Electric Charges – SS1 Physics Lesson Note (Second Term)

Electric charge is a property of matter that makes it attract or repel other charged matter, and rubbing certain materials together produces it.

• Types: positive and negative.
• Law: like charges repel and unlike charges attract.
• Electrostatics: the study of charges at rest.

Behavioural objectives

By the end of the lesson, students should be able to:

1. explain how electric charges are produced by rubbing and state the two types of charge;
2. state the fundamental law of electrostatics;
3. give examples of conductors and insulators;
4. describe how charges are distributed on a charged surface and how they are stored;
5. describe the gold-leaf electroscope and state its uses;
6. explain how a neutral insulated body is charged by contact and by induction, and how a lightning conductor protects a building.

How are electric charges produced?

Electric charges are produced by rubbing two different materials together, which makes them able to attract light objects.

• Simple test: a plastic pen rubbed on hair or a coat attracts small pieces of paper held near it.
• Charged body: a rubbed material, and the light object it attracts, is said to be charged or electrified.
• Static electricity: charge that stays on the substance where it is produced and does not flow from point to point.
• Electrostatics: the study of charges at rest.
• Cause: rubbing moves electrons from one material to the other.
• Body that gains electrons: becomes negatively charged.
• Body that loses electrons: becomes positively charged.
• Unit: the SI unit of charge is the coulomb (C).

What are the types of charges?

There are two types of charges: positive and negative.

• Opposite charges: two rods with opposite electrification attract each other.
• Similar charges: two similarly charged rods repel each other.

What is the fundamental law of electrostatics?

The fundamental law of electrostatics states that like charges repel and unlike charges attract.

What are conductors and insulators?

Materials are grouped as conductors or insulators by how easily electrons pass through them.

Conductors

Conductors are materials that allow electrons to pass through them easily.

• Metals: copper, aluminium and iron.
• Non-metals: graphite.
• Liquids: acids and salt solutions.
• Others: damp air, the earth and the human body.

Insulators

Insulators are materials that do not allow electrons to pass through them easily.

• Plastics and resins: plastic, polythene, Bakelite and ebonite.
• Natural materials: paper, dry hair, silk and wood.
• Others: oils, glass and sulphur.

How are charges distributed on a surface?

Charge on a conductor collects most at sharp points and edges, where the charge density is greatest.

• Surface density: the charge per unit area of a charged surface.
• Sharp curve or point: has a high surface density, so charge is concentrated there.
• Flat or gently curved part: has a lower surface density.
• Use: this is why a lightning conductor has a sharp spike.

How are charges stored?

Charges are stored in an electrophorus or in a capacitor.

• Electrophorus: a device for transferring and storing charges, which produces charge by electrostatic induction.
• Capacitor: another device for storing electric charges.

How is an electrophorus used?

1. Rub the insulating base plate (such as ebonite or polythene) with fur to charge it negatively.
2. Place the metal disc on the base plate.
3. Touch the disc briefly with a finger to earth it.
4. Lift the disc by its insulating handle. It now carries a positive charge.

What is a gold-leaf electroscope?

A gold-leaf electroscope is an instrument used for detecting and testing positive and negative charges.

Parts of the electroscope

• Metal rod: a brass rod with a thin gold leaf (or aluminium leaf) attached.
• Cap: a brass disc or cap on top of the rod.
• Insulation: the rod is insulated from the metal case.
• Case: an earthed metal case with glass windows, which protects the leaf from draught.

Why does the leaf rise?

• Charged cap: charge spreads over the cap, rod and leaf.
• Divergence: the rod and leaf carry the same charge, so the leaf is repelled and rises.

What are the uses of the electroscope?

The electroscope is used to test conducting properties and to identify the sign of a charge.

Testing conductors and insulators

Touch the material to the cap of a charged electroscope and watch the leaf.

• Rapid collapse: charges escape easily, so the material is a good conductor.
• Slow collapse: the material is a poor conductor.
• No collapse: the material is an insulator.

Testing the sign of a charge

Bring the unknown charge near the cap of an electroscope that has a known charge.

• Leaf diverges more: the unknown charge is similar to the charge on the electroscope.
• Leaf collapses a little: the unknown charge is opposite to the charge on the electroscope.

How is an insulated body charged?

A neutral insulated body can be charged by two methods: contact and induction.

What is the contact method?

Charging by contact is giving a charge to a neutral body by touching it with a charged object.

• Result: the body acquires the same type of charge as the charged object.

What is the induction method?

Charging by induction is charging a neutral body by placing a charged body near it without any contact.

1. Bring a positive charge close to the insulated body. The body stands on an insulated stand so the induced charge cannot leak away. Negative charges are attracted towards the inducing charge. Positive charges are repelled to the far end.
2. Earth the body by touching it momentarily. The repelled positive charge is neutralised by electrons from the earth.
3. Remove the inducing charge. The negative charge spreads over the body, leaving it negatively charged.

• Negative inducing charge: the body instead acquires a positive charge.

How do the two methods differ?

What is lightning and how does a lightning conductor work?

Lightning is a sudden discharge, or neutralising, of electric charges that occurs when charges build up in a cloud.

• Ions in the air: the atmosphere contains charged particles produced by radiation from the sun and by cosmic radiation from outer space.
• Lightning conductor: a long metal rod with a sharp spike on top of a building, connected to the earth by a cable.
• Purpose: to prevent buildings from being destroyed when struck by lightning.

How does the lightning conductor protect a building?

1. The sharp point at the top gains an induced charge opposite to that in the thundercloud.
2. The charge ionises the nearby air.
3. The charged air molecules flow upwards from the point.
4. This discharges the cloud before a lightning flash occurs.
5. Any charge that does strike the rod passes safely to the earth through the cable.

Evaluation

1. State the fundamental law of electrostatics.
2. Name two conductors and two insulators.
3. What does rapid collapse of the leaf mean when a material touches the cap of a charged electroscope?
4. Describe how a neutral insulated body is charged by induction using a positive inducing charge.
5. How does a lightning conductor protect a building?

Assignment

Describe the gold-leaf electroscope and state two of its uses. Then explain the difference between charging a body by contact and charging it by induction.