Voltage, Current and Resistance

Your first activity in this unit is to make measurements of voltage and current in a circuit with a fixed resistor. The circuit consists of a fixed resistor, a variable power supply, a multimeter set to read milliamps, and a multimeter set to read voltage. You will vary the voltage and record the voltage and the corresponding current, and then perform the data analysis.

The description of the experiment and some followup problems are located here.
The sample Excel spreadsheet is here.

Ohm's law simulation

You can repeat the experiment with different resistors. Hold current constant and vary resistance and measure voltage. Hold voltage constant and vary resistance and measure current.


Series and parallel resistances

In your next experiments, you will measure the resistances of resistors in series and parallel and compare the formulas for calculating total resistance. Series and parallel simulation.
... Resistors in series .... Rt = R1 + R2 + R3 ...
... Resistors in parallel .... 1/Rt = 1/R1 + 1/R2 + 1/R3 ...

Measuring voltage and current in circuits with multiple resistors.

You will perform these experiments using the PhET simulation below.
Some things to remember:
  • In the case of resistors in series the current will be the same through each resistor and the voltage drops across each resistor will sum to give the total voltage for that part of the circuit.
  • In the case of resistors in parallel, the voltage across each resistor is the same and the sum of the currents through each resistor will give the total current through that part of circuit.
  • The total resistance for resistors in parallel will always be less than the lowest resistance. In a parallel circuit, the current splits and the greater current takes the path of least resistance.
  • Notice that an ammeter is always part of the circuit. Current must flow through the ammeter in order to be measured. A voltmeter is not part of the circuit. The circuit will operate whether the voltmeter is present or not. The voltmeter measures the potential difference between two points in the circuit.
Circuit Construction Kit (DC Only)
Click to Run

Power in DC circuits

Power is defined as the "rate of doing work" and is measured in "watts." The power rating of every electrical device you own is printed on the device. The light output from light bulbs is rated in watts. A 100 watt bulb is brighter than a 25 watt bulb. A watt is equivalent to a joule per second (J/s). If we multiply voltage (J/C) times current (C/s) we get J/s. Therefore, P = I x V. (Note: you may notice that voltage is sometimes represent by the symbol E, which stands for "electromotive force.")
Ohm's law and power
Power in series and parallel circuits.
... We buy electricity from Progress energy or HEMC by the kilowatt-hour. Why is this a unit of energy?
... If electricity costs 10 cents per kilowatt-hour and we run five 100-watt light bulbs for 24 hours a day for 30 days, how much will it cost?

Series and parallel circuits

Tutorials, examples and problems from the Wisconsin Technical College System:

Some additional series and parallel circuits problems.
Sample DC Circuit Questions from the Physics EOC