Download 2. What exists in the region around a wire that is carrying current and

Magnetism and Electromagnets
Permanent Magnet
• A permanent magnet is a material that keeps its
magnetic properties, even when it is not close to
other magnets.
Poles
• All magnets have two opposite magnetic poles.
Attraction
Magnetic Fields
• Magnetic field lines always point away from a
magnet’s north pole and toward its south pole.
• What does the direction of magnetic field lines
tell you?
• Magnetic field lines point in the direction of the
force from the north pole.
Magnetic Declination
• The difference between the direction a compass
points and the direction of true north.
Gauss
• The unit to measure the strength of a magnetic
field.
• Earth’s magnetic field averages about 0.5 gauss
at the surface.
Electromagnets
• Magnets created by electric current flowing in
wires.
Earth’s Magnetism
• Huge electric currents flowing in the molten iron
produce the Earth’s magnetic field (like an
electromagnet!).
Right Hand Rule
Electromagnet Poles
• “right-hand-rule”
Advantages
• You can switch an electromagnet on and off by
switching the current on and off.
• You can switch an electromagnet’s north and
south poles by reversing the direction of the
current in the coil.
• The strength can be changed by changing the
amount of current in the coil.
• Can be much stronger than permanent magnets
when using large currents.
Use right-hand-rule to describe the north and
south pole.
Electromagnetic Induction
Magnetism in a Wire
• Created by moving charges
• Field lines are concentric circles with the wire at
the center
• Concentric: circles of different sizes
Magnetic Field Strength
• Directly proportional to the current, doubling
the current doubles the strength of the magnetic
field
• Inversely proportional to the distance from the
wire
▫ Stronger as you move closer to the wire
▫ Decreasing the distance to the wire by half doubles
the strength of the field
Coil
• A single wire can be looped into a coil,
concentrating the magnetic field at the coil’s
center.
• Solenoid
Electric Motors
• Convert electrical energy to mechanical energy
Electric Generators and Transformers
• Electromagnetic Induction – the process of
using a moving magnet to create electric current
or voltage.
• Magnet must be moving in order to produce
current
• A changing magnetic field is what creates
current
Faraday’s Law of Induction
• The voltage induced in a
coil is proportional to the
rate of change of the
magnetic field through the
coil.
Sub Day Questions and Answers
2. What exists in the region around a wire that is carrying current and that
exerts a force on another current-carrying wire?
A magnetic field.
3. Explain how the right-hand rule can help you determine the direction of the
magnetic field lines around a current-carrying wire.
Wrapping the right hand around a current carrying wire with the thumb
pointing in the direction of the current will cause the fingers to wrap in the direction
of the magnetic field that surrounds the wire.
4. What effect does increasing the current in a wire have on the magnetic field?
The magnetic field becomes stronger as the current increases. Field strength
is directly proportional to the current.
5. What effect does reversing the direction of the current in a wire have on the
magnetic field?
The direction of the magnetic field will be reversed if the current is reversed.
6. What happens to the magnetic field as you move farther away from a currentcarrying wire?
The strength of the field surrounding a wire will decrease as the distance
from the wire increases.
7. Why do we not use a single wire with a large current if we wish to create a
strong magnetic field?
It is easier and safer to work with a small current.
10. A motor turns _____ energy into _____ energy.
electrical, mechanical
11. Why is it necessary to use at least one electromagnet in a motor instead of only
permanent magnets?
So that the polarity of the magnets can be more easily reverses at the proper time
in the rotation of the motor.
12. What is the purpose of the commutator in a motor?
The commutator reverses the polarity of the electromagnets at the proper time in
the sequence of rotation.
13. Why must the direction of the current in a motor’s electromagnets be
switched repeatedly?
The current must be continually reverses so the electromagnets will continually
change polarity.
15. What happens as you move a magnet toward a coil of wire in terms of
electricity?
If you move a magnet through a coil of wire it will induce a potential
difference in the coil that will cause a current to flow if the coil is part of a circuit.
16. If you hold a magnet still near a coil of wire, will current or voltage be
induced? Explain your answer.
No. A magnet will only induce a voltage in a coil if the magnet is moving relative to
the coil.
17. State Faraday’s law of induction in your own words.
The voltage induced across the ends of a coil is directly related to the speed with
which a magnet must pass through a coil.