2Electromagnetic induction. Magnetic flux and flux linkage

Long-term plan unit:Magnetic field

School:

Date:

Teacher name:

Grade: 10

Number present:

Absent:

Theme of the lesson

Electromagnetic induction. Magnetic flux and flux linkage

Learning objectives that are achieved at this lesson (Subject Programme reference)

To analyse the operating principle of electromagnetic devices (electromagnetic relay, generator and transformer);

Lesson objectives

By the end of this lesson, students will be able to:

·         Define magnetic flux and the Weber;

·         Recall and solve problems using Ф=ВА;

·         Define magnetic flux linkage;

·         Infer from appropriate experiments on electromagnetic induction

that a changing magnetic flux can induce an e.m.f. in a circuit

that the direction of the induced e.m.f. opposes the change producing it

the factors affecting the magnitude of the induced e.m.f.

Assessment criteria

Knowledge

Define magnetic flux and the Weber;

Define magnetic flux linkage;

Application

Solve problems using Ф=ВА;

Analysis

Observe appropriate experiments on electromagnetic induction;

Predict what they will observe before they try the experiment;

Make a conclusion on the reasons of the deflection based on the experiments;

Language objectives

Subject-specific vocabulary & terminology

electromagnetic induction

induced e.m.f.

magnetic flux

Useful set(s) of phrases for dialogue/writing

There is an induced e.m.f. in the coil because...

The size of the induced e.m.f. increasesbecause...

The direction of the induced e.m.f. will reverse if…

Type of differentiation

Differentiated poster-session , Collaborative Learning, Progressive Task with Digital resources

Values instilled at the lesson

Safety, Consideration to others, Co-operation, Opportunity for Life-Long Learning, Academic Integrity and Transparency, Respect for Self and Others

Cross-curricularlinks

Mathematics, Chemistry

ICT skills

Presentation, the Internet

Previouslearning

Grade 8: magnetic fields; representation of fields by field lines; fields of permanent magnets and relationship between magnetic field and current in conductors

Grade 8: electrical equations: V = IR, P = IV

Grade 10: magnetic fields; flux densityand the tesla

Course of the lesson

Planned stages of the lesson

Planned activities at the lesson

Resources

Beginning

(0-3 min)

   (4-10 min)

Teacher:

-Introduces the topic of day and spelling out the learning outcome they will possess after the study.

1. Organizational moment to acquaint students with the

·         The theme of the lesson

·         The objectives of the lesson

·         The criteria of success for the lesson

·         The plan of events for the lesson

(G) Group work.

Teacher asks learners to divide into three groups and investigate the effects of moving a bar magnet into and out of a coil /  move the wire perpendicular to the field lines of a strong magnet and so on connected to a sensitive centre-zero galvanometer.

Electromagnetic induction can be demonstrated using a long loose wire / a coil / a small electric motor connected to a light beam galvanometer or sensitive ammeter. Groups do different actions:

GROUP 1:   Spin the shaft of the motor and observe the deflection of the voltmeter.

GROUP 2:   Move a bar magnet in towards the coil. Hold it still, and then remove it and observe the deflection of the voltmeter.

GROUP 3: Move the wire perpendicular to the field lines of a strong magnet (e.g. across the pole of a neodymium magnet) and observe the deflection of the light beam.

Suggest that learners make a conclusion on the reasons of the deflection based on the experiment.

For this experiment, students try to predict what factors affecting induced current depends on.

GROUP 1:   Experiment 1

GROUP 2: Experiment 2

GROUP 3: Experiment 3

Middle

  11-20 min

   21-31 min

  32-37 min

Factors affecting induced current

For a straight wire, the induced current or e.m.f.depends on:

■■ the magnitude of the magnetic flux density

■■ the length of the wire in the field

■■ the speed of movement of the wire.

For a coil of wire, the induced current or e.m.f. depends on:

■■ the magnitude of the magnetic flux density

■■ the cross-sectional area of the coil

■■ the number of turns of wire

■■ the rate at which the coil turns in the field.

Discussion and demonstration: Induction effects
The first two demonstrations involve moving a wire in a magnetic field and then a permanent magnet into and out of a small coil. In both it is important to emphasise that:

Ø  ‘electricity’ is only produced while something is moving

Ø  the faster the movement, the more ‘electricity’ we get

Cutting magnetic field lines

Start by thinking about a simple bar magnet. It has a magnetic field in the space around it. We represent this field by magnetic field lines. Now think about what happens when a wire is moved into the magnetic field (Figure). As it moves, it cuts across the magnetic field. Remove the wire from the field, and again it must cut across the field lines, but in the opposite direction. We think of this cutting of a magnetic field by a conductor as the effect that gives rise to an induced current in the conductor. It doesn’t matter whether the conductor is moved through the field or the magnet is moved past the conductor, the result is the same – there will be an induced current.

For a coil of N turns, the effect is N times greater than for a single turn of wire.

When the coil is outside the field, there are no magnetic field lines linking the coil.

When it is inside the field, field lines link the coil. Moving the coil into or out of the field changes this linkage, and this induces an e.m.f. across the ends of  the coil.

Magnetic flux and magnetic flux linkage

Magnetic flux density B is defined by the equation

B = F/IL

Now we can go on to define magnetic flux as a quantity.

We picture magnetic flux density B as the number of magnetic field lines passing through a region per unit area.

Similarly, we can picture magnetic flux as the total number of magnetic field lines passing through an area A. For a magnetic field normal to A, the magnetic flux Φ must therefore be equal to the product of magnetic flux density and the area A.

a The magnetic flux is equal to BA when the field

    is normal to the area.

b The magnetic flux becomes Bacosθ  when the field is at an angle θ to the normal of the area.

The magnetic flux Φ through area A is defined as:

Φ = BA

where B is the component of the magnetic flux density perpendicular to the area.

How can we calculate the magnetic fl ux when B is not perpendicular to A?

When the field is parallel to the plane of the area, the magnetic flux through A is zero. To find the magnetic flux in general, we need to find the component of the magnetic flux density perpendicular to the area.

Magnetic flux = (B cos θ) × A

or simply:

Magnetic flux = BA cos θ

(Note that, when θ = 90°, flux = 0 and when θ = 0°,

flux = BA.)

For a coil with N turns, the magnetic flux linkage is defined as the product of the magnetic flux and the number of turns; that is:

Magnetic flux linkage = NΦ

or

Magnetic flux linkage = BAN cos θ

The unit for magnetic flux or flux linkage is the Weber (Wb).

One weber (1 Wb) is the flux that passes through an area of 1 m² when the magnetic flux density is 1 T.

1 Wb = 1 T m².

An e.m.f. is induced in a circuit whenever there is a change in the magnetic flux linking the circuit. Since magnetic flux is equal to BA cos θ, there are three ways an e.m.f. can be induced:

■■ changing the magnetic flux density B

■■ changing the area A of the circuit

■■ changing the angle θ.

(W) Whole class work.

Figure shows a solenoid with a cross-sectional area 0.10 m². It is linked by a magnetic field of flux density 2.0 ×10⁻³ T and has 250 turns. Calculate the

magnetic flux and flux linkage for this solenoid.

Step 1 We have B = 2.0 ×10⁻³ T, A = 0.10 m², θ = 0°

and N = 250 turns. Hence we can calculate the flux Φ.

Φ = BA

Φ = 2 .0 × 10⁻³ × 0.10 = 2.0 × 10⁻⁴ Wb

Step 2 Now calculate the flux linkage.

magnetic flux linkage = NΦ

magnetic flux linkage = 2.0 ×10⁻⁴ × 250

= 5.0 × 10⁻² Wb

Ending

38-40 min

At the end of the lesson, learners reflect on their learning:

-           What has been learned

-           What remained unclear

-           What is necessary to work on

Where possible the learners could evaluate their own work as well as the work of their classmates using certain criteria.

Differentiation – how do you plan to give more support? How do you plan to challenge the more able learners?

Assessment – how are you planning to check students’ learning?

Health and safety regulations

·         Multiple Intelligences

-          Visual will watch the video

-          Analytical take information from the texts

·         Differentiation by questioning and dividing in group

·         Worksheet with varied difficulties

Assessment – how are you planning to check students’ learning?

The output for the worksheet will serve as assessment

Questions during the lesson will also serve as formative assessment.

Be careful when use the laser-coder

Reflection

Were the lesson objectives/learning objectives realistic? Did all learners achieve the LO?

If not, why?

Did my planned differentiation work well?

Did I stick to timings?

What changes did I make from my plan and why?

Use the space below to reflect on your lesson. Answer the most relevant questions from the box on the left about your lesson. 

Summary evaluation

What two things went really well (consider both teaching and learning)?

1:

2:

What two things would have improved the lesson (consider both teaching and learning)?

1:

2:

What have I learned from this lesson about the class or achievements/difficulties of individuals that will inform my next lesson?