Download Electro-magnetism

PPT
Electromagnetism
Developer Notes
Version
01
02
03
Date
2004/07/06
2004/07/07
2005/03/01
Who
sc
dk
dk
Revisions
Initial version
 expanded on the activity
 Included notes from summer 2004

Goals
1. Students should understand that a moving electric charge (current) will produce a magnetic
field.
2. Students should understand the right hand rule for moving charge producing a magnetic field.
3. Students should understand that coils reinforce current-induced magnetic fields.
4. Students should understand that bigger coils reinforce current-induced magnetic fields.
5. Students should understand that overlapping coils reinforce current-induced magnetic fields.
Concepts & Skills Introduced
Area
Physics
Technology
Concept
A moving electric charge (current) will produce a magnetic field
Electromagnets, DC motors
Standards Addressed
Time Required
Warm-up Question
Presentation
Make sure to emphasize that the magnetic effect works through space, and that work can be done
with it. This is necessary to understand electromagnetic radiation/ light.
These concepts are presented through stations. The stations are more complex than normal and
will take more time. You may need to spend several days on them.
Be sure to remind students of the field around a magnet, as they saw in the magnets activity. The
field is arbitrarily defined as going from north to south. A magnet (compass) placed in the field
will align itself with the field; north will aim in the direction of the arrows along the field lines
toward south. There are no magnetic monopoles, but if there were, a north monopole placed in
the field would have a force on it along the field lines toward the south.
147030788
1 of 12
PPT
Electromagnetism
Stations
It might be interesting at the end of the stations to have the students try to put them in logical
order of which should be taught first. They are not written in logical order.
1. Current and field in a wire
The north end of the compass should point in a clockwise circle when the + end of the
battery is connected to the top of the wire, and vice versa. This is the fundamental lesson
regarding a field around a moving charge. If follows the right-hand rule.
2. Current and field in a coil
The north end of the compass should point in a clockwise circle around one side of the coil,
and counter-clockwise around the other side. When viewing the coil so that it is wound
clockwise, and with conventional current flowing clockwise, the north end of the compass
should point away from you through the middle of the coil. This is a consequence of the
right-hand rule.
3. Solenoid
The paper clip should get pulled into the coil equally from both ends. It should end up more
or less centered in the coil since it is not magnetized itself. The double wrapped coil should
pull the paper clip with more force since the fields reinforce each other.
4. Electromagnet coils
More windings should pick up more paper clips because the field is reinforced. The nail
helps to concentrate the field.
5. Electromagnet direction
The un-energized nail may have a polarity, but it will be weak. For the other two, the right
hand rule must be applied. The direction of the field inside the coil will determine the
polarity of the nail. Reverse the current and the field reverses.
6. Electromagnet fields filings
The field should be very evident around the nail, not quite as obvious around the coil.
7. DC motor
The battery will spin using the right hand rule on the field in the coil. It will reverse
directions if the battery is reversed, and if the magnet is reversed. It will not reverse if the
coil is reversed because the direction of the coil stays the same. (Look at the coil in both
directions with the insulation up.) The field in the coil is pushed by the magnet’s field, the
motor starts to turn, the coil’s field turns off so the other side isn’t repelled the other
direction, inertia carries the coil around, and the same thing happens again. This motor
allows the students to see all the parts of a normal motor – coil, magnets, commutator, power
source – and to play with them. The key to the motor is the commutator.
8. Wire swing
The wire should swing toward one side of the magnet and stay there, although it may
oscillate a little. If conventional current runs from left to right and the north pole of the
battery is up, the wire will swing toward you.
9. Electromagnet current
With more current, the field will be stronger and the electromagnet will hold more paper
clips.
10. Electromagnet and ferrous
A ferrous core in a coil will reinforce the field.
147030788
2 of 12
PPT
Electromagnetism
Assessment
Observe – Set up the motor used in the stations and get it running.
Explain – Polarity, moving charge, field
Writing Prompts
Relevance
Answers to Exercises
1.
Answers to Challenge/ extension
1.
Equipment
Except as noted:
 Use single conductor (not stranded) 20 or 22 AWG PVC insulated wire (Radio Shack 2781219 or 278-1215). Single conductor wire holds its shape and is stiffer than stranded wire.
Strip the ends of the wire about 1 cm.
 Use 1.5 V batteries. 1.5 V batteries are common. D-cells hold a fair amount of charge.
Connecting to them is a bit of a pain, but one student can hold the wire ends on the battery
while the other touches them to the equipment. Battery holders with wires attached make it
easier (Radio Shack 270-403). Alligator clips are handy, but lead to leaving the batteries
connected, so we don’t recommend them.
1. Current and field in a wire
1
Small compass (<1” diameter)
30 cm
14-16 AWG wire
1
1.5 V D-cell battery
2
connecting wires
Mount the wire vertically so the compass works in a horizontal plane. Single strand copper
wire, 14-16 AWG, is strong enough to stay vertical on its own, but any wire will work. The
setup needs to be dressed so that the wires from the battery don’t affect the compass.
2. Current and field in a coil
1
small compass (<1” diameter).
1m
22 AWG insulated wire
1
1.5 V D-cell battery
Make a coil big enough to pass the compass through. About five turns is enough.
3. Solenoid
2
clear straws
1
small paper clip
2m
22 AWG insulated wire
1
1.5 V D-cell battery
Make a solenoid by wrapping 30 turns of wire in a single layer around the center of a clear
plastic straw. Straighten a small paper clip and put it in the straw. The clear straw is to reduce
147030788
3 of 12
PPT
4.
5.
6.
7.
Electromagnetism
friction on the paper clip and to allow students to see what is happening. Make a second
solenoid by wrapping the same number of turns of wire in a double layer to strengthen the
field.
Electromagnet coils
2
20 d (4”) common nails
1m
22 AWG insulated wire
30
jumbo paper clips
1
1.5 V D-cell battery
Wrap 15 turns of wire around a nail. Wrap 30 turns around a second nail. Clip 9 paper clips
on another to make 10. Make 3 groups of 5 paper clips, and leave 5 free.
Electromagnet direction
1
20 d (4”) common nail
0.5 m
22 AWG insulated wire
1
compass
1
1.5 V D-cell battery
Wrap about 15 turns of wire around the nail.
Electromagnet fields and filings
1
20 d (4”) common nail
1m
22 AWG insulated wire
1
1.5 V D-cell battery
1
drinking straw
Wrap about 30 turns of wire around the straw. Cut the ends of the straw off about 0.5 cm
from the ends of the coil.
DC motor
2 jumbo paper clips
1 5 cm x 5 cm corrugated cardboard
1m
22 AWG insulated wire
1 permanent marker
1 ring magnet
1 1.5 V D-cell battery
2 connecting wires
a. Supports
Bend the paper clips into an M shape with long legs and a small
center U. Bend the legs at right angles and slip them the corrugated
147030788
4 of 12
PPT
Electromagnetism
cardboard. The height of the clips can be adjusted somewhat by spreading the legs. The
width between them can be adjusted by sliding them in and out of the cardboard. You
could use bare copper wire to get better electrical contact.
b. Coil
Make a coil of about 3 cm diameter with about 10 turns of wire. Wrapping around a Dcell battery works well. Varnished coil wire is better than PVC insulated because the
varnish holds up better for a commutator (see below). 22 AWG is easy to work with. 18
or 20 AWG wire are better for strength, but 16 is stiff to work with. Loop the ends of the
wire around the coil and back under themselves so they stick out on opposite ends of the
coil. Play with the coil until it is well balanced and spins easily. Tape it together, top and
bottom – vinyl electrical tape is better than masking or transparent tape. The coil can be
round or rectangular or anything in between.
c. Commutator
Insulate one half of one lead of the coil. With the coil in a vertical position, the insulation
should be on the top half of the lead, not the side. For PVC insulated wire, coat it with
permanent marker. The permanent marker will need to be refreshed once in a while, and
may need to be scraped off the support. (Thanks to the Exploratorium for the permanent
marker idea.) For varnished wire, scrape off the varnish on one side.
d. Magnet
A bar, ring, or disk magnet will work. Label the polarity of the magnet. The stronger the
magnet, the better the motor will work. If someone wants to experiment, a second magnet
on the other side of the coil will affect the speed.
e. Power
More voltage produces more current, a stronger field, and more speed. A second battery
in series will increase the speed.
f. Setup – The closer the coil comes to the magnet, the better the motor will work.
8. Wire swing
30 cm
bare 18 AWG copper wire, cut in two 15 cm pieces
1
5 cm x 5 cm corrugate cardboard
10 cm
22 AWG wire
Make a support similar to the electric motor, but use bare copper wire to ensure good
electrical contact – there’s not much weight from the swing forcing it down. Use 18 AWG or
heavier wire for the supports (Radio Shack 278-1217, or
strip some household wire). The swing is a square U with
wings, an inverted hat section. The swing should be as
light as possible so that it deflects more – use 20 or 22
AWG wire, although 18 works, too, especially if stripped.
9. Electromagnet current
1
20 d (4”) common nails
0.5 m
22 AWG insulated wire
30
jumbo paper clips
Wrap 30 turns of wire around a nail. Clip 9 paper clips on another to make 10. Make 3
groups of 5 paper clips, and leave 5 free.
10. Electromagnet and ferrous
1
20 d (4”) common nail
1m
22 AWG insulated wire
147030788
5 of 12
PPT
Electromagnetism
5
jumbo paper clips
1
drinking straw
Wrap 30 turns of wire around the straw. Cut the ends of the straw off about 0.5 cm from the
end of the coil.
147030788
6 of 12
PPT
Electromagnetism
Background
You’ve learned about magnets and about charge and moving charge. They both consist of two
types where opposites attract and likes repel. They both can do work through space. Not
surprisingly, they’re related. Moving charge creates a magnetic field.
Problem
Investigate electromagnetism.
Materials
Stations are set up.
Procedure
In the following activities, DO NOT LEAVE THE BATTERY CONNECTED TO THE WIRE!
It will drain the battery, create a lot of heat, and the battery could leak or burst.
1. Current and field in a wire
a. With the battery disconnected, move the small compass around the vertical wire. Note the
direction of the compass needle at at least four points around the wire. Draw a diagram.
b. Momentarily connect the battery to the vertical wire. Make notes and a diagram as above.
c. Reverse the leads on the battery and do it again. Make notes and a diagram as above.
d. Disconnect the battery.
2. Current and field in a coil
a. With the battery disconnected, move the small compass around and through the coil.
Note the direction of the compass needle at at least seven points around the wire (four
places around each side of the coil, but sharing the middle). Draw a diagram.
b. Momentarily connect the battery to the coil. Note which way the coil is wound, which
way the battery is connected and make notes and a diagram as above.
c. Reverse the leads on the battery and do it again. Note which way the coil is wound,
which way the battery is connected and make notes and a diagram as above.
d. Disconnect the battery.
3. Solenoid
a. Place the straightened paper clip in the straw so that its end is inside the single coil. Can
you find an attraction between the coil and the paper clip? Try it with the double coil.
Any attraction?
b. Place the paper clip so that its end is just inside the single coil of wire. Momentarily
connect the battery. What happens?
c. Place the paper clip so that its end is just inside the double coil. Momentarily connect the
battery. What happens? Which is stronger, the single coil or the double coil (they each
have the same number of turns)?
d. Set up the same way with the coil and reverse the battery. What happens?
e. Try the paper clip from the other end of the coil. What happens?
f. Disconnect the battery
4. Electromagnet coils
a. Try to suspend paper clips from the heads of the electromagnets. How many will they
hold?
147030788
7 of 12
PPT
5.
6.
7.
8.
9.
Electromagnetism
b. Momentarily connect the battery to the electromagnet with fewer windings. Suspend as
many paper clips as you can from the head of the electromagnet. How many will it hold?
c. Momentarily connect the battery to the electromagnet with more windings. Suspend as
many paper clips as you can from the head of the electromagnet. How many will it hold?
d. Disconnect the battery.
Electromagnet direction
a. Hold each end of the nail near the compass. Does it have north and south poles? Draw a
picture of the compass and nail.
b. Momentarily connect the battery to the electromagnet and hold each end near the
compass. Note which way the coil is wound, which way the battery is connected, and
which end of the electromagnet is north. Draw a diagram.
c. Reverse the battery connections. Make notes and a diagram as above.
d. Disconnect the battery.
Electromagnet fields and filings
a. Put the nail through the wire coil/ straw. Place the paper over it. Momentarily connect the
battery. Sprinkle iron filings on the paper until you can see the pattern of the magnetic
field. Look closely, and you may see some of the filings standing on their ends.
Disconnect the battery. Draw a picture of the field. Make a U-shape with the paper and
pour the filings back in the container.
b. Remove the nail from the straw. Place the paper over the wire coil. Proceed as above.
c. Put the nail through the wire coil/ straw. Place the paper over it. Do NOT connect the
battery. Proceed as above.
d. Disconnect the battery.
DC motor
First, notice the insulation (permanent marker or varnish) on one side of one of the motor
wires. For each of the following, note the direction the coil is wound when the insulation on
the motor wire is up, the polarity of the magnet, and which way the battery is connected.
Draw a diagram.
a. Connect the battery to the motor supports (paper clips). You may need to (gently) help
get the motor started spinning. Which direction does the motor keep spinning?
b. Reverse the battery leads. Which direction does the motor spin?
c. Flip the magnet over. Which direction does the motor spin?
d. Reverse the ends of the motor. Which direction does the motor spin?
e. Disconnect the battery.
Wire swing
a. Connect the battery to the wire supports. Note which way the wire swings, which way the
battery is connected, and which pole of the magnet faces up. Draw a diagram.
b. Reverse the battery hookup. Make notes and a diagram as above.
c. Flip the magnet over. Make notes and a diagram as above.
d. Disconnect the battery.
Electromagnet current
a. Try to suspend paper clips from the head of the electromagnet. How many will it hold?
b. Momentarily connect the battery to the electromagnet. Suspend as many paper clips as
you can from the head of the electromagnet. How many will it hold?
c. Momentarily connect the two batteries in series to the electromagnet. Suspend as many
paper clips as you can from the head of the electromagnet. How many will it hold?
147030788
8 of 12
PPT
Electromagnetism
d. Disconnect the batteries.
10. Electromagnet and ferrous
a. Put the steel nail through the wire coil/ straw. Momentarily connect the battery. How
many paper clips will the coil with nail hold?
b. Put the aluminum nail through the wire coil/ straw. How many paper clips will the coil
with nail hold?
c. Remove the nail from the straw. Now how many paper clips will the coil hold?
d. Disconnect the battery.
Summary
1. Moving charge creates a magnetic field. Use what you have learned in the stations about the
direction of a magnetic field around a current-carrying wire to go back and explain what
happened at each station. Hints: Think about what would happen if you put a N or S pole at
that point in the field. Don’t forget Newton’s 3rd Law of Motion.
147030788
9 of 12
PPT
Electromagnetism
Reading
Charge comes in two types. Opposites attract, it operates through space, and it can do work.
Magnetic poles come in two types. Opposites attract, it operates through space, and it can do
work. It is not surprising that the two are related. Moving charge creates a magnetic field.
Since all magnetic fields have north and south poles, the magnetic
field created by moving charge also has polarity. As you saw with
simple magnets, a magnetic field is defined as going from north to
south. That means that a magnet (compass, for example) placed in the
field will tend to align itself with the field, with the north end pointing
in the direction of the field – toward south.
N
S
For moving charge, the field created is circular, perpendicular to the path of the moving charge.
The polarity of the field depends on the direction the charge is
moving. If positive charge (conventional current, from the + end of a
battery) is moving away from you, as in the picture, the field will be
clockwise.
In this drawing, the plus indicates a wire with conventional current
flowing vertically down into the paper (electrons flowing up). The
compasses show the direction of the field. (The plus represents the
feathers on an arrow from behind. A dot would indicate the tip of the
arrow and conventional current coming up from the paper.)
Use the right hand rule to remember the polarity of the field. Form
your right hand into a fist with your thumb sticking out (like a
hitchhiker). Your thumb indicates the direction of conventional current. Your curled fingers
indicate the direction of the magnetic field. The north end of a compass placed in the field will
point in the direction of your fingers. (You could use a left hand rule for electron flow. Just use
your left hand with fingers curled and thumb pointing in the direction of electron [eLEFTron]
flow.)
A wire with charge moving through it generates a magnetic field. What if the wire (moving
charge) is in another magnetic field? The wire will act as a magnet and will tend to move, but
which way?
The N pole in the left side of the circular field will be forced
up, like the north end of a compass. The S pole in the right side
of the circular field will be forced up, too. Therefore
(Newton’s 3rd Law), the wire will be forced down. This is the
basis for all of the phenomena you saw in the stations.
N
S
To make a field stronger, you can move more charge, move it
faster, overlap fields, or put a core in the field. It makes sense that more moving charge would
create a stronger field – no moving charge would make no field. And faster moving charge
creates a stronger field, too – if the charge isn’t moving, it doesn’t create a field. A coiled wire
147030788
10 of 12
PPT
Electromagnetism
intensifies the field by overlapping the fields of the coils. Look at the picture of a coil of wire.
Make a circle with your hand as you follow the current around. Notice that your fingers always
point through the center of the circle in the same direction. The fields at the points along the wire
reinforce each other. A ferrous material placed in a coil will also reinforce the field because the
field will be induced in the metal. That’s what makes an electromagnet work.
Any moving charge, whether it is in a wire or not, will generate a magnetic field. Earth is
protected by its magnetic field from many of the charged particles coming from the sun, called
the solar wind.
The force on a moving charge is proportional to the amount of charge, its velocity, the strength
of the magnetic field, and its angle to the field. If the charge is moving parallel to the field, there
will be no force on it. If it is moving perpendicular to the field, the force is the greatest, and it is
given by
F = qvB
Extension
The force on a moving charge is given by
F = qvBsin
where sin is the angle between the charge’s velocity and the field.
Exercises
1. Will the wire in the diagram move toward or away from you?
N
S
Conventional current is flowing in the direction of the arrow. Explain.
2. Do objects need to be in contact with each other to feel magnetic force?
3. Can magnets do work? Explain.
4. How can you make an electromagnetic field stronger? Name at least three ways.
5. Why won’t bare wire work in electric coils?
6. Why is an iron core used in electromagnets?
7. Does the wiring in your house emit an electromagnetic field?
8. In older TVs (not digital), the picture on the screen is made by electrons hitting materials that
glow when they’re energized (phosphors). The electrons are emitted from the rear of the TV
tube. Magnets control their path so they hit the screen in the right place. If you wanted an
electron to hit the top of the screen, what pole of the magnet would you put above the
electron? Draw a picture and explain.
9. If you double the amount of moving charge, what will happen to the force on it?
10. If you double the velocity of moving charge, what will happen to the force on it?
11. If a charged particle is not moving, will it have a magnetic field?
12. If a charged particle is moving with the same velocity as another object, will it have a
magnetic field relative to the other object?
13. Some waste handlers use a magnet to remove aluminum and other non-magnetic metals from
the waste pile. How could they do that?
14. Why wouldn’t the electric motor in the stations work if there were no commutator (insulation
on one side of the axle)?
15. In a dry climate, as you walk on a carpet and build up a static charge, are you creating a
magnetic field? Explain.
147030788
11 of 12
PPT
Electromagnetism
Challenge/ extension
Build your own electric motor, like in the stations. Or look on the web to find other construction
ideas.
1. Change the side of the wire that is insulated for the commutator. Does the direction change?
Why?
2. Add another magnet above the coil. Will the coil speed up or slow down if the upper coil is
opposite the lower coil? How about if it’s the same?
3. Think of a way to make the motor more powerful by using both sides of the coil.
Glossary
1.
147030788
12 of 12