Current

Google Video

Here is an animation showing electrons flowing round a circuit. Full Screen

This animation is not quite realistic. There are so many electrons in the wire that each one creeps along rather slowly. If one Amp flows for one second, one Coulomb of charge has passed. That is 6 241 509 629 152 650 000 electrons! In a torch, electrons could take a whole day to go round the circuit once.

A current is a flow of charged particles measured in Amperes (Amps).

• In a wire, it is a flow of electrons.
• In a battery, it is a flow of ions. (Ions are charged atoms or molecules.)
• In an N type semiconductor it is a flow of electrons.
• In a P type semiconductor it is a flow of holes. (A hole is a location where an expected electron is missing.)
• In a fluorescent light, it is a flow of ions.
   

The Ampere (Amp)

Current is measured in Amps or more often milliamps or microamps.

The Ampere is defined in terms of the force between two conductors.

The ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1m apart in vacuum, would produce between these conductors a force equal to 2 x 10^–7 newton per metre of length.

If the current is one Amp, one Coulomb is flowing per second.

Charge

Capacitors store energy in the form of charge.

Charge is an excess or deficiency of charged particles.
Charge is measured in Coulombs.
Neutral or uncharged objects have just the right number of negative electrons to balance the positive nuclei.

1 coulomb is the amount of electric charge carried by a current of 1 ampere flowing for 1 second.

An object that is not charged has equal numbers of positive and negative particles (Protons and Electrons).

• The negative particles are electrons.
• The positive particles are protons inside the atomic nuclei of atoms or molecules.
• To charge an object, electrons have to be added (negative) or removed (positive).
• Electrostatic charge can be created by friction between objects. Rub a balloon and stick it to the wall.
• Batteries and generators produce a potential difference (Voltage) that causes charge to flow in a circuit.

Potential Difference

Potential difference is measured in Volts.

Short Circuit

Normal circuits contain a device like a bulb or a heating element. In normal use the current through these devices is controlled and safe. This is because these devices have RESISTANCE. When there is a short circuit the resistance is very low so ... A large current flows The wire gets hot The battery gets hot There is a fire and burn risk There is an explosion risk (especially car batteries) The battery goes flat within seconds Toxic fumes from the overheating wires and / or battery / power supply Acid or alkali chemical escape (chemical burns) . A fuse can be used to protect against accidental short circuits.