# Physics

## Charge

Particles have a property named charge. It can be positive or negative.

Like charges repel, opposite charges attract.

Unit of charge is a Coulomb.

Protons and electrons have a charge. Neutrons have no charge.

e, the elementary charge, is the charge of a proton. The charge of an electron is -e.

$$C = {one Coulomb} = 6.24 * 10^{18}e$$

inverse is

$$e = 1.6 * 10^{-19} C$$

$$F_e = \frac{k_eq_1q_2}{d^2}$$

Where Fe is the static electric force in Newtons (N).

K is a constant ke, approx 9* 10^9. This is Coulomb's constant, electric force constant or electrostatic constant.

q1 is the charge of the first particle (in Coulombs).

q2 is the charge of the second particle.

d is the distance in metres.

If the number is negative, it means the the particles will be attracted to each other.

intro to charge at Khan Academy.

### Electric fields

If you have 1 charged particle, it creates an electric field.

The field can be described in Newtons per Coulomb (N/C).

Assume the charge of the particle is q, then at distance d, the electric field can be given as:

$$E = \frac{k_eq}{d^2}$$

If you put another particle, with charge q2, into the electric field, then the static electric force will be

$$F = Eq_2$$

## Units

### Energy and Work

Kinetic energy is stored in a moving object of mass m moving at velocity v.

Gravitational potential energy is stored by lifting an object higher.

#### Newton metres

Force * distance

Units would be N*m

For a moving block of mass m:

$$KE = \frac {m.v^2}{2}$$

#### Joules

J is the symbol for joules

### Force

mass times acceleration

#### Newtons

$$N = kg * (m/s)^2$$