1. Concept Overview
Electric charge follows a few basic rules that describe how it behaves in nature. These rules help explain interactions between charged bodies, how charge moves, and how we calculate total charge. The main properties are quantisation, conservation and additivity.
2. Quantisation of Charge
Charge always appears in fixed, discrete units. You can never have an arbitrary amount of charge; it is made up of whole-number multiples of a basic unit.
2.1. Definition
Quantisation of charge means that any charge \( Q \) is an integer multiple of the elementary charge \( e \).
\( Q = n e \), where \( n \) is an integer and \( e = 1.6 \times 10^{-19} \text{ C} \).
2.2. Interpretation
This means charge is not continuous. You cannot have a charge like \( 2.5 e \). It must be \( 2e \), \( 3e \), \( -5e \), etc.
2.3. Example
If an object carries a charge of \( 8 \times 10^{-19} \text{ C} \), the number of excess electrons is:
\( n = \dfrac{Q}{e} = \dfrac{8 \times 10^{-19}}{1.6 \times 10^{-19}} = 5 \).
3. Conservation of Charge
Charge cannot be created or destroyed. It may move from one object to another, but the total amount of charge always remains constant in any physical process.
3.1. Definition
Conservation of charge states that the algebraic sum of all charges in an isolated system remains constant over time.
3.2. Interpretation
If one object becomes positively charged, another connected object must become negatively charged by the same amount, keeping the total unchanged.
3.3. Example
When rubbing a balloon on hair, electrons move from the hair to the balloon. The hair becomes positive and the balloon becomes negative by equal amounts. The total charge is unchanged.
4. Additivity of Charge
When a body contains multiple charges, the total charge is simply the algebraic sum of all individual charges present on it.
4.1. Definition
Additivity of charge means that the total charge \( Q \) is the sum of all point charges:
\( Q = q_1 + q_2 + q_3 + ... \)
4.2. Example
If a body has charges \( +4 \text{ C} \), \( -7 \text{ C} \) and \( +3 \text{ C} \), then the total charge is:
\( Q = 4 - 7 + 3 = 0 \text{ C} \).
5. Attraction and Repulsion Between Charges
Objects with charge can exert forces on one another without touching. This is one of the most noticeable properties of electric charge.
5.1. Behaviour
- Like charges repel each other.
- Unlike charges attract each other.
- Neutral objects can still be attracted to a charged object due to induced charges.
5.2. Example
A negatively charged rod repels another negatively charged rod but attracts neutral paper bits.
6. Types of Charge
Charge comes in two types: positive and negative. The naming convention was given by Benjamin Franklin. The behaviour of charge in electrostatic interactions depends on these types.
6.1. Positive and Negative
An object losing electrons becomes positively charged, and an object gaining electrons becomes negatively charged.
6.2. Example
Rubbing a plastic comb through dry hair usually makes the comb negatively charged and the hair positively charged.