Evaluating Electrical Safety
Electrical appliances must be safely insulated to protect users from injury. In this lab, you will play the role of a safety engineer determining whether an electric power supply is safely insulated.
Problem How much resistance is needed in series with a known resistance to reduce the voltage by 99 percent?
Materials
9-volt battery
battery clip
multimeter
3 alligator clips
4 resistors:
1-ohm, 10-ohm,
100-ohm,
1000-ohm
For the probeware version of this lab, see Probeware Lab Manual, Lab 8.
Skills Calculating, Using Tables
Procedure
On a separate sheet of paper, make a copy of the data table shown below.
Attach the battery clip to the battery. CAUTION The circuit may become hot.
Use an alligator clip to attach one wire of the 1-ohm resistor to one of the battery clip's wires as shown.
Clip one wire of the 1000-ohm resistor to the free end of the 1-ohm resistor. Clip the other wire of the 1000-ohm resistor to the free wire of the battery clip.
The 1-ohm resistor represents the currentcarrying part of the appliance. The 1000-ohm resistor represents the insulation for the current-carrying part. Place one of the multimeter's electrodes on each wire of the 1-ohm resistor. Record the voltage difference.
Place the multimeter's electrodes on the wires of the 1000-ohm resistor. Record the voltage difference.
To model a reduction in the resistance of the insulation, repeat Steps 5 and 6, replacing the 1000-ohm resistor with a 100-ohm resistor. Disconnect the resistors from the battery clip.
Predicting Record your prediction of how increasing the resistance of the currentcarrying part of the appliance will affect the voltage difference across the insulating part.
To test your prediction, repeat Steps 3 through 7, using a 10-ohm resistor to represent the current-carrying part of the appliance.
Analyze and Conclude
Calculating When the resistance of the current-carrying part was 1 ohm and the resistance of the insulating part was 100 ohms, what was the ratio of the voltage differences across the current-carrying part and the insulating part?
Drawing Conclusions In the circuit you built, what is the voltage difference across each resistor proportional to?
Applying Concepts You know the resistance of the current-carrying part of an appliance. What should the resistance of the insulation be to reduce the voltage by 99 percent?