Download introduction to magnets and magnetic fields

Magnets and the Magnetic field - Prelab
1. In Part 1, what is the name of the equipment that will provide the electron beam? Will
this piece of equipment also supply the magnetic field? If not, what piece of equipment
will generate the magnetic field?
2. Predict what will happen when the beam of electrons passes through the magnetic
field. Will it deflect, and if so how?
3. In Part 2, what is the purpose of the the iron filings on the paper above the bar
magnet?
4. In Part 4, what is the purpose of the compasses placed on the Ampere’s Apparatus?
5. In Part 5, what kind of magnet replaces the bar magnet in the diagram?
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Magnets and the Magnetic field
Part 1: The force on a moving charge in a magnetic field; The Helmholtz Coil
Equipment:
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Helmholtz Coil
Horseshoe magnet
Dark room
Dim the lights and proceed to the
Helmholtz Coil setups . Follow
instructions regarding warm up time etc.
and obtain the blue electron beam. This
beam is a current, composed of
electrons (negative charge carriers). In
most uses of this apparatus, the coils are
electrified to provide a magnetic field.
For this exploration, the horseshoe magnet will provide the magnetic field, as it is
easier to manipulate.
1. Predict what would happen if the horseshoe magnet were positioned so
the electron beam would pass through the magnetic field of the magnet.
Would the beam be attracted, to one of the poles, repelled from one of the
poles, would there be some other deflection or would the beam not be
deflected at all?
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2. Try it! Use the horseshoe magnet to investigate how the electron beam is
deflected. What is the relative orientation of magnet and beam when the
deflection is the
greatest?___________________________________________________
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3. What is the relative orientation of magnet and beam when the deflection is
minimal (or even absent)?
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4. Explain how why the direction of deflection of the electron beam seems to
contradict the right-hand rule for force (see text for reference).
Part 2: The magnetic field of a magnet
Equipment:
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Bar magnet
Other magnets of
various shapes
Compass
Mini compasses
Iron filings
Plexiglass sheet
Plain white paper
Textbook
Caution: Do not let the iron filings come into direct contact with the magnet or
the plexiglass. The filings are very difficult to remove.
1.
Trace a bar magnet on a piece of paper, labeling north and south poles.
Put that piece of paper aside for the moment. Place the bar magnet
under the plexiglass sheet, noting where the north and south poles are.
Put a fresh sheet of paper above the plexiglass sheet. The idea is to
keep the iron filing on the paper, not on the plexiglass or the magnet.
Carefully sprinkle iron filings over the paper in the area above the magnet.
Tap the edge of the paper very gently until you see a pattern emerge.
What does this pattern
represent?__________________________________________________
_
2.
On the sheet of paper on which you traced the magnet, carefully draw the
pattern you see in the filings. Be sure to indicate on your drawing which
end of the magnet is N and which is S. How does it compare to the figure
of iron filings around a bar magnet in your text? Where is the highest
concentration of filings? As the distance away from the poles increases,
how does the pattern change? Explain carefully!
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3.
Carefully remove the paper from the magnet and return the iron filings to
their container. Replace the paper over the magnet. Place a compass on
the paper and note the effect on the compass needle as you move the
compass to various locations on the paper around the location of the
magnet. On your previously drawn sketch of the pattern of iron filings,
draw arrows showing the direction that the north (painted) end of a
compass points at a number of locations around the magnet. Be sure to
look at locations at both ends and around both sides of the magnet, and
at differing distances from the magnet. Attach your drawing to this lab.
4.
According to your text, what does the direction of the compass needle
indicate? Would you describe the compass directions as generally
parallel, perpendicular, or tangent to the field lines you have drawn? Do
your observations with the compass suggest that the magnetic field is
stronger at some locations than others? If so, where is it the strongest?
Part 3: Are static (non-moving) charges magnets? Do magnets experience
a force in an electric field?
Equipment:
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Black plastic strip
wool
Electroscope
Compass
1.
Charge the strip by rubbing it vigorously with the wool. Check to make
sure the rod is charged by looking for an effect on the electroscope. What
effect on the electroscope do you see when the rod is charged?
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2.
What kind of field causes this interaction?_________________________
3.
Move the compass close to the charged strip. Does the field from the
strip affect the compass? Are magnets affected by a static electric
field?______________________________________________________
___________________________________________________________
___________________________________________________________
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4.
For comparison, bring one end of a bar magnet close to the compass and
note the effect. Turn the magnet around so that the other pole is close to
the compass. Record your
observations._____________________________________
___________________________________________________________
___________________________________________________________
5.
What kind of field causes this effect on the compass?
6.
Now bring the bar magnet close to the electroscope (discharge
electroscope first). Does the magnetic field from the bar magnet affect the
stationary charges residing on the electroscope?
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7.
According to your observations, are static charges magnets? Explain
your answer. ______________________________________
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Part 4: Oersted’s Experiment - Are moving charges magnets?
Equipment:
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Ampere’s Apparatus
3 D-batteries, in holders (or power
supply)
5 mini-compasses
regular compass
connecting wires
switch
Connect the Ampere’s Apparatus in series
with a switch and the battery pack/power
supply as instructed. Note the direction the current will travel when the switch is
closed. Place the mini compasses on the “shelf” of the apparatus and the larger
compass under the horizontal rod. Make sure all the north (red) arrows point in
the same direction. Report any errant compasses to the authorities.
1. If current acts like a magnet, what would you expect to happen to the
compasses when the switch is closed?
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2. Try it! Close the switch. What happens? Based on the results, are
moving charges magnets? How do you know?
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3. According to the mini magnets, what is the shape of the magnetic field due
to a current carrying wire? How does that compare with the field shown in
your
text?_______________________________________________________
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___________________________________________________________
______________
4. What do you think will happen if the direction of current is
switched?___________________________________________________
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______________________
5. Try it! Switch the leads and close the switch. What happens?
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_______________________________
6. Using your text as a reference, explain how this experiment with the
Ampere Apparatus confirms the validity of the right -hand rule for fields:
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7. Is a current-carrying wire is a magnet? Explain:
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Part 5: Force and Torque on a current loop
We have seen that moving charge acts as a magnet. A current carrying wire
deflects a compass. The magnetic field of a horseshoe magnet will exert a
force on an electron beam. But what if the moving current is in the shape of
a loop and can move about some pivot point?
Equipment
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3 D-batteries, in holders (or power
supply
ring stand and clamp
wooden dowel
connecting wires
switch
horseshoe magnet (replaces bar
magnet in the diagram)
piece of non-magnetic wire to make
into a square-shaped loop
short copper wire “hangers”
1. Set up the apparatus as shown. If you put the bottom, horizontal portion
of the “loop” between the ends of the horseshoe magnet while the switch
is open, do you think the loop will swing? Explain why or why not.
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2. Do you think it will swing once the switch is closed? Why or why not?
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3. If you think the loop will swing, use the right-hand rule for force to predict
which way it will go. Prediction: ____________________
4. Do the experiment by closing the switch. Did the results match your
predictions? If not, what was
different?___________________________________________________
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_______________________________
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5. If you switch the directions of the leads, what do you think will happen?
Why? ___________________________________________________________
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________________________________
6. Try it. Did your results match your predictions? If not, why
not?_______________________________________________________
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7. Notice that the loop is pivoted about the dowel, so the force due to the
magnetic field actually caused a rotation. What do we call the tendency of
a force to cause rotation? _______________
8. Now investigate what happens when you put one of the vertical sides of
the loop in between the horseshoe magnet and close the switch. Do you
think there is a magnetic force exerted on the vertical portion of the wire
by the magnetic field? If so, does this force result in a torque? Why or
why not?
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Part 6: Force and Torque on a current loop - Exercises
The diagrams below represent pivoted loops in the presence of magnetic
fields. The pivot is the rod in the center of the loop. The directions of currents
and fields are indicated. Use the right-hand rule to determine the direction of the
force on each side of the wire and indicate the direction of those forces on each
diagram. Then describe what the overall effect on each current loop (will it rotate
and if so in what direction) in the space on the right.
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1.
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3.
4.
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5.
6.
7.
Based on your diagrams, under what conditions will a magnetic field cause
rotation torque in a current carrying loop? What must be true about the direction
of magnetic field, magnetic force and the axis of rotation relative to one
another?_________________________________________________________
________________________________________________________________
________________________________________________________________
________________________________________________________________
________________________________________________________________
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8.
What do you think would happen to the magnitude of the torque if there were
multiple loops of wire bundled together, all carrying the same current in the same
direction?
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