Electricity
What's covered
Key facts
DC flows one way; AC reverses direction periodically.
An ammeter is connected in series with the component whose current is being measured, so it carries the full current it is measuring.
A 60 W bulb left on for 2 h (7200 s): energy = 60 × 7200 = 432 000 J = 0.12 kWh.
A diode's I-V graph shows zero current below a forward-bias threshold voltage (~0.7 V for silicon), then current rises steeply.
Alternating current (AC) reverses direction periodically; direct current (DC) flows in one direction only.
Rearranging V = IR: I = V ÷ R; 12 V across 4 Ω gives 3 A.
A diode is the component that allows current to flow in one direction only.
Household lighting and socket circuits are wired in parallel so each appliance receives the full mains voltage independently of the others.
An electric field exists around any charged object and exerts a force on other charged objects nearby.
The SI unit of electric charge is the coulomb (C), equal to one ampere flowing for one second.
Sample questions
A taste of the 127 questions in this topic — answers marked. Sign up to practise the full set with spaced repetition.
What is the equation linking charge, current and time?
- •Q = I/t
- ✓Q = It
- •Q = P/t
- •Q = Vt
What does a thick line with a short line on a circuit symbol represent?
- •Bulb
- ✓Cell
- •Resistor
- •Switch
How does power dissipated relate to current and resistance?
- •P = I + R
- •P = I/R
- ✓P = I²R
- •P = R/I²
What does the I-V graph of an ohmic conductor look like?
- •Curved line bending up
- •Horizontal line at I=0
- •S-shaped curve
- ✓Straight line through origin
What colour insulation covers the live wire?
- •Black insulation
- •Blue insulation
- ✓Brown insulation
- •Green/yellow stripes
What is Ohm's Law equation?
- •V = I/R
- ✓V = IR
- •V = IR²
- •V = PR
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