Industrial Electricity

Transcription

1 Industrial Electricity Name DUE //7 or //7 (Your next lab day) Prelab: efer 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. Branch currents E T I I I I T By virtue of this fact, each individual device draws current 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 I T I I I Using Ohm s Law we can substitute I E / for each of the branch currents giving, I T E E E E 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 T flowing through it. 07 David S. Mack. All rights reserved. T to develop an expression for the total resistance of the circuit, T 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

2 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 () Power Supply () DMM; Digital Multimeter () Breadboard Components esistors: each 0Ω, 470Ω, kω () LEDs, red, green, yellow (~0) 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. Figure 4- Parallel Circuit 07 David S. Mack. All rights reserved.

3 Procedure. efer to Figure 4- on page. Use the nominal (color code) resistance values to calculate the parallel resistance combinations required for Table 4-.. Measure each resistor s value and record them in Table 4-. Note that the double slash marks in the table means parallel and are read in parallel with, e.g., // means one in parallel with two. Temporarily connect only and in parallel and measure the parallel resistance with an ohmmeter. You may want to use the breadboard to facilitate this measurement. ecord this measurement in Table 4-. NOTE: The circuit (shown below) is a little tricky in that it involves series too. Keep that in mind when fill in the table. P o w e r = = 0Ω 0Ω = 470Ω = 470Ω = = KΩ kω ed LED Yellow LED Green LED Figure 4-4. Now temporarily connect in parallel with and. Measure the parallel resistance of this three-resistor combination and record that measurement in Table 4-. emove the temporary connections at this time. 5. Construct the circuit shown in Figure 4-, including the LEDs. 6. 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-. (You should see both parallel and series sections within your circuit). Please get the lab instructor s initials before continuing Initials: 7. You will be using a 0V power supply for the circuit of Figure 4-. Calculate the theoretical current flowing through each branch of the circuit (using Ohm s Law) and the total current supplied to the circuit using I T = I + I + I.. ecord these currents in Table David S. Mack. All rights reserved.

4 8. Apply 0V to the circuit of Figure 4- and measure the voltage across a) the power supply, b) each resistor and LED combination and c) the voltages across each resistor and LED separately. ecord these measured values in Table Turn off the power supply and open the parallel branch that contains and the red LED. I would suggest removing the leg of that connects to the LED and move it to an adjacent (but vacant) hole in the bread board. 0. eenergize the circuit and configure the DMM to measure current.. Measure the current through the branch and record this value in Table 4-. Verify that the measured value is close to the calculated value.. Turn off the power supply and reconnect the circuit.. epeat steps 9 to for the other two branches. 4. Make certain that the circuit is once again properly and completely connected. 5. Measure the current being supplied to the circuit and record this value in Table With the ammeter still in place to measure the supply current, remove from the circuit. ecord the measured current value in Table With the ammeter still in place, put back into the circuit and then remove. ecord the measured current value in Table With the ammeter still in place, put back into the circuit and remove. ecord the measured current value in Table David S. Mack. All rights reserved. 4

5 Table 4-: Parallel esistance Measurements *Calculated esistance from Figure 4- (Theoretical for individual,, & ) Measured esistance from Figure 4- * // * // // Table 4-: Parallel DC Voltage Measurements Supply Voltage Theoretical Voltage Measured Voltage Voltage across (0Ω) Voltage across (470Ω) Voltage across (kω) Voltage across red LED Voltage across yellow LED Voltage across green LED Voltage across combination of & red LED Voltage across combination of & yellow LED Voltage across combination of & green LED Table 4-: Parallel DC Current Measurements Calculated Current Measured Current Supply Through Through Through Table 4-4: Changing Parallel esistance Values Measured Supply Current removed removed removed Follow-up Questions 07 David S. Mack. All rights reserved. 5

6 . 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. 4. If burned up (opened) how would the current through the other branches be affected? See Table If something caused to short circuit, what would be the most likely observed effect on the other branches? The circuit as a whole? 6. efer to Table 4-. Comment on the relationship between the voltages measured across the red LED and the 0Ω resistor separately, and the voltage measured across the combination of the two. 07 David S. Mack. All rights reserved. 6