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

## Charge and CurrentEdit

### CurrentEdit

**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)**

### ChargeEdit

**Charge** is a property that particles can have which are either positive or negative. The charge can act like an inter-molecular force between the particles either repelling or attracting. This is something you will need to be aware of for the A2 syllabus.
The SI unit for charge is **Coulombs(C)**

### Electron FlowEdit

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 ChargeEdit

Charged particles have a charge in coloumbs. An electron is a charged particle and has the charge of appoximately . 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 VelocityEdit

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 LawEdit

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

## EquationEdit

### Edit

**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

**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.