# Current Electricity

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ELECTRICAL CHARGES

When two electrically insulating materials are rubbed together, electrons are rubbed off one material and deposited on the other.  Which way the electrons are transferred depends on the materials.  Electrons have a negative charge so the material that gains electrons becomes negatively charged.  The material that has lost electrons becomes positively charged. This process ich called charging by friction.

Two objects that are oppositely charged attract each other.  Two objects that have the same charge repel each other. The greater the distance between the two objects, the weaker the force between them.

ELECTRIC CIRCUITS

Every component in a circuit has a circuit symbol.  These are put together in a circuit diagram to show how the components are connected together.

RESISTANCE

The current through a component is measured with an ammeter.  Ammeters are always placed in series with the component.  This means that they are in line with it.  The unit of current is the ampere or amp, A.  The potential difference across a component is measured with a voltmeter.  Voltmeters are always placed in parallel with the component (generally below it).  The unit of potential difference is the volt, V.

Potential difference, work done and charge are related by the equation:

V = W/Q, where V is the potential difference in V, W is the work done in J and Q is the charge in coulombs, C.  As work done is equal to the energy transferred, we can also say:

V = E/Q, where V is the potential difference in V, E is the energy transferred in J and Q is the charge in coulombs, C.  Resistance is the opposition to current flow.  The unit of resistance is the ohm.  The resistance of a component is measured using the equation:

R