Names:
In-Class Activity for Physics 7: Constructing Circuits
1. On your laptop, open a web browser and go to the PhET site (http://phet.colorado.edu/new/simulations/sims.php?sim=Circuit Cons
to find the Circuit Construction simulation. Click “Run now!” to start. The display should look like this:
Use the mouse to drag a battery into the blue area (where circuits can be constructed). Check the box next to “Voltmeter”;
this should make a black voltmeter appear on the screen. You can use this to measure voltages just like a real voltmeter, by
connecting the red and black leads to the two points whose potential difference you want to measure; the voltmeter output is
the potential at the red lead minus the potential at the black lead.
a) Use the voltmeter to measure the potential difference across the battery. Which end of the battery–black or gray–is the
positive terminal? Explain how you can tell based on the voltmeter reading.
Now build a circuit on the screen using your battery, one light bulb, and some wires (as many as you need). The exact orientation
and ordering of the components doesn’t matter, but your result should look something like this:
As soon as the circuit is complete, the “electrons” in the simulation (blue circles inside the wires, bulb, and battery) will
start to move, indicating that there is current in the circuit.
b) Give an order-of-magnitude estimate for the speed of a single “electron” on the screen, in cm/s. Explain how you came
up with your estimate. (This speed is fairly realistic–probably somewhat faster than that of real electrons in a typical circuit.)
c) About how long does it take a single electron to move from the battery to the bulb? When you connected the circuit, did
you have to wait that long before the light bulb turned on? Explain why or why not.
d) Which direction do the electrons move–into or out of the positive battery terminal? According to our convention, which
direction does current flow in the circuit? Are these two directions different or the same? Explain.
Now replace the bulb in your circuit with a resistor, to get something like the picture below.
Notes:
• To remove an object from the circuit, right-click on it and select “remove.” To break apart a junction (e.g., where a wire
connects to the battery), right-click on the junction and select “Split junction.”
• To display values of properties like resistance, right-click on an item (e.g., the resistor) and select “show value.”
Use an ammeter (not the non-contact ammeter) to measure the current on both sides of the resistor (i.e., between the battery’s
positive terminal and the resistor, and between the resistor and the battery’s negative terminal). Note that to connect the
ammeter properly, you will need to break apart your circuit at the appropriate point and insert the ammeter.
e) Was the current near the positive terminal greater than, less than, or equal to the current near the negative terminal?
Explain why this happened.
Change the resistance of the resistor from 10 to 20 Ω. (You can do this by right-clicking on the resistor and selecting “change
resistance.”
f) What happened to the value of the current when you changed the resistance? Why? Predict what the value of the current
would be if you changed the resistance to 30 Ω; be quantitative, and explain your reasoning.
g) Check your prediction from part (f). Was it correct? If not, explain what did happen and why.
h) Predict what would happen to the current in the following situations (as compared to the original case with the 10 Ω
resistor and the 9 V battery). Be quantitative.
• the voltage of the battery is doubled
• the voltage of the battery is tripled, AND the resistance of the resistor is doubled
Check your predictions, and comment on your results.
i) Use the voltmeter to read the potential difference across the resistor. What is the value of ∆V ? Why does it have that
value?
Change the resistor back to 10 Ω. Now add another 10 Ω resistor in series with the first, to get a circuit like this:
j) Use the ammeter to measure the current at two points, one just before each resistor. How do your current measurements
compare 1) to each other, and 2) to the current in the circuit with a single 10 Ω resistor? What do you think the total resistance
in the circuit is now? Explain.
k) Use the voltmeter to measure the potential difference across each individual resistor. What values did you get? What do
they add up to? Why does this make sense?
Now change one resistor to 5 Ω, and the other to 15 Ω.
l) How, if at all, has the current in the circuit changed? Explain why this result makes sense.
m) How, if at all, has the potential difference across each resistor changed? What do the two values now add up to? Explain
why your results make sense.
If you have more time left, try building some different circuits and making current and voltage measurements. Describe the
circuits you made and your observations–continue onto the back or attach extra paper if necessary.