9.6 - Charging by Friction (Electrostatics)
The experiment
Aim
To charge insulating rods by rubbing them with a cloth duster, and to test whether an object is charged and whether its charge is positive or negative.
Background
- Rubbing two insulators together transfers electrons from one to the other. The protons stay in the nuclei of the atoms, so only electrons move.
- The material that gains electrons becomes negatively charged, and the material that loses them is left with an equal positive charge.
- Like charges repel, and opposite charges attract.
- You can't see charge, but you can see its effect on something else, such as two gold leaves pushing apart or a hanging rod swinging.
- So in this practical you charge rods by rubbing them, then use a gold-leaf electroscope and a hanging rod to test whether an object is charged and which charge it has.
Variables
- Control variables - The same duster, the same number of rubs for each rod, and dry apparatus.
Method
- Charge a polythene rod - Rub it briskly along its length with a dry cloth duster. Electrons move from the duster to the rod, so the rod becomes negative and the duster positive.
- Charge an acetate rod - Rub it in the same way. This time electrons move from the rod to the duster, so the rod becomes positive and the duster negative.
- Handle the rods carefully - Hold each rod by one end only, and keep your hands dry.
- Test with the electroscope - Bring a charged rod slowly towards the metal disc on top, without touching it. If the gold leaves push apart, the rod is charged. They fall together again when you take it away.
- Hang up a rod of known charge - Put a freshly rubbed polythene rod (negative) or acetate rod (positive) in a paper cradle on a long thread, and let it settle.
- Bring up the rod you're testing - Hold it close to one end of the hanging rod, without touching. If the hanging rod swings away, the test rod has the same charge. If it swings towards it, the test rod has the opposite charge or is uncharged.
Safety
- Charged rods - The charge on a rubbed rod is far too small to give a harmful shock, but keep the rods away from eyes and faces.
- Fragile apparatus - The gold leaves tear easily, so keep the electroscope's case closed and never touch the leaves or blow on them. Bring a rod up to the disc slowly and don't knock it.
- Rods - Acetate and glass rods can snap if they're flexed or dropped, so hold each rod by one end and put it down on the bench between tests.
Common exam questions
Question 1
A negatively charged polythene rod is hung on a thread. A student brings an unknown charged rod near one end and the suspended rod swings away. What is the charge on the unknown rod, and how do you know?
It's negative. The suspended rod moved away, which is repulsion, and repulsion only happens between two charges of the same sign. The suspended rod is negative, so the unknown rod must be negative too.
Question 2
Why does attraction not prove that an object is charged?
A charged rod attracts an uncharged object too, because it pulls that object's electrons towards or away from the near side. Only repulsion shows for certain that an object carries a charge of its own.
Question 3
Explain why the gold leaves rise when a charged rod is brought near the disc, and fall when it's taken away.
The rod's charge pushes electrons along the metal towards or away from the leaves, so both leaves get the same charge and repel each other. When the rod is taken away, the charge spreads back out and the leaves fall together.
Question 4
A student can't get either rod to hold a charge on a humid day. Suggest why.
Damp air leaves a thin film of moisture on the rod, which conducts, so the charge leaks away as fast as it builds up.
Question 5
Why must the rods be made of an insulator, such as polythene or acetate, rather than a metal held in the hand?
Charge can't move through an insulator, so it stays where it was rubbed on. In a metal rod, the charge would flow through the rod and your body to earth.