College Physics Lab
PH 144
Magnetic Field Lines
Introduction:
The purpose of this lab is to clarify what is meant by magnetic field lines and how they are used.
Magnetic field lines are an important tool for describing the nature of magnetic interactions.
When a magnet is brought close to another magnet, an interaction occurs. If you are holding the
magnets typically you feel an attraction or a repulsion between the ends of the magnet that you
bring close together. If one of the magnets is suspended from a string or is balanced on a pin as
in the case of a compass, then as one magnet is brought close to the other, the suspended magnet
twists towards or away from the approaching magnet. This is clear evidence of an interaction -observed accelerations require real forces to explain them. A rule for describing interactions
between magnet poles is that like poles repel and unlike poles attract. Magnetic poles are
assigned labels of north (N) or south(S).
This interaction occurs without physical contact just as does the electrical interaction between
two charges, just as does the gravitational interaction between two masses. As in those cases it
has become useful to invent the idea of a field in order to explain the behavior of magnetically
interacting objects. According to the field conception, a magnet influences all of space around
itself. The magnet sets up a magnetic field vector at every point in space. Like all vectors, the
magnetic field has two properties: magnitude (represented as vector length) and direction. The
magnetic interaction between two magnets can be interpreted as the interaction between one
magnet and the magnetic field at that position due to the second magnet. When this interaction
occurs, the first magnet wants to align itself with the magnetic field of the second magnet.
A compass can be used to illustrate such interactions. The needle of a compass is a small light
magnet suspended on a pin. When placed in a magnetic field, the needle of a compass points in
some direction. By convention we imagine the field to be oriented so that the needle of the
compass points parallel to the field that it experiences. Also by convention we assume that the
direction that the north pole of a compass points is the direction of the field at that point. In this
lab we will use this technique to map out the magnetic field around magnet(s).
Magnetic field lines correspond to magnetic field vectors in the same way that electric field lines
correspond to electric field vectors. Magnetic field lines present an overall picture of how the
magnet influences the space around itself. By convention the arrow tip on magnetic field lines
points towards the south magnetic pole and away from the north magnetic pole. The relationship
between magnetic field lines and magnetic field vectors is as follows: Pick a point on a magnetic
field line. A straight line tangent to the field line at that point is parallel to the magnetic field
vector at that point. The relative strength of the field at that point can be inferred by the relative
separation of field lines in that region. Where field lines are relatively close together, the
magnetic field vector magnitude is relatively large. Where field lines are relatively far apart, the
magnetic field vector magnitude is relatively small.
Magnetic fields also interact with moving electric charges. The strength of the interaction is
influenced by the size of the charge, the speed of the charge, the strength of the field, and the
orientation of the motion relative to the field direction. The direction of the magnetic force is
determined by the orientation of the charge velocity relative to the magnetic field direction. The
magnetic force acting on a moving charge is described in Section 18.2 of your text (Wilson,
College Physics, Second Edition).
Purpose:
In this lab you will investigate the magnetic field around magnets and represent this information
as magnetic field lines. Then for several hypothesized charges moving through the field you will
predict the direction of the magnetic force experienced by those charges in this region of
magnetic field. Having identified the force on a charge, then predict its subsequent motion.
Materials:
2 Alnico bar magnets white paper
1 small compasses
pen or pencil
Investigations:
1.
Examine your compasses. Make sure that they generally all point in the same
direction when there are no magnets nearby. The compass needle tries to orient itself to
the direction of the Earth's magnetic field as it experiences it.
2.
Bring the north pole (N) of one of the bar magnets close to the end of the compass
needle (of one of the compasses) that had pointed to the north. Does it attract or repel the
end of the needle that pointed towards the north? What do you infer, is the end of the
compass needle which points north most likely a north magnetic pole or a south magnetic
pole? What do you infer, does the Earth's north geographic pole correspond to the north
magnetic pole of the Earth or to the south magnetic pole of the Earth?
3.
Take one large sheet of paper (or tape together two or more sheets of 8" x 11"
white paper). Place the magnet in the center of the paper along one edge the paper. Using
a pen or pencil draw an outline of the magnet. On the paper indicate which end of the
magnet is the north pole and which is the south pole.
4.
Drawing a magnetic field line for a bar magnet:
•
Place one compass close to some part of the magnet, but not touching.
•
Make a pencil mark on the paper at each end of the compass needle.
•
Move the compass so that its south pole is at the same point where the north pole
of the compass had just been (or vice versa). Make a mark on the paper at the end of the
compass needle that isn't marked.
•
Repeat this process until the compass is again nearly in contact with the magnet.
•
Connect the points on the paper with a single smooth line. Draw an arrowhead on
the line so that it is pointing towards the south pole of the magnet.
5.
Repeat step 4 for four more magnetic field lines. If the field line goes off the
paper, follow it until it returns and continue drawing it.
Compare the lines that you drew with the pattern that iron filings make around the
same magnet.
Procedure:
Take one of the rectangular plastic containers and place (open side up) it on top of
the magnet with the magnet centered.
Sprinkle iron filings from the yogurt container into the rectangular plastic
container so that the filings do not come in contact with the magnet. Observe the pattern
of iron filings that are created. Generalize your observations.
6.
Magnetic field vectors and magnetic field lines:
•
Mark three points at different locations on one of your field lines.
•
At each point make a qualitative representation of the magnetic field vector there.
7.
Interaction of magnetic fields and moving charges:
For each of the points identified in part 6, consider a charge moving in some
direction at that location. For each case identify the direction of motion and identify the
kind of charge. You should consider both kinds of charge here. You should also consider
charges moving more or less towards the magnet as well as charges moving more or less
away from the magnet.
For each situation you consider determine what would be the direction of the
magnetic force which acts on the moving charge.
8.
On the other side of the paper (on one half of the paper) investigate the magnetic
field lines around two magnets when their north tips (or their south tips -- don't do both)
are placed in opposition about 6 inches apart.
Compare your results with the pattern of iron filings that are established around
these magnets (as done in step 5).
9.
On the other half of the paper investigate the magnetic field lines around two
magnets when the north tip of one is placed opposite to the south tip of the other about 6
inches apart.
Optional:
Compare your results with the pattern of iron filings that are
established around these magnets (as done in step 5).
10.
For the drawings made in steps 8 and 9, sketch the magnetic field at two different
points.
•
Then at one point imagine a negative charge moving in or out and at the other
point imagine a positive charge moving out or in. In each case predict the direction of the
magnetic force that the moving charge would experience.
11.
On the same piece of paper or on another piece of 8" x 11" white paper
investigate the magnetic field lines around two magnets when the north tip of one is
placed at right angles to the south tip of the other about 6 inches apart. Or investigate the
field that arises when 2 north poles or 2 south poles are at right angles to each other about
6 inches apart.
Optional:
Compare your results with the pattern of iron filings that are
established around these magnets (as done in step 5).
12.
Optional: Investigate the magnetic field lines around an arbitrary configuration of
magnets. Set a few magnets down on paper. Try to predict what the field lines might look
like. Then determine field lines by the method used above.