Physics, Electricity, 13-14

Electricity 1

Electrical Power and Energy

\begin{equation} \text{Power}=\frac{\text{Energy Transferred}}{\text{Time}}\nonumber \\ \end{equation}

Power is measured in watts, and energy in joules. Alternatively, one kilowatt-hour is the energy transferred in one hour from a source of power 1000W; 3.6MJ.

Electric Current


Current is a flow of charge. Charge itself is a fundamental property of matter. Charge can be positive or negative, and opposite charges attract. Charge itself is measured in coulombs, with the charge on one electron being \(-1.6\times 10^{-19}\)C.

Conductors and Insulators

Conduction is the flow of electric charges through a material. An insulator is a material that will not readily conduct electricity, as it has no free charges - the tightly bound electrons would require a relatively large amount of energy to be freed.

By rubbing together two insulators, electrons are transferred from one material to another, leaving neither material with a neutral charge.

Metals are good conductors due to the fact that some of their atomic electrons are free to move between atoms and carry charge.


Electrostatic phenomena are when there is no flow of continuous charge. A continuous charge is known as a current.

\begin{equation} Q=It\nonumber \\ \end{equation}

The direction of conventional current is from positive to negative. However electrons, being negatively charged, flow from negative to positive.

Current is the rate of flow of charge: With a current of one ampere, one coulomb will pass a given point every second.

Electric Circuits

An electric current will be set up in a conductor if there is:

  • An energy source, such as a battery made up of cells

  • A continuous circuit

Conservation of Charge

Charge is conserved wiothin a circuit. At any given point, the input and output charge must be equal - Kirchoff’s First Law

In a series circuit, current is the same everywhere. In a parallel circuit, the sum of the current in branches is equal to the total current.