Year 7 Science · Unit 3 · Lesson 12
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Learning Goals
Match each term to its meaning
Write the letter of the matching meaning in the middle column. Each letter is used once.
| Term | Letter | Meaning |
|---|---|---|
| Electron | A. Pull each other together | |
| Like charges | B. A force that acts across a gap, without touching | |
| Unlike charges | C. The property of an object that causes a gravitational force | |
| Mass | D. Push each other apart | |
| Non-contact force | E. A tiny charged particle that rubbing moves from one material to another |
A zap in Broken Hill
Scenario: a hot, dry day
On a hot, dry day in Broken Hill, a student slides across a car seat and reaches for the metal door handle. Before her finger touches the metal, a small spark jumps across the gap and she feels a zap. A moment later she drops her phone, and it falls to the floor of the car.
1. Sliding across the seat made the student electrically charged. Explain how rubbing two materials together can give an object a charge.
2. The spark crossed a gap, and the phone fell without anything touching it. What do these two events show about the electrostatic and gravitational forces?
3. Four charged spheres, A, B, C and D, are tested in pairs. A repels B. B attracts C. C repels D. Sphere A is negative. Work out the charge on B, C and D, and explain your reasoning.
Read the data: the planets
Scenario: held by the Sun
The Sun’s gravity holds all eight planets in their orbits. Table 1 shows how far each planet is from the Sun and how long it takes to go around once.
| Planet | Average distance from the Sun (million km) | Time for one orbit (Earth years) |
|---|---|---|
| Mercury | 58 | 0.24 |
| Venus | 108 | 0.62 |
| Earth | 150 | 1.0 |
| Mars | 228 | 1.9 |
| Jupiter | 778 | 11.9 |
| Saturn | 1430 | 29.5 |
| Uranus | 2870 | 84 |
| Neptune | 4500 | 165 |
Source: published planetary data, rounded.
4. Describe the pattern in Table 1. How many times further from the Sun is Neptune than Earth?
5. Neptune is about 30 times further from the Sun than Earth, but its orbit takes 165 times longer. Give two reasons why, one of them about the Sun’s gravitational pull.
6. A student claims: “The electrostatic force and gravity are really the same thing.” Evaluate this claim.
Match each term to its meaning
Electron → E; Like charges → D; Unlike charges → A; Mass → C; Non-contact force → B.
A zap in Broken Hill
1. Rubbing does not make charge; it moves electrons from one material to the other (1). One object ends up with extra electrons (negative) and the other with too few (positive) (1). [2 marks]
2. Both forces acted across a gap, with nothing touching (1), so both are non-contact forces (1). [2 marks]
3. B is negative: it repels negative A, and like charges repel (1). C is positive: it attracts negative B, and unlike charges attract (1). D is positive: it repels positive C (1). [3 marks]
Read the data: the planets
4. The further a planet is from the Sun, the longer its orbit takes (1). Neptune: 4500 ÷ 150 = 30 times as far (1). [2 marks]
5. Neptune’s path around the Sun is much longer, about 30 times as long (1). At that distance the Sun’s gravitational pull is spread very thin, so it is much weaker (1), and Neptune moves more slowly along its path, so its orbit takes far more than 30 times as long (1). [3 marks]
Question 6
Marking guide: 1 clear position (the claim is wrong: alike in some ways, but different forces); 1 alike: both non-contact, acting across a gap; 1 alike: both get weaker as distance increases; 1 different cause: electric charge versus mass; 1 different direction: electrostatic can attract or repel, gravity only attracts; 1 an accurate example of each (balloon on a wall; a dropped pencil or the Moon’s orbit). [6 marks]
Model answer: The claim is wrong, although the two forces are alike in two ways. Both act across a gap without touching, and both get weaker as objects move apart. But they have different causes: the electrostatic force comes from electric charge, while gravity comes from mass. And the electrostatic force can attract or repel, as when two negative balloons push apart, while gravity only ever attracts, as when a dropped pencil falls. So they are two different forces.
Common error: “gravity can repel too, because it is like the electrostatic force”. Mass has only one kind, so gravity only attracts.