electro-magnetism - university of hawaii 2005-6 course materials/0... · web viewbattery holders...

PPT Electromagnetism

Developer Notes

Version Date Who Revisions01 2004/07/06 sc Initial version02 2004/07/07 dk expanded on the activity03 2005/03/01 dk Included notes from summer 2004

Goals1. 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 ConceptPhysics A moving electric charge (current) will produce a magnetic fieldTechnology Electromagnets, DC motors

Standards Addressed

Time Required

Warm-up Question

PresentationMake 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.

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StationsIt 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 wireThe 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 coilThe 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 coilsMore windings should pick up more paper clips because the field is reinforced. The nail helps to concentrate the field.

5. Electromagnet directionThe 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 filingsThe field should be very evident around the nail, not quite as obvious around the coil.

7. DC motorThe 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 swingThe 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 currentWith more current, the field will be stronger and the electromagnet will hold more paper clips.

10. Electromagnet and ferrousA ferrous core in a coil will reinforce the field.



AssessmentObserve – Set up the motor used in the stations and get it running.Explain – Polarity, moving charge, field

Writing Prompts

Relevance

Answers to Exercises1.

Answers to Challenge/ extension1.

EquipmentExcept as noted: Use single conductor (not stranded) 20 or 22 AWG PVC insulated wire (Radio Shack 278-

1219 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 wire1 Small compass (<1” diameter) 30 cm 14-16 AWG wire1 1.5 V D-cell battery2 connecting wiresMount 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 coil1 small compass (<1” diameter). 1 m 22 AWG insulated wire1 1.5 V D-cell batteryMake a coil big enough to pass the compass through. About five turns is enough.

3. Solenoid 2 clear straws1 small paper clip2 m 22 AWG insulated wire1 1.5 V D-cell batteryMake 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



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.

4. Electromagnet coils2 20 d (4”) common nails1 m 22 AWG insulated wire30 jumbo paper clips1 1.5 V D-cell batteryWrap 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.

5. Electromagnet direction1 20 d (4”) common nail0.5 m 22 AWG insulated wire1 compass1 1.5 V D-cell batteryWrap about 15 turns of wire around the nail.

6. Electromagnet fields and filings1 20 d (4”) common nail1 m 22 AWG insulated wire1 1.5 V D-cell battery1 drinking strawWrap 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.

7. DC motor

2 jumbo paper clips1 5 cm x 5 cm corrugated cardboard1 m 22 AWG insulated wire1 permanent marker1 ring magnet1 1.5 V D-cell battery 2 connecting wiresa. 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



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. CoilMake a coil of about 3 cm diameter with about 10 turns of wire. Wrapping around a D-cell 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. CommutatorInsulate 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. MagnetA 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. PowerMore 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 pieces1 5 cm x 5 cm corrugate cardboard10 cm 22 AWG wireMake 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 current1 20 d (4”) common nails0.5 m 22 AWG insulated wire30 jumbo paper clipsWrap 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 ferrous1 20 d (4”) common nail1 m 22 AWG insulated wire



5 jumbo paper clips1 drinking strawWrap 30 turns of wire around the straw. Cut the ends of the straw off about 0.5 cm from the end of the coil.



BackgroundYou’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.

ProblemInvestigate electromagnetism.

MaterialsStations are set up.

ProcedureIn 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 wirea. 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 coila. 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 coilsa. Try to suspend paper clips from the heads of the electromagnets. How many will they

hold?



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.5. 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.

6. Electromagnet fields and filingsa. 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.

7. DC motorFirst, 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.

8. Wire swinga. 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.

9. Electromagnet currenta. 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?



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.

Summary1. 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.



ReadingCharge 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.

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.

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


N S

N S


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 = qvB

ExtensionThe force on a moving charge is given by

F = qvBsinwhere sin is the angle between the charge’s velocity and the field.

Exercises1. Will the wire in the diagram move toward or away from you?

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.


N S


Challenge/ extensionBuild 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.

Glossary1.


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