Power is the rate of energy transfer, in joules per second (j/s) or watt (W).

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Electrical energy is often measured in kWh

1kWh = 3.6 x 10^6 J

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The efficiency of an electrical device is the % of electrical energy (or power) input that's transferred usefully.

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Charge, Q, is a property of matter, carried by charged particles, such as the electron and proton.

Charge is measured in coulombs (C). 

The charge carried by an electron is -1.6 x 10^-19 C.

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Current, I, is the rate of the flow of charge.

Current= change in charge/ change in time

I= Q/ t

It's measured in amperes or amps (A). 

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Conventional Current

The direction of conventional current is that in which positive charges flow from positive (+) to negative (-).

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Conservation of Charge

Charge is conserved in a circuit- the total current entering any point in a circuit equals the total current leaving that point.

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Potential Difference

The potential difference, V, between 2 points in a circuit is defined as the work done per unit charge passing between the points.

V= W/ Q

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Potential Difference

Potential difference is measured using a voltmeter connected in parallel with the component.

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Around any closed loop of circuit, the sum of the emf is provided by power sources is equal to the sum of pd's across the components.

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Electrical Power

Power = current x voltage

P= IV 

Power (W)

Current (A)

Voltage (V)

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Energy Transferred

The energy transferred, E, in a time, t, is:

Energy transferred = current x voltage x time

E= IVt

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Resistance, R, is a measure of an electrical components opposition to current.

Resistance = voltage/ current

R= V/ I

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As a current passes through a resistor, energy is transferred as heat.

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Ohm's Law

Ohm's Law holds for metal conductors provided the temperature and other physical conditions are constant.

The current through a component is proportional to the potential difference across it.

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The resistivity of a material of length, l, resistance, R, and cross-sectional area, A, is:

p= RA/ L

It's measured in Ω⋅m

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