# ELECTRIC CURRENT, RESISTANCE AND DC CIRCUITS

These revision cards come from notes made on the OCR Physics revision guide, although they may come in handy for those studying for other exam boards as well.
Hope you find them useful! =]

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• Created by: Maddison
• Created on: 20-05-09 08:51

## Charge and Current 1

Conservation of Current:
[KIRCHOFF'S FIRST LAW]
IT = I1 + I2 + I3...

• Coulombs and Amps:
Coulomb: Measure of charge. SI unit One COULOMB is the charge that passes when a current of one AMP flows for one SECOND
• Amperes (amps): Measure of current. One AMP is one COULOMB per second

CURRENT = Change in charge
Change in time

I = ΔQ
Δt

Remember: I means current, Q means charge and t means time.

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## Charge and Current 2

DRIFT VELOCITY

The conduction electrons in a metal move around rapidly, in random directions. When a voltage is applied, the electrons gain an additional velocity along the wire. The mean drift velocity, v, is the average extra velocity gained by the electrons. The freater the current I, the greater the value of V.

The current also depends on the cross-sectional area A of the conductor, and on the number density n, of electrons in the metal.

I=nAve

[e is the charge on the electron - you get this on the formula sheet.]

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## p.d & e.m.f.

Electromotive Force: The amount of energy given to each coulomb of charge passing around the circuit, including through the cell itself. (The e.m.f. is the volatage what's shown on the side of a battery)

Potential Difference: Across a component, the p.d. is the energy transferred by each coulomb of charge passing through it.

Formula to link it up:
Energy transferred = Voltage x Charge
[ W = VQ ]

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