Building Virtual Circuits with PHET
Name_______________________
In this lab, you will use two different PHET simulations to learn about the basics of
electric circuits and electrical measurements. Navigate to the PHET page
containing simulations on PHYSICS - ELECTRICITY, MAGNETS AND CIRCUITS.
Open the PHET simulation called
“BATTERY-RESISTOR CIRCUIT”
Check:

“Show cores”

“Show inside battery”
Try changing the voltage to see what
happens to the charges.
Questions
1. What do the small blue balls represent? What do the large green balls represent?
2. Which way do electrons move, relative to the positive and negative ends of battery?
(The positive end of battery is always the end that has the small bump/protrusion)
3. Electrical potential energy from the battery is converted into which form in the
resistor? How is this portrayed in the simulation?
4. Do electrons enter the resistor faster than they leave? Look and the simulation
carefully AND think about this one! run in, walk out of resistor? Can you postulate a
reason why/why not?
5. What role doe the battery ‘fairies’ play? In a real battery, what takes the place of the
fairies?
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Next, you will use a virtual
circuit kit to construct basic
series and parallel circuits.
Open the application called,
“CIRCUIT CONSTRUCTION
KIT (AC + DC)”
Part 1: Voltage sources in series and in parallel
Create the following circuit with three, 9-Volt batteries in series. Use the voltmeter to
measure the output across the entire group.
Sketch the schematic for this circuit below
Questions
1. What is true about the overall voltage when batteries are connected in series? Use
the voltmeter to measure this by touching the red and black probe ends to the ends of
your battery set.
2. How many batteries would have to be connected in series in order to achieve a voltage
of 54 volts?
3. Reverse the direction of just ONE of the batteries. Measure the total voltage across
all three. State and explain your finding.
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Create the following circuit with three, 9-Volt batteries in parallel. Use the voltmeter to
measure the output across the entire group.
Sketch the schematic for this circuit below
Questions
4. What is the overall voltage of the parallel combination of batteries? Can you state a
rule for combining voltages in parallel?
5. Can you imagine any possible advantage for connecting batteries in parallel?
6. Right-click on one of the batteries and increase its voltage to 18 Volts. Describe what
happens. Explain why.
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Part 2: Resistors in Series
Set up the circuit shown below. Change the voltage of the battery to 48 volts. Change the
values of the resistors to 20 ohms, 50 ohms and 30 ohms (right-click on items to change
their values.)
Sketch the schematic
for this circuit below
Questions
7. Measure the voltage across the battery by placing the ends of the voltmeter probes
across it_______
8. Measure the voltage across each resistor by placing the ends of the voltmeter probes
around each one, one at time.
Vdrop across 20 ohm
Vdrop across 50 ohm
Vdrop across 30 ohm
9. Which resistor uses the largest voltage?___________
10. What do these voltages add up to?__________
11. We call the voltage used by a resistor a ‘voltage drop.’ State a general rule about the
sum of the voltage drops around the resistors of a series circuit.
12. Since voltage is closely related to energy (E=qV), which conservation law in physics
do you think applies when you sum voltages around a loop in a circuit?
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13. Are you connecting the voltmeter in series or in parallel with the resistors in order to
measure their voltage drops?
14. Do you think a voltmeter should have a high or low internal resistance, relative to the
resistors in the circuit?
15. Now, look at the current measured by the ammeter. Record it here__________
16. Place the ammeter in another location in the circuit. Record it here__________
17. Does it matter where you place the ammeter in a series circuit?___________
18. The fact that current is everywhere the ____________ in a series circuit is really a
consequence of the Law of Conservation of _____________ (no, not “Energy”)
19. Are you connecting the ammeter in series or in parallel with the resistors in order to
measure the current through them?
20. Do you think an ammeter should have a high or low internal resistance, relative to the
resistors in the circuit?
21. Since the current in a series circuit is the same in all parts of the circuit, you can use
Ohm’s law to calculate the voltage drop across each resistor (V=I x R.) Do this below.
Show your work
Vdrop across 20 ohm
Vdrop across 50 ohm
Vdrop across 30 ohm
22. How do these calculated voltages compare to the voltages that you measured?
23. Use Ohm’s law to calculate the total resistance of the circuit by dividing the total
voltage by the total current.
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24. What is a good rule for determining total resistance in a series circuit?
25. Remove a resistor, but don’t complete the gap with wire. What happens to the current
in the circuit?
Part 3: Resistors in Parallel
Set up the circuit shown below. Change the voltage of the battery to 48 volts. Change the
values of the resistors to 20 ohms, 50 ohms and 30 ohms (right-click on items to change
their values.)
Sketch the schematic for this circuit
below
26. Measure the voltage drop across each resistor and record these below:
Resistor
Voltage Drop
20 ohm
50 ohm
30 ohm
27. How is this different from voltage drops in a series circuit?
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28. As shown in the figure, the ammeter will measure the total current through your
circuit (in other words, the total current that must flow through the battery.) Record
this value below.
29. Use Ohm’s Law to calculate the total resistance of the circuit, using the battery
voltage and dividing by the total current you measured above.
30. Is the total resistance you calculated larger or smaller than you expected?
31. Now, move the ammeter into each branch of the circuit and measure the currents
separately. Record the values below.
Resistor
Current
20 ohm
50 ohm
30 ohm
32. The individual currents should add up to the same total current that you measured
earlier. Do they?
33. Which resistor has the greatest current? Why? (you should also be able to see the
‘moving electrons’ in the simulation)
34. Which resistor has the least current? Why?
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35. Does this agree with the resistance that you calculated earlier using Ohm’s Law?
36. If you add more resistor branches into this circuit, what will happen to the total
current? Try it.
37. What does this result imply about the total resistance as more resistors are added in
parallel?
38. So, if you remove a resistor branch, how does this affect the total current? …the
total resistance?
Now, you will learn a quick method of calculating the total (or equivalent) resistance of a
set of resistors in parallel. This method is called the RECIPROCAL RULE, and follows the
formula below:
1/Rtotal = (1/R1) + (1/R2) + …
39. Substitute into this formula and calculate the total resistance for your circuit using
the 20, 30 and 50 Ohm resistors. Show your work below.
40. Does the resistance you calculated with this formula agree with the resistance you
determined earlier?
41. If the resistors in parallel ALL HAVE THE SAME VALUE, can you think of a simple rule
for finding the total resistance? State this below.
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Part 4: Short-Circuiting a Bulb in a Series Circuit
In PHET, build Circuits A and B as shown above. All light bulbs are identical. Each has
a resistance of 10 Ohms (default.) Set the battery voltage to 24 Volts. Use the
ammeter and voltmeter to complete the table below.
Bulb #
Current
(A)
Voltage Drop
(V)
Power (calculate)
(W)
1, 2, 3
4, 6
5
a. Which circuit (A or B) has the greater current? Why?
b. Which bulbs have the largest voltage drops across them? Why?
c. In the space below, calculate the total power used by each circuit. There are many
ways to do this!
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d. Which circuit uses more power? Which circuit produces more light? What is the
connection between light output and power?
e. Adding a wire around bulb #5 is said to be “shorting” this bulb. What happens to
the current through bulb #5?
f. Offer an explanation for this behavior.
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