To this point the PHYS141 and PHYS142 labs have focused on how to do experimental physics. Understanding how to make measurements, determine how well something has been measured, drawing appropriate conclusions from data, etc. Each of these labs has been built around doing an experiment.

This two part lab will be different. The purpose of this lab is to teach you some fundamental skills which will be necessary for future experiments that you will perform. In particular we will be focusing on the following:

• Reading basic circuit diagrams.
• Designing and constructing simple circuits.
• Making measurements of voltages and currents.
• Understanding how the act of measuring electrical quantities can alter those quantities.
• Using function generators and scopes.

These are foundational skills in the sense that so many experiments involve the use of detectors which produce an electrical signal which must be processed and displayed. You will not get far in understanding, setting up, debugging and using electronic apparatus if you are not comfortable using a DMM to make voltage and current measurements, or using a scope to look at electrical signals which vary in time.

This lab will mostly be a series of exercises where you will be tasked with figuring out how to use basic electronic components, configure and connect them properly, and then make measurements to confirm things are behaving as expected.

This lab is also different in the sense that developing these skills requires everyone in the group to work hands on with the apparatus. You will still collaborate as a group on how to do what needs to be done, but each member of the group will for example build and test their own circuits, or find and measure a signal on their own scope.

The apparatus you will use for the DC measurements are described below.

For the purposes of creating and reading circuit diagrams we will use a commonly accepted set of symbols to represent circuit elements. When you design your circuits as part of the exercises, use these symbols to draw them.

DC Power

Resistor

LED

Lamp

DMM used as a Voltmeter

DMM used as an Ammeter

There is no need to draw the circuits on the computer, that tends to take more time than it is worth. Just use paper and pencil and get used to making use of these symbols to sketch out organized, neat and readable circuit ideas. Below is an example of a perfectly acceptable circuit diagram I made of a resistor in series with a lamp and a DMM setup to measure the voltage drop across the resistor.

One potential stumbling block is the fact that the DMM's have a built in fuse protecting the current input to the ammeter functionality. There are several ways that this fuse can be blown, trying to measure too large of a current for example. Unfortunately it is not obvious when the fuse is blown because the DMM still turns on and still works in the voltage and resistance measurement modes. If you suspect that the fuse in your meter is blown you can easily use a second DMM to check the fuse on the suspect DMM.

For each of the circuits specified below:

• Work as a group to design the appropriate circuit. Each person in the group should draw their own copy of the circuit for inclusion in their out of lab summary.
• Measure the resistances of the resistors you are using with the DMM. Note that they are not all the same resistance.
• Calculate the voltage across and current through each element for a +5v output from the power supply.
• Each member of the group then builds their own copy of the circuit. You can and should work collaboratively on this. Help your lab partners to get their circuit correctly wired, but do NOT do the work for them.
• Once each member of the group has connected their circuit you can connect them to the power supply and use the DMM to measure the voltages and currents which should agree with the values which you calculated above.

Group Exercise

As a group exercise build a series circuit using two resistors whose values are approximately 10MΩs. Measure the voltage drops across each resistor individually as well as across both. You should see something puzzling about the results of the measurements, think about what is different about measuring voltage drops across large resistances.

Ohm's Law predicts a linear relationship between Voltage and Current.

$ V = IR $

Components which obey Ohm's Law are referred to as being Ohmic. However not all electrical components used in circuits have linear relationships between Voltage and Current. Such components are referred to as being non-Ohmic. For your final exercise your TA will provide you with a light bulb. One person in the group should select the resistor and another person the light bulb. The third person can enter data to the co-lab notebook.

Specifically make sure that you have done the following:

• Disconnect all components and jumper wired from your proto-board, with the exception of the two wires leading from the red and black banana jacks to the top two rows of the board.
• Test the DMM's to make sure their fuses are good. Replace any blown fuses you find.
• Turn the DMM's off.
• Put all of your resistors back in their envelope.
• Neatly organize your banana cables.
• Set the power supply to 5V and turn it off.

Not doing the above will result in an unnecessarily difficult lab experience for your fellow students.

There is not really anything to summarize for this lab which is intended as a skill building exercise. The intent is to make sure that you know how to work with voltages and currents, how to build simple circuits, how to use power supplies, and how to use DMM's to measure voltages and currents. So this Post Lab Assigment is to simply demonstrate to the TA that you were able to use Ohms law to design simple series and parallel circuits, and predict the resulting voltages and currents. Also that you were able to correctly make the connections to build the circuits and properly use the DMMs to make the required measurements.

So for this weeks Post Lab Assignment you need to: