Electric Circuits
Current, Charge and Potential Difference
Electric current (I) is the rate of flow of charge: I = Q/t, where Q is charge in coulombs C and t is time in seconds. Current is measured in amperes A using an ammeter connected in series.
Potential difference (p.d. or voltage, V) is the energy transferred per unit charge: V = W/Q (energy in joules, charge in coulombs). P.d. is measured in volts (V) using a voltmeter connected in parallel.
Resistance (R) opposes the flow of current. From Ohm's law: V = IR (volts = amps × ohms). Rearranging: I = V/R and R = V/I.
Series and Parallel Circuits
Series circuits
- Same current flows through every component: I_total = I₁ = I₂ = I₃
- Total p.d. is shared: V_total = V₁ + V₂ + V₃
- Total resistance adds: R_total = R₁ + R₂ + R₃
- If one component breaks, the circuit is broken — all components stop working.
Parallel circuits
- Total current splits: I_total = I₁ + I₂ + I₃
- Same p.d. across each branch: V_total = V₁ = V₂ = V₃
- Total resistance is less than the smallest branch resistance: 1/R_total = 1/R₁ + 1/R₂ + 1/R₃
- If one branch breaks, the rest continue working.
IV Characteristics (Required Practical)
The IV characteristic of a component is a graph of current (y-axis) against voltage (x-axis). WJEC requires you to investigate the IV characteristics of three components:
Resistor (ohmic conductor)
A straight line through the origin — current is directly proportional to voltage. Slope = 1/R. The resistance is constant regardless of voltage.
Lamp (filament bulb)
S-shaped curve — as voltage increases, current increases but the slope decreases. The filament heats up, increasing resistance. At higher voltages, the graph curves away from the straight line (resistance is not constant).
Diode
Near-zero current for negative voltages (reverse bias). A sharp turn-on at about +0.6V (forward threshold), then current rises steeply. Diodes only let current flow in one direction.
Practical Tip (WJEC Required Practical)
To obtain IV characteristics: connect the component to a variable power supply with an ammeter in series and voltmeter in parallel. Vary the voltage, reading both meters at each setting. Plot I (y-axis) against V (x-axis). Repeat with reverse connections for the diode.
⚠Common mistakes
- Ammeter and voltmeter positions swapped: ammeter goes in series (same current flows through it), voltmeter in parallel (same p.d. across it). Swapping them gives wrong readings and can blow fuses.
- Ohm's law rearrangement errors: if you double the voltage and keep R constant, current doubles (not halves). Write the formula triangle V / I R if it helps.
- Series resistance addition applied to parallel: in parallel, total R is always less than the smallest individual R.
- Confusing current and charge: current is charge per second (I = Q/t); charge is not the same as current.
- Diode IV curve drawn symmetrically: the forward and reverse branches look very different — don't draw a symmetric S-curve.
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