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An electrostatic phenomenon is anything that happens because of static charge, such as a shock from a door handle or a flash of lightning. The exam asks you to explain these in terms of the movement of electrons. Remember, only electrons move. So every explanation has the same shape: say which way the electrons move, say what charge that leaves behind, then use the rule that like charges repel and unlike charges attract.
Key terms:
A wall and a scrap of paper have no overall charge, yet a charged object attracts them. This is because the electrons inside the uncharged object can shift a little.
A rubbed balloon has gained electrons, so it is negatively charged. When it is held against a wall, its negative charge repels the electrons in the surface of the wall. These electrons are pushed a little way away from the balloon. The wall is an insulator, so they cannot travel far. The surface of the wall nearest the balloon is left with a positive induced charge. Unlike charges attract, so the balloon sticks to the wall.
A plastic comb run through dry hair becomes charged. Suppose it is negative. When it is held just above a small piece of paper, it repels the electrons in the paper, so they move to the side of the paper furthest from the comb. The near side of the paper is left positive. The attraction between the negative comb and the positive near side is bigger than the tiny weight of the paper, so the paper jumps up.
No electrons move between the two objects, so the wall and the paper are still neutral overall. The object is attracted and not repelled because the opposite charge is on the near side. The near side is closer to the charged object, so the attraction is stronger than the repulsion from the far side.
A positively charged rod pulls the electrons in the paper towards it instead. The near side of the paper becomes negative, so the paper is attracted again.
Air is an insulator, so charge can build up on an object. If the charge becomes large enough, electrons are pulled right across a small air gap to a nearby object. This sudden flow of electrons through the air is a spark. It heats the air, so you see a flash and hear a crackle. If the spark jumps to or from your skin, you feel a small shock.
As you slide off a car seat to get out, your clothes rub against the seat cover. Electrons are transferred, so you become charged. Suppose you have gained electrons and are negative. When your hand comes close to the metal door, electrons jump from your hand to the door and you feel a shock.
When you walk across a nylon carpet, your shoes rub against it and electrons are transferred. The soles of your shoes are insulators, so the charge cannot escape to the floor and it builds up on your body. When you reach for a metal door handle, electrons jump across the gap between your finger and the handle.
The exam question will usually tell you which charge the person has. If you are negatively charged, electrons jump from you to the metal. If you are positively charged, electrons jump from the metal to you. Either way there is a spark, and afterwards you are discharged. The shock comes from metal and not from a wooden door because metal is a conductor, so a lot of charge can flow in a very short time.
Lightning is the same thing on a huge scale:
A child slides down a plastic slide. Her clothes rub against the slide and electrons are transferred, so she becomes charged. Suppose electrons move from her to the slide. Electrons move away from every part of her body, including each hair, so every hair is left with a positive charge. Each hair now has the same type of charge as the hairs around it and as her head. Like charges repel, so the hairs push away from each other and from her head. They stand on end. If she had gained electrons instead, every hair would be negative and they would still repel.
Remember, an earth wire connects the metal case of an appliance to the ground. The same idea removes static charge. To earth a charged object, you connect it to the ground with a conductor, such as a metal wire. Electrons then flow until the object is neutral.
| Charge on the object | Which way the electrons flow | Result |
|---|---|---|
| Negative (extra electrons) | From the object, through the conductor, to the ground | The extra electrons leave and the object is neutral |
| Positive (missing electrons) | From the ground, through the conductor, to the object | The missing electrons are replaced and the object is neutral |
Because the electrons flow steadily through the conductor, the charge never builds up enough to make a spark.
A positively charged acetate rod is held close to a small piece of aluminium foil lying on a bench. The foil jumps up to the rod. Explain why, in terms of the movement of electrons.
Step 1, the charged object: the rod is positively charged.
Step 2, which way the electrons move: the rod attracts the electrons in the foil, so they move to the side of the foil nearest the rod.
Step 3, the charge this leaves: the near side of the foil has a negative induced charge.
Step 4, the force: unlike charges attract, so the foil is pulled up to the rod.
Writing that positive charges move. They never do; a positive charge means electrons have moved away. Writing that the paper or the wall becomes charged. It stays neutral overall, and only its near side has an induced charge. Writing "the charge jumps" without saying that electrons move and in which direction.
A boy bounces on a trampoline in his socks. Rubbing between his socks and the plastic mat gives him a negative charge.
(a) Explain, in terms of electrons, how the boy becomes negatively charged. [2 marks]
(b) The boy's hair stands on end. Explain why. [2 marks]
(c) Small pieces of dry grass on the mat are not charged, but they stick to the boy's socks. Explain why, in terms of the movement of electrons. [3 marks]
(d) The boy touches the metal frame of the trampoline, which stands on the ground. He feels a small shock. Explain what happens to the charge on the boy. [2 marks]
(a) Electrons are transferred by friction from the mat to the boy's socks (1). He has gained electrons, which are negatively charged, so he has an overall negative charge (1).
(b) Every hair has a negative charge, the same type of charge (1). Like charges repel, so the hairs push away from each other and from his head (1).
(c) The negative charge on the socks repels the electrons in the grass, so they move to the side of each piece furthest from the sock (1). The side of the grass nearest the sock is left positively charged (1). Unlike charges attract, so the grass is attracted to the sock (1).
(d) Electrons flow from the boy through the metal frame to the ground (1). He loses his extra electrons, so he is discharged and becomes neutral (1).
For each part of a question like this, write the word electrons and then a direction: from the mat to the socks, to the far side of the grass, from the boy to the ground. A sentence that names the electrons and gives their direction usually earns the first mark.