Download Electricity plays an important role in everyday life. Understanding

CLASS 23: PRACTICAL ELECTRICITY
23.1. INTRODUCTION
Electricity plays an important role in everyday life. Understanding how electricity works allows you to
deal with it in a safe manner. This chapter examines some of the applications of electricity.
23.2. GOALS
ƒ Understand the difference between AC and DC current;
ƒ Explain how a transformer works and be able to calculate the parameters involved in determining
how voltage is transformed;
ƒ Be able to explain how power is transmitted and why it is more efficient to transmit electricity at
high voltage instead of high current;
ƒ Explain how incandescent light bulbs work;
ƒ Explain what an electric ground is and how the idea of ground is used to make electrical devices
safer;
ƒ Understand how fuses, circuit breakers and ground fault circuit interrupters work and why they are
used;
ƒ Understand how electromagnets works and how they differ from regular magnets;
ƒ Know the basic principles of electrical safety; and
ƒ Understand how a motor works.
23.3. WIRING AROUND THE HOUSE
House wiring is always done in parallel, so that you don’t have to have something plugged into each
outlet for anything to work. Houses have different sets of wires that carry current to different places.
23.3.1. AC. AC stands for ‘Alternating Current’. The
‘alternating’ part comes from the fact that both the current and
Voltage
Or
the voltage change their magnitude and sign as a function of
Current
time. Note that, since resistance is usually constant, voltage and
current oscillate. Each cycle consists of an increase in current,
current going back to zero, a decrease in current, current going
back to zero, as shown in Figure 23.1. This is repeated over and
over. The voltage is usually 110 V or 120 V for residential
Time
electricity.
A battery provides DC or direct current.
The graph
corresponding to Figure 23.1 would be a straight line for DC Figure 23.1: This plot shows the
current or voltage. In DC current, the electric field moves current (or voltage as a function of
through the wire. In an AC current, there is no net motion of time.) for ac power.
the electrons (the drift velocity is zero.) The electric field
oscillates back and forth.
In the U.S., the AC coming out of a wall socket has a frequency of 60 Hz, which means that the current
goes through one oscillation every 1/60th of a second. The reason that U.S. appliances don’t work in
Europe is because Europe has 50-Hz wall outlet AC and the voltage is about 230 V or 240 V.
23.3.2. Household Wiring. An appliance, such as a refrigerator or hair dryer, has a fixed resistance.
Houses provide a fixed amount of voltage. The power rating of an appliance tells you how much current
is required for that appliance to work. Household circuits generally are fused at 15 or 20 amps. Large
appliances have their own circuits because they draw a lot of current. One circuit typically provides
from 1650 to 2400 watts of power.
23.4. TRANSFORMERS
23.4.1. Electromagnetic Induction. We know that a moving charge (a current) creates a magnetic
field. Can a moving magnetic field create a current? The answer is yes. If a magnetic field changes
near a loop of wire (or if the loop of wire is moved in a constant magnetic field), a voltage is induced in
the loop of wire. (The wire must be a loop because there must be a complete path for a current to flow.)
A voltage produced using a magnetic field and a current loop that move with respect to each other is
called the ‘induced voltage’ (and the current is called the ‘induced current’). The process of inducing
current and voltage is called electromagnetic induction. The magnitude of the induced voltage is
proportional to the number of wire loops, the strength of the magnetic field and the rate at which
magnetic field lines are cut by the wire. The direction of the induced voltage changes when the
magnetic field changes direction.
23.4.2. The Need for Transformers. Electricity is made by converting one type of energy into another.
For example, turbines can generate electricity when they are turned by water, which is how Hoover Dam
supplies power. The electrical power generated must be transported to where it will be used. A machine
that produces electricity is called a generator.
Originally, direct current (DC) generators could only be connected to loads that required the same
voltage that the generator provided because converting DC current of one voltage to another voltage was
difficult. This meant that very large currents had to go through the wires. There are two problems with
this. First, there are losses when power is transmitted. The power lost is given by
P = I 2R
(23.4.1)
where R is the resistance of the cable you are using. The further you live from the power generation site,
the more cable is required, the higher the resistance, and the more power is lost on its way to your house.
Second, carrying large currents requires very thick cables for long distances. The sheer weight of the
cables, plus the danger of high currents if power lines were downed, made this an untenable situation.
In the late 1880s, Nikola Tesla and Thomas Edison became adversaries. The electrical power grid of the
country was beginning to be established. Edison wanted the power grid to be established using direct
current (DC). Many of Edison’s patents were for devices that used direct current, and DC was the first
standard used in the United States. Tesla, however, devised a system that could be used to efficiently
transmit and utilize AC power. He partnered with George Westinghouse to commercialize the system.
Its efficiency eventually won out over Edison’s arguments for DC.
According to Equation (23.4.1), higher currents result in greater losses. Given a fixed resistance,
increasing the voltage lowers the current (Ohm’s law) and decreases the loss. It is thus more efficient to
transmit electricity at very high voltages. When the voltage reaches a house, however, it must be
decreased again to the standard 120V.
23.4.3. How Transformers Work. Transformers are used to convert voltages. Transformers are
installed at substations and used to increase the voltage from the generating plant, and then again to
decrease the voltage coming into homes. Increasing the voltage reduces the current in the transmission
and distribution lines and decreases the size of the cables required and power losses. Transformers
made it more economical to distribute power over long distances and electricity could be brought into
more and more remote regions.
A transformer has two parts: A primary coil, which is connected to a source of alternating current, and a
secondary coil, which must be fairly close to the first coil and output the transformed voltage. An
electrical current running through the primary coil creates a magnetic field. This magnetic field interacts
with the wire in the second coil and creates a voltage in the second coil via electromagnetic induction.
Remember that an ac field qualifies as a moving magnetic field. The amount of wire on each coil of the
transformer determines how the voltage can be changed. The magnitude of the voltage induced in the
secondary coil Vs is determined by the number of turns in each coil.
Vp
Np
=
Vs
Ns
(23.4.2)
where Vp is the voltage in the primary coil, Vs is the voltage in the secondary coil, Np is the number of
loops in the primary coil and Ns is the number of loops in the secondary coil. The ratio V/N is the
voltage per loop. Equation (23.4.2) tells you that the primary and secondary coils each have the same
voltage per loop. The number of loops thus controls the voltage.
A second relationship is that (in the ideal case that there are no power losses in the transformer), the
amount of power in the primary coil is equal to the amount of power in the secondary coil. In symbols:
I pV p = I sVs
(23.4.3)
where Vp is the voltage in the primary coil, Vs is the voltage
in the secondary coil, Ip is the current in the primary coil and
Is is the current in the secondary coil.
A step up or step down transformer steps up or steps down
the voltage of an alternating current according to the ratio of
wire loops in the primary and secondary coils.
In
Figure 23.2, the top transformer is a step-down transformer.
The voltage on the left side is the 120 V that would come
from the wall outlet. If you wanted to use power from the
wall outlet for something that usually runs on batteries, you
have to decrease the voltage. The side with the larger
number of turns always has the higher voltage. The ratio of
turns is 10:1, so the voltage out will be one-tenth the voltage
in. The bottom transformer is a step up transformer and
would be used to operate a piece of equipment that required
a very high voltage. The ratio of turns is 1:10, so the voltage
on the secondary is ten times the voltage on the primary.
Primary Coil
Np = 10 turns
120 V
Primary Coil
Np = 1 turn
Secondary Coil
Ns = 1 turn
12 V
Secondary Coil
Ns = 10 turns
1,200 V
120 V
Figure 23.2: A step-down (top) and
step-up (bottom)
EXAMPLE 23.1: We need to increase a voltage from 145 V to 261V. What is the ratio of turns on the
primary to turns on the secondary necessary to do this?
Draw a picture
known:
No figure is necessary
Vp = voltage on primary = 145 V
Vs = voltage on secondary = 261 V
Np
need to find:
= ratio of turns on primary to
secondary
Vp
Equation to use
Np
Np
Solve for unknown
Ns
Np
Insert numbers
Ns
Ns
=
=
Vs
Ns
=
Vp
Vs
145 V
261V
= 0.555555
Np
Answer to part b:
Ns
=0.556
Any time you are stepping up the voltage (the secondary voltage is higher than
the primary voltage), the ratio of turns on the primary to turns on the secondary
should be less than one.
Comments
EXAMPLE 23.2: We would like to increase the voltage from 145 V to 261V. If there are five turns on the
primary, how many turns on the secondary are necessary?
Draw a picture
known:
Vp = voltage on primary = 145 V
Vs = voltage on secondary = 261 V
Np = 5 turns
Vp
Equation to use
Np
Ns =
Solve for unknown
Insert numbers
Answer
Comments
N s = number of turns on secondary
need to find:
=
Vs
Ns
Vs
Np
Vp
261 V
( 5 turns )
145 V
= 9 turns
Ns =
N s = 9 turns
The number of turns on the secondary is greater than the number of turns on the
primary, which is what we expect.
23.5. LIGHT BULBS
Sir Joseph Swan of England and Thomas Edison of America (in 1878 and 1879, respectively)
independently invented the light bulb. Within 25 years, millions of people around the world had electric
lights in their homes. Although Edison usually gets most of the credit, he had a large group of very
talented employees who made important advances. For example, Lewis Howard Latimer, an AfricanAmerican draftsman and inventor who worked for Edison, invented a method of making carbon
filaments that made light bulbs more reliable. Latimer also
supervised installation of electric lights in the cities of New
York, Philadelphia, Montreal, and London.
23.5.1. Light and Heat. What happens to the power lost given
by Equation (23.4.1)? The answer is that the energy of the
current is transformed into heat. If the heat flows into the right
material, the material will become hot enough to glow and
produce light. This is the principle on which light bulbs, space
heaters and toasters work. A material with a higher resistance
will heat up more than a material with a lower resistance, and a
material will heat more if more electrons are going through it
Figure 23.3: The insides of an
than if fewer electrons are going through it.
incandescent light bulb.
23.5.2. How Light Bulbs Work. Incandescent light bulbs are fairly simple. A clear light bulb will give
you a good view of the parts of the incandescent light bulb, as shown in Figure 23.3. The most
important part of the bulb is the filament, which is a coiled wire in the center of the bulb. One end of the
filament (shown in red in Figure 23.3) travels to the side of the bulb, connecting with the metal part of
the base that screws into the socket. The other side of the filament travels to the pointy part at the very
end of the bulb. The other structures are primarily there to hold the filament in place and ensure that the
two sides of the filament don’t touch each other. The light bulb works because current goes through the
high-resistance filament and heats it hot enough (about 2200-2500 °C, which is about 4500°F) that it
glows. Light bulbs are filled with inert gas because exposure to air hastens the degradation of the
filament. The inert gas is usually argon, argon/nitrogen mixed or, in more expensive bulbs, a
krypton/xenon mix. The filament of a standard 60-watt light bulb is a tungsten wire over six feet long,
but less than one-hundredth of an inch in diameter. The wire is coiled to make
a filament just a few centimeters long so that it fits in a compact bulb. The
wire is long and thin because that shape provides the most efficient conversion
of electrical energy to light and heat.
Almost 95% of the energy that goes into a light bulb comes out as heat, so
light bulbs are very inefficient. The color emitted by a bulb depends on the
temperature to which the filament is heated. The temperature of the Sun is
5800°C (10,500°F), which produces the natural sunlight with which we are
familiar. Unfortunately, no material we know of can be heated to that
temperature without melting, so the incandescent light bulbs we use have a
different color light than natural sunlight.
23.5.3. Why do light bulbs burn out? The fundamental reason is that the
The
filament breaks. That’s what you hear rattling around when you shake a light Figure 23.4:
insides
of
a
three-way
bulb. When a material such as tungsten is heated to very high temperatures,
atoms from the surface actually leave the filament (which may cause a black light bulb. The two
film on the light bulb). The process of changing from a solid to a gas is called filaments are on the
sublimation. When enough atoms are ejected from the filament, it will become left and right.
weak and break, which means that the light bulb is now an open circuit. Tungsten sublimes around
2500°C, but the process is relatively slow.
Light bulbs are filled with inert gases because exposure to air hastens the decay of the filament. The
inert gas is usually argon, a mix of argon and nitrogen or a mix of krypton and xenon in more expensive
bulbs. These gases slow the sublimation of the tungsten filament.
23.5.4. How are halogen bulbs different than regular light bulbs? Halogen bulbs take this filamentrebuilding step even further. There is a chemical reaction between the tungsten atoms and the halogen
gasses (which usually are bromine and/or iodine). The halogen gases react with the tungsten atoms that
deposits on the glass bulb and return them to the filament. This is not a controlled process, however,
and the filament changes from being uniform to being bumpy. Eventually, one spot will become weak
and break, so the filament is only as strong as the weakest part. Recycling the filament allows halogen
lamps to operate at higher temperatures than conventional light bulbs, so the light produced is whiter
and the halogen bulb is more energy efficient. The halogen gases that are used can be toxic, which is
why these types of bulbs carry warnings about not touching their insides if the bulbs are broken.
23.5.5. How do three-way bulbs work? Three-way bulbs usually have two filaments. The three levels
of brightness are given by lighting filament 1, filament 2 or both filaments. Figure 23.4 shows one
filament on the left and one on the right. The filaments are two different thicknesses, which produce
different brightness levels.
23.5.6. Fluorescent Light Bulbs provide light using an entirely different process. The electric current
excites gas molecules that are trapped in the bulb. The gas molecules then emit ultraviolet light (which
is not visible) and the ultraviolet light excites phosphors on the inside of the tube, which convert the
ultraviolet light to visible light. Fluorescent lights are cooler and more energy efficient, but the
spectrum of light they produce is markedly on the blue side and often makes things appear very
differently than they do under natural sunlight. This is why sometime things you buy at a store appear a
different color when you get them home and look at them under natural sunlight.
23.5.7. Light Bulb Characteristics. Bulbs are rated by voltage and resistance. The batteries you use
must provide at least the amount of voltage stated to light the bulb. Christmas tree mini-lights, for
example, are 2.5 V lights with a resistance of about 7-8 Ω. They can be lit with two D-cells, or with a
9-V battery. If you use a 9-V battery, the bulb won’t last very long because they are designed to run at
2.5 V. They probably will burn out in 30-60 minutes. You can light the bulb with one D-cell, but it
won’t be very bright.
23.6. GROUNDING
Why do some plugs with two prongs have one prong
larger than the other? Why do some plugs have three
fuse
Hot wire
prongs? The answer has to do with the way circuits
are wired. In general, there is a ‘hot’ wire and a
ground wire in two-prong circuits.
Ground wire
What is ground? You can think of the wire that goes
to ground as a very low resistance path that all the
electrons would like to take. Ground usually is
somewhere where the electrons can be safely Figure 23.5: A two-prong plug.
dissipated.
A real ground is often a long metal pipe stuck in the ground. The Earth is so massive that it acts like a
source or sink of electrons. You can deposit or withdraw many electrons and not change the overall
charge of the Earth because each electron makes a miniscule contribution to the total charge. The third
prong you see on electrical plugs is a ground – it is there because, if there is a need for the electrons that
are being carried in the other two wires to leave those wires, they need go through a safe path and not,
for example, through a person. The ground plug funnels the electrons safely away from people. If
current is flowing and it has a choice of going through you or through a metal wire to ground, it will take
the wire because the resistance of the wire is less than the resistance of the human body (unless, of
course, the human body is wet).
Plugging in a lamp makes a complete circuit. The current runs from the hot wire through the appliance
cord, through the light bulb filament and back through the ground wire, as shown in Figure 23.5. Light
bulbs are not particular about which way the current goes through the filament, but some appliances
require that the current pass through them in a particular direction. Making the plug polarized (i.e. one
prong is larger than the other so that it only fits one way into the electrical outlet) allows the
manufacturer to control which way the current goes through the device.
hot
Electrons can flow through you
if you are touching the case and
there is a short to the case
hot
The third prong provides an
easy path to ground
Figure 23.6: A two-prong plug (left) compared to a three-prong plug (right).
There is a safety hazard with two-prong plugs, however. If you are using something with a metal case –
like a power tool – you have to be worried about a short circuit that provides a path for electrons that
goes through the part of the tool you hold. If the proper path is broken for some reason (like the cord is
frayed), the electrons have nowhere to go except the case. If you’re holding the case, the electrons can
go through you because you are the shortest path to ground. If there is a third prong, the case can be
grounded, providing a safe way for the electrons to leave the tool if there is a short, as shown in
Figure 23.6.
23.7. SWITCHES AND FUSES
23.7.1. Switches. A switch allows you to create an open circuit. When the switch is open, there is no
complete path for the current to take. The current sees, in effect, an infinite resistance. An item plugged
into a wall switch draws current when turned on. A switch allows us to save energy by preventing it
from drawing current.
23.7.2. Fuses. Although we want wires to heat up in a light bulb, we don’t want the wires to our lamps
to heat up because they could cause a fire. Many pieces of electronic equipment can handle only some
maximum current or they will be damaged. A fuse is a piece of wire or other metal designed to melt at a
pre-set current. The material selected and fuse characteristics are determined by calculating how much
heating will occur when the maximum current passes through the material. The fuse is designed so that
it melts when that amount of current goes through it. The melting of the fuse causes an open circuit and
the potentially damaging current doesn’t flow to any other more sensitive devices. Melting the metal
strip is called ‘blowing a fuse’.
The fuse is a one-time device: Once it is melted, you have to replace it. You can generally tell if a fuse
is good by looking to see whether it is continuous. A
fuse was one of the essential features of Edison's
electrical power distribution system. An early fuse was
positive charges
N
positive charges
said to have successfully protected an Edison
installation from tampering by a rival from a gaslighting company that wanted to sabotage his work.
23.8. ELECTROMAGNETS
A current creates a magnetic field and the strength of
the magnetic field is proportional to the amount of
current passing through the wire. If you want
S
tmagnetic field from a wire, one option is to put more
current through the wire; however, this creates
problems. The current generates heat, which can melt
wire, plus large currents are costly to produce. A Figure 23.7: The magnetic field produced by
a loop of wire.
second approach is to make a solenoid. As Figure 23.7 shows, a single wire loop has all of the fields
pointing the same way (up) inside the loop and all of the fields pointing the same way outside the loop
(down). A loop thus produces a field. If we stack a bunch of loops, all with the current going the same
Figure 23.9: The magnetic field produced by a coil of
Figure 23.8: A magnetic material strengthens
wire is strengthened when a magnetic material is
the magnetic field compared to if just the air
introduced. This is the same principle as wrapping
were present.
wire around a nail to make an electromagnet.
way, the fields from each loop add, creating a much larger field while still using the same amount of
current. All of the field lines are concentrated in the center of the solenoid, creating a strong magnetic
field. This is called an electromagnet. The advantage of an electromagnet over a regular bar magnet is
that an electromagnet can be turned on and off. Electromagnets are used, for example, in junkyards to
pick up and move cars. Turning off the electromagnet allows the car to be released.
The magnetic field can be made even stronger by using a magnetic material to further focus the
magnetic field lines. If the gap between two poles is filled with air (Figure 23.8), the magnetic field
lines make their usual pattern (top picture). If, however, you place a piece of iron between the poles, the
iron will be magnetized by the field produced by the current. The magnetic field lines are drawn into the
iron, thus strengthening the magnetic field. This can be used to make a stronger electromagnet, as
shown in Figure 23.9, but one that still can be turned on and off. The number of turns in the coil
determines how large the field is. A coil with 50 turns produces a field 50 times greater than a coil with
one turn.
23.9. CIRCUIT BREAKERS
A circuit breaker serves the same function as a fuse; however, a circuit breaker is designed to be able to
be reset.
23.9.1. Simple Circuit Breaker. A circuit breaker does the same thing as a fuse – it prevents current
from flowing if the current becomes too large. A simple circuit breaker is illustrated in Figure 23.10.
The left side shows the closed configuration, in which current is allowed to flow. The current from the
hot wire wraps around a magnet, forming an electromagnets. The wire, number or coils, etc. are chosen
so that the electromagnet is turned off if the current is less than the maximum allowable current. If the
current exceeds this value, the electromagnet becomes strong enough to pull apart the contact and break
the path for the current, as shown in the right side of Figure 23.10.
S
N
Pivot
Current out
S
N
Pivot
Spring
No Current
out – open
circuit
Current from
live wire
Spring
Current from live wire is large
enough to activate the
electromagnet
N
N
Current from
live wire
Neutral wire
N
Current from
live wire
Neutral wire
S
S
Pivot
N
Pivot
S
S
Figure 23.10: A simple circuit breaker in the usual position (left) and when tripped by too large
a current (right)
When the circuit breaker trips, there is no current running through the circuit and the electromagnet
turns off. The contact can be pushed back together (that’s what you’re doing when you flip the switch
of a circuit breaker that has been tripped) and will stay together unless the current exceeds the maximum
value again.
23.9.2. Ground Fault Circuit Interrupter (GFCI). A GFCI is a more sophisticated type of fuse, with
the goal of protecting people from electric shocks rather than protecting wiring. A GFCI works
compares the current going into and the current coming out of a device. If they aren’t equal, it means
that current must be going somewhere not in the planned circuit and the GFCI shuts off the current.
Figure 23.11 (left) shows that the current from the hot and the neutral wires both pass around
electromagnets. The gold bar in the middle is a magnetic rocker. The strength of the magnetic field is
the same in both coils because they both have the same current. If the circuit is functioning correctly,
both ends of the rocker are equally attracted by the electromagnets; however, if all the current that goes
into the circuit doesn’t come out, the magnetic field on one side of the GFCI decreases, which opens the
switch, as shown in the right-hand side of Figure 23.11.
Figure 23.11: A simple circuit breaker in the usual position (left) and when tripped by too large
a current (right)
The GFCI breaks the circuit as soon as there is any change between current in the two arms of the
device, whereas the circuit breaker waits until some preset level has been reached. The GFCI reacts
much more quickly than a conventional breaker. A GFCI can sense leaks of as little as 0.005 A and can
break the circuit in 0.025 s.
23.10. ELECTRICAL SAFETY
The human heart depends on electrical signals to tell it when to beat. If current from an external source
travels across your heart, it can cause the heart to go into fibrillation. The paddles used to get a heart
beating (defibrillators) supply a current, but it’s a specific (small) current that is meant to get your heart
beating correctly again. You will often see electricians hold one hand behind their back (usually their
left one) when working so that they are less likely to form a complete circuit.
Most people can feel a current as small at 0.0005 A. A current of 0.005 A is painful and one of 0.01 A
can cause muscle spasms that might result in your not being able to let go of the source of the current.
Current greater than 0.18 A prevents breathing. The most important rule of electrical is thus: Don’t
make yourself part of a circuit. This may not be as easy as you think because you can make yourself
part of a circuit without knowing it fairly easily.
The human body’s resistance is about 1MΩ (although this depends on the person’s size, amount of body
fat, etc.) If you’re wet, your resistance can decrease to 1000 Ω. A potential difference of 120 V will
yield a current of about (Ohm’s Law isn’t exactly valid in this case, but it is close enough to give us an
estimate).
V
R
120 V
=
1000 Ω
= 0.12 A
I=
This is a large enough current to be harmful. If
you are not grounded, you are essentially an
infinite resistance; however, if you are standing on
(or very close to) something metallic, or if your
skin is wet (which significantly decreases your
resistance), you present a more attractive path for
the current to reach ground.
This is why
bathrooms and kitchens are more dangerous in
terms of electrical hazards than other parts of the
house.
23.11. MOTORS
In a motor, electrical energy is converted to
mechanical energy. The motor has two working
parts. A stationary electromagnet called a field
N
brushes
N
S
commutator
S
Armature
Load
Figure 23.12: A simple motor.
coil
commutator
commutator
armature
commutator
axle
armature
axle
Figure 23.13: Different views of the commutator, armature and axle.
axle
magnet and a cylindrical, movable electromagnet called an armature. The armature is on an axle and
rotates in the magnetic field of the field magnet. The field magnet can be a permanent magnet or an
electromagnet. A simple motor is shown in Figure 23.12.
The armature is an electromagnet, so it can be turned on and off when desired. When the current is on,
the unlike poles of the two magnets attract and the armature rotates. If the current is DC, the armature
would stop when it aligns with the opposite pole. The commutator, however, has insulated segments
(see Figure 23.13) so when it turns halfway, the commutator segments switch brushes and the current
goes through the armature in the opposite direction. This switches in the current changes the poles in
the armature and the armature rotates further. When it had rotated another half turn, the current again
switches direction and the armature rotates another half turn. There are many different configurations,
but they all work on the same principle.
The wires from the winding of the armature are attached to the commutator – one to each half.
Figure 23.13 shows the armature/commutator assembly. Since the commutator rotates with the axle,
small stationary brushes maintain contact with the commutator and serve as electrical contacts. A DC
motor applet can be found at: http://home.a-city.de/walter.fendt/phe/electricmotor.htm
23.12. SUMMARIZE
23.12.1. Definitions: Define the following in your own words. Write the symbol used to represent the
quantity where appropriate.
1. Electromagnetic induction
2.
Induced voltage
3.
Generator
4.
Primary coil and secondary coil (for a transformer)
23.12.2. Equations: For each question: a) Write the equation that relates to the quantity b) Define each
variable by stating what the variable stands for and the units in which it should be expressed, and c)
State whether there are any limitations on using the equation.
1. The relationship between the current running through a power transmission line and the power loss
due to the resistance of the wire.
2.
The relationship between the voltages that can be produced in a transformer and the number of
turns on each coil.
23.12.3. Concepts: Answer the following briefly in your own words.
1. What does a transformer do and why are transformers necessary?
2.
How does a transformer work?
3.
Why are circuit breakers rated in terms of current and not voltage?
4.
What is the difference between a step up and a step down transformer?
5.
On what factors does the magnitude of the induced voltage depend?
6.
What is an electrical ground?
7.
Explain why electrical plugs have three wires.
8.
Explain how a fuse works and how that differs from a circuit breaker.
9.
How does an electromagnet differ from a bar magnet?
10. An electromagnet a) uses an electric current to produce a magnetic field; b) uses a magnetic field to
produce a current; c) operates only on ac power; d) has to have a magnetic core to work.
11. A transformer can change a) the voltage of ac current; b) the power of ac current; c) ac power to dc
power; d) dc power to ac power
12. Why are you more likely to experience a life-threatening shock when you are wet than you are
when you are dry?
23.12.4. Your Understanding
1. What are the three most important points in this chapter?
2.
Write three questions you have about the material in this chapter.
23.12.5. Questions to Think About
1. Explain how a motor works.
2. What happens to current when it passes through a light bulb?
23.12.6. Problems
1. A transformer is going to be used to step down voltage from 120 V to 45.0 V. (Assume 3 s.f. for
the 120 V figure.) If there are 8 turns of wire on the primary, how many turns of wire must there
be on the secondary?
2. A transformer has a current of 12.0 A in the primary and 30.0 A in the secondary. What is the ratio
of turns in the primary to turns in the secondary?
3. A transformer has 600 turns on the primary and 200 turns on the secondary. If the secondary
voltage is 80.0 V, what is the primary voltage? If the transformer delivers 300 W, what is the
primary current?
PHYS 261 Spring 2007
HW 24
HW Covers Class 23 and is due March 5th, 2007
1.
2.
3.
Explain in your own words how a light bulb works, including what has happened when a light bulb
‘burns out’.
A transformer has 600 turns on the primary and 200 turns on the secondary. If the secondary
voltage is 80.0 V, what is the primary voltage? If the transformer delivers 3.00×102 W, what is the
primary current?
Explain how a fuse works. What are the differences between fuses and circuit breakers?
23.13. RESOURCES
Moving magnet and wire coil connected to ammeter.
Current flow applet: AC vs. DC
http://www.ndt-ed.org/EducationResources/CommunityCollege/EddyCurrents/Physics/currentflow.htm
Electrical
generator
applet:
http://www.sciencejoywagon.com/physicszone/lesson/otherpub/wfendt/generatorengl.htm
23.14. ELECTROMAGNETIC SWITCHES
Electromagnetic switches can be used to control larger (often high voltage) pieces of equipment. The
switch itself can be relatively low voltage. These are often called ‘relays’.
If you put two metals, which each expand at a different rate when the temperature changes back to back
and form them into a coil, the result is a coil that winds or unwinds when there is a change in
termperature. A glass tube filled with mercury is attached to the coil, and attached to the glass tube are
two pieces of wire, as shown below. When the coil is at its ‘off’ position, the circuit is not complete.
When the coil expands or contracts, the glass vial tips and the liquid mercury (which is a conductor)
completes the circuit.
No circuit
Off position
Complete circuit
Now attach to this a circuit such
as the one shown at right (from
your text). The electromagnet is
a coil of wire with an iron core (a
solenoid). When the circuit with
the mercury switch is completed,
the electromagnet is charged and
pulls the spring-loaded piece of
iron toward it, thus making a
complete circuit.
On position
23.15. GENERATORS
In a motor, you provide a current through the wire and the interaction between the magnetic field and
the current-carrying wire produces motion. This is a conversion of electrical energy to mechanical
energy.
A generator provides mechanical energy and produces electrical energy. The set up is similar to that of
a motor: there is an armature (here shown as a wire loop) that rotates in a magnetic field. The rotation
was originally provided by things waterwheels or turbines. As the wire moves through the magnetic
field, a current is generated. The current generated may be alternating current (AC) or Direct Current
(DC).
There are two configurations for generators. In a DC generator, all of the current (or voltage) is
positive, although it does vary in magnitude. An electronic circuit called a rectifier can be used to
average out the current or voltage and produce a direct current (one that doesn’t change sign). The
figure below is from your book and shows your author’s diagram for a DC motor. The graph below the
picture shows the output voltage from the DC generator. Contrary to what you might expect, the voltage
out is not constant; however, the voltage does not change signs at any point. (There are electronic ways
of averaging out the bumps in the voltage to provide what is essentially a constant voltage.)
Alternating current is useful for many things, but in some cases, DC current is required. In a DC
generator, all of current is positive, although it does vary in magnitude. An electronic circuit called a
rectifier can be used to average out the current or voltage and produce a direct current (one that doesn’t
change sign). Figure 22.4 shows a DC motor. The graph below the picture shows the output voltage
from the DC generator. Contrary to what you might expect, the current/voltage out is not constant;
however, the current/voltage does not change sign.
The difference between the DC and the AC generator is the connection that is used. A single, split ring
called a commutator is used for a DC generator. (Your book’s Figure 22.6b does not show the
difference very clearly.) The commutator is split, so that every half turn, there is a break in the circuit
(this is when the voltage/current out goes to zero). When it starts up again, it is essentially as if the two
connections switched sides, so instead of the voltage/current going negative, you get another positive
loop.
We can also look at an applet that shows us a DC generator:
http://home.acity.de/walter.fendt/phe/generator_e.htm
http://www.sciencejoywagon.com/physicszone/lesson/otherpub/wfendt/generatorengl.htm
23.15.1. DC Generators
In general, multiple loops of wire are run with different positions relative to the field. The sum of all
these different voltages can be made to approximate a constant voltage, as shown in
Voltage
time
Voltage
time
Figure 23.1: Combining multiple loops that start at different points to get something
approximating a constant voltage
23.15.2. AC
AC stands for ‘Alternating Current’. The ‘alternating’ part comes from the fact that both the current and
the voltage change their magnitude and sign as a function of time. Each cycle consists of an increase in
current, current going back to zero, a decrease in current, current going back to zero. This is repeated
over and over.
Voltage
Or
Current
Time
In the US, the AC coming out of your wall has a frequency of 60 Hz. This means that the current goes
through this oscillation every 1/60th of a second.
The time it takes for the current to go through one oscillation is called the period and is represented by
T. The period and frequency are related:
f =
1
T
(23.15.1)
where f is in s-1 or Hz and T is in s.
Note that, since resistance is usually constant, voltage
and current oscillate.
The difference between and AC and a DC generator is in
how the current is extracted from the generator. When a
loop of wire is physically rotated, the long sides of the
wires cut across the lines of force and a current is
induced. The greater the rate at which the lines are cut,
the greater the amount of current induced (i.e. the faster
the loop spins, the more current.) Figure 20.1 illustrates
this process. The lower graph is a plot of the
voltage/current out as a function of the position of the
wire with respect to the magnetic field. The current is
zero when the wire loop is perpendicular to the magnetic
field lines, becomes increasingly positive, maximizes
when the loop is parallel to the field lines, then
decreases, and reaches zero again when the loop is
perpendicular. When the loop crosses over to the
second half of its rotation, the forces are in opposite
directions and the current is negative.
Focus on a single charge in the wire: When the
armature moves, the charge feels a force due to the field
that pushes it along the wire – this is a current. The
same thing happens on both sides of the loop, creating a
current that goes all the way around the wire, and thus
creating a potential difference between the two rings that
are used for connectors. One end of the loop is
connected to one of the rings and the other end to the
other ring.
In an AC generator, the commutator is replaced by two
continuous rings. This produces an alternating voltage
that changes sign.
Figure 23.2:
An AC generator (from
Tillery’s Physical Science text)
The same applet can be used to see how the AC
generator works.
http://home.a-city.de/walter.fendt/phe/generator_e.htm
23.16. ALTERNATORS
Figure 23.3:
A DC generator (from
Tillery’s Physical Science text)
Alternators are used in cars. The basic set up is
that a rotor (armature) is driven by the alternator pulley. The current to the armature (which creates a
magnetic field) is provided by the battery. A stator, which is another set of coils, is fixed in position
outside the rotor. When the rotor turns, it induces a voltage in the stator coils. There are three sets of
stators, located 120 degrees apart around the rotor. This means that each coil has an induced voltage
that varies in time, but the voltages are out of phase so that at any given time, one of the coils is near its
maximum voltage.
This is AC current, and a car works on DC, so the output is run through diodes (devices that allow the
current to only flow in one direction), to produce something that looks more like dc current (shown in
top of figure below). Since there are three voltages, the net result is something that looks enough like
DC to run the car.
The alternator plays an additional role because it is connected through a feedback loop to sense the how
fully your battery is charged. When you run your wipers, play your radio, use your heater, etc., it drains
the battery. When the battery voltage decreases, the alternator senses this and ramps up the field current,
increasing the voltage output of the alternator. Conversely, if the battery voltage goes up, then the
alternator decreases the current in the field and doesn’t put out as much voltage. The output of the
alternator is used to charge the battery: this is why if you idle for a long time, you can wear down your
battery, and why your battery re-charges when you drive.