You already know the two big ideas in a circuit: voltage is the push, and current is the flow of charge that the push creates. Resistance is the third idea. It measures how much something in the circuit fights against that flow. Ohm's Law joins all three in one short formula: V = I x R. V is voltage, measured in volts. I is current, measured in amps. R is resistance, measured in ohms, written with the symbol Ω.
The formula bends three ways, so if you know any two values you can always find the third. To find voltage, multiply: V = I x R. To find current, divide: I = V divided by R. To find resistance, divide the other way: R = V divided by I. Division also tells you something useful. Keep the battery the same and raise the resistance, and the current falls. Double the resistance and the current is cut in half.
The class torch runs on a 6-volt battery pack and its bulb has a resistance of 2 ohms. To find the current, use I = V divided by R: 6 divided by 2 is 3, so 3 amps flow. A toy motor pulls 3 amps from a 12-volt battery. To find its resistance, use R = V divided by I: 12 divided by 3 is 4, so the motor has 4 ohms. And if 2 amps flow through 6 ohms, then V = I x R gives 2 x 6 = 12 volts.
What makes resistance big or small? Three things: the material, the length, and the thickness. Copper lets charge pass easily, so its resistance is low. Rubber fights hard against charge, so its resistance is very high. A long wire gives charge a longer path to struggle through, so resistance grows. A thick wire gives charge more room to move, so resistance shrinks. That is why a long, thin wire has more resistance than a short, thick wire made of the same material.
Water in pipes is a handy picture. The battery is like a pump that pushes the water along, the current is the flowing water, and a resistor is a narrow stretch of pipe that slows everything down. Make the narrow part longer or tighter, and less water gets through each second, just like raising resistance cuts the current.
V = I x R. Know any two of voltage, current, and resistance, and rearranging the formula gives you the third. More resistance, or a longer, thinner wire, means less current for the same battery.
Adapted from OpenStax College Physics (CC-BY 4.0), openstax.org · license
Last time you clipped an ammeter into the class torch's loop and a voltmeter across its bulb, then read the dials to find out what was happening. Today you find out before you switch on. The pack is marked 6 V. The bulb is marked 2 Ω.
You want the current. Which line of Ohm's Law gives it to you?
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