Online Lab: Magnetic Fields

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Instructor: Section:

Background:

All magnets have two opposite magnetic poles: a North Pole and a South Pole. Magnets canexert forces which can either attract or repel. If two opposite poles face each other, the magnetsattract (pull towards). If two of the same poles face each other, the magnets repel (pull away).Two magnets create a force on each other at a distance much larger than the size of the magnets.This force can be represented by drawing the magnetic field around a magnet. The magneticfield is a magnetic force that surrounds a magnetic object at all points in space.

The magnitude of the Earth’s field varies over the surface of the Earth. The horizontal componentof the Earth’s magnetic field points toward the Magnetic North Pole (which must therefore havea South polarity), is about 50 microtesla on the surface of the Earth. The north end of a compassneedle is attracted to the south end of the Earth’s magnetic field. So the pole which is referredto as "North Magnetic Pole" is actually a south magnetic pole.

It is known that magnetism is caused by the motion of electrons. Electric currents, the flow ofcharged particles, are sources of magnetic fields. In a solenoid (loops of conductive wire), it ispossible to generate an electromagnet, which behaves similarly to a permanent magnet madeof iron. In the diagram above, the current I, flows through the solenoid. The magnetic fieldproduces at the central axis of the solenoid can be estimated with

B = µoN

LI (1)

where N is the number of turns, L is the length of the solenoid, µo is the permeability of freespace (1.2566 × 10−10 N/A2). The second part of this lab, an electromagnetic coil (solenoid),is connected to an electric power source (battery). The battery generates current, I that runsthrough the coil which turns the coil into a magnet. The solenoid has a north pole (N) and asouth pole (S) like a bar magnet.

Developed by Melissa Butner, ETSU 1

Properties of Magnets

Below there are three different interactions between two magnets. For each, indicate in the blankprovided if they Attract or Repel each other.

Earth’s Magnetic Field

1. Label the North and South Geographic poles, and the North and South Magnetic Poles onthe Diagram below.

Magnetic PoleGeographic Pole

Geographic PoleMagnetic Pole

Developed by Melissa Butner, ETSU 2

Online Experiment Setup Instructions1. Go to the following website:

https://phet.colorado.edu/sims/cheerpj/faraday/latest/faraday.html?simulation=magnets-and-electromagnets

2. Click the Bar Magnet tab on the PHeT simulation.The figure below shows what you should see on your screen.

3. Use the Compass to find the direction of the magnetic field.4. The Field Meter is used to measure the Total Magnetic Field (B̄),

the horizontal B̄x and vertical components B̄y of the magnetic fieldand the direction of the field expressed in degrees Θ. (See figure on the right.)

Observations1. Drag the compass around to the bar magnet. Observe the red arrow of the compass needle.

What do you notice about the needle color as you drag it around the magnet?

2. Click the Flip Polarity button in the control panel to reverse its polarity.What are the ways in which the sim reflects this polarity reversal?

Developed by Melissa Butner, ETSU 3

Magnetic Field Strength around the Earth

1. Click on the “Field Meter”. Place the cross-hairs (+) of the meter at the point labeled A onthe diagram below. Measure and Record the magnetic field B̄ in the corresponding blank.NOTE: Don’t forget to include UNITS .

2. Repeat Step 1 for points B, C, D and E.

A.

B.

C.

D.

E.

AB

C

DE

Observations

1. Place the compass over the United States. Which part of the compass (red is North, whiteis South) is attracted to the Earth’s geographic North Pole?

2. Where is the magnetic field strongest?

3. As you move away from the poles what happens to the field strength?

4. Which of the magnet’s poles do the red arrows point away from?

5. Which of the magnet’s poles do the red arrows point toward?

6. Why do you think the red part of the compass needle always points to Earth’s North Pole?

Developed by Melissa Butner, ETSU 4

7. Place the Field Meter on top of the North Pole until the direction Θ reads close to -90◦.

Record the horizontal component of the magnetic field. B̄x =

Record the vertical component of the magnetic field. B̄y =

8. Calculate the Total Magnetic Field B̄ using the Pythagorean Theorem. B̄ =

Magnetic Field Strength as a Function of Distance1. Click on the “Field Meter”. Place the cross-hairs (+) of the meter at the point labeled A on

the diagram below. Measure and Record the magnetic field B̄ in the corresponding blank.NOTE: Don’t forget to include UNITS .

2. Repeat Step 1 for points B and C.

A.

B.

C.

A B C

Observations

1. Based on your data above, what do you observe about the strength of the magnetic fieldas you move further from Earth? Is this what you expected? Explain.

Developed by Melissa Butner, ETSU 5

Magnetic Field Strength in an Electromagnet1. At the top left of the PHeT simulation window, Click the Electromagnet Tab.

You should see a battery connected to a wire with loops that has current running throughit as shown on the figure below.

2. Move your compass around the electromagnet. Answer the Following Questions.

3. Is the left side of the electromagnet the North end or the South end? How do you know?

4. Move the meter around the electromagnet. Does the field strength increase or decrease asyou move the meter closer to the electromagnet?

Developed by Melissa Butner, ETSU 6

Magnetic Field Strength with Varying Loops

1. Use the right menu to adjust the number of loops in your electromagnet.

2. Set the Battery Voltage to 6V.

3. Set the number of loops to 4.

4. Place the meter about one inch (on your screen) from the left side of the electromagnet.As shown in the Figure below.

5. Measure the magnetic field strength and Record the B̄ value in the table below.

6. Repeat for loops 3, 2 and 1.

Table 1: Magnetic Field Strength with Varying Loops

# Field StrengthLoops B̄ (G)

4321

B̄

Observations

1. Based on your data above, does the amount of field strength increase or decrease as youincrease the number of loops in an electromagnet? Is this what you expected? Explain.

Developed by Melissa Butner, ETSU 7

Magnetic Field Strength with Varying Voltage

1. Set the number of loops for your electromagnet back to 4.

2. Place your field meter one inch from the left side of the coils.

3. Use the sliding bar on the battery to adjust the Voltage in your electromagnet to 10 V.

4. Measure the magnetic field strength and Record the B̄ value in the table below.

5. Repeat for Voltages 8, 6, 4, 2 and 0.

Table 2: Magnetic Field Strength with Varying Loops

Voltage Field Strength(V) B̄ (G)1086420

B̄

Observations

1. Based on your data above, what is the relationship between voltage and field strengthin an electromagnet? Is this what you expected? Explain.

Developed by Melissa Butner, ETSU 8