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.
The torch, one step at a time. Write down the two you know, pick the bend that leaves the missing letter on its own, then do the sum.
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
Ohm's Law is V = I R. Voltage in volts equals current in amps times resistance in ohms. A battery provides voltage. Wires and components have resistance. Current flows when the circuit is complete. If you know any two you can find the third. A 6 V battery with a 2 ohm resistor gives 3 A. Resistance opposes current. Longer wires have more resistance. Thicker wires have less. Different materials have different resistivity. Copper has low resistivity so it makes good wires.
Ohmic materials follow V = I R. Their resistance stays constant as voltage changes. A filament lamp is non-ohmic. Its resistance increases as it gets hot. In a simple circuit a battery pushes current through a resistor. The voltage across the resistor equals the battery voltage. You can measure voltage with a voltmeter in parallel. You measure current with an ammeter in series. Rearrange V = I R to I = V/R or R = V/I depending on what you need to find.
Voltage is the push that makes charges move. Current is the flow of charge per second. Resistance is how hard it is for charge to flow. The unit of resistance is the ohm. One ohm means one volt produces one amp. Resistance depends on material length and cross-section. Copper wire has low resistance. Nichrome wire has high resistance and is used for heating elements. Temperature affects resistance. For most metals resistance increases with temperature.
In series current is the same everywhere. Resistances add up: R_total = R1 + R2 + R3. In parallel voltage is the same across each branch. Currents add up. The total resistance is less than the smallest branch. For two resistors in parallel 1/R_total = 1/R1 + 1/R2. Household wiring uses parallel so each appliance gets full voltage. If one bulb blows the others stay on. Old Christmas lights used series so one broken bulb turned off the whole string.
To find current in amps divide watts by volts for DC circuits. A 60 W bulb at 120 V draws 0.5 A. You can also use Ohm's Law I = V/R if you know voltage and resistance. Or measure directly with an ammeter in series. These methods help you check circuit calculations. For example if a 12 V battery is connected to a 4 ohm resistor the current is 12/4 = 3 A. The power is 12 × 3 = 36 W.
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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14 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.