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Charge: Measured in Coulombs

There are two sorts of charge

Current: Measured in Amps or Amperes

For the Falstad Circuit Simulation, CTRL+Click A Simple Direct Current Circuit
In options, check European Resistors and uncheck Conventional Current.
Click the switch to turn it on or off.

Alternatively view Current.txt.
Save or copy the text on the web page. Import the saved or copied text into the Falstad simulator.

Here is the new HTML5 Simulator Site.


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

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.

Adding Currents at a Junction

The sum of the currents entering a junction is equal to the sum of the currents leaving the 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!


Conventional Current and Electron Flow

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.

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Thanks xkcd.

Alternating Current

Brain Teaser


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.

Short Circuits


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 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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