Basics of Electric Fields
1. Electric charge
Learning outcomes
- I can describe electric charge as a fundamental property of matter.
- I can distinguish between positive and negative electric charges.
- I can explain how like charges repel and opposite charges attract.
- I can describe how electric charge is transferred through friction, conduction, and induction.
- I can explain the principle of conservation of charge in physical systems.
Introduction
Have you ever rubbed a balloon on your hair and watched it stick to a wall? Or noticed a small shock after walking across a carpet and touching a metal door handle? These everyday experiences are caused by electric charge.
Electric charge is one of the most fundamental properties of matter. It is responsible for electric forces, electric fields, lightning, and many of the technologies we use every day. Understanding electric charge is the first step toward learning about electric fields, electric circuits, magnetism, and modern electronics.
What Is Electric Charge?
Electric charge is a fundamental property of matter that causes objects to experience electric forces.
Every atom contains charged particles:
- Protons, which have a positive (+) charge.
- Electrons, which have a negative (–) charge.
- Neutrons, which have no charge.
An object's overall charge depends on the balance between its protons and electrons.
Definition:
Electric charge is a fundamental property of matter that causes objects to attract or repel one another through electric forces.
Positive and Negative Charge
There are two types of electric charge:
- Positive (+)
- Negative (–)
Normally:
- Protons remain fixed inside the nucleus.
- Electrons can move from one object to another.
An object becomes:
- positively charged if it loses electrons,
- negatively charged if it gains electrons,
- electrically neutral if it has equal numbers of protons and electrons.
| Object | Charge |
|---|---|
| More protons than electrons | Positive |
| More electrons than protons | Negative |
| Equal protons and electrons | Neutral |
Worked Example 1
A neutral atom loses two electrons.
Question
What is the overall charge of the atom?
Solution
The atom now has two more protons than electrons, so it has a +2 charge.
How Charged Objects Interact
Charged objects exert forces on one another.
The rules are simple:
- Like charges repel.
- Unlike (opposite) charges attract.
This can be summarised as:
| Charges | Force |
|---|---|
| Positive and Positive | Repel |
| Negative and Negative | Repel |
| Positive and Negative | Attract |
These forces act even when the objects are not touching.
Why Do Charges Attract and Repel?
Charged particles produce electric fields around themselves.
When another charged particle enters this field, it experiences an electric force.
For example:
- Two negatively charged balloons push apart.
- A positively charged object attracts a negatively charged object.
- Lightning occurs because enormous electric charges build up in clouds and then discharge.
The strength of the force depends on:
- the amount of charge,
- the distance between the charges.
(You will study this in more detail when learning Coulomb's Law.)
Transferring Electric Charge
Electric charge is usually transferred by the movement of electrons, not protons.
There are three main methods of charging an object:
- Friction
- Conduction
- Induction
Charging by Friction
When two different materials are rubbed together, electrons may move from one material to the other.
One object:
- loses electrons and becomes positively charged.
The other:
- gains electrons and becomes negatively charged.
Example
Rubbing a balloon on your hair:
- electrons move from your hair to the balloon,
- the balloon becomes negatively charged,
- your hair becomes positively charged.
The opposite charges attract, causing your hair to stand up or the balloon to stick to a wall.
Charging by Conduction
Conduction occurs when a charged object touches another object.
Electrons flow between the objects until the charges are redistributed.
Example:
- A negatively charged metal rod touches a neutral metal sphere.
- Some electrons move onto the sphere.
- Both objects become negatively charged.
Conduction requires physical contact.
Charging by Induction
Induction allows an object to become charged without touching the charged object.
A nearby charged object causes electrons inside a conductor to move.
If the conductor is grounded while the charged object is nearby, electrons can enter or leave the conductor. Removing the ground and then the nearby charged object leaves the conductor with a net charge.
Induction is widely used in:
- electrostatic painting,
- photocopiers,
- lightning protection.
Comparing the Three Methods of Charging
| Method | Contact Required? | How Charge Is Transferred |
|---|---|---|
| Friction | ✔ Yes | Electrons move during rubbing |
| Conduction | ✔ Yes | Electrons move through direct contact |
| Induction | ✘ No | Charges rearrange due to a nearby charged object, often with grounding |
Conservation of Charge
One of the most important laws in physics is the Law of Conservation of Charge.
It states:
Electric charge cannot be created or destroyed. It can only be transferred from one object to another.
When one object gains electrons:
- another object loses the same number of electrons.
The total electric charge in a closed system remains constant.
Worked Example 2
A neutral metal sphere gains 5 electrons.
Question
What is its new charge?
Solution
Each electron carries one negative elementary charge.
The sphere now has 5 extra electrons, so its overall charge is –5e, where e is the elementary charge.
Worked Example 3
A student says:
"When an object becomes positively charged, it has gained protons."
Question
Is this correct?
Solution
No.
In ordinary charging processes, protons remain in the nucleus.
An object becomes positively charged because it loses electrons.
Everyday Examples of Electric Charge
Electric charge is responsible for many familiar phenomena.
Examples include:
- lightning,
- static shocks after walking on carpet,
- clothes sticking together after drying,
- balloons sticking to walls,
- dust sticking to television screens,
- electrostatic air filters,
- photocopiers and laser printers.
Understanding electric charge helps explain these everyday observations.
Real-World Connection
Electrostatic charging is used in many industries. Spray painters use charged paint droplets so that paint is attracted evenly to metal surfaces, reducing waste. Air purifiers and electrostatic precipitators remove tiny dust particles from the air by charging them and collecting them on oppositely charged plates. Engineers also design electronic devices to prevent damage from static electricity, which can destroy delicate microchips.
Did You Know?
A single lightning bolt can transfer tens of coulombs of charge and carry currents of tens of thousands of amperes. The lightning channel can reach temperatures of about 30,000°C, which is roughly five times hotter than the surface of the Sun!
Key Terms
- Electric charge — a fundamental property of matter that causes electric forces.
- Positive charge — the type of charge associated with protons or an object that has lost electrons.
- Negative charge — the type of charge associated with electrons or an object that has gained electrons.
- Neutral object — an object with equal numbers of protons and electrons.
- Friction — a method of charging by rubbing materials together.
- Conduction — charging by direct contact, allowing electrons to move between objects.
- Induction — charging without direct contact by causing electrons to redistribute.
- Conservation of charge — the principle that electric charge cannot be created or destroyed, only transferred.
- Electric field — the region around a charged object where another charge experiences an electric force.
Key Takeaways
- Electric charge is a fundamental property of matter arising from the presence of protons and electrons.
- There are two types of charge: positive and negative.
- Like charges repel, while opposite charges attract.
- Objects become charged through the movement of electrons, not protons.
- Electric charge can be transferred by friction, conduction, and induction.
- The Law of Conservation of Charge states that total electric charge remains constant and can only be transferred between objects.
- Understanding electric charge provides the foundation for studying electric fields, electric forces, circuits, and magnetism.