NORTHERN ILLINOIS UNIVERSITY PHYSICS DEPARTMENT

Physics 211 – E&M and Quantum Physics

Fall 2016

Lab #4: Electronic Circuits II Lab Writeup Due: Mon/Wed/Fri, Oct. 10/12/14, 2016

Background The concepts of current flow, voltage and resistance in actual electrical circuits are confusing when first introduced to students. There are several misconceptions about electricity that may be the source of student’s general confusion about how an electrical circuit actually works. The purpose of this lab is to explore the concepts of series and parallel electrical circuits by applying Ohm’s law, resistor addition rules and current junction rules to circuit diagrams and then to actual circuits

1. Overview Current is defined as the rate at which charged particles move through a material, copper wire as an example. Voltage is defined as the electromotive “force” that pushes the charged particles through the conducting material. Resistance is defined as the conducting material’s ability to slow down or limit the flow of current. Comparing an electrical circuit to a pressurized water system helps relate these concepts to something we already understand. Think about the water system in a typical home. The water supply that comes into the house has a certain pressure (created by pumps or a water tower) so that when you open a faucet you get a nice stream of water. This water pressure is comparable to the voltage supplied by a battery or power supply to a circuit. If all the water outlets in a house are closed there is no water flowing through the pipes, but the pipes are still full of water due to the pressure. Again, this is comparable to an electrical circuit that is full of electrons, which are ready to light a lamp as soon as the light switch is turned on. Finally, the water system in a house is made up of different sized pipes that offer different “resistances” to the flow of the water, e.g. a small pipe restricts the water flow more than a large pipe.

Let’s start with a simple series circuit, which means all the components of the circuit are connected in a single loop, or “end-to-end”. The required components are a voltage source (battery or power supply); a resistor, something that the will carry the generated current, and a switch to open and close the circuit. A small light bulb will be used for the resistor because it gives a visual indication that current is flowing in the circuit. Ohm’s Law relates voltage, resistance and current in a circuit at a constant temperature:

V

IR

The resistance of a specific component in a circuit is determined primarily by the material it is made of and temperature. An extreme example is an incandescent light bulb, which radiates light due to high temperatures of the tungsten filament in the bulb. The increase in temperature of the tungsten filament can increase the resistance of the bulb by a factor of 1015 compared to its “cold” resistance value. Ohm’s law implies that if we have a constant voltage, which is usually the case in direct current circuits, then a higher resistance will result in a lower current and vice versa. In a series circuit, current will be constant throughout the circuit, and the voltage will be different through each component but sum to the total voltage of the power source. The total resistance in a series circuit will be the numerical sum of the resistances of all components. In a parallel circuit Kirchhoff’s Rules tell us that current branches at junctions in parallel circuits just like water flows when pipes split at a junction:

I1

I2

I3

Parallel circuits contain these junctions or branches, where current is split, but the voltage is constant. The equivalent resistance of resistance in the parallel branches is calculated as follows:

1 REq

1 R1

1 R2

1 ... R3

2

2. Procedure During the experimental portion of this lab you will construct combinations of parallel and series circuits using both simulation software and wires & components. The TAs will explain how to use the voltmeters and ammeters to measure the voltage and current in the circuits. Apparatus includes: . Power supply (black) . 3 Bulbs . Voltmeter . Ammeter . Rheostat (variable resistor) . Fixed power resistor . Wires, alligator clips . Simulation software on lab computer

A. Activity #1: Set the Rheostat to 5 Ohms Set the DMM (Digital Multimeter) to measure resistance and attach it the Rheostat. Adjust the rheostat knob until the DMM reads 5 ohms. NOTE: DO NOT MOVE THE RHEOSTAT KNOB, SINCE YOU WILL USE IT AS A “CONSTANT” RESISTANCE DEVICE FOR THE REST OF THE EXPERIMENT

For each circuit below, 2 members of the group will use the simulation software to “construct” the circuit and the other 2 will physically construct it using the supplied wires & components. These “sub-teams” should compare their circuits in 2 ways: 

Visually, as far as connections, etc.



Measurements of voltage, current as stated in the sections below

Note: Switch “sub-teams” for each circuit below. 3

B. Activity #2: Control Circuit B2 Build a circuit using the lab circuit components with the rheostat in series with the power supply, ammeter and a fixed “power” resistor (your TA will provide). Make sure the power supply is OFF.

The circuits you built in Electronic Circuits I lab all used light bulbs, which provided visual evidence of the different current levels flowing in the circuit. However, you noticed that the current you measured in the light bulb circuits was significantly different than the current levels you recorded from the circuit simulation software. You were given some hints as to what could be the reason for these differences. This circuit will provide you with current data using a fixed resistor instead of a light bulb, and you will compare graphs of voltage vs. current for the series circuit using 1 light bulb and this “control” series circuit using a fixed resistor in place of the light bulb.

Make sure the power supply is turned off. Build the circuit, leaving the switch open and have your TA check it prior to closing the switch. 1. Set the power supply to 3 volts.

2. Measure the total current of the circuit _______________A. 3. Use Ohm’s law to calculate the total resistance of the circuit:

R  ______________ ohms (note that this resistance is due to the fixed resistor and the rheostat (set to 5 ohms) in the circuit with some small contribution from the wires & power supply).

Repeat steps 1-3 for voltages of 5, 7, 9, 11 volts and record on your data sheets.

4

C. Activity #3: Circuit C1—2 Bulbs in a Parallel Branch, in Series with a Rheostat and an Ammeter

A. Simulation Build the circuit, set the value for the battery (3 volts) and two bulbs in a parallel branch. (a) Set the battery voltage to 3 volts, the resistor to 5 ohms and the bulb resistances to 40 ohms (b) Measure the current (I) in the circuit ______________A. (c) Use Ohm’ law to calculate the total resistance ________________ohms.

Repeat steps 4-6 for voltages of 5, 7, 9, 11, 13 volts and record on your data sheets.

B. Lab Component Circuit Make sure the power supply is turned off. Build the circuit, leaving the switch open and have your TA check it prior to closing the switch. (a) Set the power supply to 3 volts and use the DMM to make sure the rheostat is still set to 5 ohms. (b) Measure the total current of the circuit _______________A. (c) Use Ohm’s law to calculate the resistance of the circuit. _______________ohms (note that this resistance is due to the bulb and the resistor in the circuit with some small contribution from the wires & power supply).

Repeat steps 7-9 for voltages of 5, 7, 9, 11, 13 volts and record on your data sheets. 5

D. Activity #4: Circuit D—2 Bulbs in a Parallel Branch, in Series with a 3rd bulb and an Ammeter

Only do the lab components circuit version. This circuit does NOT include the rheostat, so you can remove it. You will need a voltmeter in addition to the ammeter—have your TA show you how to use it. In this circuit we want to look at how Ohm’s law applies in a combination of parallel and series components of a circuit (a) Set the power supply to 7 volts. (b) Measure the voltage drop across each light bulb:

(parallel 1)_______V

(parallel 2) ________V

(series 1)_________V.

(c) Measure the total current in the circuit _____________ A. (d) What is the relative brightness of the 3 bulbs in circuit D compared to the bulbs in circuit B and C (1 = brightest, 3 = dimmest)

(parallel 1)______________

(parallel 2)______________

(series 1)_______________

(e) Understanding that the relative brightness of a bulb is proportional to the current flowing through the bulb, use the voltages you measured in each branch and rank the relative resistance of each bulb ( 1 = highest resistance, 3 = lowest) (parallel 1)______________ (parallel 2)_______________

(series 1)______________ 6

(f) Apply Ohm’s law INDIVIDUALLY to each of the parallel bulb branches and the bulb in series. Based on your voltage and total current measurements, make an educated prediction of the current and resistance of each bulb.

(parallel 1) I = _______A

R = __________ ohms

(parallel 2) I = _______A

R = __________ ohms

(series 1)

R = __________ ohms



I = _______A

Your data section should include a circuit diagram of each circuit and the data for both the lab components circuit and the simulation data (except Circuits B2 & D, which are only lab components circuit). 

Your Results section should contain graphs of voltage vs. total circuit current using your data for each circuit and sample calculations using the appropriate formulas. Comments of what the graphs show and your answers to the questions in the procedure. Note: It is more common to plot current vs. voltage since current is usually dependent on voltage, but in this case we want to see the resistance as the slope of the voltage vs. current graph. Specifically, o Circuit C1—graph of voltage vs. current for all voltages, one plot for simulation & one for lab components circuit on the same axes, with line fits (include line fit equations). o Circuit D – measured and calculated values. o Circuit B2 & A1(from Elec Circuits Lab 1)—graph of voltage vs. current (lab components circuit only) for all voltages of both sets of data on the same graph.

7

DATA TABLES

CIRCUIT A1 (**FROM ELEC CIRCUITS 1 LAB)—Series circuit: 1 bulb, measured rheostat resistance (should be close to 5 ohms)

LAB COMPONENTS CIRCUIT Power supply - 3 volts 5 volts 7 volts

9 volts

11 volts

13 volts

Measured Current (A) Calculated Total resistance: R V / I

CONTROL CIRCUIT B2—with the rheostat in series with the power supply, ammeter and a fixed “power” resistor Power supply -

3 volts

5 volts

7 volts

9 volts

11 volts

13 volts

Measured Current (A) Calculated Total resistance: R V / I

CIRCUIT C1—2 bulbs in a parallel branch, in series with a rheostat (set to 5 ohms) and an ammeter SIMULATION CIRCUIT Power supply - 3 volts 5 volts

7 volts

9 volts

11 volts

13 volts

11 volts

13 volts

Measured Current (A) Calculated Total resistance: R V / I

LAB COMPONENTS CIRCUIT Power supply - 3 volts 5 volts 7 volts

9 volts

Measured Current (A) Calculated Total resistance: R V / I 8

CIRCUIT D—2 bulbs in a parallel branch, in series with another bulb at a power supply setting of 7 volts Voltage drop across each light bulb:

(parallel 1)_______V

(parallel 2)________V (series 1)_________V.

Total current in circuit ____________A

Calculated total resistance in the circuit ___________ohms

TA SIGNATURE _______________________ 3. Questions

1.

How did the actual intensities of the bulbs you found when doing the experiment compare to what you thought the relative intensities would be (as you described them in your prelab write up)?

2.

Compare the graphs of voltage (total from power supply) vs. current (total in circuit) for the simulation vs. lab components circuit in circuit C1. Comment on the differences in these graphs.

3.

Compare the plots of voltage vs. current (lab components circuit data) for circuit B2 and A1 (graph them on the same graph). Explain why the plots are different using the concepts of Ohm’s law, specifically the resistance values at increasing voltages. Look up the approximate operating temperature for a tungsten filament incandescent light bulb and include in your lab report.

4.

For circuit D explain the relative brightness of the bulbs in the parallel branches vs. the single series bulb at a constant voltage.

5.

In circuit D what did you learn about how Ohm’s law applies to a circuit with both parallel and series components? Comment on how Ohm’s law can be applied individually to each circuit component based on Kirchhoff’s rules. Calculate the total resistance of this circuit using the appropriate resistance addition rules, then calculate the resistance of each of the 3 bulbs. Explain in your own words why the voltage, current and resistance values in each bulb are what they are. 9