Ohm's law: how push, flow and squeeze connect

You now know three ideas: the push (voltage), the flow (current) and the squeeze (resistance). In 1827 a German teacher named Georg Ohm found that they are linked by a rule short enough to fit on a sticky note.

The rule

Voltage = current × resistance

V = I × R

Here, V is in volts, I is the current in amps (physicists use the letter I), and R is in ohms.

Read it in plain words: the push needed equals the flow you want times the squeeze it has to go through. A bigger squeeze needs a bigger push to get the same flow.

Turn it around and you get the two forms you'll use most:

• I = V ÷ R (how much current will flow?)
• R = V ÷ I (how big is this resistance?)

A worked example

A battery of 6 V pushes current through a lamp with 12 Ω of resistance. How much current flows?

I = V ÷ R = 6 ÷ 12 = 0.5 A

Try it below. Set 6 V and 12 Ω and read the current. Then change one number at a time and predict the answer before you look.

Three patterns to spot

1. Double the voltage, double the current (the resistance stays the same).
2. Double the resistance, halve the current (the voltage stays the same).
3. Double both, and the current doesn't change.

These patterns are why the Many ways mission in Current Control has more than one answer.

Why an LED needs a resistor

An LED is a tiny, efficient light. It is also delicate: it takes whatever current the supply gives it, so too much current burns it out in an instant. LEDs are always used with a resistor to limit the current.

An LED keeps about 2 V for itself. Suppose it runs from a 5 V supply and wants 0.020 A (20 mA). The resistor must handle the rest:

• Voltage across the resistor = 5 − 2 = 3 V
• R = V ÷ I = 3 ÷ 0.020 = 150 Ω

You'll do exactly this in the LED guard mission.

Detective work

Ohm's law also lets you find things you can't see. Put 6 V across a hidden component and read 0.25 A on a meter. Then R = 6 ÷ 0.25 = 24 Ω. That's the Mystery resistor mission.