# OCR A-Level Physics/Electrons, Waves and Photons NEW SPECIFICATION/Electric current

## Charge and Current

[edit | edit source]### Current

[edit | edit source]**Current** is the flow of charged particles around a circuit. The electrons are usually the particles that flow. Metals are good conductors because they have **free electrons** that are able to flow.
The SI unit for current is **Amperes (A)**

### Cross Sectional Area

[edit | edit source]The cross-sectional area of the wire is the area of a circle of radius r: A=πr2=π(d2)2, where d is the diameter of the wire. The SI unit for area is ms^2.

### Electron Flow

[edit | edit source]Before it was known that it was electrons that were the **charge carriers**, it was believed that current flowed from positive to negative. This became convention and is still used under the name **Conventional Current** even after the discovery of free electrons.
We now know that electrons have a negative charge. This means they are repelled from the negative terminal and is attracted to the positive terminal.

Remember this - Electron flow is always opposite to conventional current.

### Elementary Charge

[edit | edit source]Charged particles have a charge in coloumbs. An electron is a charged particle and has the charge of approximately . This is known as the **elementary charge** and is represented with an **e** symbol. Protons also have an approximate charge of e. The difference being is that electrons are negatively charged so strictly speaking they have a charge of **-e** and protons have a charge of **+e**.

### Drift Velocity

[edit | edit source]When electrons flow in a circuit, they have a velocity. we have an equation that relates this velocity and current together

```
```

From this equation we know that the current is affected by **the cross-sectional area of the wire, A** as well as the **number density, n** of electrons. This is just the number of electrons per unit volume. The number of electrons are as many as there are atoms. Current also is dependent on **elementary charge, e** and the **mean drift velocity, v**
If we rearrange the top equation we can see a relationship between the values.

```
```

We can then deduce these three relationships.

The elementary charge, e is constant.

## Kirchoff's First Law

[edit | edit source]**Kirchoff's First Law**- The sum of the currents entering any point is equal to the sum of the currents leaving that same point.

## Equation

[edit | edit source]### [edit | edit source]

**Charge**- Current passing a point in a given time.

**1 Coulomb**- Current of 1 Amp passes a point in the time of 1 second.

### [edit | edit source]

**Current**- Current of a cross-sectional area depends on the mean drift velocity of the charge carriers and on their number density in the material.