What you'll learn
This topic covers the fundamental principles of electric circuits that form the foundation of your AQA GCSE Physics exam. You'll understand how charge flows through circuits, what drives this flow, and what opposes it. These concepts are essential for both Paper 1 and practical investigations.
Key terms and definitions
Current (I) — the rate of flow of electrical charge, measured in amperes (A). One ampere equals one coulomb of charge flowing per second.
Potential difference (V) — the work done (energy transferred) per unit charge between two points in a circuit, measured in volts (V). Also called voltage.
Resistance (R) — the opposition to current flow in a component or conductor, measured in ohms (Ω).
Charge (Q) — a physical property of matter measured in coulombs (C). Moving charges create an electric current.
Ohm's law — for an ohmic conductor at constant temperature, current is directly proportional to potential difference (V = IR).
Ohmic conductor — a component that obeys Ohm's law, showing a constant resistance regardless of current.
Directly proportional — when two quantities increase at the same rate, their ratio remains constant.
I-V characteristic — a graph showing how current through a component varies with potential difference across it.
Core concepts
Current as the flow of charge
Electric current exists when charge carriers move through a conductor. In metal wires, these carriers are electrons that move from the negative terminal towards the positive terminal. This is opposite to conventional current direction, which flows from positive to negative (a historical convention from before electrons were discovered).
Current is measured using an ammeter connected in series with the component. The ammeter must be in series because all the current flowing through the component must also flow through the ammeter to get an accurate reading.
The relationship between charge, current and time is:
Q = It
Where:
- Q = charge in coulombs (C)
- I = current in amperes (A)
- t = time in seconds (s)
This equation tells you that charge is conserved—the total charge flowing into a point in a circuit equals the total charge flowing out. If 5 A flows for 10 seconds, then 50 C of charge has passed through that point.
Potential difference and energy transfer
Potential difference (voltage) represents energy transfer in a circuit. When charge flows through a component, energy is transferred. The potential difference tells you how much energy is transferred per coulomb of charge.
The relationship between energy transferred, charge and potential difference is:
E = QV
Where:
- E = energy transferred in joules (J)
- Q = charge in coulombs (C)
- V = potential difference in volts (V)
You can combine this with Q = It to get: E = IVt
This equation is particularly useful for calculating energy transferred by appliances over time.
Potential difference is measured using a voltmeter connected in parallel across the component. The voltmeter must be in parallel to measure the energy difference between the two points either side of the component.
In a series circuit, potential differences add up to equal the supply voltage. In a parallel circuit, the potential difference across each branch equals the supply voltage.
Resistance and Ohm's law
Resistance determines how much current flows for a given potential difference. High resistance means less current flows; low resistance means more current flows for the same voltage.
The fundamental relationship is:
V = IR
Where:
- V = potential difference in volts (V)
- I = current in amperes (A)
- R = resistance in ohms (Ω)
You can rearrange this equation depending on what you need to find:
- I = V/R
- R = V/I
For ohmic conductors (such as fixed resistors at constant temperature), the resistance stays constant regardless of the current. This means a graph of current against potential difference gives a straight line through the origin.
Factors affecting resistance
Several factors affect the resistance of a wire:
Length: Doubling the length doubles the resistance. Longer wires have more resistance because charge carriers collide with more atoms as they travel through the conductor.
Cross-sectional area: Doubling the area halves the resistance. Thicker wires have less resistance because charge carriers have more space to flow through.
Material: Different materials have different resistances. Copper has low resistance (good conductor), while materials like nichrome have higher resistance.
Temperature: For most conductors, increasing temperature increases resistance because atoms vibrate more, causing more collisions with charge carriers.
I-V characteristics of different components
Different components show different relationships between current and potential difference:
Fixed resistor (at constant temperature): Produces a straight line through the origin. The resistance is constant, so current is directly proportional to potential difference. The gradient of the line equals 1/R.
Filament lamp: Produces a curved line. As current increases, the filament heats up significantly. Higher temperature increases resistance, so the line curves, showing resistance increasing with current. The filament can reach over 2000°C during operation.
Diode: Conducts in one direction only (forward bias) and has very high resistance in the other direction (reverse bias). The graph shows current increasing rapidly once a threshold voltage (approximately 0.6-0.7 V for silicon) is reached in forward bias, but virtually no current in reverse bias.
Thermistor (NTC type): A temperature-dependent resistor. In negative temperature coefficient (NTC) thermistors, resistance decreases as temperature increases. Used in temperature sensors and thermostats. The I-V characteristic shows increasing gradient as current (and therefore temperature) increases.
Light-dependent resistor (LDR): Resistance decreases as light intensity increases. In darkness, resistance can be millions of ohms; in bright light, it can drop to hundreds of ohms. Used in automatic lighting circuits and light sensors.
Required practical: Investigating I-V characteristics
AQA requires you to investigate the relationship between potential difference and current for various components. Key points:
Method:
- Set up a circuit with the component, ammeter (in series), voltmeter (in parallel across component), and variable resistor
- Adjust the variable resistor to change the current
- Record current and potential difference values
- Plot current (y-axis) against potential difference (x-axis)
- For diodes, reverse the connections to test both directions
Key considerations:
- Take repeat readings to identify anomalies
- Use appropriate ranges on meters for accuracy
- For filament lamps, work quickly or allow cooling between readings to avoid overheating
- Plot graphs during the practical to spot patterns immediately
Analysis:
- Fixed resistors show linear relationships (straight line through origin)
- Calculate resistance using R = V/I at any point
- Filament lamps show non-linear relationships (curve)
- For curves, resistance changes, so calculate it at specific points only
Worked examples
Example 1: Calculating charge flow
Question: A current of 0.5 A flows through an LED for 2 minutes. Calculate the charge that flows through the LED in this time. [3 marks]
Solution:
- Convert time to seconds: 2 × 60 = 120 s [1 mark]
- Use equation Q = It [1 mark]
- Q = 0.5 × 120 = 60 C [1 mark]
Answer: 60 coulombs
Example 2: Applying Ohm's law
Question: A 12 V battery is connected to a 4 Ω resistor. (a) Calculate the current through the resistor. [2 marks] (b) Calculate the energy transferred when 30 C of charge flows through the resistor. [2 marks]
Solution: (a)
- Rearrange V = IR to give I = V/R [1 mark]
- I = 12/4 = 3 A [1 mark]
(b)
- Use equation E = QV [1 mark]
- E = 30 × 12 = 360 J [1 mark]
Answers: (a) 3 A, (b) 360 J
Example 3: Resistance calculation from I-V data
Question: A student investigates a fixed resistor. When the potential difference is 6.0 V, the current is 0.4 A. When the potential difference is 12.0 V, the current is 0.8 A.
(a) Calculate the resistance at each voltage. [4 marks] (b) Explain what these results tell you about the resistor. [2 marks]
Solution: (a)
- At 6.0 V: R = V/I [1 mark]
- R = 6.0/0.4 = 15 Ω [1 mark]
- At 12.0 V: R = 12.0/0.8 [1 mark]
- R = 15 Ω [1 mark]
(b)
- The resistance remains constant [1 mark]
- This shows it obeys Ohm's law / is an ohmic conductor [1 mark]
Answers: (a) Both 15 Ω, (b) The resistor is ohmic
Common mistakes and how to avoid them
Confusing current and charge: Current is the rate of flow; charge is what flows. Remember current is measured in amperes, charge in coulombs. Use Q = It to link them.
Using the wrong equation triangle: Don't rely on triangles—learn to rearrange equations properly. For V = IR, divide both sides by I to get R = V/I, or divide by R to get I = V/R.
Ammeter and voltmeter positioning: Ammeters MUST be in series (current flows through them); voltmeters MUST be in parallel (measuring difference between two points). Reversed connections will give incorrect readings or damage meters.
Forgetting unit conversions: Time must be in seconds, not minutes or hours. 5 minutes = 300 seconds. Energy can be in joules or kilojoules—check the question carefully.
Stating "resistance increases with voltage": Incorrect. For non-ohmic components like filament lamps, resistance increases with temperature (caused by increased current), not directly with voltage.
Mixing up LDR and thermistor behaviour: LDRs respond to light (resistance decreases with increasing light intensity). Thermistors respond to temperature (NTC thermistors decrease resistance with increasing temperature).
Exam technique for "Current, potential difference and resistance"
"Calculate" questions: Show your working clearly. Write the equation, substitute values with units, then give the answer with the correct unit. This ensures method marks even if your final answer is wrong.
Graph questions: For I-V characteristics, current goes on the y-axis, potential difference on the x-axis. Draw smooth curves for filament lamps (not dot-to-dot). Use a ruler for straight lines. Always label axes with quantities and units.
"Explain" command word: Give reasons, not just descriptions. For example, "The resistance increases because the higher current heats the filament, making the atoms vibrate more, causing more collisions with charge carriers."
Practical questions: Describe specific details—name of equipment (ammeter, voltmeter), where connected (series/parallel), variables (independent, dependent, control), and how to improve accuracy (repeat readings, identify anomalies).
Quick revision summary
Current is the rate of charge flow (Q = It), measured in amperes using an ammeter in series. Potential difference is energy transferred per unit charge (E = QV), measured in volts using a voltmeter in parallel. Resistance opposes current flow (V = IR), measured in ohms. Ohmic conductors have constant resistance; filament lamps show increasing resistance with current due to heating. I-V graphs reveal component characteristics. Know the required practical method and be able to calculate using all three main equations.