LAB: Resistors in Parallel
Please fill the blanks for full credit.
PURPOSE
Investigate resistance and determine the equivalent resistance of resistors in paralell. I suppose that you all know how to measure the current in a circuit and the voltage across element.
DISCUSSION
word bank: current, parallel,V1, V2, the same, increases, I1, I2, decreases, effective, resistors
When the conductors are arranged as in the following figure:
with the ends of the _____________ connected to common points, so that the total ________ divides among the resistors, they are said to be ___________. For resistors connected in parallel:
The total ____________ is the sum of the separate current: I = ___ + ______. The volatage across each resistor is ____________ as that across any other resistor in the group:
V = ___ = _____. The total resistance (or____________) of the group Reff is related to the separate resistance by 1/Reff = 1/R1 + 1/R2 or Reff = 1 / (1/R1 + 1/R2)
It is important to observe that the connection of additional resistors in series ________ the total resistance, while inserting resistors in parallel ____________ the total resistance. if you don't know the answer, take an example.
Suppose R1 =2 and R2=5. Which Reff is smaller : Reff = R1+R2 (series circuit) or Reff = 1/ (1/R1 + 1/R2) (parallel circuit)
power supply or dry cells, 8 connecting wires, voltmeter (0-5V), knife switch,
3 resistors (1K, 100K, 150K) , 0.5W or greater
milliammeter, 0-50mA or 0-100mA.
PROCEDURE
1) Connect 3 resistors in parallel. That is connect their end points.
Connect this group to the battery switch and milliammeter.
Measure the main current. Place the multimeter between one end point and the battery. I = __________
2) Measure the current in each of the parallel branches. You need to insert the multimeter between one resistor and one end point.
I1 = _______________, I2 = _______________, I3 = _____________. Compute I1 + I2 +I3 = _______________
discuss
3) Measure the voltage drop across each resistor (V1, V2, V3) and the voltage drop cross the group of resistors (V)
V1 = ___________, V2 = ____________, V3 = _____________, V = _____________________
Discuss the results
4) By using V(from 3) ) across the battery and I the main current (from 1) ) calculate the effective resistance Reff.
Reff = V / I = ___________ (ohm's law)
Find its reciprocal: 1/Reff = _____________
5) Calculate the value of each resistance using Ohm's law and I1, I2 and I3 found in 2)
R1 = V1/I1 = ___________, R2 = V2/I2 = ________________, R3 = V3/I3 = ________________
Compute 1/R1 + 1/R2 + 1/R3 = ______________________
compare to 1/Reff found in 4). Do you get the same thing? ____________
GOING FURTHER
1) A 5 ohms, 4 ohms and a 10 ohm resistors are connected in parallel. The group is connected to a 6 volts battery.
A) Draw the circuit diagram
B) Find the current in the main circuit
C) Find the current in each brach
2) If the three resistors above are connected in series, find the current in the circuit.
Investigate resistance and determine the equivalent resistance of resistors in paralell. I suppose that you all know how to measure the current in a circuit and the voltage across element.
DISCUSSION
word bank: current, parallel,V1, V2, the same, increases, I1, I2, decreases, effective, resistors
When the conductors are arranged as in the following figure:
with the ends of the _____________ connected to common points, so that the total ________ divides among the resistors, they are said to be ___________. For resistors connected in parallel:
The total ____________ is the sum of the separate current: I = ___ + ______. The volatage across each resistor is ____________ as that across any other resistor in the group:
V = ___ = _____. The total resistance (or____________) of the group Reff is related to the separate resistance by 1/Reff = 1/R1 + 1/R2 or Reff = 1 / (1/R1 + 1/R2)
It is important to observe that the connection of additional resistors in series ________ the total resistance, while inserting resistors in parallel ____________ the total resistance. if you don't know the answer, take an example.
Suppose R1 =2 and R2=5. Which Reff is smaller : Reff = R1+R2 (series circuit) or Reff = 1/ (1/R1 + 1/R2) (parallel circuit)
power supply or dry cells, 8 connecting wires, voltmeter (0-5V), knife switch,
3 resistors (1K, 100K, 150K) , 0.5W or greater
milliammeter, 0-50mA or 0-100mA.
PROCEDURE
Do not leave the switch close for more than a few seconds at a time.
1) Connect 3 resistors in parallel. That is connect their end points.
Connect this group to the battery switch and milliammeter.
Measure the main current. Place the multimeter between one end point and the battery. I = __________
2) Measure the current in each of the parallel branches. You need to insert the multimeter between one resistor and one end point.
I1 = _______________, I2 = _______________, I3 = _____________. Compute I1 + I2 +I3 = _______________
discuss
3) Measure the voltage drop across each resistor (V1, V2, V3) and the voltage drop cross the group of resistors (V)
V1 = ___________, V2 = ____________, V3 = _____________, V = _____________________
Discuss the results
4) By using V(from 3) ) across the battery and I the main current (from 1) ) calculate the effective resistance Reff.
Reff = V / I = ___________ (ohm's law)
Find its reciprocal: 1/Reff = _____________
5) Calculate the value of each resistance using Ohm's law and I1, I2 and I3 found in 2)
R1 = V1/I1 = ___________, R2 = V2/I2 = ________________, R3 = V3/I3 = ________________
Compute 1/R1 + 1/R2 + 1/R3 = ______________________
compare to 1/Reff found in 4). Do you get the same thing? ____________
GOING FURTHER
1) A 5 ohms, 4 ohms and a 10 ohm resistors are connected in parallel. The group is connected to a 6 volts battery.
A) Draw the circuit diagram
B) Find the current in the main circuit
C) Find the current in each brach
2) If the three resistors above are connected in series, find the current in the circuit.