Ohm’s Law Lab

Objectives

·        Determine the mathematical relationship between current, potential difference, and resistance in a simple circuit.

·        Compare the potential vs. current behavior of a resistor to that of a light bulb.

  Warning:  Never ever hook up a multimeter which has its dial set to the resistance setting (W) and a LIVE circuit (one with a battery hooked up to it).  WHY?  Because this setting has your multimeter sending out a current from the 9.0 Volt battery from inside the multimeter to test the resistance of a resistor.  You will wreck the multimeter if you also send a current through the circuit from another source (such as another battery outside of the multimeter).  This setting is to test a resistor which has no current flowing through it.

Materials:

1.  200 in 1 kit with a total of 6 AA batteries

2.  Set of Secular Holiday light strings.  You should have one bulb, two bulb series, three bulb series, four bulb series…etc.  The more the merrier (get it?!)

3.  Multimeter

4.  Have GA fired up on the computer ready to go.

Pre-Lab Questions:  Answer these before class

1.  When you hook up 4 AA batteries in series, what is the voltage?

2.  What is the slope of a regression line of V/I going to tell you?

3.  True or False:  If we consider that the resistors in the 200 in 1 kit are perfectly OHMIC in nature, then we should see that when we increase the number of batteries in series that the current increases at a constant rate.

4.  What do you think will happen to the resistance in a light bulb as you attach more and more batteries in series?  Will the resistance:  a) stay constant regardless of the voltage     b)  increase with increasing voltage     c) decrease with increasing voltage?  (Hint:  Light bulb filaments are metal).

5.  What do you think will happen to the current in the light bulb as you add more batteries in series?  a) stay constant regardless of the voltage     b)  increase with increasing voltage     c) decrease with increasing voltage? 

6.  What do you think will happen if you hook up more and more light bulbs to the same voltage – light bulbs in series?  (In questions 4 and 5, we increased the voltage on ONE light bulb.  Now, I want you to increase the number of light bulbs – in series - on ONE voltage).  Would we find the resistance to be:  a) greater than expected     b)  less than expected     c)  the same as expected.

7.  What do you think will happen if you hook up more and more light bulbs to the same voltage – light bulbs in series?  (In questions 4 and 5, we increased the voltage on ONE light bulb.  Now, I want you to increase the number of light bulbs – in series - on ONE voltage).  Would we find the current to be:  a) greater than expected     b)  less than expected     c)  the same as expected.  

8.  What should you never-ever do with the multimeter?  (see above warning)

Pre-Lab Procedure:

     First, you are going to figure out how to hook up your battery sources so that you get the voltage you want across your resistor.  To do this, you are going to need your multimeter set on 20V and the red probe in the W/V plug of the multimeter.

     Next, put a wire from 57-58.  This will put all six batteries in series.

     Finally, put the black probe in 55 and the red probe in 56.  Do you get a reading close to 1.5V?

     Okay, this next part is tricky…   You are given a Pre-Lab data table here, and what I want you to fill out is which number springs you need to get which voltage.  For example, you have already discovered that 55 and 56 gives you 1.5V (or something relatively close to it).  So, I have filled in the first data table box for you (oh, how nice of me).  But, What I haven’t done is fill out the actual voltage given off by 55 and 56 because that might vary from group to group.  You need to fill that in (as with all of the other ones).  This is the voltage your multimeter told you was coming from 55/56.

Pre-Lab Data Table:

     This data table will help you as you do the procedures below.  When I ask for you to put 1.5V across a resistor, you can just simply look at this table and know which springs to use.  Please make sure that when you plot your actual graphs, you use the actual voltages you measured with your multimeter, not the ones I listed in the data tables (For example, I might ask for 1.5V, but your multimeter read 1.48V.  You should use 1.48V in all of your graphs and analysis).

Procedure:

1.  Hook up the batteries such that just 1 AA battery is going across a 100W resistor (in other words, 1.5V)

2.  Use your multimeter to measure the current (I) flowing through this resistor.  Record it in Data Table 1 below. (Go back to “Getting to Know Your Multimeter” if you don’t remember how to measure current through  a resistor)

3.  Increase the voltage to 3.0V.  Repeat steps 2-3.

4.  Increase the voltage to 4.5V.  Repeat Steps 2-3

5.  Continue this process until you reach 9.0V.  You should be recording the currents (I) you get in the Data Table 1 below.

6.  Now, do the same thing with the SINGLE secular holiday light bulb.  Start with 1.5V and increase it to 9.0V.  Remember to record V and I each time you increase it.  Record this in data table 2 below.  Since the light bulb has no resistance marked on it (unlike the resistors in the 200 in 1 kit) you will have to use your multimeter to determine its natural resistance.  There is a place for this information on the data table 2.

     Tip:  These light bulbs are not supposed to burn as brightly as they will when you start putting 7.5V and 9.0V across them.  Don’t leave them burning too long or they will burn out.

7.  Use GA to make 2 lines of Voltage vs. Current (I) on a single graph. 

TIPS FOR GA:

·        Label x:  “current” with units of Amps  (double click on the X in the X column if you don’t remember how to do this).  Note, it is important that you call x:  "current" just as I have written it (no "" marks, however)

·        Label y:  “Voltage 100W” with units of Volts.  Use the actual volts recorded by your multimeter, not the voltages listed on the batteries.

·        At DATA, you should NEW DATA SET for the light bulb.  Label x:  “current”.  Label y:  “voltage light bulb”.           Note, it is important that you call x:  "current" just as I have written it (no "" marks, however).  REASON:  The x-axis will only allow you to plot one item (such as current or time or whatever).  But the y-axis will allow you to plot many different things vs. x.  So, it is important whenever you want to plot more than one line on the graph that ALL of your x-axis data tables are labeled EXACTLY the same!

·        Plot them on the graph so that you have 2 lines on there simultaneously.  HOW DO YOU DO THAT?  Simple, click on the word “Voltage” on the y-axis of the graph itself.  There you can choose which values of y you wish to plot.  Just click on all of the voltages and they will plot.

8.  Once you have made your graph, you can answer data table 3’s questions.

9.  Now, use 9.0V and start putting light bulbs in series (you will NOT alter the voltage, just the number of light bulbs in series.)  First use one light bulb, then two, then three…etc.  Record your values for resistance and current in data table 4.  Realize that you will need to use your multimeter to determine the resistance across one, and then two and then three…etc light bulbs when they have NO CURRENT RUNNING THROUGH THEM.  My advice:  determine the resistances of one, two, three….etc light bulbs BEFORE you start putting 9.0V across them and record these resistances in the Data Table 4.

     Why are you doing this?…..  Think about it.  Those resistors on the 200 in 1 kit don’t change resistance that much when current flows through them.  Light bulb filaments, on the other hand, get very hot when a current flows and this will effect their resistance.

10.  In Data Table 5, you are going to determine the resistance PER light bulb.  How do you do this?  Well, as you know, R = V/I.  So, all you have to do is divide the actual voltage by the current AND then divide by the number of light bulbs in the series.

     For example:  Let’s say my multimeter reads 8.94V for the 9.0V series of batteries and I get a value of 0.20 amps for 3 light bulbs in series.  I would do as follows:

(8.94V/0.20amps)/3light bulbs = 14.9W/light bulb

In other words, each light bulb in that string of 3 bulbs has an effective resistance of 14.9W.(the total resistance of the series is 44.7W)  Record this in Data Table 5 next to the current registered from data table 5

11.  Plot a graph of Resistance vs. Current from the data in data table 5 and paste this graph below Data Table 5

DATA TABLE 1

DATA TABLE 2

What is the resistance of the light bulb? (Use your multimeter and hook it up to the light bulb)

        Resistance of Light bulb _________________W

DATA TABLE 3

*How do you find the slope between two specific points on GA?  Simple:  highlight the points you are interested in by dragging the mouse across them.  Then, click on the LINEAR FIT button at the top right side of the screen.  It will ask you which line you are interested in (highlighted in Blue highlight).  The one highlighted is the one it will give you the slope for. 

Data Table 4

Data Table 5

Post-Lab Questions:

Note:  Some of these are worded funny, I tried to help you by putting the main part of the question in blue type.

1.   As the voltage potential across the resistor increased, the current through the resistor increased. If the change in current is proportional to the voltage, the data should be in a straight line and it should go through zero. In these two examples how close is the y-intercept to zero? Is there a proportional relationship between voltage and current? If so, write the equation for each run in the form potential = constant current. (Use a numerical value for the constant.).  Your constant should be the slope of your line.

2.   Resistance, R, is defined using R = V/I where V is the potential across a resistor, and I is the current. R is measured in ohms ( ), where 1   = 1 V/A. The constant you determined in each equation should be similar to the resistance of each resistor. However, resistors are manufactured such that their actual value is within a tolerance. For most resistors used in this lab, the tolerance is 5% or 10%. Question:  Calculate the range of values for each resistor. Does the constant in each equation fit within the appropriate range of values for each resistor?  (In other words, the 100W Resistor could be anywhere from 90W-110W because that is +/- 10% of 100W)

3.   Do your resistors follow Ohm’s law – just the resistors, not the light bulb? Base your answer on your experimental data. A simple yes or no will do.

4.   Describe what happened to the current through the light bulb as the potential increased. Was the change linear? Since the slope of the linear regression line is a measure of resistance, describe what happened to the resistance as the voltage increased. Since the bulb gets brighter as it gets hotter, how does the resistance vary with temperature?

5.         Does your light bulb follow Ohm’s law? Base your answer on your experimental data. A simple yes or no will do.

6.  In the part of the lab where you made a series of light bulbs all at 9.0V, what did you notice about the resistance of the individual light bulbs as they had more current running through them?  Did the resistance increase, decrease or stay the same.  Look at data table 5.  Notice that in data table 5 you have divided the total resistance by the number of bulbs in series.

7.  Finish this statement:  As you add more light bulbs to a series of light bulbs (and keep the voltage the same), the CURRENT running through the series _________ (increases, decreases or stays the same)

8.  Finish this statement:  As you add more light bulbs to a series of light bulbs (and keep the voltage the same), the RESISTANCE of the individual bulbs in the series __________(increases, decreases or stays the same).  Individual means you have to divide the total resistance by the number of bulbs in series.  Look at data table 5

What is due for this lab?

1.  Pre-Lab Questions. In RED type.

2.  Pre-Lab data table in RED type.

3.  Data Tables 1-3  with your data in RED type.

4.  GA Graph should follow Data Table 3.  Remember, I should see two lines on the same graph.  Line #1 is the 100W resistor at varying voltages.  Line #2 is the light bulb at varying voltages.

5.  Data table 4 with your data in RED type.

6.  Data table 5 with your data in RED type

7.  The other GA Graph (the one with the series of light bulbs all lit with the same voltage) should follow data table 5.

8.  Answer the Post-Lab questions in RED type.

This is an IB lab.  Therefore, those students in IB will have an additional week to write it up.  Use my questions as a guide to you as you compose your lab report.  Realize that what IB is looking for is for you to show how you hypothesized what would happen to resistance as voltage/current increased in a regular resistor vs. a light bulb filament.

     The whole idea of an IB style lab is to illustrate how, through self-discovery, you came to the intended conclusions.  It is not important that your original hypothesis was correct.  If you honestly thought that there would be no difference in the slope of the lines of a regular resistor and that of a light bulb, then you should put that in your report.  The important thing is, in your conclusion, you were led to the idea that hot resistors DON’T obey Ohm’s law.  It is simply NOT important to IB that you got your original hypothesis correct and then proved it to yourself.  That actually looks a bit faked.  Think about it, how would you have known that resistors don’t obey Ohm’s law when they get hot?  You’d only know it if I told you so in class or you did this lab.