Electric Charge
Positive and Negative Charges
Electric charge is a fundamental property of matter, existing in two types: positive and negative.
NoteProtons carry a positive charge, while electrons carry a negative charge of equal magnitude.
Example- When you rub a plastic rod with a wool cloth, electrons transfer from the wool to the rod.
- The rod becomes negatively charged, while the wool becomes positively charged.
Conservation of Charge
The total charge in an isolated system remains constant.
Example- If two spheres with charges of +4 μC and -2 μC touch and then separate, the total charge remains +2 μC.
- Each sphere ends up with +1 μC.
Quantization of Charge
Charge is quantized, meaning it exists in discrete units of the elementary charge $e = 1.6 \times 10^{-19} \, \text{C}$.
NoteThe charge of any object is always an integer multiple of $e$.
Coulomb’s Law
- Coulomb’s law describes the electric force between two point charges.
- The force $F$ between two charges $q_1$ and $q_2$ separated by a distance $r$ is given by:
$$F = k \frac{q_1 q_2}{r^2}$$
where $k$ is the Coulomb constant:
$$k = \frac{1}{4\pi \epsilon_0} \approx 8.99 \times 10^9 \, \text{N m}^2 \ \text{C}^{-2}$$
Hint$\epsilon_0$ is the permittivity of free space, with a value of $8.85 \times 10^{-12} \, \text{C}^2 \ \text{N m}^{-2}$.
Properties of the Electric Force
- Attractive or Repulsive:
- Like charges repel, opposite charges attract.
- Vector Quantity:
- The force acts along the line joining the two charges.
- Inverse Square Law:
- The force decreases with the square of the distance between the charges.
