Lab 02: DC Ci rcu i tsINTRODUCTION

Electricity is so common that it goes unremarked in everyday life. So common, in fact, that most people don't know or think about how it works (used to be that if you drove a horseless carriage, you had to know how an internal combustion engine worked, too...). Household electricity is AC, or alternating current. Anything you plug into an outlet is designed to run on this alternating current. But anything that runs on batteries uses DC, or direct current. DC circuits are very straightforward, and can be understood using Ohm’s Law. We will build several and examine their properties and uses.

OBJECTIVES๏ Demonstrate the principles of DC circuits๏ Observe qualitative differences between series and parallel circuits๏ Verify Ohm's Law by finding the resistance in a series circuit ๏ Observe the difference between ohmic and non-ohmic behavior in an

incandescent bulb

EQUIPMENT

If the bulbs are the same, why aren’t they equally bright?

๏ DC power supply๏ LabPro interface

๏ Vernier circuit board๏ Electrical wires

๏ Current and voltage probes๏ Spare light bulbs

EXPERIMENTAL PROCEDURE

๏ The current and voltage probes are extremely sensitive, and can be permanently damaged by overloading them. The current probe will be literally destroyed by currents greater than 0.600A. The voltage probe is limited to 6V. It is slightly more forgiving, but will also be rendered permanently inoperable by exposure to voltages greater than 7V.

๏ It is necessary to set the power supply to limit the maximum current output. We will always use the power supply in constant voltage (CV) mode. The power supply has both coarse and fine adjustments for both current and voltage. It also has a HI–LO range selector.

๏ When the selected range is LO, the maximum current in CV mode will be 0.200A. With the power supply off, zero all of the control knobs to the left (anti–clockwise). Now adjust the fine control of the current to maximum (clockwise). The coarse control remains at zero. When you switch the power supply on, you will adjust only the voltage to power your circuit. The current will never exceed 0.200A. Note that you are not voltage–limited, so you must still be careful not to exceed 6V.

๏ To increase the maximum current, power off and toggle the range button from LO to HI. Zero all knobs, then adjust fine current to maximum, leaving the coarse control at zero. This increases your maximum current to 0.600A. Please only use this HI setting when absolutely necessary, as it dramatically increases the risk that you will break both the current and voltage probes simultaneously.

SERIES AND PARALLEL CIRCUITS

๏ These circuits require the power supply to be in HI mode. You do not need detailed numeric data, so do not wire the current or voltage probe into these circuits. When you are wiring your circuits, make sure that the voltage controls are zeroed. Do not power up until the circuit is complete.

๏ Connect the power supply to the input terminals of the circuit board. Complete the circuit through a single light bulb.๏ Power up to 3V (read the display on the power supply; do not wire in the voltage probe).๏ Record the current by reading the display on the power supply (do not wire in the current probe).๏ Power down, wire a second bulb in series with the first. Record the current at 3V and note and qualitative changes.๏ Add a third bulb in series. Again record the current at 3V and observe how the brightness changes.๏ Rewire so that you have two bulbs in parallel. Record the current at 3V and note and qualitative changes.๏ Add the third bulb in parallel with the first two. Record the current at 3V and note and qualitative changes.

Same battery, same bulbs, different wiring.Which pair of bulbs will be brighter?

PHYS 1420: College Physics II Summer 2018

Course Web: http://faculty.uca.edu/njaustin/PHYS1420/Laboratory page 01/02

QUESTIONS1. Document each circuit that you construct using a neat sketch. Use the standard symbols illustrated for you in class to

create your diagrams.

2. What happens in a series circuit when one bulb is removed, leaving the other two in place? Parallel?

3. Demonstrate the concept of equivalent resistance, using the voltage and current values you recorded. Note that a light bulb is not ohmic, so you may not see a perfect correlation between Ohm’s Law and your values.

OHM’S LAW

We will be using the LabPro to automate data collection for the remaining exercises. Both current and voltage data will be collected automatically by the LabPro, so you must switch the power supply to the LO setting. Do not leave the power supply toggled to HI. Switch to LO right now!

๏ Plug in the LabPro power supply, and attach the USB from the LabPro to the computer. Connect the voltage probe to Channel 1, and the current probe to Channel 2.

๏ Construct the circuit shown in the diagram. Both the ammeter (A = current probe) and voltmeter (V = voltage probe) have the additional wire (not shown) connecting them to the LabPro.

๏ Begin with the 10Ω resistor on the circuit board. Note the color and order of the bands on the resistor.

๏ Measure and record the current through and the voltage across the resistor. In the LabPro software window, click Collect. Simultaneously, using the fine voltage control, gradually increase the voltage at the power supply, continuing until the data collection interval elapses.

๏ Your data should graph automatically, but you must apply the curve fit. Make sure that you are plotting voltage V on the y–axis and current I on the x–axis. Also make sure to save each trial before continuing to the next. Label each data set with the known resistance.

๏ Repeat the experiment, using a 51Ω resistor (note color bands), and again using a 68Ω resistor.

๏ Wire two 10Ω resistors in series and repeat the data collection.

QUESTIONS4. Verify Ohm’s Law (V = IR). Are the graphs of voltage as a function of

current linear? Note the slopes and intercepts.

5. Compare the slopes of your graphs to the actual resistances. Calculate the percent error in your slopes.

6. For the series resistors, does the graph accurately determine the equivalent resistance?

7. Note whether your slope values are within the range indicated by the tolerance on the resistor band. If not, does your graph give you an

indication of the extent of your random experimental error? Did you collect sufficient data, or would your results be improved by enlarging your data set? How would you go about enlarging the data set?

NON—OHMIC BEHAVIOR

Replace the resistors in your previous circuit with a single light bulb. Using the same method as above, collect voltage and current data for the bulb. You should still be current–limited to 0.200A, but please make sure not to exceed 5V (keep one eye on the power supply). The bulb may not glow brightly, but even if the bulb filament is not incandescent there is still a complete circuit drawing current.

8. Prepare a graph of voltage as a function of current for the bulb.

9. Is there a region over which the bulb is ohmic? Does it start out linear and become non–ohmic, or does your graph start out non–linear and become ohmic? Can you tell from your graph when the bulb filament starts to glow visibly?

The electrons don’t care if the wire is blackor red, but color-coding the wires helps

you see the direction of the circuit.

PHYS 1420: College Physics II Summer 2018

Course Web: http://faculty.uca.edu/njaustin/PHYS1420/Laboratory page 02/02