Potential difference, resistance and IV characteristics
Key definitions
Potential difference (p.d. / voltage), V: the work done per unit charge moved between two points. Measured in volts (V) using a voltmeter connected in parallel.
Resistance, R: the opposition to current flow. Measured in ohms (Ω).
Current, I: the rate of flow of charge. Measured in amperes A using an ammeter connected in series.
Ohm's law
$$V = IR$$
Where V = potential difference (V), I = current A, R = resistance (Ω).
A component is ohmic if its resistance is constant (independent of current/voltage) — it obeys Ohm's law and its IV graph is a straight line through the origin.
Rearrangements:
- I = V/R (current = voltage ÷ resistance)
- R = V/I (resistance = voltage ÷ current)
Series and parallel circuits
Series
- Same current flows through all components: I_total = I₁ = I₂
- Voltage splits: V_total = V₁ + V₂
- Resistance adds: R_total = R₁ + R₂
Parallel
- Voltage the same across all branches: V_total = V₁ = V₂
- Current splits: I_total = I₁ + I₂
- Resistance: 1/R_total = 1/R₁ + 1/R₂ (total resistance is less than the smallest branch)
IV characteristics
An IV characteristic graph plots current (y-axis) vs potential difference (x-axis).
Ohmic resistor (fixed resistor)
Straight line through origin. Constant gradient = constant resistance (R = V/I).
Filament lamp
S-shaped curve. As V increases, current increases but the resistance increases too (filament heats up → atoms vibrate more → more collisions → higher resistance). The gradient decreases at higher voltages.
Diode
Only allows current in one direction (forward bias). Very high resistance in reverse direction (graph flat at zero). In forward bias, threshold voltage ~0.7 V then current rises steeply.
Light-dependent resistor (LDR)
Resistance decreases as light intensity increases (more photons → more charge carriers released). Used in automatic lighting circuits.
Thermistor (NTC)
Resistance decreases as temperature increases. Used in temperature sensors, thermostats.
Core Practical — IV characteristics
Core Practical 14: Investigate how the current through a component varies with potential difference.
Circuit setup: variable resistor (rheostat) to change voltage; ammeter in series; voltmeter in parallel across the component. Reverse connections to get negative values.
Procedure:
- Set up the circuit with the component (resistor/lamp/diode).
- Vary the resistance of the rheostat to change V across the component.
- Record I and V at each setting.
- Reverse connections; repeat to get negative V values.
- Plot IV graph; identify shape (linear = ohmic; curved = non-ohmic).
⚠Common mistakes
- Ammeter in parallel / voltmeter in series: this is always wrong. Ammeter must be series; voltmeter parallel.
- Resistance of parallel circuits: always less than the smallest individual resistance.
- Filament lamp resistance increases at higher temperatures — it is non-ohmic.
- Diode in reverse bias: resistance is very large (not infinite), but for GCSE treat as blocking current.
AI-generated · claude-opus-4-7 · v3-edexcel-combined-science