Mental model
Imagine water in a pipe.
A battery is like a pump. It creates pressure that can move charge around a complete path. A narrower pipe resists that movement.
Voltage creates the push.
Current is the resulting flow.
Resistance limits the flow.
The quantities
Three ideas run the circuit.
Voltage
Difference in electric potential energy per unit charge. Think: how much push is available?
- Symbol
- V
- Measured with
- Voltmeter
Current
The rate electric charge passes a point. One ampere is one coulomb of charge per second.
- Symbol
- I
- Measured with
- Ammeter
Resistance
How strongly a material opposes current. Longer, thinner wires usually have more resistance.
- Symbol
- R
- Unit
- Ω (ohm)
Ohm’s law
The relationship
that ties it together.
For many components, current increases with voltage and decreases with resistance.
Find voltageV = I × R
Find currentI = V ÷ R
Find resistanceR = V ÷ I
Worked example
A 9 V battery across a 300 Ω resistor.
I = 9 V ÷ 300 Ω = 0.03 A, or 30 mA.
Interactive circuit lab
Change the circuit.
Watch the numbers respond.
Adjust the battery and resistor. The current and power update using Ohm’s law.
Try doubling the voltage. With resistance fixed, the current doubles too.
Power & circuits
Power is how fast
energy is transferred.
Power is measured in watts (W). A 12 V device drawing 2 A uses 24 W. That energy may become light, motion, sound, heat—or a mixture.
Series
One path
- Current is the same everywhere
- Voltages divide across components
- Rtotal = R₁ + R₂ + …
Parallel
Multiple paths
- Voltage is the same across branches
- Current divides between branches
- Total resistance becomes smaller
Check yourself
Make the model stick.
Answer all four. You can retry as many times as you like.
The pocket version
Push. Flow. Opposition. Work.
Voltageis the push
Currentis the flow
Resistanceopposes the flow
Poweris energy per second