Year 7 Science · Unit 3 · Lesson 12
Challenge Worksheet
Learning Goals
Find the errors
A student wrote these revision notes. Three of the five lines contain one error each. For each line, write “correct” or rewrite it so it is true.
| Student’s note | Correct, or your correction |
|---|---|
| 1. Two negatively charged balloons push apart, even without touching. | |
| 2. Rubbing a balloon on hair makes brand-new electrons on the balloon. | |
| 3. Gravity is a stronger force than the electrostatic force, which is why it runs the Solar System. | |
| 4. The gravitational pull between two masses gets weaker as they move apart. | |
| 5. A planet contains huge numbers of charged particles, so it has a huge electrostatic force. |
Two hanging balloons
Scenario: balloons on strings
Two balloons are both rubbed on the same jumper, so they pick up the same kind of charge. They hang side by side on strings from one hook. Instead of hanging straight down, they swing apart and hang still at an angle.
1. Sketch the two hanging balloons. On the right-hand balloon, draw and label arrows for its weight and for the electrostatic force. Then explain why the balloons hang apart.
2. Both forces act across a gap and both weaken with distance. Explain why the electrostatic force can repel, as it does here, but gravity never can.
Read the data: how long does it stick?
Scenario: the classroom wall
On five different days, a class rubs a balloon the same way and sticks it to the same wall. They record the humidity (how much water vapour is in the air) and time how long each balloon stays up before it falls.
| Humidity (%) | 30 | 45 | 60 | 75 | 90 |
|---|---|---|---|---|---|
| Time on wall (min) | 52 | 31 | 14 | 6 | 1 |
Source: sample class results.
3. Describe the trend in Table 1, using numbers. Use it to explain why static shocks are more common on dry days in inland Australia.
4. A student says: “The balloon falls because gravity gets stronger the longer it stays on the wall.” Evaluate this claim. Explain why the balloon sticks at first and why it eventually falls.
5. Imagine that overnight the Sun’s gravitational force on the planets became a push instead of a pull. Nothing else changes: Earth’s gravity still pulls on you and on the Moon. Predict and explain what would happen. Then explain why real gravity could never do this.
Find the errors
Lines 1 and 4 are correct. Line 2: rubbing moves existing electrons from the hair onto the balloon; it makes none (1). Line 3: gravity is far weaker; it runs the Solar System because mass only adds up, with nothing to cancel it (1). Line 5: a planet’s positive and negative charges cancel, so it has almost no overall charge or electrostatic force (1). [3 marks]
Two hanging balloons
1. Weight arrow pointing straight down (1); electrostatic force arrow pointing away from the other balloon (1). Both balloons carry like charges, and like charges repel across the gap, pushing them apart (1). [3 marks]
2. Charge comes in two kinds, positive and negative (1), so two objects can have like charges (repel) or unlike charges (attract) (1). Mass comes in only one kind, so there is no “unlike mass” to push away from, and gravity only attracts (1). [3 marks]
Read the data: how long does it stick?
3. The higher the humidity, the shorter the time: 52 min at 30% but 1 min at 90% (1). In dry air, charge leaks away slowly, so it builds up on people and gives shocks (1). [2 marks]
4. The claim is wrong: the balloon’s mass does not change, so its weight stays the same (1). It sticks because its charge pulls the wall’s unlike charges toward the surface, so balloon and wall attract (1). Its charge leaks into the air, faster when humid, so the attraction weakens (1). When it can no longer hold the balloon against its weight, gravity wins and it falls (1). [4 marks]
Question 5
Marking guide: 1 now the Sun’s pull holds each planet in its orbit; 1 with a push instead, the planets would leave their orbits and move away from the Sun; 1 Earth would move away too, getting less light and warmth; 1 on Earth, objects still fall and weights are unchanged, because Earth’s gravity is unchanged; 1 the Moon still orbits Earth, held by Earth’s pull; 1 real gravity cannot push, because mass has only one kind, so there are no “unlike masses” to repel. [6 marks]
Model answer: Today the Sun’s gravity pulls every planet toward it, which holds each one in its orbit. If it pushed instead, the planets would be driven out of their orbits and away from the Sun, and Earth would get less light and warmth. On Earth, dropped objects would still fall and the Moon would still orbit, because Earth’s own gravity is unchanged. Real gravity cannot push: it is caused by mass, which has only one kind, so gravity only ever attracts.
Common error: “gravity gets stronger”. The weight is constant; the attraction fades.