Electronics

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

Electrical energy is often measured in kWh

1kWh = 3.6 x 10^6 J

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

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.

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). 

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

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

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

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

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.

Power = current x voltage

P= IV 

Power (W)

Current (A)

Voltage (V)

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

Energy transferred = current x voltage x time

E= IVt

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

Resistance = voltage/ current

R= V/ I

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

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.

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