Electricity
What's covered
Key facts
Charge and Current
The SI unit of electric charge is the coulomb (C), equal to one ampere flowing for one second.
Circuit Components and Symbols
An ammeter is connected in series with the component whose current is being measured, so it carries the full current it is measuring.
Electrical Power
A 60 W bulb left on for 2 h (7200 s): energy = 60 × 7200 = 432 000 J = 0.12 kWh.
I-V Characteristics
A diode's I-V graph shows zero current below a forward-bias threshold voltage (~0.7 V for silicon), then current rises steeply.
Mains Electricity
Alternating current (AC) reverses direction periodically; direct current (DC) flows in one direction only.
Resistance and Ohm's Law
Rearranging V = IR: I = V ÷ R; 12 V across 4 Ω gives 3 A.
Resistance in Components
A diode is the component that allows current to flow in one direction only.
Series and Parallel Circuits
Household lighting and socket circuits are wired in parallel so each appliance receives the full mains voltage independently of the others.
Static Electricity
An electric field exists around any charged object and exerts a force on other charged objects nearby.
Charge and Current
Conventional current flows from positive to negative terminal; electrons (negative carriers) flow the opposite way.
Practice questions with answers
A taste of the 110 questions in this topic, answers marked. Sign up to practise the full set with spaced repetition.
What is current?
- •Energy carried per charge
- •Force on a charge in a field
- ✓Rate of flow of electric charge
- •Total amount of charge stored
What does a long thin line next to a short thick line represent on a circuit diagram?
- •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 are the colours of the three wires in a UK mains plug?
- •Live: blue; Neutral: brown; Earth: green/yellow
- •Live: blue; Neutral: green/yellow; Earth: brown
- ✓Live: brown; Neutral: blue; Earth: green/yellow
- •Live: green/yellow; Neutral: brown; Earth: blue
What is Ohm's Law equation?
- •V = I/R
- ✓V = IR
- •V = IR²
- •V = PR
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