Current, Potential Difference and Resistance (P2.1)
Charge, current and time
Electric current is the rate of flow of charge:
Q = It
Q = charge (coulombs, C), I = current (amperes, A), t = time (s).
One ampere = one coulomb per second.
Conventional current flows from positive to negative terminal (opposite to electron flow — electrons move from − to +).
Potential difference (voltage)
Potential difference (p.d.) is the energy transferred per unit charge between two points:
V = W/Q
V = p.d. (volts, V), W = energy (J), Q = charge C.
A cell/battery maintains a p.d. between its terminals by doing work on charges.
Resistance and Ohm's law
Resistance is the opposition to current flow:
V = IR (Ohm's law)
V = potential difference (V), I = current A, R = resistance (ohms, Ω).
A component obeys Ohm's law if its resistance remains constant as voltage changes — it is an ohmic conductor. The I–V graph is a straight line through the origin.
I–V characteristics
| Component | I–V graph shape | Notes |
|---|---|---|
| Resistor (ohmic) | Straight line through origin | R = constant; Ohmic |
| Filament lamp | Curved (S-shape) | R increases as T increases |
| Diode | Asymmetric | Current flows in one direction only; very high resistance in reverse |
| Thermistor | — | R decreases as temperature increases |
| LDR | — | R decreases as light intensity increases |
Circuit symbols you must know
- Cell, battery, switch, resistor, variable resistor, lamp, diode, LED, capacitor, fuse, voltmeter, ammeter, thermistor, LDR.
Measuring resistance — required practical
Set up a circuit with ammeter in series, voltmeter in parallel across the component. Vary the p.d. and record I and V. Calculate R = V/I. Plot I–V graph.
Common exam errors
- Confusing current and charge — charge is Q (coulombs); current is rate of flow I (amps).
- Saying the filament lamp obeys Ohm's law — its resistance changes (non-ohmic).
- Reading resistance from an I–V graph by using the gradient — slope gives I/V (= 1/R), not R; use R = V/I directly.
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