PHY 2049L
Lab 3

Circuits used in the Lab

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Updated: 1/1/2009
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See the main lab page for a picture showing the resistor box. The blue numbers on the circuit diagram refer to numbers by each of the banana-plug connectors on the resistor box. The notation 'K+' and 'K-' refers to the positive (red) and common (blue or black) outputs on the Kenwood power supply, while 'C+' and 'C-' refer to the outputs from the Cenco power supply.

The 'A' in a square box shows where you will insert the ammeter to make measurements of the three currents. Only one of these is used at a time. The number next to each location shows which current it is measuring. Do you see why?

Note that the ammeter is used in series. You will just unplug a cable from the circuit and connect it to the input of the ammeter then use a second cable from the ammeter to connect to where you removed the cable. Look closely at the pictures below and compare to the diagrams.

You should solve these circuits on a work sheet before coming to lab. The average values of the resistances given here are the same as what you will use to more than 2 sig figs, but your final calculations should use the R values for the box you were given. This work is not wasted because (a) two sig figs is enough to tell you if your measurements are reasonable and (b) it is extremely easy to repeat one of these problems with slightly different numbers if you have good notes on the steps to follow.


 

Circuit Zero

This circuit hooks up only one of the two loops needed for part 1. Since the result is a case where it is easy to calculate the magnitude and direction of the current, you can use this to check if you know where and how to hook up the ammeter to measure a current with the same sign (and magnitude) as what you have calculated.

half of the circuit for part 1

You should solve this circuit before lab; otherwise you will need to solve it (or one with the actual resistances in your box) during the lab to be sure you are on the right track. Note that the junction rule gives you the obvious result that I1 = I2. The only reason I give them different labels in this diagram is so that they match up with the other diagrams below.

Current measurement for circuit zero:
The photo below shows circuit zero set up to measure I1. Notice how the C- to K- connection was made with the black wire? Look closely at how the ammeter was inserted between C+ and 5 (position "A1" in the circuit diagram) to measure a current that is positive if current flows from C+ to 5 and through the resistor to 6. How was this done? See below:

current measurement for circuit zero

This was done by unplugging the short red wire from 5, connecting it to the red input on the meter, and then using the black lead from the meter to connect to 5 and complete the original circuit. You will see that this gives a positive reading as expected for this circuit and you should find that you have correctly predicted its value.

Important: The meter displays a positive value for the current when the current is flowing into the red connection (HI) and out of the black connection (LO) on the multimeter. (See the photo on another page showing a closeup of the meter.)

Current flowing from 6 to 4 is measured in a similar way. Disconnect the plug from 4 and then hook it up so current would flow from 6 to the red input and then from the black input to 4 if the current is positive. You should get the same sign and magnitude here.

Beware: Some of the Cenco power supplies do not regulate the voltage to the same value at zero current (while being set) and when in use. Students who "save time" by not bothering to calculate the expected current and not listening when I warn them to do this check have ended up (a) finding out they have junk data a week later or (b) taking twice as long to do the lab when they discover this after they are done and then try the calculations. If you find that you are not getting the right magnitude for the currents in Circuit Zero, the fix is trivial: You take the ammeter out of the circuit, switch it to voltmeter mode, and reset the power supplies to the proper voltage while there is current flowing in the circuit.


 

Circuit One

This circuit is identical to the one in Figure 34.5 in the lab manual except for the different resistances you are given.

circuit for part 1

Connecting a parallel loop:
This photo shows how to add the second loop to make circuit one. The part I call circuit zero has been wired up with red cables (except for the C- to K- connection) just as it was in the photo above before making the current measurement. The second loop has been added by using two black cables so they stand out clearly in the picture. You can see the new connections from 6 to 2 and from 1 to K- that are called for by the circuit diagram.

connections for circuit one

Note: You will have a variety of cables to work with, and you can use the two colors any way you wish to make it easy to keep track of the connections you will have to work with to make the measurements in the lab.

I do not show the current measurements being made. The measurement of I1 at point 1 is made the same way as shown above for circuit zero, hooked up so it will read a positive value for current flowing from C+ to 5. The others are done the same way, being careful to hook up your ammeter so you get a positive reading when current flows from 6 to 4 (point 2) or from 6 to 2 (point 3).


 

Circuit Two

This circuit is significantly different from the one given in Figure 34.6 in the lab manual. We only have two loops rather than three; the only change from part 1 is to replace the resistor in the second loop with a much smaller one.

circuit for part 2

The current measurements are made the same way as in circuit 1. Be careful to hook up your ammeter correctly so you know if the sign of the current is positive or negative.

 
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