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10 Magnetism Motors Generators Johnson County Community College
PSCI 120
Student Guide (Light & The Electromagnetic Spectrum )
What Causes Magnetism:
MAGNETISM IS CAUSED BY A MOVING CHARGE!!!!!!
The Two Types of Magnets:
Ferro Magnets:
All atoms are composed of a dense nucleus surrounded by electrons. These electrons are in motion. The
two motions exhibited by the electrons are Spin & Orbit.
The spin and orbital motion of the electrons creates a magnetic field. Why then aren't all substances mag-
netic?
In most substances the magnetic field created by elements spinning one way, are canceled out by those
spinning in the other. The magnetic field created by the orbital motion of the electron is cancelled out by
electrons orbiting in the other direction. In the case of Iron there is an imbalance which results in the iron
atom having an overall magnetic field.
In a sample of iron, the atoms group together to form small microscopic sized magnets called Domains.
These domains are what results in iron having its unique magnetic properties.
What happens to Iron if it is heated?
What happens if a magnet is dropped?
How do we magnetize a piece of iron?
When we heat a substance we speed the atoms up. This increased speed results in the atoms crash-
ing into one another and thus, the domains are either destroyed (if hot enough) or they get out of
alignment.
If you drop a magnet the physical trama or vibrations can cause the domains to get out of alignment.
This weakens the strength of the magnet.
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S Electromagnets:
If a current is moving through a wire, electrons are moving through the wire. This movement of the
charged particles (electrons) results in a magnetic field around the wire.
In the case of an electromagnet, the magnetic field exists only when there is a current flowing through the
wire. This is different that a ferromagnet in that the magnetic field is always present if the domains are
aligned properly.
What is the Magnetic Field?
The region of space near a magnetized body within which magnetic forces can be detected.
Direction: The direction of a magnetic field is always described as the direction in which the north-seek-
ing pole of a compass points.
Electric Motors:
An electric motor works on the principle that like magnetic poles repel.
Induced Current:
If current passing through a wire creates a magnetic field, then a magnetic field can create a current.
Demonstration:
There must be relative motion or simulated relative motion between the wire and the magnetic field in
order for an induced current to exist. Induced Current and Generators:
If you move a magnet close to a coil of wire and then move it away, this will induce a current in the wire.
We do exactly that in an electrical generator.
We basically spin a coil of wire inside a magnetic field.
What does a generator closely resemble?
Types of Current:
Direct Current:
In this case the current flows in only one direction.
Alternating Current: In alternating current the electrons change direction of motion. They basically
move back and forth. In this country we utilize 60 Hz (sixty cycles per second) alternating current.
Demonstration:
Transformers: Current is carried over great distances utilizing very large Voltage and very low Amperage
(current). When this power reaches our home we do not want to bring it into the home at such high volt-
age so we change or transform the voltage to lower voltage and at the same time raise the current.
To do this we utilize either Step-up or Step-down transformers.
The power must remain the same (Power = Volts X Amps). From this you can see that is you lower the
voltage, you must raise the amperage.
Question: If the Power coming into a transformer is 6 Volts at 3 amps and the voltage is lowered to 2
Volts, what is the value of the new amperage?
A generator is basically the same as an electric motor. The difference is that in the case of a
motor we input the electrical energy and this results in an interation between the magnetic fields
producing mechanical energy. In the case of a generator, we imput the mechanical energy and
produce the electrical energy. We can generate electricity with a motor.
In this case the volts times the amps is (3) (6) = 18. So the volts times the amps in this system must
eual 18. (2) (X) must equal 18 so solver for the value of X
(2)X) = 18
X = 18 ÷ 2 = 9 Question: If the Voltage on the left is 220 and the amperage is 4 amps, what is the voltage and amperage
on the right. The left coil is the Primary Coil and the right is the secondary.
If there are more windings on the primary than there are on the secondary coil, you have a
step down transformer. In this case we have half the number of wraps on the right as we do
on the left. Therefore, we will have half the voltage on the secondary coil. This menas that
the voltage will drop from 220 down to 110. This also menas that the current must double.
Remember the Volts times the amps must remain constant. The amps will be 4.
To chech your answer take 220 times 2 = 440
110 times 4 = 440
The power is the same on both sides of the coil. Questions: Motors, Generators, & Transformers
1.
What causes a magnetic field?
All magnetic fields are cause by a moving charge. In the case of a Ferro magnet, this moving charge is
the spin and orbit of the electrons. In the case of the Electromagnet, it is the flow of electrons through a
wire.
2.
If you could imagine a large bar magnet through the Earth such that it was in line with the magnet-
ic axis, the end of the bar magnet near the geographic north pole would be what kind of magnetic
pole? (North Seeking or South Seeking) Explain
Remembering that opposite magnetic poles attract, if the north seeking pole of a bar magnet points north,
it must be attracted towards the opposite polarity. Therefore, the north magnetic pole is must be the same
magnetic polarity as the south-seeking pole of a bar magnet.
3.
How can you magnetize a piece of iron?
To magnetize a piece of iron, you must orient the domains in the same direction. This can be done by
placing the piece of iron inside a strong magnetic field. If this is a soft piece of iron, however, the
domains can easily be "jumbled" and the magnetic property around the piece of iron will diminish.
4.
Why aren't all metals magnetic?
To be magnetic, there must be an imbalance in the spin and orbit of the electrons. In most substance, there
is a balance of spin and orbit resulting in no magnetic field around the atom. In iron, however, there is an
imbalance resulting in each atom having a magnetic field. These atoms subsequently group together is
clusters called domains which are the micro magnets in a piece of iron.
5.
How do you determine the direction of a magnetic field?
The direction of a magnetic field is determined by the direction in which the north end of a compass nee-
dle would point.
6.
Describe ways in which the overall magnetic property of a bar magnet can be damaged.
Heating: This speeds up the atoms and could result in the domains in being jumbles out of alignment.
Dropping: The physical vibrations sustained by the fall could shake the domains out of alignment.
Placing near other Magnet: If a magnet is stored in the incorrect orientation near another magnet, the
domains can be forced out of alignment by the opposing magnetic poles.
7.
What makes an electric motor work?
In the case of an electric motor, a coil of wire is placed inside a magnetic field. When current is passed
through this coil it also develops its own magnetic field. The interaction of the two magnetic fields results
in a force which causes the coil to spin. This is the production of Kinetic Energy from Electrical energy. 8.
How is an electric motor and an electrical generator similar to one another?
In the case of the generator the same coil of wire is placed in the magnetic field. In this case, however we
do not supply the electrical energy but rather the Kinetic Energy. We physically turn the coil. As the
wires of the coil move through the magnetic field, a force is applied on the electrons causing them to
move. This motion is electric current. In general we supply the generator with Kinetic Energy and in
return get electrical energy.
9.
What type of current do we utilize in a normal household in this country and how is it produced?
In this country we utilize Alternating current. This means that the electrons change direction of flow 60
times every second. The Alternating current is a result of the wires of the generator passing through the
magnetic field in different directions. When a wire move in a different direction through a magnetic field
the direction of the force on the electrons in the wire changes direction. This results in the electron chang-
ing direction of flow through the wire.
10. If the Voltage coming into the primary coil of a transformer is 10 Volts and the amperage is 2 amps,
what is the amperage is the Voltage on the secondary coil is 5 Volts?
The power must be the same in both the primary and secondary coil. Power is Volts X amps or in this
case (10)(2) = 20 watts. Therefore, in the secondary coil (5)(X) must equal 20 wats. Solve for the value
of X and you find the amperage must be 4 amps. If the Voltage is cut in half, the amperage must double.
11. Why do we need to use transformers when bringing electrical energy from the production site to
you home?
Electrical current can be transported move efficiently at high Voltage at l