• Quick retrieval from earlier learning (Static electricity and charges; Newton's law of gravitation and field strength). Students answer on mini whiteboards.
• Show answers together and note who needs support in the main task.

1. An object with a − charge is brought near an object with a − charge. Do they attract or repel? (attract / repel) → repelPrompt: Like charges repel; opposite charges attract.
2. One of two masses is multiplied by 0.5 and the distance stays the same. By what factor does the gravitational force change? (F = G m₁ m₂ ÷ r²) → 0.5Prompt: Force is proportional to each mass.
3. A balloon is rubbed on a woolly jumper (sweater) and becomes negatively charged. Why? (The air charged it / Protons moved onto the balloon / The balloon made new electrons / Electrons moved from the jumper onto the balloon) → Electrons moved from the jumper onto the balloonPrompt: Only electrons can move easily between materials.

• Show a circuit with a gap and ask why the bulb doesn't light.
• Share the objective: Coulomb's law: force between charges. Read the success criteria together.

• Work through the example out loud, saying why you do each step, not just what you do.
• Example: Two small charged spheres carry 3 µC and 2 µC and are 0.5 m apart. What is the size of the force between them, in N? (F = k q₁ q₂ ÷ r², k = 8.99 × 10⁹ N m²/C², 1 µC = 10⁻⁶ C; give 2 decimal places)
• Step 1: F = 8.99 × 10⁹ × 3 × 10⁻⁶ × 2 × 10⁻⁶ ÷ 0.5² = 0.216
• Step 2: So the answer is: 0.22 N
• Answer: 0.22 N
• Pause after each step and ask a student to say what comes next.

• Do these together. Students try each step on whiteboards before you reveal it.
• Use the hints as prompts rather than giving the answer.

1. Two small charged spheres carry 2 µC and 1 µC and are 0.2 m apart. What is the size of the force between them, in N? (F = k q₁ q₂ ÷ r², k = 8.99 × 10⁹ N m²/C², 1 µC = 10⁻⁶ C; give 2 decimal places) → 0.45 NPrompt: Convert µC to C.
2. The distance between two charges is multiplied by 3. By what factor does the electrostatic force change? (Give 4 decimal places.) → 0.1111Prompt: F ∝ q₁ q₂ ÷ r²

• Students work on the student sheet. Circulate and check the first two answers of every student early.
• Students who finish: write a question of their own for a partner, with the answer.

The 8 questions are on the student sheet below, with answers in the key.

• Two questions on their own. Sort answers into got it / nearly / not yet to plan the next lesson.

1. Both charges are multiplied by 2 and the distance is unchanged. By what factor does the force change? → 4Prompt: F ∝ q₁ q₂ ÷ r²
2. How is the electrostatic force different from gravity? (It does not depend on distance / It can attract or repel; gravity only attracts / It only acts on metals / It is always weaker) → It can attract or repel; gravity only attractsPrompt: There are two kinds of charge.

Free online practice with hints and printable worksheets for this skill: eurekami.com/science/grade-12.