Current, Charge and the Ampere - Electronics Tutorials

Current

Google Video

Some key facts ...

Here is an animation showing an electric current. Click the switch too turn it off and on.


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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.

Electrons going round a circuit is very like cars going along a road. If there is a junction and some cars/electrons go one way, it is easy to calculate how many cars/electrons went the other way.

When electric current was first discovered, scientists had to guess its direction. When electrons were discovered, to everyone's lasting annoyance, it was discovered that they flowed the opposite way. So most circuit diagram arrows point the opposite way to the electron flow and the electrons flow uphill on circuit diagrams. Some text books and teachers use conventional current (Mainly USA) and others use electron flow (Mainly Europe). So there is confusion all round.

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

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.

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).

Potential Difference

Potential difference is measured in Volts.

Short Circuit

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 and laptop 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.

Don't try this at home! YouTube Short Circuit video and another one which is thoroughly dangerous and stupid to try.

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