Ohm's Law: Find the Resistance

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 Ohm’s Law (Unknown Resistance)

Concept: Ohm’s Law relates voltage, current, and resistance in an electrical circuit

Overall goal: to calculate the resistance based on voltage and current, and then check by measuring the resistance directly with the meter 

V = I × R, where V is Voltage, I is current, and R is resistance

We can rearrange this to solve for an unknown resistance:

R = V / I

Need

  • Multimeter
  • Battery (e.g., 9V)
  • 1–2 resistors (at least one unknown or not labeled)
  • Connecting wires

Safety

  • Never place a multimeter in current (A) mode directly across a battery. This creates a short circuit.
  • Always double-check meter settings before connecting to a circuit.
  • Do not exceed safe voltage levels for your equipment.

Procedure

  1. Measure and record the actual voltage of the battery. (It may not be exactly the labeled value.)
  2. Build a simple series circuit with the battery and one resistor.
  3. Set the multimeter to measure current (A) in amps and place it in series with the circuit.
  4. Measure and record the current flowing through the circuit.
  5. Set the multimeter to measure voltage (V). Measure the voltage by putting a probe on each side of the resistor, in parallel. 

  6. Use Ohm’s Law to calculate resistance:

    R = V / I

  7.  Measure the actual resistance of the resistor (this is usually possible with a different setting on the multi-meter hooked up to the resister without the battery).
  8. Compare your calculated resistance to the measured resistance.

Observations / Data Table

  • Battery voltage: ______ V
  • Current: ______ A
  • Voltage across resistor: ______ V
  • Calculated resistance: ______ Ω
  • Measured resistance: ______ Ω

Math Check

Use your measured values:

R = V / I

Interpret your result:

  • Does it match the labeled resistor value (if known)?
  • If not, consider possible sources of error (meter accuracy, connections, converting a unit incorrectly etc.)
  • Personal note: we found our answer to be off by a factor of ten, alerting us that we had converted something wrong, which we then corrected

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