Download Chapter 37 - Electromagnetic Induction Outline

Chapter 37 – Electromagnetic Induction
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Once it was discovered that moving charges produced magnetism, scientists began to question if moving
magnets could produce electricity. In 1831, two scientists (Michael Faraday in England and Joseph
Henry in the U.S.) proved that you can indeed use moving magnets to make electricity.
31.7 – Electromagnetic Induction
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The production of voltage in a wire by a magnetic field depends on the relative motion of the magnets
with respect to the magnetic field (this means that either the magnet or the wire itself can be moving, it
doesn’t matter).
The faster the motion, the larger the voltage produced.
The greater number of loops there are in the wire, the larger the current produced ( however, it is also
true that the more coils there are the harder you have to work to create the movement, otherwise it would
violate the law of conservation of energy).
This process of creating current and voltage in a wire by moving a magnet is called electromagnetic
induction.
37.2 – Faraday’s Law
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We can summarize the rules for current and voltage production in a wire through the use of a magnet
with Faraday’s Law
o This law says that the voltage produced in a coil is proportional to the product of the number of
loops and the rate at which the magnetic field is changing.
In plain English this means that to find the voltage produced in a wire I take the number
of loops and multiply it by how fast I am moving the magnet and another number that we
call a proportionality constant (remember how we had G for gravity and k for electricity
– those were proportionality constants too).
It is important to note that Faraday’s Law gives us VOLTAGE not current. In order to find the current
produced we need to know the resistance of the coil and the circuit that it is connected to. Once we
know this then we can solve Ohm’s Law using the known resistance and the voltage found with
Faraday’s Law (please note this is a two step problem, so you should be extra careful with units so that
you are putting the right things in the right places).
37.3 – Generators and Alternating Current
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A generator is the opposite of a motor (remember a motor took the electric energy in the coil and used a
magnetic field to create mechanical energy) because it takes mechanical energy and converts it to
electric energy.
A generator can either have a moving magnet or a moving wire, but either way as the movement occurs
the field increases to a maximum, decreases to zero then continues to decrease to a minimum and then
returns to the maximum and repast the cycle. Therefore it always creates an alternating current (Fig
37.6-7 pg 581)
o A common generator set up is found on page 580 – it has a modified horseshoe magnet with a
coil of wire inside it that is connected to a crank and a light bulb. As you turn the crank the wire
moves within the magnetic field. This produces current and voltage that is able to power the
light bulb.
In America the standard generator produces a current that changes magnitude and direction 60 times per
second (this is a frequency of 60 Hz…remember when we learned about Hz and frequency in chapter
25?)
The generators in power plants use a turbine powered by wind, water or steam instead of a hand crank,
use very powerful electromagnets, and wrap the coil of wire around a piece of iron.
o It cannot be ignored that whether we use steam, wind, water or human arm strength to power
generators, we are still using some form of outside energy, and so the law of conservation of
energy is still being obeyed, we are simply changing the energy’s form.
37.4 – Motor and Generator comparison
o Both the motor and the generator work based on the same principle. The deflection of a current carrying
wire when it is placed in a magnetic field.
o We call the deflection of the wire the motor effect and the law of induction the generator effect.
37.5 – Transformers
o If I have a pair of coils side by side – my first or primary coil attached to a power source (input), and my
second or secondary coil attached to a galvanometer (output), as soon as current beings to pass through
the primary coil, a current begins to flow in the secondary coil, even though there is no connection
between the two. The current in the secondary coil is very brief, but if I turn off the voltage source and
therefore stop the current in the primary coil, a current will again briefly flow in the secondary coil, this
time in the opposite direction (note this sounds a lot like an alternating current!) (fig 37.10 – pg 583)
o This happens because the current in the first coil produces a magnetic field. This change in
magnetic field causes the current in the second coil based on electromagnetic induction
(following the rules of Faraday’s Law).
o If we put an iron bar through the center of both coils, it intensifies the effect.
o If I hook the primary up to a source of alternating current I don’t need to keep turning it on and off to get
the desired effect. When I hook to coils up to a magnet like this and then attach it to an alternating
current source, I have a transformer.
o I can change the voltage in a transformer by changing the number of primary and secondary
loops. The relationship between voltage and turns is:
Primary voltage/# of primary turns = secondary voltage/# of secondary turns
o The law of conservation is followed because:
Power into primary = power out of secondary
o This means that:
(voltage x current)p = (voltage x current)s
o If the transformer is increasing the voltage, there will be a decrease in the amount of current, and if the
transformer is increasing the current there will be a decrease in the amount of voltage.
37.6 – Power transmission
o When electrical power leaves a power plant it has a very low current and a very high voltage (this saves
energy because high currents produce heat, which is basically a loss of energy). As the power enters the
city it goes through a transformer where its voltage is “stepped down” to a lower level (this means that it
current goes up), and then when it get to your home it is stepped down again to it final voltage of 120V.
37.7 – Induction of Electric and Magnetic Fields
o Instead of looking at inducing currents and voltages, it is better to look at it as inducing electric fields.
o An electric field is created in the region of space in which the magnetic field changes over time.
The magnitude of the electric field is proportional to the rate of change of the magnetic field over
time. The electric field created is at right angles to the magnetic field. If there is an electric
charge in the area then it will experience a force…if it is in a wire then this will make it start to
move – we call this a current.
o We can use similar logic with magnetic fields.
o A magnetic field is created in the region of space where an electric field is changing over time.
The magnitude of the field created is proportional to the rate of change, and it is at right angles to
the electric field.
37.8 – Electromagnetic Waves
o This section serves as a review of what we learned earlier in the year about electromagnetic waves (see
chapter 27). Please feel free to read this section and review chapter 27 in preparation for the test.