Current Electricity and Conventional Current

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Current Electricity and Conventional Current

Current electricity is about moving charged particles. If you allow the charge that builds up in static electricity to flow, you get a current.

Current is the rate of flow of charge; it is the amount of charge flowing per second through a conductor.

The equation for calculating current is:

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I = current (amps, A)

Q = charge flowing past a point in the circuit (coulombs, C)

t = time taken for the charge to flow (seconds, s)

So a current of 1 amp is 1 coulomb of charge flowing past a point every second.

Likewise a coulomb is the same as an ampere-second!

(Note: if you plot a graph of current flowing against time, the area under the graph will equal the charge that has moved.)

Well, first you need to have a conductor for it to flow through and then you need to attract or repel the charged particles to make them move. The amount of attracting or repelling you do is measured in volts and is called the voltage or the potential difference (p.d. for short).

Work is being done on these charged particles to make them move, so the voltage is a measure of the amount of energy that is provided per coulomb of charge.

1 volt = 1 joule per coulomb.

The equation for calculating voltage is:

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W = amount of energy (joule, J)

V = voltage (volt, V)

Q = charge (coulomb, C)

As the charged particles flow around a circuit they don't get used up; it is the energy that the charged particles carry that decreases as they move around the circuit.

(Runners going around the 400m track run all the way round, but they lose energy as they run).

So current is not used up - if you have 12 amps leaving the battery, there will be 12 amps in the circuit and 12 amps returning to the battery.

Voltage changes as the charge moves around the circuit. The potential energy given to the charge is changed into heat energy in the circuit. An electron may leave a battery with 6 V, but will return to the battery with 0 V. This gives a change in potential of 6 V, hence the words 'potential difference'.

There are two main types of circuits you need to know about and each has two rules that make calculations simpler:

Series circuits:

Current Electricity diagrams

In a series circuit...

  • the current is the same all the way around the circuit.
  • the voltage is divided between the components in the circuit.

Parallel circuits:

Current Electricity diagrams

In a parallel circuit...

  • the current divides to travel along each loop.
  • the voltage is the same across each loop.

Originally scientists believed that it was positively charged particles that flowed in circuits and so circuits are always labelled with the current flowing from the positive to the negative terminal of a cell in a circuit. We call this current the conventional current. The electrons are actually flowing in the opposite direction!

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Conventional current is the flow of positive particles. All references to current in diagrams and questions at A-level refer to conventional current, unless it's specifically stated otherwise in the question.

To measure current we use an ammeter. It is placed in series in a circuit to measure the amount of charge flowing through it per second. (You can compare it to a turnstile counting people into a stadium.)

To measure voltage we use a voltmeter. It is placed in parallel to compare the potential at two different points, either side of a component. It can then measure the potential difference, or voltage across the component.

Current Electricity diagrams

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