Investigation 18B:  Magnetic field around a magnet

Essential questions: How does a force field indicate the forces acting on an object?

What is the shape of the magnetic field around a bar magnet?

We use magnets and magnetized objects every day, often without realizing it.  All around us is a magnetic field—the Earth's magnetic field.  How do these magnetic forces and fields work?  In this investigation, you will explore the magnetic field surrounding a bar magnet and compare it to the Earth's magnetic field.  Magnetic fields are measured in units of teslas (T).

Part 1:  Tracing the magnetic field of a bar magnet

1.      Launch the magnetic field interactive simulation and click to select the bar magnet option.

2.      Pass your mouse over the diagram to reveal the magnetic field line vectors.

3.      Find a few positions close to the north magnetic pole of the magnet from which you can follow the field arrows all the way to the south magnetic pole.  Sketch the paths you followed on the figure below.

Questions

a.       What does the direction and length of each field arrow indicate?

b.      What is the strength and direction of the magnetic field at a location:

o   5 cm to the right of the bar magnet’s middle?

o   5 cm to the left of the bar magnet’s middle?                 

o   5 cm above the bar magnet’s middle?                               

c.       If you increase the strength of the bar magnet, what happens to the magnetic field lines?

d.      What is the meaning of the paths from north to south magnetic pole that you sketched out?

Part 2:  Earth’s magnetic field

1.      In the interactive simulation, click on the button to select the Earth's magnetic field option.

2.      Pass your mouse over the diagram to reveal the magnetic field line vectors.

3.      Find Earth's north and south magnetic poles.

Questions

a.       What is the strength of the Earth’s magnetic field:

o   near one pole (location #1 in the figure)?

o   near the other pole (#2)?

o   near the equator (#3)?

b.      Where are Earth’s north and south magnetic poles?  Justify your answer in terms of the magnetic field lines in the simulation.

c.       How does the strength of the Earth’s magnetic field near its surface compare to the strength of the magnetic field you measured in part 1, at a location 5 cm away from the bar magnet?

d.      Research online the strength of Earth's magnetic field and compare to your results in (a) above.

Part 3:  Magnetic field of a bar magnet using iron filings

1.   Place a bar magnet underneath a piece of poster board.

2.   Sprinkle iron filings on top of the poster board.

Questions

a.       How can you use iron filings to locate the poles of the bar magnet? 

Applying new knowledge

1.      Define force field and give two examples of forces that can be depicted with a force field.

2.      Use the figure below for the following questions:

a.       Draw the direction of the magnetic force field at locations #1-4.

b.      Draw a curve for a magnetic force field line that passes through each of these four locations.  (Be sure to include an arrow on your field line.)  In the case of location #1, draw a field line all the way between the north and south magnetic poles of the magnet.

3.      Using a compass:

a.       Describe how to use a compass to determine direction.

b.      What corrections must be made to a compass' direction?

c.