PHY_10_3_V2_TG_Measuring magnetic flux density

Teacher Guides of the Lesson

Theoretical material for the lesson, definitions for concepts

Figure below shows a simple arrangement that can be used to determine the flux density between two magnets. The magnetic field between these magnets is (roughly) uniform. The length L of the current-carrying wire in the uniform magnetic field can be measured using a ruler. When there is no current in the wire, the magnet arrangement is placed on the top pan and the balance is zeroed. Now, when a current I flows in the wire, its value is shown by the ammeter. The wire experiences an upward force and, according to Newton’s third law of motion, there is an equal and opposite force on the magnets. The magnets are pushed downwards and a reading appears on the scale of the balance. The force F is given by mg,

where m is the mass indicated on the balance in kilograms and g is the acceleration of free fall (9.81 m s⁻²).

Knowing F, I and L, the magnetic flux density B between the magnets can be determined using the equation:

B =F/IL

You can also use the arrangement in Figure below shows that the force is directly proportional to the current. A system like this in effect ‘weighs’ the force on the current-carrying conductor, and is an example of a current  balance.

Instructions for demonstrations and safety

Warning: experiments with electricity should be performed under the supervision of teachers or adults familiar with electricity safety procedures.

Additional guidelines for organizing a lesson 

1.      Organization moment. Establishing emotional state.  Checking for absent students.

2.      Teacher introduces the topic and objectives of the lesson, assess criteria.

3.      Teacher starts lesson by asking:  What is the physical meaning of magnetic flux density? •           

a.       How to measure the value of magnetic flux density?

4.      Teacher asks learners in pairs do the experiment to Measure the magnetic flux density by using a current balance. Then they to set up apparatus then follow the instructions of experiment and take measurements.

5.      Students share their knowledge with other pairs, presenting their results in a class. They

a.       may compare the methods by analyzing of measurements.

6.      Teacher asks learners individually to solve problem on the topic of the lesson.

7.      At the end of the lesson students are encouraged to reflect on what they have learned and

a.       what they need to improve.

8.      Recommendations for formative assessment

        Activity1. Students discuss learning objectives and assess criteria.

        Activity2. Students define the magnetic flux.

         Activity3.  Learners in pairs do the experiment to Measure the magnetic flux density by using

                           a current balance. Then they to set up apparatus then follow the instructions of  

                           experiment and take measurements.          

         Activity4. Individual students were called on to calculate the magnetic flux density and share

                        their own solutions.

Activity5. At the end of the lesson students are encouraged to reflect on what they have learned

                and what they need to improve.

Answers, criteria for assignments, additional materials for the lesson

Assessment criteria

Knowledge

Define magnetic flux density and the tesla;

Know that the size of the force also depends on the size of the magnetic field;

Understanding

Understand how the force on a current-carrying conductor can be used to measure the flux density of a magnetic field using a current balance;

Application

Can set up apparatus then measure the flux density;

Analysis

Describe the methods of measuring magnetic flux density;

      a    F = mg = (103.14 – 102.00) ´ 10^–3 ´ 9.81 so F = 1.12 ´ 10^–2 N  1.1 ´ 10^–2 N     [1]

b   B =                                                                                                                             [1]
B =                                                                                                                [1]
B = 2.73 ´ 10^-2 T (27 mT)                                                                                          [1]

List of useful links and literature 

https://tap.iop.org/fields/electromagnetism/411/page_46911.html

Douglas  C. Giancoli, Physics Principles with Applications, Seventh edition  2014.

http://dev.physicslab.org/Document.aspx?doctype=5&filename=Magnetism_ForcesWires.xml

https://www.juliantrubin.com/bigten/ampereexperiments.html

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