## University Physics Electromagnetics Lab

Lab 2     Circuits

In this lab Ohm's law is verified and current flow in series and parallel resistive circuits is studied.

Equipment

• Circuit boards
• resistors
• DC Power Supply
• Voltmeter
• Ammeter

Resistors

 Figure 1   Resistors are labeled using colored bands
Resistors are labeled using colored bands. The first two bands are digits. The third band is the number of zeros. The fourth band gives the tolerance. For tolerance; no band represents 20%, silver represents 10%, gold represents 5%, and red represents 2%.

For example, consider the resistor shown in Figure 1. The color bands are brown, black, red, silver (1 0 2 10%). This is a 1000 Ohm resistor with 10% tolerance.

 Color Number Black 0 Brown 1 Red 2 Orange 3 Yellow 4 Green 5 Blue 6 Violet 7 Gray 8 White 9

Table 1   Resistor color codes

Ohm's Law

Ohm's law states that the voltage across a resistor is proportional to the current flowing through the resistor.

Ohm's law does not hold in all situations. Resistors are designed using linear materials, so that resistors follow Ohm's law. Other useful devices such as diodes and transistors are nonlinear. They do not obey Ohm's law.

• Wire up the circuit shown in Figure 1 to measure the voltage across a resistor and the current flowing through the resistor. Two meters are required. One is set to measure current (Ammeter). The other measures voltage (Voltmeter). Use a 1000 Ohm (1K) resistor.

Take a series of measurements. Plot the current as a function of voltage.

Figure 1   A circuit to verify Ohm's law is shown. Current flows through the ammeter and through the resistor. The voltmeter has a high input resistance and draws very little current. It measures the voltage across the resistor.

• Select three resistors. Measure their resistance using the multimeter. Compare measured values with values given by the color bands.
• Construct the circuit shown in Figure 2. Verify that the equivalent resistance of resistors in series is the sum of the individual resistors.

Figure 2   Resistors in series.
• Use the three resistors to construct the parallel circuit shown in Figure 3. Verify that the reciprocal of the equivalent resistance of the parallel combination is the sum of the reciprocals of the individual resistors.

Figure 3   Resistors in parallel.
• Construct the series-parallel circuit shown in Figure 3. Verify that equivalent resistance of the series parallel combination is,

Figure 3   Resistors in series and in parallel.