Electromagnetic Field – SS3 Physics Lesson Note (Second Term)

An electromagnet is a temporary magnet whose magnetic field is created by an electric current. Its strength can be changed by varying the current, and its polarity can be changed by reversing the direction of the current. Electromagnets are used in cranes, motors, generators, headphones, loudspeakers and many home appliances such as electric fans and doorbells.

Behavioural objectives

By the end of the lesson, students should be able to:

1. Explain the concept of electromagnetic field.
2. State the applications of electromagnets.
3. List the uses of electromagnetic fields in home appliances.

Instructional resources

• A chart showing electromagnets

Lesson presentation

Step 1: Identification of prior ideas

Mode: The entire class

Teacher’s role: Ask the students the following question: What are electromagnets?

Students’ role: Respond to the teacher’s question.

Step 2: Experimentation

Mode: The entire class

Teacher’s role: The teacher uses the chart to explain the electromagnetic field.

Students’ role: The students listen and observe the activities.

Step 3: Discussion

Mode: Whole class

Teacher’s role: Using the questions below, the teacher guides the students in discussing the activities.

1. What are electromagnets?
2. State two home appliances that use an electromagnetic field.
3. List three uses of electromagnets.

Students’ role: The students answer the questions by contributing their ideas to the discussion. The teacher then summarises the lesson topic as follows.

What is an electromagnet?

An electromagnet is a kind of magnet where the magnetic field is created by an electric current. Electromagnets can be considered temporary magnets that function with the help of an electric current.

The magnetic strength of an electromagnet can be easily altered by varying the amount of electric current. Its polarity can be changed by varying the direction of the electric current.

What are the uses of electromagnets?

Electromagnets are used for various purposes on a day-to-day basis. For example, they are used in large cranes and in numerous electromechanical and electronic devices. Some common uses are given below.

1. Generators, motors and transformers
2. Electric buzzers and bells
3. Headphones and loudspeakers
4. Relays and valves
5. Data storage devices like VCRs, tape recorders, hard discs, etc.
6. Induction cookers
7. Magnetic locks
8. MRI machines
9. Particle accelerators
10. Mass spectrometers

How are electromagnets used in home appliances?

Most of the electrical appliances in the home use electromagnetism as the basic working principle. Some electromagnet uses in the home include the electric fan, electric doorbell, induction cooker and magnetic locks.

• Electric fan: the electromagnet keeps the motor rotating, which makes the blade of the fan rotate.
• Electric doorbell: when the button is pressed, electromagnetic forces energise the coil and the doorbell sounds.

Applications of electromagnets

The teacher guides the students to mention the following applications of electromagnets.

1. Headphones
2. Loudspeakers
3. Motors
4. Generators

Evaluation

1. What is an electromagnet?
2. State two home appliances that use an electromagnetic field.
3. List three uses of electromagnets.
4. How can the strength of an electromagnet be changed?
5. How can the polarity of an electromagnet be changed?

Assignment

Study the medical uses of the electromagnetic field for the next class.

Reference materials

1. New System Physics for Secondary School by Dr Charles Chew et al.
2. New School Physics by M. W. Anyakoha
3. Internet facility