Ohm's law: push, resistance, and what flows

Voltage pushes, resistance resists, current is what results: V = I × R.

Hone is a place to practise a career, one idea a day. This is one of its lessons, written out in full and free to read without an account.

What it is for

A sensor loop reads nothing and the technician has a meter and this one law. Volts across the resistor, ohms of the resistor, and the current that should be there is one division away. Every circuit on the FE exam and most of them on the plant floor start here.

How to think about it

Write the law. Cover the letter you do not know with a finger; the other two show the arithmetic. Put the units on before the number, and turn milliamps into amps before anything is multiplied.

Worked example

V = I × R

The law. Any two give the third.

A 9 V battery across a 30 Ω lamp

Two knowns, one unknown.

I = V / R = 9 / 30 = 0.3 A = 300 mA

Rearranged for the current, then the arithmetic.

Check: 0.3 A × 30 Ω = 9 V

Multiply back. If it does not give the source, the division went the wrong way.

Your turn

A 5 V supply across 250 Ω. Write the current.

I = 5 /  = 0.02 A

The trap

Reading milliamps as amps. A 20 mA loop current typed as 20 into V = I R gives a voltage a thousand times too big, and the datasheet will not warn you.

Practise Ohm's law on HoneA question on it now, a coding challenge where there is one, and it is remembered for review. Free, no email needed.