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TwiddleLab / Physics / Circuits You Can Build

Resistance and Ohm's Law

Lesson 4 of 10 · 7 min · Free, no account needed

What you will learn

  • Resistance (R) is how hard a part makes it for current to flow. It is measured in ohms (Ω).
  • Ohm's law: V = IR, so I = V ÷ R. Twice the resistance, half the current.
  • For a resistor at constant temperature, I against V is a straight line through zero. A bulb's line bends as its filament heats up.

Start the lesson

How the lesson goes

  1. LEDs burn out in a flash without one tiny part. First: 6 V across 6 Ω gives 1 A. What about 12 Ω?

    Drag the marker to your guess for the current through 12 Ω.

  2. Turn up the voltage across a 4 Ω resistor. Plot at least 4 points.

  3. Same test, two parts: a resistor and a bulb. Which line keeps doubling?

    Plot 3 points for each, then tap the line where doubling V doubles I.

    A resistor at constant temperature has a fixed R, so I = V ÷ R gives a straight line through zero. A filament heats up, its R grows, and the line bends towards the voltage axis.

  4. What the resistor follows is Ohm's law.

    Resistance (R) is how much a part resists current. Unit: the ohm (Ω).

    Ohm's law: V = IR. So I = V ÷ R, and R = V ÷ I.

    It holds for a resistor at constant temperature. Parts that heat up, like bulbs, change their R as they go.

  5. Pick a resistor so this 6 V battery drives exactly 0.5 A.

    Drag a resistor into the slot and read the ammeter.

    From V = IR: R = V ÷ I = 6 V ÷ 0.5 A = 12 Ω.

  6. Game: LED saver. Each LED must light with at most 20 mA. Burn as few as you can.

    An LED uses about 2 V. Pick the resistor for the rest. Drag it into the slot.

  7. Math: on an I against V graph the slope is 1 ÷ R. Tap the line of the smallest resistance.

    From I = V ÷ R, the graph of I against V is a line with slope 1 ÷ R. Small R, steep line.

  8. A 9 V battery drives current through a 3 Ω resistor. What is the current?

    I = V ÷ R = 9 V ÷ 3 Ω = 3 A.

  9. Before and after: an LED on 6 V without a resistor, and with yours.

Previous lesson: Voltage: The Push Behind the Flow · Next lesson: Series: One Path · Part of the course: Circuits You Can Build

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