Theoretical material for the lesson, definitions for concepts

Ampere’s Force Law

Ampere’s Force Law states that the force of attraction or repulsion between two wires carrying currents is proportional to their lengths and the intensities of current passing through them. If the currents flow in the  same direction, repulsion takes place. If currents are flowing in opposite directions, attraction takes place. The law is based on these two basic concepts of electrostatics:

·         Biot-Savart Law states that every current carrying wire produces a magnetic field around it, as shown in Figure 1.

·         Lorentz force refers to the force that every magnetic field exerts on any electric charge moving in its field.

 Figure 1: Thumb Rule to Find Magnetic Field Around a Current Carrying Wire

Foundation of Electromagnetism

Based on the Biot-Savart Law and Lorentz force, there is a relationship between magnetic field and electric charge/current. It is this relationship that Ampere sought to establish through experiments. The most basic of these experiments was to study the force between two current-carrying wires, as shown in Figure 2. This experiment and subsequent theories to explain its results laid the foundation of electromagnetism as a field in physics by itself.

Figure 2: Magnetic Field Between Current Carrying Wires

Ampere, the SI unit of electric current, is defined as the electromagnetice force per unit length between two wires of infinite length, having negligible diameter and placed 1 m apart in vacuum. The basic assumption here is that the wires are in free space, i.e. there is no matter present that can be magnetized. If any matter present in the environment gets magnetized, it will exert its own magnetic force that will have to be taken into account, so this assumption has to be made.

Applications of Force Law

Using Ampere’s Force Law, magnetic field around an infinite wire, infinite sheet, toroid, solenoid, or any other regular shape can be calculated, as shown in Figures 3 and 4 below.

Figure 3: Magnetic Field Around a Solenoid             Figure 4: Magnetic Field Around a  Toroid

Ampere’s Force Law proved to be such a fundamental law that after him, many physicists like James Clerk Maxwell, Wilhelm Weber, Bernard Riemann, etc. extended it further to find a basic definition of force itself. Coming back to Ampere’s work, Force Law states that force between the current-carrying wires is proportional to their length and intensity of current flowing. This means that higher the current, greater is the attraction or repulsion between the wires.

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. Acquainting 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.

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.

Learners would share their experiences with left-hand rule, Ampere force, and Lorentz force. Watch videos. Perform demonstrations and simulations as explained by their teacher.

Teacher using ppt explains left-hand rule, describes, discuses, and explains the left-hand rule, Ampere force, and Lorentz force, gives questions on a worksheet.

Learners would discuss their experiences with left-hand rule, Ampere force, and Lorentz force. Watch videos on left-hand rule, Ampere force, and Lorentz force.  Answers questions about on the videos.

Attempt questions on the worksheet.

This BBC Bitesize resource is useful for revision of ideas about magnetic fields and their representation using field lines:

http://www.bbc.co.uk/schools/gcsebitesize/science/ocr_gateway_pre_2011/living_future/5_magnetic_field1.shtml

This video shows a simple demonstration of the ‘motor effect’ and shows how to apply Fleming’s left hand Rule:

http://www.youtube.com/watch?v=U9RezsWnPYs

This simple demonstration could be used as a class experiment into the motor effect:

http://www.youtube.com/watch?v=9Wby4aHyXJQ

This clip shows a simple d.c. motor with brushes and a commutator:

http://www.youtube.com/watch?v=Ue6S8L4On-Y&feature=related

Animation of ‘Left Hand Rule’:

https://www.vascak.cz/data/android/physicsatschool/templateimg.php?s=mag_fleming&l=ru

This short video shows how the image on an old CRT TV is distorted by the presence of a magnetic field:

http://www.youtube.com/watch?v=Se8UtAflums

This video shows the deflection of an electron beam in a uniform magnetic field produced by a pair of Helmholtz coils:

http://www.youtube.com/watch?v=3McFA40nP0A

Interactive model of the particle motion in a magnetic field:

https://www.vascak.cz/data/android/physicsatschool/templateimg.php?s=mag_wehnelt&l=ru

Teacher would highlights key concepts, definitions, and equations learnt using the concept map. Asks students to do questions on the worksheet provided. Looks forward to the next lesson.

Students would 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.

Generalization: Students will be asked if they have achieved the lesson’s objectives.

At the end of the lesson, learners ask question if they have.

The teacher collects the reflections.

At the end teacher gives homework: complete the specified thinking tasks for this lesson.

Additional multilevel (on differentiation) tasks

The teacher  assigns questions 1, 2, 3, 4, 5, 6 to weak students, questions 7, 8, 9, 10, 11 to the average students, questions 4, 7 (a, b), 12, 13  to the strong students.

Recommendations for formative assessment

Students will do this work by themselves after that teacher will show answers on the board, learners will check themselves.

Answers, criteria for assignments, additional materials for the lesson

Permanent Magnets

The Force that a Magnetic Field Exerts on a Moving Charge

1.      E

2.      D

3.      E

4.      A

5.      B

6.      A

7.      C

8.      D

9.      C

10.  A

11.  E

Ampere’s Law. Magnetic Materials

12.  D

13.  B

Answers and worked solutions

1(a)     F = BIL

  (b)     B=F/IL

B = [2.0 x 10^-3 x 9.81]/ [4 x 0.15] = 0.0327 T (Remember to change grams to kg and centimetres to metres)

2          Weight of mass required = BIL

Therefore: 1.5 x 10^-3 x 9.81 = 0.5 x I x 0.06 giving I = 0.49 A

3          F = BIL = 0.2 x 2 x 0.5 = 0.2 N

4          F =  2.5 x10^-5 N

5          For 20 cm length

6.         If each wire carries 3.0 A this is the same as effective current of 9.0 A,

so force F = 3 * 0.625 = 1.9 N.

7(a)      The wire of voice coil is at right angles to the field. When a varying current from an amplifier current flows in the coil it experiences a force that moves the coil backwards and forwards along the soft iron core.

  (b)     F = nBIL = 0.013 N

(Note: cm to m and mA to A)

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