Simple circuits and Ohm's law
Key quantities
- Current (I) — rate of flow of charge, measured in amperes A using an ammeter placed in series. Q = I × t (charge in coulombs).
- Potential difference (V) — energy transferred per unit charge, measured in volts (V) using a voltmeter placed in parallel across the component.
- Resistance (R) — opposition to current, measured in ohms (Ω).
Ohm's law
V = I × R
For an ohmic conductor at constant temperature, V is directly proportional to I — the resistance R is constant. A graph of V (y) vs I (x) is a straight line through the origin.
Series vs parallel circuits
| Series | Parallel | |
|---|---|---|
| Current | Same everywhere | Splits between branches |
| p.d. | Shared between components | Same across each branch |
| Total R | R₁ + R₂ + ... | Less than smallest R |
| If one breaks | Whole circuit stops | Other branches keep working |
Household lighting uses parallel so that switches work independently and one bulb failing doesn't break the rest.
IV characteristics
- Fixed resistor at constant T → straight line through origin (ohmic).
- Filament lamp → S-shaped (curved) — as current rises, lamp heats up, R increases, line bends away from straight.
- Diode → current flows in one direction only (forward bias above ~0.7 V); zero current in reverse.
OCR PAG P5
Investigate the IV characteristic of a fixed resistor and a filament lamp. Use a low-voltage power supply, ammeter in series, voltmeter in parallel, vary V (or use a variable resistor), record I.
✦Worked example
A 12 V supply drives 3 A through a heater. R = V/I = 12/3 = 4 Ω.
OCR exam tip
When asked why the lamp's IV graph isn't straight, say "the filament heats up, atoms vibrate more, electrons collide more, resistance increases as current rises". Three steps, three marks.
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