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Unlock This CourseThis practical is examined in Paper 2 only.
The aim is to investigate how insulating materials can be charged by friction. You rub different insulators with a dry cloth, then test them to see whether they are charged and what type of charge they have. You may not have a laboratory at home, so this lesson describes exactly what is done and what is seen. That is all you need to answer the exam questions.
Remember, rubbing transfers electrons, so a polythene rod becomes negatively charged and an acetate rod becomes positively charged.
Apparatus:
Key terms:
In the exam you may be asked to describe the set-up. Draw or describe it like this: a clamp stand on the bench, a thread hanging down from the clamp, and a cradle at the bottom of the thread with a rod lying level in it. The rod must be able to swing round freely. The thread is nylon because nylon is an insulator, so the charge cannot flow away along it.
| Test | What is seen | What it shows |
|---|---|---|
| Rubbed polythene near suspended polythene | The suspended rod swings away | The two rods have the same type of charge |
| Rubbed acetate near suspended acetate | The suspended rod swings away | The two rods have the same type of charge |
| Rubbed acetate near suspended polythene (or the other way round) | The suspended rod swings towards the other rod | Polythene and acetate have different types of charge |
| Rubbed rod above pieces of paper | The paper jumps up and sticks to the rod | The rod is charged |
| Rubbed balloon on a wall | The balloon stays on the wall | The balloon is charged |
| Rubbed metal rod held in the hand | No paper is picked up | The metal rod has not stayed charged |
The results show that rubbing charges insulators, and that there are two types of charge. Polythene and acetate end up with different types, because they attract each other but each repels a rod of its own kind.
These tests tell you that an object is charged. They do not tell you which type of charge it has. A positive rod and a negative rod both pick up paper. Why a charged object attracts things that are not charged is explained in the lesson called Explaining Electrostatic Phenomena.
This test tells you the type of charge. If an object repels a suspended polythene rod, the object is negative. If it repels a suspended acetate rod, the object is positive.
A charged rod attracts an object with the opposite charge. It also attracts an object with no charge at all, just as it attracts the paper. So attraction on its own cannot prove what charge an object has. Repulsion only happens when both objects are charged with the same type of charge. To be certain, look for repulsion.
A student runs a plastic comb through her dry hair several times. She brings the comb close to a suspended, charged polythene rod and the rod swings away. She then brings the comb close to a suspended, charged acetate rod and the rod swings towards the comb. What is the charge on the comb?
The polythene rod is negatively charged and it is repelled. Repulsion needs the same type of charge on both objects, so the comb is negatively charged. The second test agrees: the acetate rod is positive, and it is attracted to the negative comb.
You can also compare how strongly different materials are charged. Count how many pieces of paper each rubbed rod picks up. For this to be fair, only the material of the rod should change.
| Variable | In this practical |
|---|---|
| Independent variable (what you change) | The material of the rod |
| Dependent variable (what you measure) | The number of pieces of paper picked up |
| Control variables (what you keep the same) | The number of rubs, the same dry cloth, the size of the paper pieces, the distance the rod is held above the paper |
Rub each rod the same number of times, because more rubbing transfers more electrons. Repeat each test and check that you see the same thing each time.
The rods, the cloth and your hands must all be dry. On a damp day a very thin layer of moisture forms on the surface of the rod. Water conducts, so the charge leaks away through the moisture and the rod is soon discharged. The practical works best in a warm, dry room.
Remember, metal and the human body are both conductors. Any charge given to the metal rod flows through the rod and through your hand straight away, so it never builds up. A metal rod can only keep a charge if it is held by an insulating handle.
Safety: this is a low-risk practical. Make sure the clamp stand is steady so it cannot topple off the bench, and do not blow the balloon up so far that it bursts near your face.
Writing that attraction proves two objects have opposite charges. It does not, because a charged rod also attracts objects that are not charged. Letting the rods touch, or handling the rubbed end, which can remove some of the charge. Rubbing one rod more times than another, so the comparison is not fair. Hanging the rod so that it cannot turn freely. Forgetting to say that the cloth and rods must be dry.
A student investigates how insulating materials can be charged by friction. She rubs one end of a polythene rod with a dry cloth and hangs the rod from a nylon thread so that it is free to turn.
(a) Describe how the student could use this suspended rod to show that a second rubbed polythene rod is charged. State what she would see. [2 marks]
(b) The student rubs a rod made from an unknown plastic and brings it close to the suspended polythene rod. The suspended rod moves towards the unknown rod. Explain why this result does not prove that the unknown rod is positively charged. [2 marks]
(c) The student wants to compare how strongly two different plastic rods are charged, using small pieces of paper. State two things she should keep the same. [2 marks]
(d) The student repeats the practical on a damp day and the rods do not stay charged. Explain why. [2 marks]
(a) Bring the rubbed end of the second rod close to the rubbed end of the suspended rod, without touching it (1). The suspended rod moves away from the second rod (1).
(b) A charged rod also attracts an object that is not charged (1). So the unknown rod could be uncharged or positively charged; only repulsion would show for certain that it is charged (1).
(c) Any two from: the number of rubs (1), the same dry cloth (1), the size of the pieces of paper (1), the distance of the rod above the paper (1). [maximum 2 marks]
(d) Moisture from the damp air forms a thin layer of water on the rods (1). Water is a conductor, so the charge leaks away through it (1).
In any question about testing a charged object, write the word repel or repulsion when you want to prove the type of charge. Then name the known rod: repelled by polythene means negative, repelled by acetate means positive.