# PHYS 2212 Module 8 Class Activities

## Magnets and Electromagnets

To access the simulation for this lab, click here: Magnets and Electromagnets Simulation. It will open the simulation in a new window.

### I. Exploring the Magnetic Field of a Bar Magnet

Under the Bar Magnet tab, in the menu on the right, uncheck the box next to Show Field. Everything should look like this:

Before you do anything in the simulation, answer this question to predict what you think you will see…

For a bar magnet like this, what would the compass needle look like if you placed the compass at position A?

Now using the simulation, move the compass around the bar magnet. Which pole of the magnet does the red compass needle point towards? Click “Flip Polarity” in the right menu.  Now which pole of the magnet does the red needle point towards? Does it still point toward the same pole? What does this tell you about the polarity of the compass needle? What are the similarities between the compass needle (magnetism) and a test charge (electricity)?

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

Check the box “Show Field Meter” in the right menu. A blue box should appear. This measures the magnetic field around the magnet. The magnetic field is measured in Gauss (G). Move the field meter around the magnet. Does the field increase or decrease as you move the meter closer to the magnet? Move your meter so that it is about one inch (on your computer screen) away from the North end of your magnet.  What is the magnitude of field strength in Gauss? Now move the meter the same distance away from the South end of your magnet. Is the amount of magnetic field the same for both North and South ends of a magnet?

In this simulation, magnetic field is expressed in the units of gauss (G). Using 1 T = 10,000 G, convert your values to teslas (T).

### II. Exploring the Magnetic Field of an Electromagnet

Switch to the Electromagnet tab at the top of the simulation.

In the menu on the right, uncheck the boxes next to Show Field and Show Electrons. Set the number of loops to 1. Everything should look like this:

Place the compass on the left side of the coil so that the compass center lies 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. Describe what happens to the compass needle. Move the compass along a semicircular path below the coil until you’ve put it on the opposite side of the coil. Describe what happens to the compass needle. Move the compass needle from left to right through the coil and describe what happens to the compass needle.

How many complete rotations does the compass needle make when the compass is moved once around the coil? Why do you think this is? 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.

Based on your observations, summarize the similarities between the bar magnet and the coil. What happens to the current in the coil when you set the voltage of the battery to zero?  What happens to the magnetic field around the coil when you set the voltage of the battery to zero? Play with the voltage slider and describe what happens to the current in the coil and the magnetic field around the coil. What is your guess as to the relationship between the current in the coil and the magnetic field?

Now, consider this set up but don’t use the simulation yet. If you put the compass at point A, what would the compass needle look like?

After answering this question, go ahead and test it with the simulation. How did you do? Were you correct in your prediction? If yes, explain your reasoning. If no, explain where you think you went wrong.

Check in with an LA or instructor before proceeding.

### III.  Graphing Relationships: Field Strength vs. Position

Using the Electromagnet simulation, click on “Show Field Meter.” In the menu on the right, check the box next to Show Field and uncheck the box next to Show Compass. Set the battery voltage to 10 V where the positive terminal is on the right of the battery. It should like like this:

Along the axis of the coil and at the center of each compass needle starting five points 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 at least 11 measurements at positions along the line through the center of the coil. NOTE: Be sure to take all of your values along this central 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.

What happens to the value of magnetic field strength as you move the sensor inside the coil?

Graph the position of the B-field measurement on the x-axis and magnetic field magnitude on the y-axis. The units for the position are arbitrary since we don’t have a way to measure distance. It’s ok to just use numbers like -5, -4, …, 0, 1, … 5 for the values of the position. We want to just be able to visualize the relationship between the magnetic field strength and position near the electromagnet.

Is your graph symmetric? What is the relationship between magnetic field strength and position?

Check in with an LA or instructor before proceeding.

### IV. Graphing Relationships: Field Strength vs. Number of Coils

Before using the simulation, consider this question. Which would be a stronger magnet?

Design an experiment to test how field strength varies with the number of coils. Collect data in a table and graph your results. Then comment on those results. What is the relationship between the field strength and the number of coils?

Check in with an LA or instructor before proceeding.

### V. Graphing Relationships: Field Strength vs. Current

Design an experiment to test how field strength varies with the current. (Recall that voltage is directly proportional to current….Ohm’s Law.) Collect data in a table and graph your results. Then comment on those results. What is the relationship between the field strength and the current in the coils?