Electromagnetic Fields and Their Applications for SS 1

Electromagnetic Field

Performance Objectives

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

1. explain the meaning of electromagnetic field and identify the directions of electric and magnetic fields;
2. State and explain the Faraday’s law of electromagnetic induction;
3. State and explain the implication of Lenz’s law;
4. State how charge conservation principle is involved in both Faraday’sand Lenz’s laws;
5. Explain the action of a loop of wire in a magnetic field.
6. Use a coil, magnet, and galvanometer to demonstrate electromagnetic induction.

Introduction to Fields

In Physics, a field is a region in space where a force can be experienced. For example, a gravitational field is the region around a mass where other masses experience a gravitational force. Similarly, electric charges create electric fields, and magnets create magnetic fields.

Meaning of Electromagnetic Field

An electromagnetic field is a physical field produced by electrically charged objects. It is a combination of an electric field and a magnetic field that are linked together.

This means that a changing electric field produces a magnetic field, and a changing magnetic field produces an electric field. These two fields are not separate but are two aspects of a single electromagnetic field.

Electric Fields

An electric field is a region around an electric charge where another electric charge would experience a force. Electric field lines show the direction of the force on a positive test charge. They originate from positive charges and terminate on negative charges.

Magnetic Fields

A magnetic field is a region around a magnet or a current-carrying conductor where magnetic forces can be detected. Magnetic field lines show the direction of the magnetic force. They emerge from the North pole of a magnet and enter the South pole, forming continuous loops.

Interaction Between Electric and Magnetic Fields

The interaction between electric and magnetic fields is fundamental to understanding electromagnetism. This interaction can be observed in two main ways:

1. Moving Charges Create Magnetic Fields: An electric current, which is a flow of electric charges, produces a magnetic field around it. This was discovered by Hans Christian Ørsted. The direction of this magnetic field can be determined using the Right-Hand Grip Rule.

   To use the Right-Hand Grip Rule, imagine gripping the current-carrying wire with the right hand, with the thumb pointing in the direction of the current. The fingers then curl in the direction of the magnetic field lines around the wire.

2. Changing Magnetic Fields Create Electric Fields: A changing magnetic field induces an electric field, which in turn can drive an electric current in a conductor. This phenomenon is known as electromagnetic induction, discovered by Michael Faraday.

Electromagnetic Induction

Electromagnetic induction is the production of an electromotive force (e.m.f.) across an electrical conductor in a changing magnetic field.

Faraday’s Law of Electromagnetic Induction

Faraday’s Law states that the magnitude of the induced electromotive force (e.m.f.) in a circuit is directly proportional to the rate of change of magnetic flux linkage through the circuit.

Magnetic flux linkage refers to the total number of magnetic field lines passing through a coil or circuit. A change in magnetic flux linkage can be caused by:

1. Moving a magnet near a coil.
2. Moving a coil near a magnet.
3. Changing the current in a nearby coil.
4. Changing the area of a coil in a magnetic field.

The faster the magnetic flux changes, the greater the induced e.m.f. If a coil has more turns, the induced e.m.f. will also be greater for the same rate of change of magnetic flux.

Lenz’s Law

Lenz’s Law states that the direction of the induced current (or induced e.m.f.) is always such that it opposes the change in magnetic flux that produced it.

This law is a consequence of the principle of conservation of energy. If the induced current did not oppose the change, it would mean that the system could generate electrical energy without any external work being done, which would violate the conservation of energy. For example, if you push a North pole of a magnet towards a coil, the induced current will create a North pole at the end of the coil facing the magnet, thus opposing the motion. Work must be done to overcome this repulsion, and this work is converted into electrical energy.

Charge Conservation Principle in Faraday’s and Lenz’s Laws

The principle of charge conservation states that electric charge can neither be created nor destroyed; it can only be transferred from one place to another.

1. In Faraday’s Law: When an e.m.f. is induced, it causes free charges (electrons) in the conductor to move, resulting in an induced current. This movement of charges does not create or destroy charge; it merely sets existing charges into motion. The total amount of charge in the system remains constant, but its distribution or state of motion changes due to the induced e.m.f.

2. In Lenz’s Law: Lenz’s Law determines the direction of the induced current. This direction is crucial for ensuring that energy is conserved. The flow of induced current means that charges are moving in a specific direction. This directed movement of charges is consistent with charge conservation, as the charges themselves are not created or destroyed, but simply moved within the circuit. The law ensures that the energy required to move these charges (electrical energy) comes from the work done against the opposing force (mechanical energy), upholding the overall energy conservation principle.

Action of a Loop of Wire in a Magnetic Field

When a loop of wire is placed in a magnetic field, its action depends on whether there is a change in the magnetic flux passing through it.

1. Stationary Loop in a Constant Magnetic Field: If a loop of wire is stationary in a constant magnetic field, and the magnetic flux through it is not changing, no e.m.f. or current will be induced in the loop. The charges within the wire are not experiencing a net force that would cause them to move in a continuous flow.

2. Moving or Changing Loop in a Magnetic Field: If the loop of wire moves through a magnetic field, or if the magnetic field passing through the loop changes, an e.m.f. will be induced. For instance:

   1. Moving a loop into or out of a magnetic field: As the loop enters or leaves a magnetic field, the number of magnetic field lines passing through it changes, inducing an e.m.f. and current.

   2. Rotating a loop in a magnetic field: When a loop of wire rotates within a uniform magnetic field, the magnetic flux passing through it continuously changes. This continuous change in flux induces a continuous e.m.f. and current in the loop. The direction of the induced current reverses every half rotation, producing an alternating current. This is the basic principle behind electric generators.

   3. Changing the strength of the magnetic field: If the strength of the magnetic field passing through a stationary loop changes (e.g., by moving a magnet closer or further away), an e.m.f. will be induced.

Demonstrating Electromagnetic Induction

Electromagnetic induction can be easily demonstrated using a coil, a bar magnet, and a sensitive center-zero galvanometer.

Materials Required:

1. A coil of insulated copper wire (induction coil)
2. A strong bar magnet
3. A sensitive center-zero galvanometer
4. Connecting wires

Procedure:

The procedure involves the following steps:

1. Connect the two ends of the coil to the terminals of the sensitive center-zero galvanometer using connecting wires. Ensure the connections are firm.
2. Hold the bar magnet and the coil steady. Observe the galvanometer. It should show no deflection, indicating no current.
3. Move the North pole of the bar magnet quickly into the coil. Observe the galvanometer. It will show a momentary deflection in one direction.
4. Hold the magnet stationary inside the coil. Observe the galvanometer. It should return to zero, indicating no current.
5. Move the North pole of the bar magnet quickly out of the coil. Observe the galvanometer. It will show a momentary deflection in the opposite direction compared to step 3.
6. Repeat steps 3-5 using the South pole of the bar magnet. You will observe similar deflections, but the directions will be reversed compared to using the North pole.
7. Repeat steps 3-5, but this time move the magnet more slowly. Observe that the deflection of the galvanometer is smaller, indicating a smaller induced current.
8. Repeat steps 3-5, but this time move the magnet more quickly. Observe that the deflection of the galvanometer is larger, indicating a larger induced current.

Observations and Explanation:

1. No relative motion: When the magnet and coil are stationary relative to each other, there is no change in magnetic flux through the coil, so no e.m.f. or current is induced. The galvanometer reads zero.

2. Relative motion: When the magnet is moved into or out of the coil, the magnetic flux passing through the coil changes. This change in flux induces an e.m.f., which drives an induced current through the coil and the galvanometer. The galvanometer deflects, indicating the presence and direction of this current.

3. Direction of deflection: The direction of deflection depends on the direction of motion of the magnet and its pole. Moving the North pole into the coil causes deflection in one direction, while moving it out causes deflection in the opposite direction. This demonstrates Lenz’s Law, as the induced current opposes the change in flux.

4. Magnitude of deflection: The magnitude of deflection depends on the speed of relative motion. Faster motion leads to a greater rate of change of magnetic flux, resulting in a larger induced e.m.f. and current, hence a larger deflection. This demonstrates Faraday’s Law.