Physics 001.003.001 Electric Charge

Alignment

Learning Intentions

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

• Describe electric charge as positive or negative.
• Identify protons as positively charged, electrons as negatively charged, and neutrons as neutral.
• Describe charged objects as having an imbalance of positive and negative charge.
• Predict whether two charged objects will attract, repel, or have no electrostatic interaction.

Success Criteria

By the end of the lesson, students have successfully:

• Correctly classified particles and objects as positive, negative, or neutral.
• Used the terms positive charge, negative charge, neutral, attraction and repulsion accurately.
• Explained that an object becomes negatively charged by gaining electrons and positively charged by losing electrons.
• Applied the rule that like charges repel and unlike charges attract.

Syllabus Reference

• Unit 1: Thermal, Nuclear and Electrical Physics
• Topic 3: Electrical Circuits
• Current, Potential Difference and Energy Flow
• Describe electric charge as positive or negative.

Phenomenon

When a plastic ruler is rubbed with a cloth and brought near small pieces of paper, the paper may jump toward the ruler. Similarly, a balloon rubbed on hair can stick to a wall or cause hair to stand up.

These effects occur even though there is no visible contact force pulling the objects together. The interaction is caused by electric charge.

Key Idea

Electric charge is a property of matter that can be positive or negative. Objects with the same type of charge repel each other. Objects with opposite types of charge attract each other.

Concept

Matter is made of atoms. Atoms contain:

• protons, which have positive charge
• electrons, which have negative charge
• neutrons, which have no electric charge

A neutral atom or object has equal amounts of positive and negative charge. This does not mean it has no charge particles; it means the positive and negative charges balance.

An object becomes charged when there is an imbalance between positive and negative charge. In most everyday situations, this occurs because electrons move from one object to another.

If an object gains electrons, it has more negative charge than positive charge, so it becomes negatively charged.

If an object loses electrons, it has less negative charge than positive charge, so it becomes positively charged.

The basic interaction rules are:

• Positive and positive charges repel.
• Negative and negative charges repel.
• Positive and negative charges attract.
• A neutral object may be attracted to a charged object due to charge separation within the neutral object.

Convention

Electric charge is represented using the symbols and .

The SI unit of electric charge is the coulomb, symbol .

The charge on a proton is .

The charge on an electron is .

The elementary charge has magnitude:

Therefore:

A neutral object has zero net charge, even though it contains many positive and negative charges.

The net charge of an object depends on the balance between positive and negative charges.

For example:

• more electrons than protons gives a negative net charge
• fewer electrons than protons gives a positive net charge
• equal electrons and protons gives zero net charge

Misconceptions

Common misconceptions students have regarding the concept when applying to various situations and solving problems. It could be a conceptual, mathematical or logical misconception.

• Neutral objects contain no charge.
• Positive charge means an object has gained protons.
• Negative charge is stronger or more dangerous than positive charge.
• A charged object can only attract another charged object.
• Electrons are positive because they move in circuits from positive to negative.

Further Reading

• Static electricity and charge transfer
• Conservation of electric charge
• Electric current as moving charge
• Conventional current and electron flow
• Coulomb’s law as a later extension

Explicit Instruction

Begin by asking students:

“Why can a balloon stick to a wall after being rubbed on hair?”

Use this as a diagnostic question. Students may suggest magnetism, air pressure, glue, or friction. Clarify that the effect is caused by electric charge.

Define electric charge as a property of matter that can be positive or negative.

Explain that the names positive and negative are conventions. They do not mean good and bad. They are labels used to describe two opposite types of electric charge.

Introduce the three subatomic particles:

Particle	Charge
Proton	Positive
Electron	Negative
Neutron	Neutral

Explain that protons are found in the nucleus and usually do not move between objects during everyday charging. Electrons are found around the nucleus and can be transferred more easily.

Use the example of rubbing a balloon on hair:

Before rubbing:

• the balloon is neutral
• the hair is neutral

During rubbing:

• electrons transfer from the hair to the balloon

After rubbing:

• the balloon has gained electrons, so it is negatively charged
• the hair has lost electrons, so it is positively charged

Then introduce charge interactions:

Pair of charges	Interaction
Positive and positive	Repel
Negative and negative	Repel
Positive and negative	Attract

State the rule:

Like charges repel. Unlike charges attract.

Worked Examples

Worked Example 1

A plastic rod gains electrons when rubbed with a cloth.

Describe the charge on the rod.

Solution:

Electrons have negative charge.

If the rod gains electrons, it now has more negative charge than positive charge.

Therefore, the rod becomes negatively charged.

Answer:

The rod is negatively charged.

Worked Example 2

A glass rod loses electrons when rubbed with silk.

Describe the charge on the glass rod.

Solution:

Electrons have negative charge.

If the glass rod loses electrons, it loses negative charge.

The rod now has more positive charge than negative charge.

Therefore, the glass rod becomes positively charged.

Answer:

The glass rod is positively charged.

Worked Example 3

Two objects are brought near each other.

Object A is negatively charged.

Object B is also negatively charged.

Describe the interaction between the objects.

Solution:

Object A is negative.

Object B is negative.

The charges are the same.

Like charges repel.

Answer:

The objects repel each other.

Check for Understanding

Check 1

A student says:

“A neutral object has no charges inside it.”

Is this correct?

Expected answer:

No. A neutral object contains positive and negative charges, but the amounts balance. Its net charge is zero.

Check 2

A balloon becomes negatively charged after being rubbed on hair.

What happened to the balloon?

Expected answer:

The balloon gained electrons.

Check 3

Complete the table:

Object A	Object B	Attract or repel?
Positive	Positive
Negative	Negative
Positive	Negative
Negative	Positive

Expected answer:

Object A	Object B	Attract or repel?
Positive	Positive	Repel
Negative	Negative	Repel
Positive	Negative	Attract
Negative	Positive	Attract

Investigation (Alternative to Explicit)

Hypothesis

If two objects have the same type of charge, they will repel. If they have opposite types of charge, they will attract.

Data Collection

Materials:

• balloons
• wool cloth or hair
• small pieces of paper
• aluminium foil pieces
• plastic ruler
• hanging charged strip or pith ball if available

Method:

1. Rub a balloon with wool or hair for several seconds.
2. Bring the balloon near small pieces of paper without touching them.
3. Record the observation.
4. Rub two balloons with the same material.
5. Bring the two balloons near each other.
6. Record whether they attract or repel.
7. Bring the charged balloon near a neutral wall.
8. Record the observation.
9. Repeat trials to check consistency.

Data table:

Trial	Object A	Object B	Charge situation	Observation
1	Charged balloon	Paper	Charged and neutral
2	Charged balloon	Charged balloon	Same charging method
3	Charged balloon	Wall	Charged and neutral

Analysis

Students should use observations to infer the presence of electric charge.

Guiding questions:

1. What evidence suggested that the balloon became charged?
2. Did charged objects always attract?
3. Why can a charged object attract a neutral object?
4. Why did two balloons rubbed in the same way repel?

Expected reasoning:

The balloon became charged because it exerted a force on other objects without contact. Two balloons rubbed in the same way likely gained the same type of charge, so they repelled. A neutral object can still be attracted because charges inside it can shift slightly, making one side more attracted to the charged object.

Evaluation

Students should consider:

• whether each object was charged consistently
• whether humidity affected the results
• whether objects touched accidentally and transferred charge
• whether repeated trials gave similar observations
• whether observations were qualitative or quantitative

Problems

The following problems are designed to practise describing electric charge as positive or negative.

1. State the charge of each particle: a. proton b. electron c. neutron

2. A neutral object gains electrons. Describe its final charge.

3. A neutral object loses electrons. Describe its final charge.

4. Two positively charged rods are brought near each other. Describe the force between them.

5. A negatively charged balloon is brought near a positively charged rod. Describe the force between them.

6. A student says, “An object becomes positively charged when it gains protons.” Explain why this is usually incorrect for everyday static electricity.

7. A neutral metal sphere has equal amounts of positive and negative charge. Is the sphere uncharged or does it contain no charged particles? Explain your answer.

8. Complete the sentence:

   Like charges __________ and unlike charges __________.

9. A cloth becomes positively charged after being rubbed with a plastic rod. What type of charge did the rod most likely gain?

10. Explain why the words positive and negative are best understood as labels for two types of charge, not as meanings of good and bad.

Followup

Self-check

Students should be able to answer the following without notes:

1. What are the two types of electric charge?
2. Which particle has positive charge?
3. Which particle has negative charge?
4. What does it mean for an object to be neutral?
5. What happens when an object gains electrons?
6. What happens when an object loses electrons?
7. What happens when like charges are brought together?
8. What happens when unlike charges are brought together?

Next Topic

Describe electric current as carried by discrete electric charge carriers.