Part 1: Voltage, Current and Resistance

Electromagnetism

ELECTROMAGNETISM

OBJECTIVE: To investigate the concepts of Voltage and Current, see how they are related, and see how they can be generated.

Part 1: Voltage, Current and Resistance

THEORY:

Current is defined as the flow of electricity (flow of charges). It is measured in amps (A). Voltage is defined as the energy per charge at a particular position and is measured in volts (V). Current tends to flow from a place of high voltage to a place of low voltage - just like water tends to flow from a place of higher elevation to a place of lower elevation. (Hence, elevation in water flow is analogous to voltage in current flow.) Resistance is simply how much the material resists the flow of charges and is measured in ohms (). (In our water analogy, water flowing through large pipes encounters very little resistance, while water flowing through a sponge or through sand encounters a higher resistance.) A voltage source (such as a battery or a power supply) is like a water pump: a water pump can move water from a lower place to a higher place; the voltage source moves the charge from a place of low voltage (negative terminal) to a place of high voltage (positive terminal). A Circuit is simply a path for the charges to flow from the high voltage position to the low voltage position, just like pipes or rivers can form a "circuit" for water flow.

METHOD:

First we will see how current and voltage are related. This means that we will have to keep the resistance the same and see how the current responds when we change the voltage. Second, we will keep the voltage the same and vary the resistance (the circuit) and see how current and resistance are related.

NOTE:

In using D.C. instruments, care must be taken to connect them up with the proper polarity. The terminal marked (+) should always be connected to the positive terminal of the power supply and the terminal marked () to the negative terminal. They need not be connected directly, but you should be able to trace back to the proper terminal. On many instruments only one terminal is marked, and it is understood that the other is the opposite polarity. On meters with more than one scale, the number on the terminal refers to the MAXIMUM value that can be measured on that scale.

CAUTION: DO NOT PLUG IN THE POWER SUPPLY UNTIL YOUR COMPLETE CIRCUIT HAS BEEN CHECKED BY THE INSTRUCTOR. THIS IS TO PROTECT THE INSTRUMENTS.

PROCEDURE:

Connect the apparatus as shown in Fig. 1, using the 10 ohm resistor on the wooden block for R, one DMM for the voltmeter, and the other DMM for the ammeter. Although the resistor is stamped 10, it may not be exactly 10 ohms when measured. Resistors are guaranteed accurate only within a certain percentage (10% in this case) which is usually indicated on the resistor in some way. The power supply (PS) has a control (left side) so that by rotating it the voltage difference across the resistor can be varied from zero to some maximum value. [Note: the right control should be set between ½ to full clockwise.]

Adjust the voltage control on the PS so that it increases from zero to five volts in intervals of 1 volt. Take readings of voltage and current at every 1 volt interval.

In your written report the data from step 2 will be plotted to see whether current and voltage are proportional. Can you estimate from your data right now if this is so? What is the constant of proportionality (that is, what do you have to multiply current by to get the voltage)? Is it similar to your value of 10 for the resistor? Is this constant the same for all voltages measured?

Replace the 10 resistor with one light bulb. Slowly increase the voltage from zero to 5 volts in one volt intervals again and note how both the current and the brightness of the light bulb vary. Can you estimate what the resistance of this light bulb is? Does the light bulb's resistance change with increasing voltage (and current)? Note that as the current increases, the wire in the light bulb gets brighter: it heats up and starts to glow. [In this case, the resistor (light bulb wire) gets very hot, and this shows that resistance increases with temperature.]

Replace the one light bulb with two light bulbs connected in series. (That is, connect the left end of light bulb #1 to the + voltage terminal, connect the right end of light bulb #1 to the left end of light bulb #2, and then the right end of light bulb #2 to the + terminal of the ammeter. In other words, make the current flow through both resistors - do not give it a choice.) Now set the voltage of the PS to 5 volts and note both the current and the brightness of both bulbs. Is the current from the PS flowing completely through light bulb #1, partially through #1 and partially through #2, completely through #2, or completely through both? What do you have to do (or is it even possible) to the voltage to make both light bulbs bright? What happens to the current when you do this?

Unhook the two light bulbs of step 5 and re-hook them in parallel. (That is, connect the left ends of light bulbs #1 and #2 together and to the + terminal of the PS, and connect the right ends of the light bulbs together and to the + terminal of the ammeter. In other words, give the current a choice of flowing either through light bulb #1 or light bulb #2.) Now set the voltage of the PS to 5 volts, measure the current, and note how bright each light bulb is. Is the current from the PS flowing completely through light bulb #1, completely through #2, split between #1 and #2, or completely through both?

REPORT: Answer all questions posed above in the Procedure. In addition:

Plot a graph of voltage (ordinate or vertical axis) and current (abscissa or horizontal axis) using the data gathered in step 2 of the procedure. Is it a straight line? What does this mean about how current and voltage are related? Compute the slope (V/A) of the best straight line through the points. According to our relations between units, 1 V/A = 1. Does your slope value approximately equal your resistance? Can you infer how voltage, current, and resistance are all related?

By placing more resistors in the circuit in a series fashion, does the overall (effective) resistance increase, decrease, or stay the same. Can you explain this?

By placing more resistors in the circuit in a parallel fashion, does the overall (effective) resistance increase, decrease, or stay the same. Can you explain this?

Power used in the circuit can be found by multiplying the current and the voltage together. Power is measured in Watts (W). (Note that 1 W = 1 AV.) For the same voltage, which circuit (parallel or series) takes more power? For the same voltage, which circuit (parallel or series) has the lower effective resistance? Which circuit generated the most light from the light bulbs? In your household wiring, do you connect appliances in series or parallel to the outlet?

Part 2: Generating Electricity

METHOD:

In this part we will see a very surprising result: we can generate electricity by moving a magnet in an electrical circuit.

PROCEDURE:

(In order to be sure that your observation of an effect was correct, you may repeat any step in this procedure as many times as you wish.)

1)Bar magnet at rest inside the coil.

a)Place the N pole of the bar magnet inside the coil.

b)Connect the coil to the voltmeter and note any reading that occurs as the last connection is made.

2)Bar magnet moved.

a)Move the N pole of the magnet out of the coil quickly (with a jerk). Did you get a reading while you were removing the magnet?

b)Now insert the N pole of the magnet into the coil with a quick movement. Did you get a reading while you were inserting the magnet? Note the sign +/- of the reading. Was it the same as in step a when you removed the N pole?

c)Next insert the S pole of the magnet (as in step b) and note whether you get a reading. Did the +/- sign change or stay the same for inserting the S pole compared to inserting the N pole?

3)Change the voltmeter into an ammeter and repeat steps 1 and 2 above.

4)Now switch the DMM from DC to AC (there is a button for this). Now move the magnet in and out very quickly and repeatedly. Can you generate much of a current? Would this current be enough to light the light bulb? Try it!

NOTE: By oscillating the magnetic field (via the magnet), a current was produced. It is also true that by oscillating a current, you can produce an oscillating magnetic field. This is the basis for understanding light (or radio waves or any other type of light) as an electromagnetic wave.

REPORT: In your report, answer the questions posed in the Procedure.