1, Ampere’s experiments

Teacher Guides of the Lesson

Theoretical material for the lesson, definitions for concepts 

We have seen that a wire carrying a current produces a magnetic field. Also, a current-carrying wire feels a force when placed in a magnetic field. Thus, we expect that two current-carrying wires will exert a force on each other. Consider two long parallel wires separated by a distance d, as in Fig. 1–a.

They carry currents I₁ and I₂  respectively. Each current produces a magnetic field that is “felt” by the other, so each must exert a force on the other.

If we use right-hand-rule-1, we see that the lines of are as shown in Fig. 1-b. Then using right-hand-rule-2, we see that the force exerted on will be to the left in Fig. 1-b. That is, exerts an attractive force

on (Fig. 2–a). This is true as long as the currents are in the same direction.

If I₂is in the opposite direction from I₁ right-hand-rule-2 indicates that the force is in the opposite direction. That is I₁, exerts a repulsive force on I₂ (Fig. 2-b). Reasoning similar to that above shows that the magnetic field produced by I₂exerts an equal but opposite force on I_{1 .}We expect this to be true also from Newton’s third law. Thus, as shown in Fig. 2, parallel currents in the same direction attract each other, whereas parallel currents in opposite directions repel.

                             Figure – 1-ab                                               Figure -2-ab

Forces between currents

Any electric current has a magnetic field around it. If we have two currents, each will have its own magnetic field,

and we might expect these to interact.

Explaining the forces

There are two ways to understand the origin of the forces between current-carrying conductors. In the first, we draw the magnetic fields around two current-carrying

conductors (Figure 26.22a). Figure 26.22a shows two unlike (anti-parallel) currents, one flowing into the page, the other flowing out of the page. Their magnetic fields circle round, and in the space between the wires there is an extra-strong field. We imagine the field lines squashed together, and the result is that they push the wires apart.

The diagram shows the resultant field, and the repulsive forces on the two wires.

Figure 26.22b shows the same idea, but for two like (parallel) currents. In the space between the two wires, the magnetic fields cancel out. The wires are pushed together. The other way to explain the forces between the currents is to use the idea of the motor effect. Figure 26.24 again shows two like currents, I₁ and I₂, but this time we only consider the magnetic field of one of them, I₁. The second current I₂ is flowing across the magnetic field of I₁; from the diagram, you can see that B is at right angles to I₂. Hence there will be a force on I₂ (the BIL force), and we can find its direction using Fleming’s left-hand rule. The arrow shows the direction of the force, which is towards I₁. Similarly, there will be a BIL force on I₁, directed towards I₂.These two forces are equal and opposite to one another.

They are an example of an action and reaction pair, as described by Newton’s third law of motion.

Instructions for demonstrations and safety

These activities allow qualitative investigation of the forces on currents.

You will need:

Ø  aluminium foil

Ø  scissors

Ø  large horseshoe magnet

Ø  two clip component holders

Ø  four long 4 mm leads

Ø  power supply, 0–12 V plus rheostat (4 A) (or Westminster Very Low Voltage power supply)

You will be making some general observations about forces on currents. As force is a vector, you will be interested to see how large the force is and in what direction it acts. Systematic observations will be essential.

Practical advice

The main point is that currents experience forces in a magnetic field, which can be due to either a permanent magnet or another current. The size of the force depends on the sizes of the current and the magnetic field.  Depending on your circumstances this may well be revision of pre-16 level work. You might like to extend the tasks. How does the force between currents depend on distance? What happens when alternating current is put through the foil in the field? What has this got to do with a possible loudspeaker or microphone?

Technician's note:

Westminster power supplies are ideal for the first two experiments. If these are not available it may be necessary to put a high-current rheostat in the circuit to prevent the cut-out operating.

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 reminds students that current-carrying conductors create a magnetic field around them and asks the following question:

·         If we place two current-carrying conductors close to each other, will they interact?

·         If yes, will the nature of interaction differ in case the currents flow in one direction and in opposite directions?

4.      Teacher asks learners in pairs to set up apparatus then observe the attraction and repulsion between two parallel currents using the equipment. They answer the questions step by step after finishing the experiment.

5.      The study of the new material is based on the explanation of the topic by the teacher accompanied by a presentation.

6.      Teacher asks learners individually to answer qualitative questions on the topic of the lesson.

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

they need to improve.

Recommendations for formative assessment

Activity1. Students discuss learning objectives and assess criteria.

Activity2. Students in pairs to set up apparatus then observe the attraction and repulsion between two parallel currents using the equipment. They answer the questions step by step after finishing the experiment.

Activity3. Individual students are called on to respond to qualitative questions and share their own opinions/thoughts.

Activity4. 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

Activity2.

a.      The force on a current in a magnetic field

Set the power supply to 1 V, switch on, and observe what happens to the foil.

2.         Increase the power supply to 2 V. What happens?

3.         Now reverse the connection to the power supply. What happens?

4.         Finally, reverse the magnet. What happens to the foil?

What you have seen

1.         The size of the force on a current in a magnetic field depends on the size of the current.

2.         The direction of the force depends on the direction of the current and the direction of the magnetic field.

3.         The direction of the force is at right angles to the current and to the magnetic field.         (Fleming’s left-hand rule.)

4.         You have not seen that the size of the force also depends on the size of the magnetic field. But you can show that the magnetic field is essential to the force by removing the magnet!

b.      The force between two currents

1.        First, arrange the connections so the current is passing down both strips of foil. Set the power supply to 1 V. Switch on and describe what happens.

2.         Increase the power supply to 2 V. What happens now?

3.         Arrange the connections so that the current is passing up both strips of foil. What            happens?

4.         Finally arrange the connections so the current is passing up one foil and down the other. What is the result?

What you have seen

1.         The size of the force between currents depends on the size of the current.

2.         The direction of the force depends on the current directions. If they are in the same direction they attract. If they are in opposite directions, they repel. Like currents attract,            unlike currents repel.

Activity 3

Conceptual Questions

1. What is the nature of force between two parallel current carrying wires in the same direction?

The magnetic force expression can be used to calculate the force. The direction is obtained from the right hand rule. Note that two wires carrying current in the same direction attract each other, and they repel if the currents are opposite in direction.

2. Why do two parallel wires repel?

The magnetic field of the left-hand current causes a force on the right-hand current that is a repulsion so we know -- from Newton's Third Law -- that the two currents repel each other.

3. Do perpendicular wires exert magnetic force each other?

Yes. Two wires repel each other when carrying current in opposite directions—the force on each wire is equal in magnitude and opposite in direction. Parallel wires exert magnetic forces on each other. ... correct Perpendicular wires exert no net force on one another, but there will be a torque.

4. How does current affect magnetic field?

Current is directly proportional to magnetic force for a straight current carrying conductor in a uniform magnetic field. So the force is directly proportional to the size of the current. ... If the current is is reduced to a third of the initial value the force will also reduce to a third of its initial value.

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.

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