Download Magnetism (Part 1)

Topic #5: Magnetism & Electricity “IN LOOPS”
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Magnetic Field of a current carrying loop.
Field lines resemble those of a bar magnet.
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Solenoids.
Many coiled loops.
The magnetic field inside the solenoid increases with current and with the number
of coils.
You can increase the magnetic field inside tremendously by inserting an iron rod,
thus producing an electromagnet.
The magnetic domains are aligned as long as the material is “ferromagnetic”.
Field lines in the solenoid are nearly parallel, uniformly spaced, and close
together.
The field outside is much weaker and non-uniform.
To find direction of the magnetic field, wrap your right hand around the coils in
the direction of the current. Your thumb points in the north direction.
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The “Nail” electromagnet
In the loop of wire at the right,
draw the magnetic field.
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What is a “coiled” loop of wires called when current flows through it? ___________________
In the coiled wire at the right,
draw the magnetic field.
What kind of material should be put inside the coils in order to create an
“electromagnet”?
__________________________
Why does placing a nail inside the coiled loop create a stronger magnetic field inside the
coil?
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If the battery is hooked up as shown in the picture below, which end of the magnet will
be the NORTH end? Label it.
Which color (red or black) on the compass needle should the tip of the nail attract? _____________
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MULTI-COIL DC MOTOR
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N
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N
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When a coil of wire has current running through it, we call this coil a
_______________. When a ferromagnetic material (such as the nail above)
is placed inside the coil, an ___________________ is created. When placed
in an external magnetic field, the nail should _______________ so that its
North-end attracts towards the _________-end of the fixed magnetic field.
The nail, the coiled wire, and the “structure” that holds this all together is
called the ______________ of the motor. This “structure” is most often a
shaft called the __________, since it “rotar-tates”. The “split ring” in the
picture above is known as the _____________. It rubs against the brushes,
thus allowing electrical current to pass from the source through the armature.
When current passes through the armature (which contains the solenoid), a
magnetic field is created. One end of the electromagnet becomes the Northend, while the other end becomes the South-End. The North-end then
rotates towards the South-end of the fixed magnetic field.
But wait!!!! What happens when the nail rotates past the “split” in the
commutator???
Look at one of the two brushes. As the split-ring rotates, it will eventually
lose contact with this brush and make contact with the other brush instead.
What will happen in the armature?
________________________________________________________
What then happens to the nail?
________________________________________________________
The nail will then rotate again, another _______________, until its Northend aligns with the fixed magnetic field’s South-end. This process will
continue to repeat itself, until either the battery runs out or the current source
is turned off.
Day #5 Homework
1. In the picture at the right, the tip of the nail would attract the
TAIL / TIP (circle one) of a compass.
2. The nail above is called an “electromagnet”. Explain what the
three necessary parts of an electromagnet are.
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3. Name 3 clear ways to increase the strength of the electromagnet. You may swap
out any parts for similar parts with different characteristics.
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4. Using the picture at the right, which wire (top or bottom) would electrons
flow OUT OF if the tip of the nail acts like the North end of a magnet.
5. Explain the differences between the magnetic field inside a solenoid and
the field outside the solenoid.
6. A Solenoid with ends marked A and B is suspended by a thread so that the core
can rotate in the horizontal plane. A current is maintained in the coil so that the
electrons move clockwise when viewed from end A toward end B. How will the
coil align itself in the Earth’s magnetic field?
7. A conductor carrying a current is arranged so that electrons flow from North to
South If a compass is held on the east end of the wire, in what direction is the
needle deflected (assuming it can point any direction it wants to)?
8. Is it possible to orient a current-carrying loop of wire in a uniform magnetic field
so that the loop of wire doesn’t rotate? Explain.
9. If a solenoid were suspended by a string so that it could rotate freely, could it be
used as a compass when it carried a direct current? Could it be used as a compass
if alternating current were used?
10. You are looking directly into one end of along solenoid. The magnetic field at its
center points directly at you. What is the direction of the current in the solenoid,
as viewed by you? CW, CCW, directly toward you, directly away from you?
11. There are two solenoids, one with 100 turns and the other with 200 turns. If both
carry the same current, will the one with more turns necessarily produce a
stronger magnetic field at its center?
12. The wire below has 5A of current flowing through it. At a location 8 cm above
the center of the wire, find the strength and direction of the magnetic field.
13. The Earth’s magnetic field 8 cm above the wire is 0.00002165 T directed to the
left. Find the direction that a compass needle would point if placed at this
location (8 cm above the wire). Hint: Vectors?