronics Exam Tips — Voltage, Current & Resistance:
1. For Voltage, Current & Resistance questions, use correct electronic symbols and terminology
2. Always show your working in calculations, including units at each step
3. When analysing circuits, state which law or rule you are applying first
4. For evaluation questions on Voltage, Current & Resistance, compare component choices and consider cost, reliability and tolerance
5. Draw circuit diagrams neatly with conventional symbols
1. For Voltage, Current & Resistance questions, use correct electronic symbols and terminology
2. Always show your working in calculations, including units at each step
3. When analysing circuits, state which law or rule you are applying first
4. For evaluation questions on Voltage, Current & Resistance, compare component choices and consider cost, reliability and tolerance
5. Draw circuit diagrams neatly with conventional symbols
⚠️ Common Errors
✗ Confusing electron flow direction with conventional current direction. ✓ Conventional current flows positive to negative; electrons flow negative to positive.
✗ Connecting an ammeter in parallel with a component. ✓ An ammeter must be connected in series to measure current correctly.
✗ Forgetting to include units in the final answer. ✓ Always include the correct SI unit: V, A, Ω or C.
✗ Assuming all components obey Ohm's Law. ✓ Only ohmic conductors have a constant resistance; filament lamps and diodes are non-ohmic.
✍️ Model Answer
Full-Mark Response
Explain the relationship between voltage, current and resistance, and describe how each is measured in a circuit.
Voltage, current and resistance are related by Ohm's Law: V = IR. Voltage (potential difference) is the driving force that pushes charge through a circuit and is measured in volts using a voltmeter connected in parallel across the component. Current is the rate of flow of charge (I = Q/t), measured in amperes using an ammeter connected in series with the component. Resistance opposes current flow and is measured in ohms; it can be calculated from voltage and current measurements using R = V/I. For ohmic conductors, resistance remains constant regardless of the applied voltage, producing a straight-line V-I graph through the origin.
📊 AO Deep Dive
Assessment Objective Analysis
AO1 (Knowledge & Understanding): Demonstrate knowledge and understanding of voltage, current & resistance, including electronic components, circuit theory and systems concepts relevant to WJEC Eduqas C690QS.
AO2 (Application): Apply knowledge and understanding of voltage, current & resistance to analyse, design and construct electronic circuits and systems.
AO3 (Evaluation): Evaluate electronic circuits and systems, making reasoned judgements about design choices, performance and practical considerations, constructing supported arguments.
📝 Exam Questions by Topic
🎬 Video Resources
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