# Current, Voltage and Power

**Electricity**is the**flow of electric charge**. We can describe the flow of electric charge in several ways. These include the quantities**Current**,**Voltage**and**Power**.

## Current

**Current**(**I**) is the**rate of flow**of**Charge Carriers**,*such as electrons*. Current is usually thought of as moving in the direction of**positive charge**, so from the**positive**power supply to the**negative**. However, since in metals it is**electrons**that carry electric charge, the actually flow is opposite to the way in which we think of it.

- Current it the the amount of
**Charge, Q**that passes a point in a set**time, t**. It is measured in**Amps**(**A**), and charge is measured in**Coulombs**(**C**). Since Amps are SI base units, Coulombs are defined as A×s, As.

## Voltage

**Voltage**(**V**) or**Potential Difference**(**p.d.**) is a measure of the**Energy**transferred per**Charge Carrier****between two points**.

- Voltage is the
**Energy**,**E**per**Charge**,**Q**. Voltage is measured in**Volts**(**V**), which is defined as**one Joule per Coulomb**. Voltage can be defined in base units as Kgm^{2}s^{-3}A^{-1}.

## Power

**Power**(**P**) is the**rate of Energy transfer**. It is measured in**watts**(**W**), where one watt is defined as**one Joule per Second**. Hence watts can be expressed in base units as Kgm^{2}s^{-3}

- From this definition of Power, we can substitute the
**algebraic definitions**above to produce a variety of other formulae, including ‘**Power = Current × Voltage**’

**Ohm’s Law**states that ‘**Voltage = Current × Resistance**’. We can use this to produce two more definitions of Power.