Electrical quantities · revision map · IGCSE 0625, Topic 4.2
TOPIC 4.2 · ELECTRICAL QUANTITIES
CAMBRIDGE IGCSE PHYSICS 0625 · ONE-PAGE REVISION MAP
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ELECTRICAL
QUANTITIES4.2
QUANTITIES4.2
Charge
There are two kinds of charge: positive and negative.
- Like charges repel; unlike charges attract.
- Rubbing transfers electrons: one object becomes negative, the other positive.
- Conductors let charge flow; insulators do not.
Current
Current is the rate of flow of electric charge, measured in amperes (A) by an ammeter in series.
I = Q / t
Extended
I = current (A), Q = charge (C), t = time (s)
- d.c. flows one way; a.c. keeps reversing.
Potential Difference & Resistance
Potential difference (p.d.) is the energy transferred per unit charge, in volts (V), read by a voltmeter in parallel. Resistance opposes the current, in ohms (Ω).
R = V / I
V = I R
R = resistance (Ω), V = p.d. (V), I = current (A)
- A longer wire has more resistance; a thicker wire has less.
Current–Voltage GraphsEXTENDED
I–V graphs show how a component behaves.
- A fixed resistor at constant temperature gives a straight line (constant resistance).
- A filament lamp curves: resistance rises as it heats.
- A diode conducts one way only.
Electrical Power
Electrical power is the energy transferred each second, in watts (W).
P = V I
P = I² REXTENDED
P = power (W), V = p.d. (V), I = current (A), R = resistance (Ω)
- 1 watt = 1 joule per second.
- P = I²R explains why thick supply cables waste less energy.
Electrical Energy & the kWh
The energy transferred grows with power and with the time it runs.
E = I V tE = energy (J), I = current (A), V = p.d. (V), t = time (s)
- Home energy is metered in kilowatt-hours (kWh).
- 1 kWh is a 1 kW device running for 1 hour.
- cost = energy in kWh × price per kWh.