PHY_10_65_V2_TG_Electromagnetic devices

Theoretical material for the lesson, definitions for concepts

Electromagnets are used in a number of everyday devices.

One useful characteristic of an electromagnet is the fact that you can vary its magnetic force by changing the amount and direction of the current going through the coils or windings around it. Loudspeakers and tape recorders are devices that apply this effect.

Some electromagnets can be very strong and its power can be readily turned off and on. Junk yard electromagnets, common doorbells and electromagnetic locks are examples.

Electromagnets can also be used to create continual motion when opposed by other electromagnets or permanent magnets. Examples include electric motors and maglev trains.

Questions you may have include:

·         What devices use variations of electromagnetic force?

·         How can turning the magnetic field on and off be used?

·         What devices create continual motion?

Devices using varying field

You can change the strength of an electromagnet's magnetic field by varying the electrical current that passes through the wires wrapped around it. If you change the direction of the electrical current the polarity of the magnetic field reverses.

These effects can be used to move a loudspeaker cone back and forth, creating sound according to the electrical current through the wire. They also can be used to create magnetic fields in a magnetic tape or computer hard drive, such that it stores information.

Loudspeaker

The loudspeakers in your radio, television or stereo system consists of a permanent magnet surrounding an electromagnet that is attached to the loudspeaker membrane or cone.

By varying the electric current through the wires around the electromagnet, the electromagnet and the speaker cone can be made to back and forth. If the variation of the electric current is at the same frequencies of sound waves, the resulting vibration of the speaker cone will create sound waves, including that from voice and music.

Cutout of a loudspeaker

If you examine the back area of a loudspeaker, you should be able to see the permanent magnet and coil of wire for the electromagnet. Some loudspeakers use an electromagnet without the iron core, which is called a solenoid.

Tape recorder

When a Mylar tape covered with fine iron dust passes near a small electromagnet that has a varying magnetic field, according to an electrical signal, the dust become magnetized in different directions. The electrical signal could be from a radio or microphone.

The tape then is a record of the electrical signal. When it passes by another small electromagnet, it creates an electrical signal, duplicating that of the original signal. This signal can be amplified and played back through loudspeakers.

Devices using turning magnetic field on and off

The magnetic strength of an electromagnet depends on the number of turns of wire around the electromagnet's core, the current through the wire and the size of the iron core. Increasing these factors can result in an electromagnet that is much larger and stronger than a natural magnet. For example, there is no known natural magnet that is able to pick up a large steel object such as a car, but industrial electromagnets are capable of such a task.

Also, if the core of the electromagnet is made of soft iron, its magnetic force can be turned off by turning off the electricity to the electromagnet.

Picking up and dropped junk cars

Thus, an electromagnet can be used to pick up a piece of iron and then drop it someplace else.

Strong electromagnets are often used in areas of heavy industry to move large pieces of iron or steel. They are commonly employed in junkyards, where a crane with a huge electromagnet is used to pick up, move and drop old, junked cars.

Electromagnetic lock

An electromagnetic lock be used to lock a door by creating a strong field in an electromagnet that is in contact with a magnetic plate. As long as there is current through the electromagnet, the door remains closed and locked.

Another type of electromagnetic lock uses an electromagnet to extend a plunger between the doors, making it nearly impossible to open the door until the electromagnet releases the plunger.

Doorbell ringer

An old-fashioned doorbell used an electromagnet that was rapidly turned on and off to pull a clangor against a bell.

Devices creating steady motion

Clever use of electromagnetic forces can create steady motion.

Electric motor

An electric motor is another application of electromagnets. Suppose you put some electromagnets on a wheel and put some permanent magnets around the wheel. The electromagnets could be made to attract and repel the surrounding magnets, causing the wheel to turn. By varying the current, the speed of the motor can be made to vary.

Look at an electric motor and see the internal wheel made of electromagnets and the outer shell made of permanent magnetic material.

Maglev trains

A maglev (magnetic levitation) train works without wheels and is propelled by electromagnetic forces.

This type of train usually consists of a set of magnets along the bottom of the train and a series of electromagnets on the tracks or guide-way for the train. The electromagnets are adjusted to have the same polarity as the train's magnets, though complex computer controls. Since the magnetic poles repel, the train is levitated or floats slightly above the track. Guides on the sides prevent the train from sliding off.

Depending on the position of the train, the polarity of the electromagnets is adjusted, causing the train to move forward. Maglev trains can reach speeds over 260 mile per hour or 430 kilometers per hour.

Summary

Electromagnets are used in a number of devices, such as loudspeakers and tape recorders. Some electromagnets can be very strong and its power can be readily turned off and on, such as in junk yard electromagnets and electromagnetic locks.

Electromagnets can also be used to create continual motion such as with electric motors and maglev trains.

Additional guidelines for organizing a lesson 

Lesson starts with introducesing the topic of day and spells out the learning outcomes they will possess after the study. Acquaint students with the following issues:

•      The theme of the lesson

•      The objectives of the lesson

•      The criteria of success for the lesson

•      The plan of events for the lesson

•      Pre-teach the subject specific vocabulary.

Learners will share their experiences with the operations of magnetic relays, generators, and transformers.

Then students can deduce topic of the lessen and objectives, for clarification you can show topic and the learning objectives on the presentation.

Then Subject-specific vocabulary & terminology will be presented to the students and their activities during the research work will be explained.

Then Teacher:

•      Describes, discuses, and explains the principles of the operations of the generator, electromagnetic relay, and transformer.

•      Uses both videos and animations to explain the principles of the operations of electromagnetic devices.

•      Asks learners to perform tasks with use of animations and video footage.

Self-induction: https://www.youtube.com/watch?v=IqblMu50tnE

Calculation of induction: https://www.youtube.com/watch?v=kTKwbPUi0e4

Video showing the induction heating: https://www.youtube.com/watch?v=jPvXzmyl4UM

This animation compares a simple a.c. generator using slip rings with a simple d.c. generator using a split-ring commutator. http://www.youtube.com/watch?v=0huObSCPK14

This animation is useful when explaining how a simple a.c. generator works: http://www.youtube.com/watch?v=i-j-1j2gD28

Bicycle generator: https://yandex.kz/video/search?text=велосипедный%20генератор&path=wizard&noreask=1&filmId=

Useful video showing the operational principle of transformers: http://www.youtube.com/watch?v=ZjwzpoCiF8A

Then Learners:

•      Discuss their experiences with electromagnetic induction and its laws

•      Demonstrate and explain the principles of the operations of electromagnetic devices in groups/pairs

•      Perform tasks with use of animations and video footage.

•      Summarize their observations and conclusions findings.

•      Answers questions about the videos.

Teacher:

•      Highlights key concepts, definitions, and equations learnt using the concept map.

•      Looks forward to the next lesson.

Students:

•      Attempt the questions given by the teacher.

•      Summarize the main concepts, definitions, and equations learnt.

•      Reflect on their own learning.

•      Evaluate their own work and the work of their classmates.

Extension Work:

•      Complete the flipped reading and research assignment before the next lesson.

Homework:

Complete the specified thinking tasks for this lesson. Complete worksheet

Additional multilevel (on differentiation) tasks

The teacher  assigns questions 1- 10 to weak students, questions 11-20 to the average students, questions 21- 26 to the strong students.

Recommendations for formative assessment

Students think of everything they already know about the importance of electricity for our life, and record this information in the first column of the table given.

Students discuss learning objectives and success criteria.

Students work in groups. Each group fill out one line of the table provided by the teacher.

Students share their knowledge with neighboring pairs, filling in the table. Students make a list of electric charge properties significant for our daily life.

Students perform a short quiz on the topic of the lesson.

Answers, criteria for assignments, additional materials for the lesson

Electricity Magnetism        Answer Section

MODIFIED TRUE/FALSE

            1.   F, An insulator

            2.   F, electrons

            3.   T                              

            4.   F, volts

            5.   F, ampere

MULTIPLE CHOICE

            6.   A

            7.   C

            8.   D

            9.   A

          10.   C

          11.   C

          12.   C

          13.   D

          14.   D

          15.   C

          16.   A

          17.   A

MATCHING

          18.   D

          19.   A

          20.   C

          21.   E

          22.   B

COMPLETION

          23.   A

          24.   B

          25.   B

          26.   A

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