Current Within A Circuit Loop - PhET


Experimental Procedures

Part I: the theory of voltmeters and ammeters

The above simulator we'll use repeatedly in our study of circuits as we move through this latter part of the course; we'll use it to observe how current in a circuit behaves, and it can model the answers we get to real problems.

What I would like to use it to do here, however, is model the reasons why we use voltmeters and ammeters in the way that we do.

You will use the “Lab” app within this simulator for this purpose; when you get to the front page of the simulator, click the “Lab” icon twice. That will bring you to this screen:

Screenshot of the PhET simulation right after you click 'Lab'

On the left side of the screen you see the devices – the circuit components. On the right side of the screen you see the meters – the way we'll measure voltage and current. We'll get items from both sides of the screen to use in the simulator.

The first thing I would like for you to set up in the simulator is a battery connected to two wires. Take a battery out of the device box on the left; place it at the top center of the screen. Pull out a wire, and connect it to the left side of the battery; draw the wire straight down to roughly midscreen. Pull out a second wire, and repeat this on the right side.

Then take out the voltmeter and the rightmost ammeter from the meter box, and place them both underneath.

When you everything you need in place, your screen will look like this:

Screenshot of the PhET simulation showing voltage meter and incomplete circuit with 2 wires connected to a battery

First connect the voltmeter to the exposed wires.

What is the voltage of the battery?

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Do you see any evidence that current is moving in the circuit? Explain what you do or do not see that might indicate that there is or isn't current.

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Then, disconnect the voltmeter and connect the ammeter to the two exposed wires in the circuit.

What current does the ammeter show?

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Do you see any evidence that current is moving in the circuit? Explain what you do or do not see that might indicate that there is or isn't current.

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If this was a real ammeter, it would have broken a while ago, and possibly drained the battery to an unusable point as well. Since this is a fake ammeter, you can calmly break the current flow by clicking and holding one of the connections for a second, and then clicking the scissors that appear next to that connection.

Based on what you know about resistance, which of these meters is a high-resistance meter that is difficult to break?

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What does this simulation indicate that you have to be careful of when you connect an ammeter?

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Now, to the exposed wire on one side, connect a switch; to the other side's exposed wire, connect a bulb. The new appearance of your simulator should look like what you see on the next page.

Screenshot of the PhET simulation showing voltage meter and circuit with 2 wires connected to a battery, a switch, and a light bulb

You can close the circuit by clicking on the switch.

When you close the circuit, do you see any evidence that current is moving in the circuit? Explain what you do or do not see that might indicate that there is or isn't current.

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With the circuit closed, connect the voltmeter to either side of the bulb.

What voltage is shown across the bulb?

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How does the observed voltage compare to the voltage of the battery?

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If the switch is opened, and the light is made to go out, what is the voltage across the bulb then? What other object in the circuit will show the voltage with the switch open when probed with the voltmeter that the the bulb showed when the circuit was closed (besides the battery)?

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At this point, you should break and reform connections in the circuit that will include the ammeter in the circuit. When you have done so successfully (assuming you haven't changed any of the settings of the battery or the bulb in the simulator), you will show a current of 0.90 A when the circuit is closed.

Describe how you include the ammeter in the circuit in a way that is distinct from how the voltmeter measures voltage across a bulb. To the best of your understanding right now, why are these two distinct methods of using these meters necessary?

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