1. Concept Overview
Electric charge is one of the basic properties of matter, just like mass. It is responsible for electric forces—pushes or pulls—that objects exert on one another. Even tiny charged particles can create noticeable effects. Many everyday events, such as a spark from a metal surface or a balloon sticking to a wall, happen because of electric charge.
2. Definition
Electric charge is a fundamental property of matter that causes it to experience an electric force when placed near other charged matter. In symbols, charge is denoted by \( q \) or \( Q \).
3. Types of Electric Charge
Electric charge exists in two forms. Their behaviour forms the basis of all electrostatic interactions:
3.1. Positive Charge
Objects that lose electrons become positively charged. A common example is a glass rod rubbed with silk.
3.2. Negative Charge
Objects that gain extra electrons become negatively charged. For example, a plastic comb rubbed through dry hair usually becomes negatively charged.
3.3. Interaction Between Charges
- Like charges repel each other.
- Unlike charges attract each other.
- Neutral objects can still be attracted to charged objects due to induced charge effects.
4. Quantisation of Charge
Charge does not take any random value. It comes in fixed units.
4.1. Definition
Quantisation of charge means that charge always appears as an integer multiple of the elementary charge \( e \).
The smallest unit of charge is the charge of an electron:
\( e = 1.6 \times 10^{-19} \text{ C} \).
4.2. Formula
Any charge \( Q \) can be written as:
\( Q = n e \), where \( n \) is an integer.
5. Conservation of Charge
Electric charge can neither be created nor destroyed. It can only be transferred from one body to another.
5.1. Simple Interpretation
If two objects interact and one gains charge, the other must lose the same amount. The total charge before and after a process always remains constant.
5.2. Example
When a balloon is rubbed on hair, electrons move from hair to the balloon. The balloon becomes negatively charged, and the hair becomes positively charged, but the net charge of the balloon + hair system stays the same.
6. Additivity of Charge
When a body has several point charges, the total charge is the algebraic sum of all individual charges.
6.1. Definition
Additivity means that if an object carries charges \( q_1, q_2, q_3, ... \), the total charge is:
\( Q = q_1 + q_2 + q_3 + ... \)
6.2. Example
If a body has three charges \( 2 \text{C}, -5 \text{C}, 3 \text{C} \), the total charge is:
\( Q = 2 - 5 + 3 = 0 \text{ C} \)
7. Everyday Meaning of Charge
Although we cannot see charge directly, we observe its effects all around:
7.1. Objects Attracting or Repelling
A plastic ruler attracting small paper pieces after being rubbed is due to charge transfer.
7.2. Small Sparks
A spark when touching metal is due to sudden movement of accumulated charge.
7.3. Clothes Sticking After Drying
Friction between fabrics makes them charged, causing them to cling to each other.
8. Physical Interpretation
Think of charge as something that creates an invisible influence around itself. When another charge comes nearby, this influence causes a push or a pull. This idea leads naturally to the concept of an electric field, which describes how charge affects the space around it.