2.4.2Electricity
Core Practical: Investigating Charging by Friction
5 min read 4 parts Lesson 16 of 17 Unit quiz
Section 1 of 4Planning the Investigation
Planning the Investigation
Aim
- To find which charges are produced on rods of different insulating materials when they are rubbed with a cloth.
Independent variable
- The material of the rod being rubbed, for example acrylic, acetate, glass or polythene.
Dependent variable
- The force on a suspended polythene rod, seen as attraction or repulsion, which shows the sign of the charge.
Control variables
- Rub for the same length of time, use the same type of cloth and use rods of the same length.
Section 2 of 4Method
1
Rub a polythene rod with a dry cloth and hang it in a paper or wire cradle on a nylon thread so that it can swing freely.
2
Rub an acrylic rod with a different cloth and bring one end close to one end of the hanging rod, without touching.
3
Record whether the hanging rod moves towards the acrylic rod (attraction) or away from it (repulsion).
4
Wait until the hanging rod has settled and, if needed, touch it to remove its charge before reusing it.
5
Repeat the test with each of the other rods, recharging the hanging rod each time.
Section 3 of 4Interpreting Results
Rubbing
- Electrons are transferred by friction from one insulator to the other, so one becomes negative and the other positive.
Repulsion
- If the two rods repel each other they carry the same type of charge.
Attraction
- If the rods attract each other they carry opposite charges.
Evaluation
- Humid air, dirty rods and uneven rubbing change the charge, so results are qualitative and repeats help confirm them.
| Observation | Conclusion |
|---|---|
| Hanging rod moves towards test rod | Charges are opposite |
| Hanging rod moves away from test rod | Charges are the same |
| No movement | Test rod is uncharged or the charge has leaked away |
Section 4 of 4Exam Checklist & Tips
2.4.2 – Key Points for the Exam
- Friction The force between two surfaces that rub together, here causing electrons to transfer.
- Charge
- A property of matter;
- electrons carry negative charge and the protons in the nucleus positive.
- A property of matter;
- Attraction A force pulling oppositely charged objects together.
- Repulsion A force pushing objects with the same charge apart.
- Control variable A quantity kept constant so that only the independent variable affects the result.
Common Mistakes
- Saying that protons move when an insulator is charged; only electrons are transferred.
- Touching the rods together, which can transfer charge and spoil the result.
- Concluding the sign of the charge from one test only; compare with a rod of known sign.
- Forgetting that a charged object also attracts an uncharged one, so only repulsion proves the same charge.