Voltage, Current and Resistance
The three words every electronics fan uses, and the simple rule that connects them.
The question
A 9-volt battery and a car battery are both “batteries.” Why can one start a truck and the other only power a smoke alarm?
What you’ll discover: the three words every electronics fan uses (voltage, current and resistance) and the simple rule that connects them.
In simple terms
BeginnerPush, flow and squeeze
A water hose is a handy way to picture electricity. It isn’t perfect, but it helps:
| Electricity | What it means | Hose picture | Unit |
|---|---|---|---|
| Voltage | How hard the charges are pushed | Water pressure | volt (V) |
| Current | How much charge flows each second | How much water flows past each second | ampere (A) |
| Resistance | How much something slows the flow | Squeezing the hose | ohm (Ω) |
More push (voltage) means more flow (current). More squeeze (resistance) means less flow. That’s really the whole idea.
An AA battery gives 1.5 V. Put three in a row and you get 4.5 V, a good, safe voltage for the projects in Electronics School.
So why can the car battery start a truck?
A car battery is about 12 V, not much more than a 9 V battery. The big difference is how much current it can deliver: a car battery can push hundreds of amps for a few seconds to turn the starter motor. A small 9 V battery can only supply a tiny fraction of that. Voltage isn’t the whole story; how much current a source can safely provide matters too.
Go deeper
IntermediateOhm’s law
In 1827, German scientist Georg Ohm published the rule that connects all three:
V = I × R
Voltage (V, in volts) equals current (I, in amps) times resistance (R, in ohms). Rearranged: I = V ÷ R and R = V ÷ I.
Worked example: a 4.5 V battery pack is connected across a 220 Ω resistor. The current is I = 4.5 ÷ 220 = 0.0205 A, or about 20.5 mA.
Power, the rate energy is used, is P = V × I, in watts (W). In that example, P = 4.5 × 0.0205 ≈ 0.09 W. A common ¼-watt resistor handles that easily.
AdvancedWhen Ohm’s law doesn’t hold
Ohm’s law describes ohmic materials, like ordinary resistors, whose resistance stays constant. Many parts aren’t ohmic:
- An LED conducts almost nothing until the voltage across it reaches its forward voltage (about 2 V for red), then current rises steeply. That’s why LEDs always need a resistor (Lesson 5).
- A filament bulb’s resistance rises as it heats up, so its cold resistance is much lower than its hot resistance.
Resistance of a wire depends on its material, length and thickness: R = ρL ÷ A, where ρ (rho) is the material’s resistivity. Copper’s low resistivity is why it’s used for wiring. Resistance of most metals also rises with temperature.
Quick check
If you add more resistance to a circuit with the same battery, what happens to the current?
It goes down. More resistance means less current for the same voltage.
What is the current through a 1,000 Ω resistor connected to 4.5 V?
I = 4.5 ÷ 1000 = 0.0045 A, which is 4.5 mA.
Is a car battery’s voltage much higher than a 9 V battery’s?
Not much: about 12 V. The big difference is how much current it can deliver.
Words to know
- Voltage (V)
- The electrical push, measured in volts.
- Current (I)
- The flow of charge, measured in amperes (amps).
- Resistance (R)
- How much a material resists the flow, measured in ohms (Ω).
- Ohm’s law
- V = I × R.
Learn more
- Science World (Vancouver): Current Electricity: eight hands-on circuit activities
- Try it on the Virtual Workbench: change the voltage and resistance and watch the current
- Next: Complete circuits
Electronics School is part of the Discovery Zone. Every activity here runs on small batteries or a USB cable: never wall outlets. The one plug-in tool is the optional soldering iron, set up by an adult.