In this lab you will learn to use a multimeter to measure DC voltage differences, resistances, and currents. The associated five-page Guide to Building Circuits contains all necessary instructions. Read it and this page carefully before coming to lab. Please follow the guide carefully the first time you do each different measurement and then until you have mastered the technique. Parts C2, D5, and E4 need only be done once, but results are to be recorded in the lab book.

In both parts of this lab you will investigate the behavior of the electric current in a simple circuit. A simple circuit is one that contains only one resistor and one battery (or source of voltage difference) connected in a single loop (see the diagram in Part A of the Guide to Building Circuits, etc.). You will be given a selection of different battery voltages (if you are unsure how to combine single batteries to make larger voltages, ask the instructor or lab assistant) and resistors (use 150 < R < 15,000 ), a multimeter, and a breadboard and are asked to determine the relation between current, battery voltage, and resistance in the circuit. We suggest that you acquire at least 6 sets of data points for each of the two parts of the lab. Make sure that you measure all battery voltages and all resistances with the multimeter! Do not assume they have the values listed on them.

Your goal in this part is to measure the current as a function of resistance in a simple circuit and to discover the relationship between these two quantities. Design a method for doing this and write a brief but clear description of it in your lab book. Hint: is there a quantity that should remain constant during this part?

You might choose to measure all your resistances first, and then measure each of the resulting currents, in order to save switching the multimeter back and forth. Be especially careful about measuring the current properly.

Your goal in this part is to measure the current as a function of battery voltage in a simple circuit and to discover the algebraic relationship between these two quantities. Design a method for doing this and write a brief but clear description of it in your lab book.

1) look at your data in order to decide how to plot your data so that it can be fit by a straight

line; you may have to re-express your data in order to obtain a linear relationship.

2) graph the dependent variable (current, in AMPS in each case) on the vertical axis versus the

independent variable (use your TI calculator; you will need to print a copy to turn in), (graphs must

have titles, labels and units on axes, correct sig figs and units on fit parameters)

3) compare your graphical results (slope and intercept for each of the two cases) with

those expected. Read Giancoli [section 18(3)] for help on what to expect.

Summarize what you did, how you did it, and your results.

The "breadboard" consists of a block of white plastic with two sets of five rows of holes. Each set of rows looks like

. . . .

. . . .

. . . . etc.

. . . .

. . . .

where each dot represents a hole. These holes are receptacles in which the ends of a resistor or the ends of a battery holder can be placed to construct a circuit.

Each of the 5 holes in a vertical column is connected to the others in that same column by a(n invisible) wire inside the white plastic. Each of the 5 holes in a vertical column is therefore at the same voltage.

There are two distinct sets of these rows of 5-hole columns. The upper set is NOT connected to the lower set.

Suppose that we want to use the breadboard to construct the circuit

diagrammed below:

(Placing the opposite ends of a battery in holes of the same column is the same as directly connecting the two ends of the battery, because of the wire connecting all holes in a vertical column. This is called "shorting out" or "short-circuiting" the battery; it costs the battery power and reduces its useful lifetime.)

Now place one end of the resistor (labeled "C" above) in one of the holes in the same column as the positive end of the battery (the row doesn't matter); because the vertical wire connecting all holes in a column, point A is now electrically connected to point C, as the circuit above requires. Finally, place the remaining end of the resistor (labeled "D") into a hole in the same column (but any row) as that of the negative end of the battery.

The Fluke (brand name) multimeters are capable of measuring AC voltage differences, DC voltage differences, resistances, AC current, and DC current. The top section of the multimeter allows the user to choose the measuring function. The multimeter face looks like the diagram on the left; the corresponding function symbols are described on the right.

turn the dial to this position

in order to

OFF turn the multimeter off

V ~ measure AC voltage difference

_____

V - - - - - measure DC voltage difference

(greater than 0.3 volts)

_____

300 mV - - - - - measure DC voltage difference

(dial) (less than 0.3 volts)

measure resistance

--> + ))) test diodes

A ~ measure AC current

_____

A --------- measure DC current

The bottom portion of the multimeter contains the two plug-in leads (one is red, the other is black) that will be used to measure voltage, current, and resistance. The bottom of the multimeter face looks like the diagram below; the symbol "" has been used to represent a hole via which the leads are connected to the instrument.

10 A V

300 mA COM

Plug the black lead into the hole to the right of COM (which stands for "common" or ground; if you are measuring voltage differences, "ground" means the place of zero voltage, i.e., wherever you place the black lead is defined to be zero voltage; the multimeter then reads how much higher or lower the voltage is at the place where you put the red lead).

Where the red lead is plugged depends on what you are measuring. If you wish to measure voltage or resistance, plug the red lead into the hole to the right of the V symbol. If you wish to measure current, the red lead is plugged into either of the other two holes. Since we will be using large resistances (greater than 1000 ) and small voltages (less than 10 volts), the resulting currents will be less than 300 mA (milliamps); therefore, plug the red lead into the hole to the left of the 300 mA symbol when measuring current.

1) Choose a resistor that falls in the range given in the lab instructions.

2) Do this part only for the first resistor you use: Use the color code (Giancoli: page 464, 3rd

edition) to read the value of the resistance. Record the color code and the resistance value

in your lab book.

3) Now measure the resistance with the multimeter:

a) turn the multimeter dial to the appropriate setting (see Part B)

b) plug the red and black leads into the appropriate multimeter holes

c) touch one lead from the multimeter to one end (it doesn't matter which) of the

resistor, and touch the other lead of the multimeter to the other end of the resistor.

Schematically, what you are doing looks like

4) Record the value of the resistance measured with the multimeter in you lab book.

How does the measured value compare with that read from the color code? (This only needs

to be answered for the single resistor whose color code you read above.)

1) Construct the simple circuit shown below on the breadboard with one 1.5-volt battery

and the resistor whose resistance you measured above. Refer back to Part A,

necessary.

USE CARE WHEN PLUGGING THE WIRE ENDS OF THE BATTERY HOLDER IN THE BREADBOARD HOLES; THE WIRES ENDS ARE FRAGILE AND EASILY DAMAGED.

3) Suppose that we want to measure the voltage difference V_A - V_B (refer again to the figure in Part A).

Notice that since A is at the positive end of the battery and B is at the negative end, we expect a

measured voltage difference of about 1.5 volts. To actually make the measurement, touch the black

lead (the "ground") to point B (the black wire emerging from the negative end of the battery or any

piece of metal touching it) and touch the red lead to point A (the red wire emerging from the positive

end of the battery or any piece of metal touching it). Schematically, what you have done with the

multimeter looks like

5) Perform and record the following measurements in your lab book ONLY for the very first

circuit you build; you may assume the results apply to all the others.

a) V_C - V_A ; why is this voltage difference zero?

b) V_B - V_A ; why is this voltage difference negative?

The multimeter is used in a very different way to measure current. Unlike the method for measuring voltage difference (where the multimeter was placed in the circuit "in parallel"), the method for measuring current requires the multimeter to be placed in the circuit "in series". More about series and parallel later.

Suppose that we want to measure the current at point P in the circuit shown below, left.

To do so, the circuit must be "broken" (or opened up at P) as shown in the diagram above, right. This is best done by removing both the battery wire connected to A and the C end of the resistor from the breadboard.

1) Break the circuit at point P

2) Now connect the multimeter in series at point P as shown below; turn the multimeter

dial to the appropriate current setting. Did you remember to plug the red lead into the 300 mA port?

Do you see the difference between using this diagram and the second diagram on the previous page? If not, stop and ask for help.

3) Record the measured current in your data table.

4) Do this part only for the very first circuit that you build; assume others behave the same.

Is the current the same everywhere in a simple circuit like this? To find out, measure the

current at point Q, using the method above. Record the measured value of the current at Q

in your lab book. Is it the same as the current at point P?

Later we will learn that current can only change when there is a branch in the circuit. Since

this circuit has no branches, the current is the same everywhere in the circuit.