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Lab Experiment #
Electric Circuits: Parallel Connection
Summary of relevant concepts :
Parallel Circuits
Devices, which have all the same beginning and all the same end point, are wired in parallel. Since they start at the same point and end at the same point, they have the same voltage drop across them. The current through each device is different and the total current is the sum of the currents:
R 3
R 2
R 1
V
According to Ohmโs law, current is voltage divided by resistance: ๐ฐ๐ฐ =
๐ฝ๐ฝ ๐น๐น Rewriting the equation (1): ๐ฝ๐ฝ ๐น๐น (^) ๐๐๐๐
๐ฝ๐ฝ ๐น๐น (^) ๐๐
๐ฝ๐ฝ ๐น๐น (^) ๐๐
๐ฝ๐ฝ ๐น๐น (^) ๐๐
Or:
Reciprocal of the equivalent resistance is equal to the sum of reciprocal resistances wired in parallel.
Resistors wired in parallel have a lower resistance than any single resistor in the circuit simply because there are more paths the current can follow and so current can flow more easily.
Experiment
Step 1:
In this experiment, you will examine parallel circuits. You will first find the resistance of each of the three resistors.
- Make sure the power supply is off, and that the voltage V and current I knobs are turned all the way counter-clockwise to zero
- Connect the + terminal of the power supply to the positive terminal of the ammeter. Connect the negative terminal of the ammeter to one side of the first resistor with an alligator clip. Connect the other side of the resistor to the negative terminal of the power supply.
- Connect the voltmeter across r
Schematic Diagram: R
V
R
- Turn on the power supply (PS) and increase slowly the current by turning clockwise the V and I knobs on the PS until you get about 5V across the resistor (as before do this CAREFULLY, feel the resistor and make sure it does not get hot). Measure V in volts and I in amps. Then take 4 other measurements (V, I) with decreasing voltages to V=1V.
- Plot you results (V vs I) in Excel and get the resistance from the slope as before.
- Repeat above steps for the other 2 resistors
You get:
R1=________________
R2=________________
R3=________________
A
Step 3 :
Weโll measure now the current in each branch of the parallel circuit (i.e. in each resistor) and then in the main branch of the circuit and verify that I=I1+I2+I
- Turn off PS and set V and I knobs at zero
- Relocate the ammeter to measure I in the first resistor.
- Turn on PS set the voltage at about 5V. Measure and record I
- Turn off the PS with the switch but WITHOUT TURNING THE V AND I KNOBS. Youโll keep the PS at this setting for the rest of this part so as to not alter the current through the circuit.
- Now relocate the ammeter in order to measure the current in the next resistor R
- Turn on the PS with the switch but WITHOUT TURNING THE V AND I KNOBS! And measure and record I2 in that resistor
- Repeat for the last resistor. Measure and record I
- Turn off the PS and reconnect the circuit to measure the current I in the main branch of the circuit. Measure and record I.
Compute I1+I2+I3 and compare to I.
I1+I2+I3= _______________
I=______________
Conclusion:
Questions
Four resistors of 3ฮฉ, 4ฮฉ, 2ฮฉ and 6ฮฉ are connected in parallel:
a) What is their equivalent resistance?
b) If the resistors mentioned above are connected to an 8V battery, what would be the total current supplied?
c) What is the voltage drop across the 2ฮฉ resistor? across the 6ฮฉ resistor?
d) What would be the current through each resistor?
e) What power does each resistor dissipate?
f) Is it the same power that dissipated by all resistors and that delivered by battery? Calculate.
