☰ Year plan · Science

Term 1 review: reactions, electricity and heredity

Week 15 · Term 1 review (Units 1–4) and Darslab's own review tasks · 3 lessons
2

Review: electricity and magnetism

Science Prep 3 Term 1, Units 1–4: Review
GoalReview electric current, Ohm’s law, power and electromagnetism with new mixed tasks. No new facts.
New words
Ohm’s law · V = I × R: the voltage equals the current times the resistancepower · P = V × I; the energy used each secondelectromagnet · a coil with a current that becomes a magnet only while the current flows
Explanation

Remember: current I = Q ÷ t (coulombs per second, amperes); V = I × R; resistors in series add (R = R₁ + R₂), in parallel the total is less than the smallest; P = V × I and energy = P × t. A current in a coil makes an electromagnet, and a motor and a generator are the same parts used in opposite ways: a motor turns electrical energy into motion, a generator turns motion into electrical energy. Use only cells and batteries for experiments, never the mains.

12 V+−4 Ω2 Ω
Worked examples
Series circuit: 4 Ω and 2 Ω with a 12 V battery. R = 4 + 2 = 6 Ω, so I = 12 ÷ 6 = 2 A. The voltage across the 4 Ω resistor is 2 × 4 = 8 V, and across 2 Ω it is 2 × 2 = 4 V. Check 8 + 4 = 12 V.
Power and energy: a lamp of 6 V and 0.5 A has P = 6 × 0.5 = 3 W. In 2 hours (7200 s) it uses 3 × 7200 = 21 600 J, which is 3 W × 2 h = 6 Wh.
A charge of 90 C passes a point in 30 s. I = 90 ÷ 30 = 3 A.
Class activity

Circuit practice with a grown-up (on paper, plus one 1.5 V AA cell for the nail). Draw the circuit with a 9 V battery and two resistors of 3 Ω and 6 Ω in series, work out the current and the voltage across each resistor, and check that the voltages add up to 9 V. Then wind a wire around an iron nail, connect it to one AA cell for a few seconds only and see if it picks up a paper clip. Never use mains electricity.

Practice
1
Two resistors of 3 Ω and 6 Ω in series. What is the total resistance?
2
A 9 V battery is connected to 9 Ω. What is the current?
3
A device of 10 V and 2 A: its power?
4
What is the difference between a motor and a generator?
5
What happens to an electromagnet when the current stops?