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objectives
objectives
Electric Charge
Electric Charge
- Electric charge, or ‘electricity’, can come from batteries and generators.
- But some materials become charged when they are rubbed.
- Their charge is sometimes called electrostatic charge or ‘static electricity’.
- It causes sparks and crackles when you take off a pullover, and if you slide out of a car seat and touch the door, it may even give you a shock.
- This person has been charged up. Her hairs all carry the same type of charge, so they repel each other.
Negative and Positive Charges
Negative and Positive Charges
- Polythene and Perspex can be charged by rubbing them with a dry, woollen cloth.
- When two charged polythene rods are brought close together, as shown below, they repel (try to push each other apart).
- The same thing happens with two charged Perspex rods. However, a charged polythene rod and a charged Perspex rod attract each other.
- Like charges repel; unlike charges attract. The closer the charges, the greater the force between them.
Experiments like this suggest that there are two different and opposite types of electric charge. These are called positive (+) charge and negative (-) charge:
Where do Charges come from?
Where do Charges come from?
- Everything is made of tiny particles called atoms. These have electric charges inside them. A simple model of the atom is shown on the left. There is a central nucleus made up of protons and neutrons. Orbiting the nucleus are much lighter electrons:
- Electrons have a negative (-) charge.
- Protons have an equal positive (+) charge.
- Neutrons have no charge.
- Normally, atoms have equal numbers of electrons and protons, so the net (overall) charge on a material is zero. However, when two materials are rubbed together, electrons may be transferred from one to the other. One material ends up with more electrons than normal and the other with less.
- So one has a net negative charge, while the other is left with a net positive charge. Rubbing materials together does not make electric charge. It just separates charges that are already there.
When polythene is rubbed with a woollen cloth, the polythene pulls electrons from the wool.
When Perspex is rubbed with a woollen cloth, the wool pulls electrons from the Perspex.
Conductors and Insulators
Conductors and Insulators
- When some materials gain charge, they lose it almost immediately. This is because electrons flow through them or the surrounding material until the 2 dance of negative and positive charge is restored.
- Conductors are materials that let electrons pass through them. Metals are the best electrical conductors. Some of their electrons are so loosely held to their atoms that they can pass freely between them. These free electrons also make metals good thermal conductors.
- Most non-metals conduct charge poorly or not at all, although carbon is an exception.
- Insulators are materials that hardly conduct at all. Their electrons are tightly held to atoms and are not free to move - although they can be transferred by rubbing. Insulators are easy to charge by rubbing because any electrons that get transferred tend to stay where they are.
- Semiconductors* These are ‘in-between’ materials. They are poor conductors when cold, but much better conductors when warm.
- The 'electricity' in a cable is a flow of electrons. Most cables have copper conducting wires with PVC plastic around them as insulation.
activity - questions
activity - questions
Complete the Questions on the worksheet and hand it in on google classroom
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