Ohm's Law

Design Challenge 5: Inferring Current From Voltage

Ohm's Law:

Design Challenge 5: Inferring current from voltage


Measuring current directly tends to be tedious—you generally need to break open your circuit in order to insert the ammeter into the circuit. Voltage measurements tend to be considerably easier, so it is common to determine the current in a circuit by measuring the voltage across a known resistor and using Ohm's law to estimate the current through the resistor. In this exercise, we will use this approach to estimate an unknown resistance in a circuit.

This exercise uses concepts introduced in our experiment on Ohm's law. A link to this experiment is provided at the right.

Step 1

Construct the circuit shown below. Use either the waveform generator or the voltage instrument on the Analog Discovery™ to implement the source. Choose two resistors for the circuit:

  1. A 1 kΩ resistor.
  2. An arbitrary resistor from your parts kit (R in the circuit below) whose third color band is red.

Step 2

Measure the two voltages V1 and V2, as shown on the circuit schematic.

Step 3

  1. Use the voltage V1 across the 1kΩ resistor and Ohm's law to estimate the current I in the circuit.
  2. Use the current I, the measured voltage V2, and Ohm's law to estimate the unknown resistance R.
  3. Use the color bands on the unknown resistor to determine its nominal resistance value. Calculate a percent error between the nominal value and the measured value from the formula below:

\[{\rm{PercentDifference = }}\frac{{{\rm{MeasuredValue - ExpectedValue}}}}{{{\rm{ExpectedValue}}}} \times 100\]

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