Learning Library · Circuit basics · 1

Ohm's law: voltage, current, and resistance

Every circuit you will ever measure obeys one relationship. Push harder with voltage and more current flows. Add resistance and less does. Drag the sliders to see it happen.

The relationship

Voltage (volts, V) is the push. Current (amperes, A) is the flow. Resistance (ohms, Ω) is the opposition to that flow. Know any two and you can calculate the third. Double the voltage across a fixed resistor and the current doubles. Double the resistance at a fixed voltage and the current halves.

Try this

• Set 12 V and 1 kΩ. The current is 12 mA. Now set 100 Ω: the current jumps tenfold to 120 mA.
• Watch the resistor colour. At 12 V and 100 Ω it dissipates 1.44 W, almost six times what a ¼ W part is rated for. On a real bench, that resistor gets hot fast.
• Drop the voltage to 0 V. No push, no flow, whatever the resistance.

Power: where the energy goes

A resistor turns electrical energy into heat. The rate is power, in watts:

Because power rises with the square of voltage, a small change in supply can push a part past its rating. Common through-hole resistors are rated at ¼ W or ½ W. Designers usually keep dissipation at half the rating or less for margin.

Measuring it on the bench

A digital multimeter (DMM) measures all three quantities, but each one is connected differently:

• Voltage: leads go across the component, in parallel, with the circuit powered.
• Current: the meter goes in series, so the circuit has to be opened and the meter inserted into the path. Move the red lead to the meter's current jack first.
• Resistance: power off. The meter supplies its own small test current, and any other voltage in the circuit corrupts the reading.

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