The Electricity and Magnetic Connection

Learning objective:

Map a magnetic field by using a magnetic compass.

Look for a relationship between an electric current and a magnetic field.

Something to think about:

The first compass was invented in 247 BC in ancient China

Investigation

Part 1

You will use a bar magnet to explore magnetic forces between magnets and magnetic materials. You will also use a compass to explore the shape of a magnetic field around a bar magnet.

Using two bar magnet with the north end labeled N and the south end labeled S.

a) Explore whether there are attractive forces or repulsive force when the magnets are placed near each other.

In what arrangements are the magnets repulsed?

In what arrangements are the magnets attracted?

b) Determine the strength of one of the bar magnets by measuring how many paper clips the magnet can lift

c) The needle of a compass is a small bar magnet that is placed on a point so that it can rotate easily. It can be used as a magnetic field detector. Any magnet or magnetic material will affect the compass. When the compass is not near any magnet or magnetic material it aligns itself with Earth’s magnetic field and indicates which direction is north.

Note the direction of the compass needle. Rotate the casing; did the compass needle change direction?

d) Set the magnetic compass on the table and bring another type of magnet, such as a bar magnet, into the area near the compass needle. But do not get them too close because a strong magnet can ruin a compass needle if it gets too close.

Describe your observations

What happens to the dependable north pointing property of the compass?

How dependable is the compass is the compass at pointing north when placed in a region where there are other magnetic effects, in addition to Earth’s magnetic field?

e) You will now make a map of the magnetic field of a rectangular bar magnet. Place the magnet on a piece of paper and trace its position. Label which end of the bar magnet is N pole and which is S pole. Keep the compass a centimeter or two away from the magnet to avoid ruining the compass. Place the compass at one location and mark the direction it points. Remove the compass.

Sketch a small arrow at the location from which you removed the compass to show the way it pointed. Pay attention to which end of the compass needle points to which pole of the magnet.

Place the compass at a location near the tip of the first arrow you sketched. Remove the compass and sketch another small arrow in the location to show the way the compass pointed.

Repeat the process at 10 or more locations to get a map of the magnetic field of a bar magnet. Part 2

Magnetic fields and electric currents.

Wrap a wire around the magnetic compass a few times to form a coil. Wrap the wire across the north-south marking. Hold the wire in place with tape. Use sandpaper to remove the insulation from a short section of the wire ends. Connect a DC hand generator, a light bulb, and compass/coil in a series circuit. Rest the compass/coil so the compass is horizontal, with the needle pointing north and free to rotate. Also, turn the compass with in, if necessary so that the compass needle is parallel to the turns of the wire. Crank the DC generator and observe the compass needle.

a) Record your observation

Reverse the direction of the current in the wire by reversing the direction you crank the DC generator

b) Describe the results

c) What evidence do you now have that electric currents can produce magnetic fields.

Notice that the compass does not need to be in contact with wire to experience a force due to the current in the wire. This illustrates the general phenomenon of “action at a distance”

Part 3

Go to http://phet.colorado.edu. Click on “play with sims” this will take you to the list of new sims. On the left side of the screen is a list of categories. Click on physics, then electricity, magnets and circuits. Then open and run magnets and electromagnets. The program opens up a with a bar magnet and a compass. Move the compass slowly along a semicircular path above the bar magnet until you’ve place the compass on the opposite side of the magnet.

a) Describe what happens to the compass needle. How do these results compare to the “real” experiment performed earlier?

b) How is the strength of the force on the compass needle indicated?

Move the compass along a semicircular path below the bar magnet until you’ve put it on the opposite side of the bar magnet.

c) Describe what happens to the compass needle. How do these results compare to the real experiment performed earlier?

d) How many complete rotations does the compass needle make when the compass is moved once around the bar magnet?

Click “flip polarity” and repeat the steps above after you’ve let the compass stabilize.

Click on the electromagnet tab. Place the compass on the left side of the coil so that the compass center lies along the axis of the coil. (The y- component of the magnetic field is zero along the axis of the coil.) Move the compass along a semicircular path above the coil until you’ve put it on the opposite side of the coil.

e) Describe what happens to the compass needle. How do the results compare to bar magnet in PHeT portion of the activity.

Move the compass along a semicircular path below the coil until you’ve put it on the opposite side of the coil.

f) Describe what happens to the compass needle. How do the results compare to bar magnet in PHeT portion of the activity.

g) How many complete rotations does the compass needle make when the compass is moved once around the coil?

Use the voltage slider to change the direction of the current and repeat the steps above for the coil after you’ve let the compass stabilize.

h) Based on your observations, summarize the similarities between the bar magnet and the coil.

i) What happens to the current in the coil when you set the voltage of the battery to zero?

j) What happens to the magnetic field around the coil when you set the voltage of the battery to zero? k) Play with the voltage slider and describe what happens to the current in the coil and the magnetic field around the coil.

l) What is your guess as to the relationship between the current in the coil and the magnetic field?

Part 4

Graphing Field Strength vs. Position

Using the Electromagnet simulation, click on “Show Field Meter.” Set the battery voltage to 10V where the positive is on the right of the battery. Along the axis of the coil and at the center of each compass needle starting 5 to the left of the coil, record the value of B. Move one compass needle to the right and record the value of B. Repeat until you’ve completed the table below. NOTE: Be sure to take all of your values along the axis of the coil. You’ll know you’re on the axis because the y component of the magnetic field is zero along the axis.

Compass position (arbitrary units) Magnetic Field Strength (fill in units) -5 -4 -3 -2 -1 0 1 2 3 4 5

a) What happens to the value of magnetic field strength inside the coil?

Graph the compass position on the horizontal axis and magnetic field magnitude on the y axis. Print your graph. Make sure to label the axes and title the graph.

b) Is your graph symmetric?

c) Using your graph, what is the relationship between magnetic field strength and position?

Part 5

Using the simulation to design an experiment, be sure to collect data and graph your results. Be prepared to share your results with the class

Some things that could be tested

Field Strength vs. Number of Coils

Field Strength vs. Current