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Lab 37: Magnetic Field ; Magnets
- Drawing magnetic fields
- Magnetic poles
- Forces between magnets
1) The following simple magnet configurations were shown to you in class - draw the
magnetic field lines for each case (at least 8 lines per diagram), marking the directions
of the field lines with arrows.
a) bar magnet
b) horseshoe magnet
c) opposite poles of 2 magnets (other ends not shown)
d) like poles of 2 magnets (other ends not shown)
e) Why do you think b) is a popular shape for a magnet?
A horseshoe magnet is just a bar magnet bent around so that the N- and Spoles are near the same location. The strongest parts of the field are near
the poles, so this puts the strongest part of the magnet all in one location
(so that it’s easier to pick up magnetic objects).
2) Here are some more complex configurations of magnets - using the same basic rules
as you did in 1) , label the magnetic field lines. (Hint: check the online lectures for a set
of rules in drawing these field lines.)
a) 2 bar magnets side by side,
opposite-aligned
b) 2 bar magnets side by side,
like-aligned
c) 2 perpendicular
bar magnets
d) bar and
horseshoe magnets
e) 3 bar magnets
f) Earth
Recall that
the North
Pole is
actually an S-pole, and the the South Pole is
actually an N-pole. Also, recall that the
magnetic poles are slightly offset from the
actual poles.
Lab 38: Magnets and Magnetic Materials
- Interaction of magnets with other matter
- Magnetic poles of the earth
- Broken magnets
3) When an object is attracted to a magnet, does it actually become a magnet? List
some observations that support your answer.
Yes - a magnetized object actually becomes a magnet. When you
magnetized paper clips, and removed the original magnet, the paper
clips retained their magnetic fields (and attracted other paper clips).
Also, when you hung two paper clips from an N-pole, their opposite
ends repelled, because they were also both N-poles.
4) If you break a magnet with an N and S pole, is it possible to get an N pole or an S
pole by itself? Is it ever possible to break a magnet into small enough pieces to do this?
What would a magnetic field diagram for a lone N-pole look like? Would this diagram
break any of our “rules” for drawing magnetic field diagrams? If so, which rule(s)?
No! If you break magnets into smaller and smaller pieces, it just
becomes smaller and smaller magnets, all with both N and S poles.
A magnetic field diagram for a lone N-pole (shown here) would break
the rule requiring that all magnetic fields lines connect from an N-pole
to an S-pole.
Note that the shown field isn’t really
physically possible, because N-poles
are ALWAYS paired with an S-pole.
5) For diagrams c) and d) in #1, explain for both cases how the force of the left magnet
on the right magnet compares to the force of the right magnet on the left magnet. How
would your answer change if you pushed or pulled the left magnet?
c) According to the Law of Interaction, the force on the left magnet is
ALWAYS exactly equal to the force on the right magnet.
d) According to the Law of Interaction, the force on the left magnet is
ALWAYS exactly equal to the force on the right magnet (again).
Lab 39: Electromagnets
- Relation between electricity and magnetism
- Magnetic field produced by a coiled wire
- Building a simple motor
6) In this lab activity, you learned that an electric current produces a magnetic field.
a) Why was the wire coiled around the straw or nail? Do you think any wire with a
current running through it will produce a magnetic field?
Yes - as we saw with the aluminum foil, ANY electric current produces a
magnetic field; coiling the wire places small segments of the wire carrying
current parallel to one another, so it increases the strength of the field.
b) Part of this activity involved you scraping the insulation off the end of the wires with
sandpaper. Why not just use non-insulated wire to save you the trouble?
If non-insulated wire was wound into a tight coil, the current would flow
between every point the wires touched, instead along the path of the coiled
wire.
c) Does the electromagnet you produced with the coiled wire have an N and S pole, or
just one of these?
The electromagnet has an N and S pole, just like a bar magnet.
7) When you built your motor, you “blackened” part of the exposed wire with a magic
marker.
a) Explain what purpose this served, and what would have happened if you didn’t do
this step.
Blackening added insulation to the back half of the wire, so that the motor
wouldn’t turn “backward” halfway through, and end up stuck in one place
(or bouncing back and forth....)
b) Give a 1-2 sentence summary on how your motor was able to work.
Running a current through your small loop turned it into a magnet - this
magnet interacted with the magnetic field of the permanent magnet to give
it a “push”. When the ring turned over to the “blackened” side of the end
wires, inertia kept it spinning until it rotated back to where the wire
conducted again, giving it another “push”.
c) Many people doing this activity noticed their setup became quite hot. Why did this
happen?
The setup is actually a short circuit - in this case, the short circuit is
actually wanted, because short circuits have very large currents, and a
large current creates a large magnetic field.
d) Gary suggests that adding a light bulb to the circuit in series might solve the heat
problem. Is he correct? Why? What problems might this cause in this experiment?
This would solve the heat problem, since adding a load (or resistance)
reduces the electric current. However, the problem is that there now might
not be enough current to generate a strong magnetic field.
13. Billy claims that all magnets are electromagnets, with the “wires” and the “battery” hidden
inside. How would the “wires” have to run, to produce a bar magnet like the ones we have
been using? (It might be useful to make a drawing)
The coil would have to be along the
length of the magnet, as shown, to
create an N- and S- pole. (Reversing the
current would reverse the N and S poles.)
14. If we wrapped two coils the same way around a single straw and had currents going around
them in the same direction, would they attract or repel?
Explain your answer.
As long as the electric currents run in the same direction, as shown, the S
pole of the left magnet would meet up
with the N pole of the 2nd magnet, and
they would attract. (If one of the currents
was reversed, they would repel, since
two like poles would be together. If BOTH
currents were reversed, both pairs of
poles would reverse, and they would
again attract.)