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Introductory Electromagnetism Experimental Laboratory
Circuits
Goals: Observe the behavior of current and voltage in resistors and lamps. Use a digital multimeter to measure volts, amps and ohms.
APPARATUS A circuit is connected set of electrical components. The components are connected to form one or more closed conductive loops to allow current to flow. In this laboratory we will use a number of different components including a power supply, switch, resistors, lamps and a digital multimeter (DMM). The DMM is designed to measure either volts, amps, or ohms and is therefore can be used as a voltmeter, ammeter or ohmmeter.
A circuit diagram is a representation of the components of the circuit and their connec- tions. It does not indicate the physical orientation of the components. Each type of com- ponent has its own symbol. A series of alternating short and long lines represents a power supply. A diagonal line connected on one end repersents a switch. A zig-zag line represents a resistor. Meters are represented by circles with the symbol of the unit mea- sured inside. Components like power supplies and resistors are frequently labeled with a specific value or with variable name for reference. Figure 1 is a typical circuit diagram.
FIGURE 1. Circuit diagram to measure Ohm’s Law.
Power Resistor: RL
A
Supply: V
Ammeter
Voltmeter V
Switch
2 Circuits
Building a circuit from a diagram begins by finding the needed components. Each com- ponent is connected to other components with wires or other conductors. The points where lines connect the symbols on the diagram indicate where the components are con- nected in the actual circuit. Note that the components have at least two ends and it mat- ters how the ends are connected. If both ends of a component are not connected, no current can flow through that component. In place where conductor line connect on the diagram, the conductors should also connect in the circuit. These connections between wires can occur any place along the wire since all points of the conductor are at the same potential.
There are two fundametally different ways of connecting components together. In a series connection, like the lamps in Figure 2, one terminal of one lamp is connected to one terminal of the other lamp. In a parallel connection, like the lamps in Figure 3, each terminal of one lamp is connected to a terminal of the other lamp so that both terminals of both lamps are connected to one and only one terminal of the other. Note that orienta- tion of the lamps on the figures do not change their meanings.
FIGURE 2. Circuit diagram with two lamps in series.
FIGURE 3. Circuit diagram with two lamps in parallel.
Circuits often don’t show the position of the meter to make a measurement. An ohmme- ter is used on a component that is not connected in a circuit, otherwise the mesurement
V
Lamp: A 1 Lamp: A 2
V A
1 A 2
4 Circuits
4. Record the current measured by the ammeter. 5. Repeat steps 2 and 3 for voltage values of 0.5 V, 1.0 V, 2.0 V and 5.0 V. 6. Remove the resistor and replace it with a different valued resistor. Repeat steps 2 through 4 for this second resistor. 7. Remove the resistor and replace it with a lamp. Repeat steps 2 through 4 for the lamp.
SERIES AND PARALLEL CIRCUITS
8. Keep in mind the instructions in steps 1 and 2 and connect the components to build the circuit in Figure 2. Set the voltage V equal to 5.0 V and close the switch. 9. Use the DMM to measure the voltage drop across lamp A 1 and record the result. Remember that a voltmeter measures in parallel to the component. 10. Use the DMM to measure the voltage drop across lamp A 2 and record the result. 11. Use the DMM to measure the resistance of two resistors R 1 and R 2 and record the values. 12. Replace the two lamps in the circuit in step 8 with the two resistors from step 11 and measure the voltage drop across each resistor as in steps 9 and 10. 13. Open the switch and insert an ammeter in series with the switch. Close the switch and record the current. 14. Repeat steps 8 through 10 for the circuit in Figure 3. 15. Replace the lamps in the circuit from step 14 with the two resistors and repeat steps 13 and 14.
DATA ANALYSIS 16. Place the data from steps 4 and 5 in a table and plot the current vs. the voltage from that data.
17. Find the slope of the line I / V for from the plot and use the inverse of the slope to find the resistance R according to EQ 1. 18. Repeat steps 15 and 16 for the data from step 6. 19. Repeat step 15 for the lamp data from step 7. 20. Add the value of the voltage source in step 8 and subtract the voltage drops mea- sured in steps 9 and 10 to get the total voltage change around the circuit loop. 21. Rearrange EQ 1 to solve for the current and use the two values of the resistors and the voltage drop in step 15 to find and record the current through each resistor. 22. Add the current measured by the ammeter in step 15 and subtract the two currents found in step 21 to find the total current into the junction of the resistors.
Circuits 5
OBSERVATIONS For each of these questions make your observation and support it by answering the question “Why?”.
According to your data and the graphs in steps 17 and 18, did the resistors obey Ohm’s Law?
At what voltage did the lamp first begin to glow in step 7?
Based on your graph and analysis in step 19, is the lamp an ohmic device?
How did the brightness of the lamps compare within the circuits from Figure 2 and 3 and how did they compare in brightness between those two circuits?
According to your data in step 20, did the circuit obey Kirchoff’s Voltage Law (EQ 2)?
According to your data in step 22, did the circuit obey Kirchoff’s Current Law (EQ 3)?
