D. Physics: Principles and Applications of Electricity · Electricity: Charges, Circuits and Energy

Charging Objects: Friction, Contact and Induction

D2.1 A1.2 A1.5About 75 minutes

Learning goals

I can use the electrostatic series to predict which object becomes positive and which becomes negative when two materials are rubbed together.
I can explain charging by friction, by contact and by induction in terms of electron movement.
I can explain why a charged object attracts a neutral object (polarization).
I can explain grounding and electric discharge, including lightning.
I can draw labelled charge diagrams to explain the results of an investigation.

Words to know

charging by friction
Charging two different materials by rubbing them together. Electrons move from one material to the other, so they end up with equal and opposite charges.

electrostatic series
A list of materials ordered by how strongly they hold on to electrons. Also called the triboelectric series.

charging by contact
Charging a neutral object by touching it with a charged object. The neutral object gets the same sign of charge as the charged object.

charging by induction
Charging a neutral conductor using a nearby charged object that never touches it, plus a temporary ground. The conductor gets the opposite sign of charge.

induced charge separation (polarization)
The shifting of electrons to one side of a neutral object when a charged object comes near. The object is still neutral overall.

grounding
Connecting an object to the Earth so that electrons can flow to or from it until it is neutral. The Earth is so large that it can give or take electrons without noticeably changing.

electric discharge
The rapid movement of built-up charge from one place to another, such as a spark or lightning.

electroscope
A device used to detect electric charge. In a metal-leaf electroscope, two thin leaves spread apart when they carry the same charge.

Why does the balloon stick?

Rub a balloon on your hair and hold it against the wall. It stays there! Bring it near your hair and the strands lift towards it. Hold it near a thin stream of tap water and the water bends.

In the last lesson you learned that objects become charged by gaining or losing electrons. Today you will learn the three ways to charge an object: friction, contact and induction. Each one is just a different way of moving electrons.

1. Charging by friction

When two different materials are rubbed together, many points on their surfaces touch. One material holds on to its electrons more strongly than the other, so some electrons are pulled across.

The material that gains electrons becomes negative.
The material that loses electrons becomes positive.
The two charges are equal in size and opposite in sign. Charge is not created or destroyed; it is only moved. This is the law of conservation of charge.

Charging by friction (a simplified model). Before rubbing, each object is neutral. After rubbing, 2 electrons have moved from the wool to the balloon: the wool is +2 and the balloon is −2. The total charge is still zero.

The electrostatic series

How do you know which material will gain electrons? Scientists rub pairs of materials and test the charges. The results are put in order in the electrostatic series.

When two materials are rubbed together, the one higher on the list loses electrons (becomes +) and the one lower on the list gains electrons (becomes −). The farther apart they are, the more charge usually moves.

A short electrostatic series. Rub any two: the higher material becomes positive, the lower one becomes negative. Different sources order some materials slightly differently, and humidity affects results.

Worked example

Example 1: Use the electrostatic series

A glass rod is rubbed with silk. Which becomes positive? Which way do electrons move?

Find both materials on the series: glass is above silk.
The higher material loses electrons, so glass loses electrons and silk gains them.
Electrons move from the glass to the silk.

The glass rod becomes positive and the silk becomes negative, with equal amounts of charge.

Why a charged object attracts a neutral object

Bring a negative balloon near a neutral wall. The balloon's negative charge repels electrons in the wall's surface, pushing them slightly away. The side of the wall nearest the balloon is left slightly positive; the far side is slightly negative.

The wall is still neutral overall, but now its positive side is closer to the balloon. Because electric force is stronger at shorter distances, the attraction to the near positive side is stronger than the repulsion from the far negative side. Result: the balloon sticks. This shifting of charge is called induced charge separation or polarization.

2. Charging by contact

When a charged object touches a neutral object (especially a metal), some charge is shared between them.

A negative rod touching a neutral metal sphere: some extra electrons move from the rod onto the sphere. The sphere becomes negative.
A positive rod touching a neutral sphere: some electrons move from the sphere onto the rod. The sphere becomes positive.

Rule: charging by contact gives the object the same sign as the charging object.

Worked example

Example 2: Charging an electroscope by contact

A negatively charged ebonite rod touches the metal knob at the top of a neutral electroscope. Describe what happens to the leaves and explain.

Extra electrons on the rod repel each other, so some move onto the metal knob.
Metal is a conductor, so the electrons spread through the knob, the stem and down into both leaves.
Both leaves are now negative. Like charges repel.

The leaves spread apart and stay apart after the rod is removed, because the electroscope is now negatively charged (same sign as the rod).

3. Charging by induction

Induction charges a neutral conductor without touching it with the charged object. You also need a ground: a path to the Earth, such as a wire or your finger.

Bring a charged rod near (but not touching) a neutral metal sphere. Charges separate inside the sphere.
Touch the far side of the sphere with your finger (ground). Charge flows through you to or from the Earth.
Remove the ground first, while the rod is still near.
Then remove the rod. The sphere keeps a charge opposite to the rod's charge, spread evenly over its surface.

Rule: charging by induction gives the object the opposite sign to the charging object.

Charging a metal sphere by induction with a negative rod. (2) The rod repels electrons to the far side. (3) Grounding lets those electrons escape to the Earth. (4) Remove the ground, then the rod: the sphere is left positive, the opposite sign to the rod. Protons never move. (To keep it simple, only a few of the sphere's charges are drawn.)

Worked example

Example 3: Induction with a positive rod

A positive rod is brought near a neutral metal can on an insulating stand. You touch the far side of the can, remove your finger, then remove the rod. What is the final charge on the can? Explain with electrons.

The positive rod attracts electrons in the can towards the side nearest the rod. The far side is left positive.
When you touch the can, electrons are attracted up from the Earth, through your finger, into the can.
You remove your finger first, so those extra electrons are trapped on the can.
Then the rod is removed. The extra electrons spread out over the can.

The can is negative: opposite to the rod, just as the induction rule says.

Method	Does the charged object touch?	Final charge on the object being charged	What moves?
Friction	Rubbed together	Equal and opposite charges on the two materials	Electrons, from the higher to the lower material in the series
Contact	Yes	Same sign as the charging object	Electrons, between the objects
Induction	No (a ground is touched instead)	Opposite sign to the charging object	Electrons, between the object and the ground

The three ways to charge an object.

Grounding and discharge

Charge does not stay on objects forever. When charge moves quickly from one object to another, it is called an electric discharge.

Touch a metal doorknob after shuffling across a carpet: the extra charge on you jumps to the doorknob as a spark.
Lightning is a giant discharge. Charges separate inside a storm cloud as ice particles collide. When enough charge builds up, it discharges between parts of the cloud, between clouds, or between a cloud and the ground.
A lightning rod is a pointed metal rod on top of a building, connected by a thick wire to the ground. If lightning strikes, the charge has a safe path to the Earth instead of through the building.
Fuel trucks are grounded before fuelling, so that a spark cannot ignite fuel vapour.

Charge diagrams: the rules

Show the same number of + signs in every step; protons never move.
Move only − signs (electrons).
A neutral object needs equal numbers of + and − signs.
Label each step and use arrows to show which way electrons move.

Hands-on activity

Balloon investigations

You need: a balloon, your hair or a wool sweater, a few tiny bits of paper, an empty aluminium pop can, a kitchen tap.

Predict using the electrostatic series: which will become negative, the balloon or your hair? (The balloon, because rubber is lower than hair.)
Rub the balloon on your hair 10 times. Hold it 2 cm above the bits of paper. Record what happens.
Lay the empty can on its side on a smooth floor. Hold the charged balloon near it without touching. Record what happens.
Turn on the tap to a very thin, steady stream. Bring the charged balloon close to the stream without touching it.
For each test, draw a labelled charge diagram showing why the attraction happens.

What to look for: the paper jumps up, the can rolls towards the balloon, and the water stream bends towards the balloon. Each one is a neutral object attracted by induced charge separation: the side nearest the negative balloon becomes slightly positive.

Safety (grown-up note): Keep balloons away from young children (choking hazard), and use non-latex balloons if anyone has a latex allergy. Wipe up any water on the floor so no one slips, and keep water away from electrical outlets.

Just for fun

The word electricity comes from the Greek word elektron, which means amber. Over 2000 years ago, people noticed that amber rubbed with fur could pick up bits of straw and feathers. Amber is not really a rock: it is fossilized tree resin. So the science of electricity started with a lump of ancient tree sap and a fuzzy rubbing cloth.

Your turn

Wool is rubbed with a PVC pipe. Which becomes negative? Use the electrostatic series.

When an object is charged by friction, how do the charges on the two materials compare?

A positive rod touches a neutral metal sphere. What charge does the sphere get, and which way do electrons move?

A negative rod is used to charge a sphere by induction. What is the sphere's final charge?

Why does a charged balloon stick to a neutral wall?

What is a lightning rod, and how does it protect a building?

More practice: warm-up

Name the three ways to charge an object.

Which particles move when an object becomes charged?

Charging by contact gives the same or the opposite sign?

What does an electroscope detect?

What is the name for a quick movement of built-up charge, like a spark?

More practice: on your own

Human hair is rubbed with a plastic bag (polyethylene). Predict the charges and explain.

In induction, why must you remove the ground before removing the charged rod?

A charged electroscope has its leaves spread apart. You touch the knob with your finger. What happens and why?

A negatively charged electroscope has spread leaves. A rod is brought near (not touching) and the leaves spread farther. What is the rod's charge? Explain.

Explain the doorknob spark you get after shuffling across a carpet in socks in winter.

Challenge

You have a neutral metal sphere on an insulating stand and a positive glass rod. Describe two different ways to make the sphere positive and one way to make it negative.

Two identical metal spheres on insulating stands carry charges of −6 nC and +2 nC. They touch and are separated. What is the charge on each? (nC means nanocoulombs.)

Draw (or describe in words) a set of charge diagrams to explain why a charged balloon makes a thin stream of water bend, and say whether the balloon's sign matters.

Check your understanding

Quiz

Glass is rubbed with silk. Glass is above silk in the electrostatic series. What happens?

A neutral sphere is charged by contact with a negative rod. The sphere becomes:

A neutral sphere is charged by induction with a negative rod. The sphere becomes:

Why is a neutral piece of paper attracted to a charged comb?

What does grounding do to a charged object?

In charging by friction, the total charge of the two objects after rubbing is:

Which is the best example of an electric discharge?

For parents and teachers

Big idea: This lesson continues D2.1 (behaviour of electric charges in static electricity). Students should explain every result using electron movement only, and support explanations with labelled charge diagrams, which is a key Grade 9 skill.

Common difficulties:

Mixing up contact and induction. Memory hook: contact = copy (same sign), induction = invert (opposite sign).
Moving + signs in diagrams. Remind your teen that protons never move; only − signs should move between pictures.
Order of steps in induction. The ground must be removed before the charged rod.

How to help: Do the balloon tests together. Ask your teen to predict using the electrostatic series, then draw the diagram. Balloons are a choking hazard for younger children; keep water away from outlets and wipe up spills.