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Transcript
Student ________________________
Lab Date _____________
Lab # _____
Due Date _____________
ELECTRIC CIRCUITS
PURPOSE: To study the principles of series and parallel circuits, voltage, current flow,
electrical resistance and power that are used in electricity
MATERIALS: Snaptricity lamp board, ammeter, digital voltmeter, wiring, switch
PROCEDURE: The electric boards we will use in this lab show the effect of different
kinds of series and parallel electric circuits via the brightness of light bulbs acting as
resistances. To use the digital voltmeter make sure you use these settings: the red lead of
the multimeter has to go into the opening next to V,  and the dial of the multimeter
should point to 20 V (DC  , not AC ~). Most importantly, a voltmeter is always
connected in parallel to a part of a circuit. The picture of this connection looks like this:
To use the multimeter as an ammeter make sure you use these settings: the red lead of the
multimeter has to go into opening next to A, 10 A and the dial of the multimeter should
point to 10 A (DC , not AC ~). We always an ammeter connect in series with a part of a
circuit. In other words, to measure the electric current in a bulb you should break the
circuit at a point next to the bulb and connect the ammeter as part of the circuit. The
picture of this connection looks like this:
SERIES CIRCUIT
The first principle to understand about series circuits is that the amount of current is the
same through any component in the circuit. This is because there is only one path for
electrons to flow in a series circuit. From the way that the battery is connected, we can
tell which direction the current will flow. We know that charge flows from positive to
negative, by convention. We know that in a series circuit the current has to be the same
in all components. So we can write: 𝐼 = 𝐼1 = 𝐼2 = 𝐼3 . We also know that total voltage of
the circuit has to be equal to the sum of the voltages over all three resistors. So we can
write: 𝑉 = 𝑉1 + 𝑉2 + 𝑉3. When there is more than one resistor in a circuit, we are usually
able to replace all resistors with a single resistor whose effect is the same as all the
1
resistors put together. The resistance of the single resistor is known as equivalent
resistance. For 3 resistors in series the equivalent resistance is: 𝑅𝑒𝑞 = 𝑅1 + 𝑅2 + 𝑅3
PARALLEL CIRCUIT
The first principle to understand about parallel circuits is that the voltage is equal across
all components in the circuit. This is because there are only two sets of electrically
common points in a parallel circuit, and voltage measured between sets of common
points must always be the same at any given time. For a parallel circuit the following is
true: 𝑉 = 𝑉1 = 𝑉2 = 𝑉3. The second principle for a parallel circuit is that all the currents
through each resistor must add up to the total current in the circuit: 𝐼 = 𝐼1 + 𝐼2 + 𝐼3
1
1
1
1
For 3 resistors in parallel, the equivalent resistance is: 𝑅 = 𝑅 + 𝑅 + 𝑅
𝑒𝑞
1
2
3
PART A - SERIES CIRCUIT
1. Build a circuit that consists of three light bulbs, a switch and 4.5 V battery. What is the
direction of the electron current? _______________________
DO NOT flip the switch until the teacher checks your circuit!
2. Connect voltmeter in parallel. Measure the voltage in the battery and in each bulb.
Vbattery (V)
V1 (V)
V2 (V)
V3 (V)
3. Connect ammeter in series. Measure the total current in the circuit and in each bulb.
Itotal (A)
I1 (A)
I2 (A)
I3 (A)
4. Take a paper clip and extend it so it becomes a long wire. Make two contacts around
the middle bulb. Explain what you see. __________________________________________
5. Disconnect the middle bulb from the circuit. What happens? ______________________
______________________________________________________________________________
PART B
1. Build a circuit that consists of three light bulbs, a switch and 4.5 V battery. What is the
direction of the conventional current? ___________________
DO NOT flip the switch until the teacher checks your circuit!
2. Connect voltmeter in parallel. Measure the voltage in the battery and in each bulb.
Vbattery (V)
V1 (V)
V2 (V)
V3 (V)
2
3. Connect ammeter in series. Measure the total current in the circuit and in each bulb.
Itotal (A)
I1 (A)
I2 (A)
I3 (A)
4. Take a paper clip and extend it so it becomes a long wire. Make two contacts around
the middle bulb. Explain what you see. __________________________________________
5. Disconnect the middle bulb from the circuit. What happens? ______________________
______________________________________________________________________________
CALCULATIONS & CONCLUSIONS
PART A - SERIES CIRCUIT
1. What is your conclusion about the total voltage of a battery and the voltage drops in
each resistance for a series circuit? Explain any discrepancies.
Vtotal = V1 V2 V3
______________________________________________________________________________
______________________________________________________________________________
2. What is your conclusion about the total voltage of a battery and the voltage drops in
each resistance for a series circuit? Explain any discrepancies.
Itotal = I1 I2 I3
______________________________________________________________________________
______________________________________________________________________________
3. Use Ohm’s law to calculate the resistance in the circuit and in each bulb.
Req ()
R1 ()
R2 ()
R3 ()
Req = R1 R2 R3
______________________________________________________________________________
______________________________________________________________________________
4. Use power formula to calculate the power developed by the battery and by each bulb.
Ptotal (W)
P1 (W)
P2 (W)
P3 (W)
What is your conclusion about the total power developed by the battery and the power
developed in each bulb? Explain any discrepancies.
3
Ptotal = P1 P2 P3
______________________________________________________________________________
______________________________________________________________________________
5. Which conservation law seems to apply for series circuits?
______________________________________________________________________________
______________________________________________________________________________
PART B - PARALLEL CIRCUIT
1. What is your conclusion about the total voltage of a battery and the voltage drops in
each resistance for a parallel circuit? Explain any discrepancies.
Vtotal = V1 V2 V3
______________________________________________________________________________
______________________________________________________________________________
2. What is your conclusion about the total voltage of a battery and the voltage drops in
each resistance for a parallel circuit? Explain any discrepancies.
Itotal = I1 I2 I3
______________________________________________________________________________
______________________________________________________________________________
3. Use Ohm’s law to calculate the equivalent resistance (of the circuit) and in each bulb.
Req ()
R1 ()
R2 ()
R3 ()
1
𝑅𝑒𝑞
=
1
1
1
𝑅1
𝑅2
𝑅3
______________________________________________________________________________
______________________________________________________________________________
4. Use power formula to calculate the power developed by the battery and by each bulb.
Ptotal (W)
P1 (W)
P2 (W)
P3 (W)
Ptotal = P1 P2 P3
5. Which conservation law seems to apply for parallel circuits ? Is that different from
series circuits?
______________________________________________________________________________
______________________________________________________________________________
4