Basic Electronic Formulae

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Important basic electronic formulae for the beginner:

Ohms law.
The voltage across a resistance is equal to the current passing through the resistance (in Amps) multiplied by the value of the resistance (in Ohms).

Voltage = Current x Resistance or V=IR

V=IR or I=V/R or R=V/I

Power:
The power (in Watts) dissipated by a resistance is equal to the current (in Amps) passing through the resistance multiplied by the voltage (in Volts) across it.

Power = Current x Voltage or P=IV

Using Ohms Law to substitute the voltage term we get:

P = I x (IR) or P = I²R

Using Ohms Law to substitute the current term we get:

P = (V/R) x V or P = V²/R

As a practical example of this -
Q - Why would a person moving home from the UK to the USA be well advised to buy a new electric kettle?

A - The kettle in the UK is rated at 2000 Watts on a voltage of 230V.
Applying the formula P = V²/R we can find the resistance of the kettle:

R = V²/P = 230²/2000 = 26.45 Ohms

If the kettle were to be plugged into a 120V USA supply, its power would be:

P = V²/R = 120²/26.45 = 544.4 Watts

So, the kettle would produce around a quarter of the heat and would take roughly four times as long to boil.
If you halve the voltage, you get a quarter of the power.

Resistors in Series:

The series connection is a "daisy-chain" arrangement - as shown.

R_total= R_a + R_b

So, if Ra was 10k (10,000 Ohms) and Rb was 22k (22,000 Ohms), their combined total resistance would be:

10,000 + 22,000 = 32,000 Ohms or 32k

It does not matter how many resistors are connected in series, their resistances are simply added together to get a total resistance.

Resistors in Parallel:

In the parallel arrangement, the ends are connected together - as shown.