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# Current

This is the AQA version closing after June 2019. Visit the the version for Eduqas instead.

## Charge: Measured in Coulombs

There are two sorts of charge

• The negative particles are electrons.
• The positive particles are protons inside the atomic nuclei of atoms or molecules.
• An object that is not charged has equal numbers of positive and negative particles (Protons and Electrons).
• 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 an Electromotive Force (Voltage) that tends to cause charge to flow in a circuit.
• In a coulomb, there are 6.24 x 1018 electrons.

## 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.

## Circuit

• Current only flows if there is a complete unbroken circuit.
No current flows.
• One amp is flowing when one Coulomb of charge passes a point each second.
• 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.
• 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.

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.

Thanks xkcd.

## Alternating Current

• DC: Batteries provide a steady direct current (DC).
• AC: The mains power supply uses alternating current (AC) because, unlike DC, the voltage can be stepped up or down using a transformer. This makes the high voltage national distribution grid possible and it's easy to design power supplies for applications needing different low voltages.

## 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 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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