Current, Charge and the Ampere - Electronics Tutorials 

Current

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Some key facts ...

• Current is a flow of electric charge.
• In a solid conductor, charge consists of of free electrons.
          A negative charge occurs when there are MORE than the usual number of electrons.
          A positive charge occurs when there are FEWER than the usual number of electrons.
• A current is free electrons flowing round the wires. (in liquids and gases, it is a flow of ions.)
• There must be a complete circuit for current to flow (no dead ends).
• If the switch is opened, there is no longer a complete circuit and no current flows.
• Charge is measures in Coulombs.
• Current is measured in Amperes or Amps.

Here is an animation showing an electric current. Click the switch to 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).

• 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

• Charge is an excess or deficiency of charged particles (often electrons).
• Charge is measured in Coulombs.
• Neutral or uncharged objects have just the right number of negative electrons to balance the positive nuclei.
• In a coulomb, there are 6.24 × 10¹⁸ electrons.

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.

Current at a Junction

Electrons in wires are just like cars on roads.

It is really easy to understand that if ten cars enter a junction and four turn left then six must have turned right. If ten amps flows into a junction and four amps turn left than the other six amps must have turned right. Neither cars nor electrons can vanish into thin air!

Here is a brain teaser. Assume all the components are ideal and work out the current in each ammeter.

The current through each resistor is one amp.

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 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.