The problems are on the static electricity worksheet. Try them on your own first, then compare with the answers below.
Quick answer: Static electricity is electric charge that builds up on insulators when electrons are transferred by rubbing. These worked answers correct the 20 problems of the Physics 9-10 worksheet on static electricity, from attract or repel to rod and discharge: each solution shows the formula, the substitution and the final answer with its unit.
Full answers follow each problem in the same order. Check that you converted minutes to seconds and prefixes to base units, and that you describe charging as a movement of electrons, never protons.
Level: Physics grades 9-10 (GCSE · IGCSE · O Level · Class 9-10) · Curriculum: GCSE P2, HS-PS2-4
Problem 1 Attract or repel
1. Repel (like charges).2. Attract (unlike charges).
3. Repel (like charges).
Problem 2 Rubbing a rod
1. Electrons.2. From the cloth to the rod.
3. Positive, because the cloth has lost electrons.
Problem 3 Charge from current
1. Q = I × t.2. Q = 2.0 × 5.0 = 10 C.
Problem 4 Milliamps and minutes
1. 250 mA = 0.25 A and 2.0 min = 120 s.2. Q = 0.25 × 120 = 30 C.
Problem 5 Conductor or insulator
Conductors: copper, steel, aluminium. Insulators: polythene, rubber, glass. Metals have free electrons; the others do not.Problem 6 Add the arrows
1. Towards the negative charge.2. Each arrowhead points inwards, towards the charge.
3. The lines spread out, so the field gets weaker with distance.
Problem 7 Counting electrons
1. N = Q ÷ e.2. N = 4.0 × 10⁻⁹ ÷ 1.6 × 10⁻¹⁹ = 2.5 × 10¹⁰ electrons.
Problem 8 How long does it take
1. t = Q ÷ I.2. t = 36 ÷ 1.5 = 24 s.
Problem 9 Current from charge
1. 3.0 min = 180 s.2. I = Q ÷ t = 90 ÷ 180 = 0.50 A.
Problem 10 The balloon on the wall
1. Negative, because it gained electrons.2. The balloon repels electrons in the wall surface, leaving the nearest part of the wall with a positive charge. Unlike charges attract, and the nearer positive charge is closer, so the force of attraction is stronger than the repulsion. The balloon is attracted.
Problem 11 Charging graph
1. 36 µC.2. Gradient = 48 ÷ 40 = 1.2 µC/s.
3. It is the current, 1.2 µA, because current = charge ÷ time.
Problem 12 Refuelling safely
1. Fuel flowing through the pipe rubs against the pipe and the lorry’s surfaces, transferring electrons, so charge builds up.2. The wire lets the charge flow safely to the ground, so no spark can ignite the petrol vapour.
Problem 13 Spray painting a bicycle frame
1. The droplets carry like charges, so they repel one another and spread out.2. The charged droplets induce the opposite charge on the frame, so they are attracted and coat all sides, even the back.
Problem 14 Gold-leaf electroscope
1. The rod repels electrons in the cap, which travel down to the stem and leaf. Both then hold extra negative charge, so they repel and the leaf rises.2. The electrons return to the cap and the leaf falls back, because the electroscope was never charged overall.
Problem 15 Spot the errors
1. Electrons move; protons stay in the nuclei.2. Like charges repel; unlike charges attract.
3. Q = I × t = 5 × 2 = 10 C, not 2.5 C (he divided instead of multiplying).
Problem 16 A lightning flash
1. I = Q ÷ t = 6.0 ÷ 2.0 × 10⁻⁴ = 3.0 × 10⁴ A.2. N = 6.0 ÷ 1.6 × 10⁻¹⁹ = 3.8 × 10¹⁹ electrons.
3. The current and the energy released are millions of times larger, enough to heat air to thousands of degrees and kill.
Problem 17 Van de Graaff dome
1. t = 50 × 10⁻⁶ ÷ 2.5 × 10⁻⁶ = 20 s.2. N per second = 2.5 × 10⁻⁶ ÷ 1.6 × 10⁻¹⁹ = 1.6 × 10¹³.
3. The spark carries the charge away through the air, so the dome discharges.
Problem 18 Electron between plates
1. Upwards, from the positive plate to the negative plate.2. The electron is negative, so it feels a force opposite to the field, downward, towards the positive plate (it is also repelled by the negative plate).
3. A proton is positive, so it is pushed up to the negative plate, in the direction of the field.
Problem 19 Dust in a chimney
1. Q = 20 × 1.6 × 10⁻¹⁹ = 3.2 × 10⁻¹⁸ C.2. The particles are negative, so the plate is positive.
3. Opposite charges attract, so dust collects on the plate; tapping the plate shakes the dust into a hopper.
Problem 20 Rod and discharge
1. Q = 5.0 × 10¹⁰ × 1.6 × 10⁻¹⁹ = 8.0 × 10⁻⁹ C.2. I = 8.0 × 10⁻⁹ ÷ 0.040 = 2.0 × 10⁻⁷ A (0.20 µA).
3. The current is far too small to be felt or to harm a person.
