## PHYS 1402 Lab – 6: Series and Parallel Circuits

Name: _____________________

### Objectives:

· To understand what currents will flow in various parts of simple DC circuits, and why.

You will be using PHET simulation Circuit Construction Kit: DC – Virtual Lab (https://phet.colorado.edu/en/simulation/circuit-construction-kit-dc-virtual-lab)

### Activity 1: Current in a Simple Circuit with Light Bulbs

Connect the circuit with two bulbs. Again, measure the voltage across the battery and the current flowing through each light bulb with the switch closed.

**Voltage across the battery: ____________**

**Current flowing through each light bulb: ____________**

**Question 1-1: **When you added the second bulb to the circuit, did the current through the battery change significantly (i.e., by more than a few percent)? Did the voltage change significantly?

**Question 1-2**: Does the battery appear to be a source of constant current, constant voltage, or neither when different elements are added to a circuit?

**Question 1-3:** Based on your observations, how is the brightness of the bulb related to the current flowing through it?

### Activity 2: Resistors Connected in Series

1. Set the resistance of each light bulb to the following: R1 = 5 W, R2 = 10 W, R3 = 15 W.

2. Connect the light bulbs in series as shown:

3. Connect the circuit shown below.

Set the battery voltage to 20 V. Measure the total voltage: VTotal = __________

4. Use the voltmeter to measure the across each of the light bulbs. Measure the voltages across each bulb.

V1 = ________

V2 = _________

V3 = _________

VTotal = V1 + V2 + V3 = ___________

Does VTotal in Step 3 equal VTotal in Step 4?

5. Use the ammeter to measure the current through each of the light bulbs. Write down the values of these currents below.

I1 = ________

I2 = _________

I3 = _________

6. Now measure the total current flowing from the battery by placing the ammeter next to the battery.

Itotal (from the battery) =________

7. Use the Ohm’s law to calculate the total resistance of the light bulbs (assuming that the wires are of negligible resistance). Show all your work below.

**Resistance of the light bulb: = **

8. The light bulbs are connected in series.

When resistors are in series, total resistance Req = R1 + R2 + R3

Calculate the total resistance. **Rtotal (calculated) = ___________**

Does RTotal in Step 7 equal RTotal in Step 8?

**Question 2-1: **Do you notice any pattern in the currents through the light bulbs connected in series? Explain.

**Question 2-2:** Is there any relationship between the voltages across each light bulb and its resistance? Explain.

**Question 2-3:** Is there any relationship between the battery voltage and the voltages across the light bulbs? Explain.

9. Put a 4th bulb in series with the other bulbs. R4 = _____________ W

**Question 2-4:** Do the bulbs seem to burn as bright as before?

### Activity 3: Resistors Connected in Parallel

1. Now build the circuit shown below using three light bulbs, battery, and some wires.

R1 = 10 Ω, R2 = 20 Ω, and R3 = 30 Ω

2. Click on the battery and set the battery voltage to 12 volts.

3. Use the ammeter to measure the current through each of the light bulbs (by placing the ammeter on wire either before or after the light bulb. Write down the values of these currents below.

I1 = ________

I2 = _________

I3 = _________

4. Now measure the total current flowing from the battery by placing the ammeter next to the battery.

Itotal (from the battery) =______

5. Use the voltmeter to measure the across each of the light bulbs. Write down the values of these voltages below.

V1 = ________

V2 = _________

V3 = _________

**Question 3-1: **Do you notice any pattern in the voltages across the light bulbs connected in parallel?

**Question 3-2:** Is there any relationship between current through each light bulb and its resistance? Explain.

**Question 3-3:** Is there any relationship between the total current flowing from the battery and the currents through the light bulbs? Explain.

6. Use the Ohm’s law to calculate the resistance of the light bulb (assuming that the wires are of negligible resistance). Show all your work below.

Calculate the total resistance: ** = **

7. These light bulbs are connected in parallel.

When resistors are connected in parallel, total resistance for any number of resistors connected in parallel is given by:

Calculate the total resistance. **Rtotal (calculated) = ___________**

Does RTotal in Step 6 equal RTotal in Step 7?

8. Remove one light bulb and calculate the total resistance: Rtotal = _________

9. Remove a second light bulb; there should now only be one bulb in the circuit.

Calculate the total resistance: Rtotal = _________

10. From Steps 7, 8, and 9, you should be able to reach a conclusion about the resistance in a parallel circuit: As the number of branches increases, the total resistance in a parallel circuit ____________________.

11. Put a 4th bulb in parallel with the others. Connect the bulbs to the batteries. Do the bulbs seem to glow as brightly as before?

R4 = _____________ W

**Analysis**: Complete the following sentences:

**Resistors in Series Circuits:**

1. The total resistance is equal to

2. The sum of the voltage drops across the individual bulbs is equal to

3. When you put a 4th bulb in the circuit, do you think the current in the circuit increased, decreased, or stayed the same?

4. Complete the following: As the current in the circuit ___________, the brightness of the bulbs ____________.

**Parallel Circuits:**

5. As you add more branches, the total resistance

6. As you add more branches, does the voltage across each branch increase, decrease, or stay the same?

7. As you add more branches, does the current across a previously existing branch increase, decrease, or stay the same?

8. If you were to decrease the number of branches to just one branch, how would the brightness of the bulb now compare the brightness when there were three bulbs in parallel?

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