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

Electric Charge and the Atom

D2.1 A1.2 A1.5About 70 minutes
Learning goals
  • I can describe the three subatomic particles (protons, neutrons and electrons) and their charges.
  • I can explain why an object becomes positive or negative by gaining or losing electrons.
  • I can state the law of electric charges and use it to predict attraction and repulsion.
  • I can calculate the net charge on an object in elementary charges and in coulombs.
  • I can tell the difference between static electricity and current electricity.

Words to know

proton
A positively charged particle in the nucleus of an atom. Protons stay in the nucleus; they do not move from object to object when you rub things.
neutron
A particle with no charge, also in the nucleus.
electron
A tiny negatively charged particle found outside the nucleus. Some electrons can be moved from one object to another.
elementary charge (e)
The size of the charge on one proton or one electron: 1.6 × 10⁻¹⁹ coulombs.
coulomb (C)
The SI unit of electric charge. One coulomb is the charge of about 6.24 × 10¹⁸ electrons.
neutral
Having equal numbers of protons and electrons, so the net charge is zero.
ion
An atom (or group of atoms) that has gained or lost electrons, so it has a net charge.
law of electric charges
Like charges repel, unlike charges attract, and a charged object attracts a neutral object.
static electricity
Electric charge that builds up and stays (is 'static') on the surface of an object.
current electricity
A continuous flow of electric charge through a conductor, such as a wire.

Remember from Grade 6

In Grade 6 you learned that static electricity is caused by tiny charged particles called electrons moving from one object to another, and that current electricity is a steady flow of charge through a circuit. In Grade 9 you will explain these ideas exactly, using the structure of the atom, numbers and units. You will also look at atoms closely in the Chemistry unit of this course, Atomic and Molecular Structures.

A crackle in January

It is a cold, dry day in January. You pull off your wool toque and your hair stands straight up. You hear a tiny crackle, and in a dark room you might even see small sparks.

Nothing was plugged in. So where did that electricity come from? The answer is hiding inside every atom of your hair and your toque.

What is inside an atom?

All matter is made of atoms. Each atom has two parts:

  • A tiny, heavy centre called the nucleus. It holds protons (positive charge, +) and neutrons (no charge).
  • A much larger region around the nucleus where electrons (negative charge, −) are found.

The charge on one proton and the charge on one electron are exactly the same size, just opposite in sign. This size is called the elementary charge, e.

Protons are held tightly in the nucleus. They do not move when you rub a balloon on your hair. Electrons, especially the outer ones, are held less tightly, and some can be pulled off one material and onto another. In solids, only electrons move. (In liquids such as salt water, whole charged atoms called ions can move too.)

ParticleChargeWhere it isMass compared with an electronCan it move to another object when you rub things?
Proton+1 eNucleusabout 1800 times moreNo
Neutron0Nucleusabout 1800 times moreNo
Electron−1 eAround the nucleus1Yes, some outer electrons can
The three subatomic particles. 1 e = 1.6 × 10⁻¹⁹ C.

Neutral, positive and negative

An atom with the same number of protons and electrons is neutral: the charges cancel, so the net charge is zero. Most objects around you are neutral.

  • If an object loses electrons, it has more protons than electrons. It becomes positively charged.
  • If an object gains electrons, it has more electrons than protons. It becomes negatively charged.

Notice: an object never becomes positive by gaining protons. It becomes positive by losing electrons.

3+−−−neutral atom3 protons, 3 electronsnet charge 03+−−lost 1 electron3 protons, 2 electronsnet charge +13+−−−−gained 1 electron3 protons, 4 electronsnet charge −1
A model atom with 3 protons in the nucleus. Losing an electron leaves it with a net charge of +1; gaining an electron gives it a net charge of −1. The protons never change.

Counting charge

Net charge = (number of protons − number of electrons) × e

Scientists measure charge in coulombs (C). One electron has a charge of −1.6 × 10⁻¹⁹ C. That is so small that even a tiny shock involves billions of electrons. One coulomb is the charge of about 6.24 × 10¹⁸ electrons (6 240 000 000 000 000 000).

Worked example

Example 1: Net charge from particle counts

A small object has 5000 protons and 5003 electrons. What is its net charge in elementary charges? Is it positive, negative or neutral?

  1. Net charge = protons − electrons = 5000 − 5003 = −3.
  2. There are 3 more electrons than protons, so the object has gained electrons.

The net charge is −3 e. The object is negatively charged.

Worked example

Example 2: Charge in coulombs

A balloon rubbed on hair gains 3.0 × 10¹² extra electrons. What is its charge in coulombs?

  1. Each electron has a charge of −1.6 × 10⁻¹⁹ C.
  2. Multiply: 3.0 × 10¹² × (−1.6 × 10⁻¹⁹ C).
  3. 3.0 × 1.6 = 4.8, and 10¹² × 10⁻¹⁹ = 10⁻⁷ (add the exponents: 12 + (−19) = −7).

The balloon's charge is −4.8 × 10⁻⁷ C. That is less than one millionth of a coulomb, even though it is trillions of electrons.

The law of electric charges

Charged objects push or pull on each other without touching. This push or pull is the electric force. It follows three rules:

  1. Like charges repel (+ and +, or − and −, push apart).
  2. Unlike charges attract (+ and − pull together).
  3. A charged object attracts a neutral object. (You will see why in the next lesson.)

The electric force gets stronger when the objects are closer together and when they carry more charge.

++repellike charges−−repellike charges+−attractunlike charges
The law of electric charges. The arrows show the direction of the force on each ball: like charges push apart, unlike charges pull together.
Worked example

Example 3: Predict the force

Rod X is positive. Rod Y repels rod X. Rod Z attracts rod Y. What is the charge on Y? What can you say about Z?

  1. Y repels X. Only like charges repel, so Y has the same charge as X: Y is positive.
  2. Z attracts Y. Attraction happens between unlike charges or between a charged object and a neutral one.
  3. So Z could be negative or neutral. We cannot be sure from attraction alone.

Y is positive. Z is negative or neutral. Repulsion is the only sure test that two objects are both charged with the same sign.

Watch out!

  • Protons do not move in solids. If something is positive, it lost electrons.
  • Attraction does not prove an object is charged; a neutral object is also attracted. Repulsion proves it.
  • Static charge is easiest to build up on dry winter days. Humid air helps charge leak away.

Static electricity and current electricity

Both kinds of electricity involve electric charges. The difference is what the charges are doing.

  • Static electricity: charge builds up and stays on the surface of an object (often an insulator such as a balloon). When it finally jumps, it moves in one quick burst: a spark or discharge.
  • Current electricity: charge flows continuously around a closed path (a circuit), pushed by a source like a battery. This is what runs your phone and your fridge.

In this unit you will first study static charge (Lessons 1 to 3), then current electricity in circuits.

Hands-on activity

Sticky-tape investigation (scientific experimentation)

You need: clear sticky tape, a table, a pencil or marker, and a balloon (optional).

Question: Do pieces of tape pulled off quickly become charged? What kinds of charge?

  1. Stick a 15 cm piece of tape to the table. Fold over one end to make a handle and label it B (bottom).
  2. Stick a second piece directly on top of B. Label it T (top).
  3. Make a second B-and-T pair the same way.
  4. Pull each pair off the table together, then quickly peel each T off its B.
  5. Hang them from the table edge and bring them close together: T near T, B near B, then T near B.
  6. Record what happens in a table with three rows: T and T, B and B, T and B.

What to look for: T and T repel, B and B repel, and T and B attract. Peeling them apart moved electrons from one tape to the other, so the T strips have one kind of charge and the B strips have the opposite kind.

Safety (grown-up note): Balloons are a choking hazard for young children; keep balloons and broken pieces away from little ones, and use a non-latex balloon if anyone has a latex allergy.

Just for fun

A neutron walks into a café and orders a hot chocolate. "How much do I owe?" it asks.

"For you," says the server, "no charge."

Your turn

1

Name the three subatomic particles and give the charge of each.

2

Which particle moves from one solid object to another when they are rubbed together?

3

An object has 120 protons and 117 electrons. What is its net charge?

4

A neutral glass rod loses electrons. What is its new charge?

5

Two negatively charged balloons are hung near each other. What happens?

6

A comb attracts a small piece of paper. Can you be sure the paper is charged? Explain.

7

What is the difference between static electricity and current electricity?

More practice: warm-up

1

Where in the atom are protons found?

2

True or false: an object becomes positive by gaining protons.

3

What do unlike charges do?

4

What is the SI unit of electric charge?

5

An atom has 8 protons and 8 electrons. What is its net charge?

More practice: on your own

1

A sock has 2.0 × 10¹⁰ more electrons than protons. What is its charge in coulombs?

2

Ball A is negative. Ball A repels ball B. Ball B attracts ball C. Ball C repels ball D. What are the charges on B, C and D?

3

Why are protons not transferred when you rub a balloon on your hair?

4

Two charged balls repel. They are moved closer together. What happens to the force between them?

5

Why does static charge build up more easily in January than in July in Ontario?

Challenge

1

A metal sphere has a charge of +8.0 × 10⁻⁹ C. Did it gain or lose electrons, and how many?

2

Explain why it is useful that the charge of a proton and an electron are exactly equal in size.

3

Design a quick test with sticky tape to find out whether a mystery object is charged, and what sign it has. Assume you know the T strips are positive.

Check your understanding

Quiz

1.

Which particle has a negative charge?

2.

A neutral object loses electrons. What is its charge now?

3.

Which pair would repel each other?

4.

An object has 40 protons and 44 electrons. What is its net charge?

5.

What is the charge of one electron in coulombs?

6.

Which observation proves that an object is charged?

For parents and teachers

Big idea: Ontario's Grade 9 expectation D2.1 asks students to explain how electric charges behave in static and current electricity, and to relate this to subatomic particles and atomic structure. This lesson gives the atomic foundation for the whole unit.

Common difficulties:

  • Thinking positive charge means 'gained protons'. Ask: "Which particle can move?" (only electrons, in solids).
  • Thinking attraction proves an object is charged. A charged object attracts neutral objects too. Only repulsion is proof.
  • Scientific notation with negative exponents (10¹² × 10⁻¹⁹ = 10⁻⁷). Students add the exponents; the Grade 9 math course practises this too.

How to help: Do the sticky-tape activity together and ask your teen to predict before each test, then explain the result using the words electron, gain, lose, repel and attract. Keep balloons away from young children (choking hazard).