DUE 2/21/17 Or 2/23/17 (Your Next Lab Day)

Industrial Electricity

Name______

DUE 2/21/17 or 2/23/17 (Your next lab day)

Prelab:Refer to the tables on Page 5. Show work neatly and completely on separate paper for any entry labeled “calculated.”You do not need to show calculations for entries marked “theoretical,” but you still need to complete those entries. You will need to show this (semi-completed) page to the lab instructor before beginning the lab exercise. Attach your sheet of calculations to the lab prior to turning it in for grading.

Lab 6: Investigating Parallel Circuits

Introduction

By definition, devices wired in parallel are configured in such a way that they all have the same voltage applied across them.

By virtue of this fact, each individual device drawscurrent from the supply based solely on that device’s resistance – independent of the other devices in the circuit. The amount of current required for the circuit is, then, the sum of the individual “branch” currents. For the circuit above, we can write

Using Ohm’s Law we can substitute for each of the branch currents giving,

The quantity in parenthesis is often referred to as the conductance of the circuit and is equivalent to the reciprocal of the total resistance of the circuit.That is,

We can solve this expression for to develop an expression for the total resistance of the circuit,

As there is only one voltage in a parallel circuit, Ohm’s Law proves that the current through an individual resistor will be inversely proportional to the value of the resistance. The highest resistance will have the lowest current flowing through it and the lowest resistance will have the highest current flowing through it.

Objectives

In this lab you will:

Establish that voltage is the same across all components of a parallel circuit
Observe the laws of current divider action in a parallel circuit
Verify the law which governs resistances connected in parallel
Verify Kirchoff’s Current Law by measurement of parallel circuit currents
Investigate the function of a photocell in an electrical circuit

Equipment

(1) Power Supply

(1) DMM; Digital Multimeter

(1) Breadboard

Components

Resistors: 1 each 330Ω, 470Ω, 1kΩ

(3) LEDs, 1 red, 1 green, 1 yellow

(~10) Jumper wires

When measuring current in a parallel circuit, care must be taken that the ammeter is connected in series, not in parallel with a resistor. The circuit path must be broken where current is to be measured. The ammeter is then connected to the two ends of the break, keeping the positive test lead of the ammeter on the side of the break closest to the positive side of the voltage source.

An open-circuited resistor in a parallel circuit will cause the current through that branch to be zero amps. Voltage will still be available across other components of the parallel circuit, however, so current will continue to flow through the other branches. No power is consumed by the open-circuited resistor and the power dissipated by the other resistors will remain constant. If one resistor in a parallel circuit becomes open, the total resistance of the circuit increases, resulting in lower total current and total power from the supply.

A short-circuited resistor in a parallel circuit will also be a short circuit directly across the power supply resulting in a blown fuse in the supply. For this reason, short circuits will not be investigated in this experiment.

The total power dissipated by the components in a parallel circuit is equal to the power supplied by the voltage source. Since the power dissipated by a resistor is the square of the voltage divided by the resistance, the power dissipated by parallel-connected resistors will be inversely proportional to resistance.

Procedure

Refer to Figure 4-1 on page 2. Use the nominal (color code) resistance values to calculate the parallel resistance combinations required for Table 4-1.

Measure each resistor’s value and record them in Table 4-1. Note that the double slash marks in the table means parallel and are read “in parallel with,” e.g., R1 // R2 means “R one in parallel with R two”

Temporarily connect only R1 and R2 in parallel and measure the parallel resistance with an ohmmeter. You may want to use the breadboard to facilitate this measurement. Record this measurement in Table 4-1.

Now temporarily connect R3in parallel with R1 and R2. Measure the parallel resistance of thisthree-resistor combination and record that measurement in Table 4-1.Remove the temporary connections at this time.

Construct the circuit shown in Figure 4-2, including the LEDs.

If you stare at the circuit that you just built, and reflect for a moment about what you have learned about series and parallel circuits you should be able to fill in the first column of

Table 4-2. (You should see both parallel and series sectionswithin your circuit).

Please get the lab instructor’s initials before continuingInitials: ______

You will be using a 10V power supply for the circuit of Figure 4-2. Calculate the theoretical current flowing through each branch of the circuit (using Ohm’s Law) and the total current supplied to the circuit using IT = I1 + I2 + I3.. Record these currents in Table 4-3.

Apply 10V to the circuit of Figure 4-2 and measure the voltage across a) the power supply, b) each resistor and LEDcombination and c) the voltages across each resistor and LED separately. Record these measured values in Table 4-2.

Turn off the power supply and open the parallel branch that contains R1 and the red LED. I would suggest removing the leg of R1 that connects to the LED and move it to an adjacent (but vacant) hole in the bread board.

Reenergize the circuit and configure the DMM to measure current.

Measure the current through the R1 branch and record this value in Table 4-3. Verify that the measured value is close to the calculated value.

Turn off the power supply and reconnect the circuit.

Repeat steps 9 to 12 for the other two branches.

Make certain that the circuit is once again properly and completely connected.

Measure the current being supplied to the circuit and record this value in Table 4-3.

With the ammeter still in place to measure the supply current, remove R1 from the circuit. Record the measured current value in Table 4-4.

With the ammeter still in place, put R1back into the circuit and then remove R2. Record the measured current value in Table 4-4.

With the ammeter still in place, put R2back into the circuit and remove R3. Record the measured current value in Table 4-4.

/ R1 / R2 / R3 / *R1 // R2 / *R1 // R2 // R3
*Calculated Resistance from Figure 4-1
(Theoretical for individual R1, R2, & R3)
Measured Resistance from Figure 4-1
/ Theoretical
Voltage / Measured
Voltage
Supply Voltage
Voltage across R1 (330Ω)
Voltage across R2 (470Ω)
Voltage across R3 (1kΩ)
Voltage across red LED
Voltage across yellow LED
Voltage across green LED
Voltage across combination of R1 & red LED
Voltage across combination of R2 & yellow LED
Voltage across combination of R3 & green LED
Supply / Through R1 / Through R2 / Through R3
Calculated Current
Measured Current
R1 removed / R2 removed / R3 removed
Measured Supply Current

Follow-up Questions

It is said that “the total resistance in a parallel circuit is lower than the lowest resistance value in circuit.” Is that true for this circuit? Use your measured data to prove or refute this statement.

As more branches are added to a parallel circuit, will the total resistance increase or decrease? Support your answer with a reasoned explanation and/or a simple example.

If any single resistance in a parallel circuit were increased, what would be the effect on the total resistance of the circuit? What about the current supplied by the power source? Support your answers with reasoned explanation/s and/or simple example/s.

If R3 burned up (opened) how would the current through the other branches be affected? See Table 4-4.

If something caused R3 to “short circuit,” what would be the most likely observed effect on the other branches? The circuit as a whole?

Refer to Table 4-2. Comment on the relationship between the voltages measured across the red LEDand the 330Ω resistor separately, and the voltage measured across the combination of the two.