Ammeters and Voltmeters

Ammeters and Voltmeters

by John C. Boehringer and David Bezinque. The production of this lab was funded in part by a grant from the McKinney Education Foundation, Spring 2006.

Ammeters are devices that are used to measure the electric current in a circuit. Voltmeters are used to measure the potential difference, or relative electric potential, between two points in a circuit. Both devices must be used while the circuit is operating or on, that is, any switches must be closed allowing for a complete circuit and current flow.

In this laboratory exercise you will attempt to draw some conclusions about voltmeters and ammeters and how they must operate in an electric circuit without interfering with the operation of that circuit. In this activity you will actually employ digital voltage and current probes, rather than analog meters, but the principles to be learned are applicable to either.

Problem

How are ammeters and voltmeters used to measure the respective current and voltage in an electric circuit?
How are they designed so as to not interfere with the operation of a circuit?

Preliminary Questions

1) (AP ONLY) Voltmeters read the potential difference between two points in a circuit, usually with some resistor in between. What should the potential difference be of two points on an ideal wire?

2) (AP ONLY) Ammeters read the current flowing in a circuit. What should the internal resistance of an ammeter be (approximately) so that the device does not interfere with the operation of the circuit? Explain.

Materials

2 D Cell batteries (or equivalent)4 alligator clips

2 wire leads1 voltage probe

1 current probe1 Windows PC w/ LoggerPro

1 LabPro data interface1 resistor of known value (>25)

Procedure

SAFETY FIRST!

NEVER hook up an ammeter in parallel with another circuit. This can damage the ammeter and possibly harm you!

Part I: Voltmeter in Series

1)Turn on your PC. Connect your LabPro via the USB port. Be sure that the LabPro is plugged in. Connect your voltage probe to Channel 1 and your current probe to Channel 2.

2)Start LoggerPro. You will not load a pre-programmed experiment file for this lab. You will take data from the status bar near the top of your home screen. There is no need to click Collect during this experiment. If the LabPro unit does not automatically detect and register the probes in their respective channels, let your lab instructor know immediately.

3)You will need to zero your probes. While you are doing this have your lab partners skip to step 4 and begin setting up your circuit. Click the Zero button to the right of the Collect button. A small window will appear. Confirm that both probes/channels are highlighted. Click OK. Your probes are now zeroed to relative values. Confirm this on the status bar at the top of the graphs. The readings may be fluctuate slightly, but they should be very close to zero. If this does not occur, attempt to zero the probes again. Use you best judgment. If you have problems ask your instructor for assistance.

4)Set up, but DO NOT CONNECT the circuit below. Make certain that the voltmeter is in series with the resistor and the battery.

5)When your instructor has approved your set up, connect the circuit and take a reading from the voltmeter. Record this value in your data table.

6)Now take a reading from the ammeter. Be sure not to confuse the values. Record this number in your data table.

Part II: Voltmeter in Parallel with a Wire

1)Set up, but DO NOT CONNECT, the circuit below. Be certain that the voltmeter is connected in parallel from the battery to the resistor. Be certain that the ammeter is still in series with the resistor and the battery.

2)When your instructor has approved your set up, connect the circuit and take a reading from the voltmeter. Record this value in your data table.

3)Now take a reading from the ammeter. Be sure not to confuse the values. Record this number in your data table.

Part III: Voltmeter in Parallel with a Resistor

1)Set up, but DO NOT CONNECT, the circuit below. Be certain that the voltmeter is connected in parallel across the resistor. Be certain that the ammeter is still in series with the resistor and the battery.

2)When your instructor has approved your set up, connect the circuit and take a reading from the voltmeter. Record this value in your data table.

3)Now take a reading from the ammeter. Be sure not to confuse the values. Record this number in your data table.

Part IV: Voltmeter in Parallel with Ammeter & Resistor

1) Set up, but DO NOT CONNECT, the circuit below. Be certain that the voltmeter is connected in parallel across the resistor AND the ammeter. Be certain that the ammeter is still in series with the resistor and the battery.

2)When your instructor has approved your set up, connect the circuit and take a reading from the voltmeter. Record this value in your data table.

3)Now take a reading from the ammeter. Be sure not to confuse the values. Record this number in your data table.

Part V: Voltmeter in Parallel with Ammeter only.

2) When your instructor has approved your set up, connect the circuit and take a reading from the voltmeter. Record this value in your data table.

3) Now take a reading from the ammeter. Be sure not to confuse the values. Record this number in your data table.

Part

/ Voltage (V) / Current (A) (always in SERIES)
I / Voltmeter in SERIES w/ battery & resistor
II / Voltmeter in PARALLEL w/ wire
III / Voltmeter in PARALLEL w/ resistor
IV / Voltmeter in PARALLEL w/ resistor & ammeter
V / Voltmeter in PARALLEL w/ ammeter ONLY

Drawing Conclusions

1) Based on your data, what conclusion can you draw about the proper placement of a voltmeter in determining the potential difference between two points in an electric circuit? Explain your reasoning.

2) Since voltmeters measure the potential between two points in a circuit, what can you say about the potential between any two points on a single wire? Under what conditions can this conclusion be faulty in practice?

3) Given what you already know about the nature of series circuits, an ideal ammeter should have what internal resistance? If this were not so, what effect would this have on the circuit?

4) In this lab ammeters were always placed in series with the circuit components. Based on your answer to question 3, explain how an ammeter can be “burnt out,” or “short out” a circuit if it is placed in parallel with circuit components.

5) Ideally, what should the internal resistance of a voltmeter be so that if properly placed, its presence will not significantly affect current flow in the circuit?

Extending

1) Design an experiment or a test demonstration to determine the actual resistance of a voltmeter. Your proposal may include any common electronics lab equipment in your design. There are a number of possible solutions.