Lenz' Law

Hands-On Lab 9

Using the Laws of Induction

Reference: RHK 36-1/4 and handout on motors

Record data and answers in your lab journal. Make LIBERAL use of LARGE diagrams.

Part A. Induced Current in a Coil

Equipment: Multimeter, coil of wire, cylindrical magnet, compass

The initial configuration of a cylindrical magnet and the coil of wire is shown. "R" denotes the side of the spool that is colored red. Sketch the magnetic field of the cylindrical magnet. Be sure to include arrows on the field lines.

As the N pole of the magnet is pushed into the coil, what do you predict will be the induced magnetic field of the coil? Sketch this field. Indicate which end of the coil will be the N pole. Explain your choice.
Based on the induced magnetic field that you sketched, predict the direction of induced current in the coil. Draw a diagram showing whether the current goes into or out of the page at both the top and bottom of the coil. Explain how you determined the current direction.
Suppose you connected a multimeter to read the induced current. If you connected the coil’s center lead to the (+) terminal of the meter and the outer lead to the (-) terminal, would the meter reading be positive or negative? Explain.
Now check your predictions experimentally. Use the multimeter on the 300 ma scale. Be sure that the poles of your bar magnet are correctly marked. Check them with a compass but first be sure you know which end of the compass needle is an N pole. (How can you find out? Don’t trust any markings!)
Was your prediction of the direction of the induced current correct? If not, find out what you did wrong. Describe your mistake.
Now repeat 2-7 for the case that bar magnet is initially inside the coil and is pulled out, the S pole coming out last from the red end of the coil.

Part B. Genecons

Equipment: oscilloscope, light bulb, D battery, selection of resistors, Genecon

Set up your oscilloscope as follows:

Both red-top knobs turned to CAL'D position.

Knob at far upper right pushed in.

DC/AC on DC (left side)

Volts/Div on 2 (left side)

Time/Div on 2 ms (right side)

Push GND (left side under AC/DC) in. This grounds the vertical deflection plates so that the electron beam scans across the screen at V=0. This will be your time axis. Use the vertical control (far upper left) to move the time axis so that it splits the screen horizontally. Once this is done, push GND again so that the button comes out.

What do the vertical and horizontal scales of the oscilloscope measure?
Connect the battery pack to the Genecon leads. What happens? What type of device is a Genecon? Since it operates on direct current, what does this tell you about the internal construction of the Genecon?
Connect the leads of the Genecon to the light bulb and turn. What happens? What kind of device is the Genecon?
Does the Genecon produce direct or alternating current? Can you tell by looking at the light bulb? Try connecting the Genecon leads to the oscilloscope. Turn the handle at a rate of about 1 rotation per second. Sketch the entire screen, depicting accurately the number of peaks and placing numerical labels, with units, on the scales.
Your oscillosope experiment showed you that the Genecon produces direct current. (While the current isn’t constant, it never changes direction.) How might it possible for a generator to do this? Consider the internal construction.
Note that while turning at a steady rate, the current never reaches zero. How is this possible? Doesn’t the induced current have to be zero when the coil is perpendicular to the source magnetic field?
While turning at a steady rate of about 1 turn per second, measure the period of the voltage (2 peaks). If you have trouble getting a steady pattern, try a time scale of 5 or 10 ms/div. Calculate the frequency of the voltage in units of s^-1. How does this compare to the frequency of turning? Examine the inner workings of the Genecon as best you can and then try to explain why the voltage frequency is so much greater than the rotation frequency of the crank.
Connect the leads of the Genecon to a 10 W resistor. Turn very slowly and then increase gradually. Avoid turning extremely fast, as this may strip the gears. How does the mechanical opposition to turning change? Touch the resistor. What do you feel? What can you conclude?
Repeat the previous test using electrical resistances of 0 and infinite ohms. Describe your results.
Explain your observations in 8 and 9 using Faraday’s and Lenz’ Laws.