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Magnetic Effects of Electric Current

Chapter: Magnetic Effects of Electric Current

A wire carrying electric current behaves as a magnet. We can say that magnetism and electricity are related to each other. This magnetic effect of electric current is called as electromagnetic effect.

Properties of a magnet:

  1. A freely suspended magnet always points towards north and south direction.
  2. The pole of magnet which is pointing towards north direction is called North Pole.  It is also called north seeking.
  3. The pole of magnet which is pointing towards south direction is called South Pole. It is also called south seeking.
  4. Opposite poles of magnet attract each other
  5. Like poles of magnet repel each other.

Magnetic field and field lines

We can observe that when a compass is brought near a current carrying wire/ conductor the needle of compass deflects. The deflection is because of flow of electricity. This shows that electric current produces a magnetic effect.

Magnetic field is the influence of force which is surrounding a magnet. This force exerted by the magnet can be detected by a compass or a magnet.

Field lines are the imaginary lines of magnetic field around a magnet. Let us consider an experiment. Get some iron filings and place around a magnet. They get arranged in a pattern which depicts the magnetic lines. Magnetic field is a vector quantity. It has both magnitude and direction.

Direction of Field Line:

The direction of magnetic field line outside the magnet is considered from North Pole to South Pole. And inside the magnet, the direction is considered from South Pole to North Pole.

Strength of magnetic field:

The closer lines show stronger magnetic field and vice versa.

Magnetic Field Due to a Current-Carrying Conductor

Magnetic field due to a Current through a Straight Conductor:

A current carrying straight conductor has magnetic field in the form of concentric circles around it. The direction of magnetic field through a current carrying conductor depends upon the direction of flow of electric current. The direction of magnetic field gets reversed in case of a change in the direction of electric current.

For example, a wire is suspended vertically and the current is flowing from south to north. Here the direction of magnetic field will be anticlockwise. The direction of magnetic field will be clockwise if the current is flowing from north to south.

Right hand Thumb Rule

Right hand thumb rule is also called as Maxwell’s Corkscrew law.

 

 

Let us hold a current carrying conductor in your right hand in such a way that the thumb is pointing towards the direction of current. Then the direction of magnetic lines is the direction in which our fingers wrapped around the conductor. This is called as Right hand thumb rule.

Magnetic field due to current through a circular loop:

In case of a circular current carrying conductor, the magnetic field lines would be in the form of concentric circles around every part of the periphery of the conductor. The magnetic field would be stronger near the periphery of the loop as the magnetic lines remain close near the conductor. The magnetic field lines would be at a distance from each other when we move towards the center of the current carrying loop. The arcs of big circles would appear as straight lines at the center.

Right Hand Thumb’s Rule can be used to know the direction of magnetic field. If current is moving in anti-clockwise direction, the magnetic field would be in clockwise direction and vice versa.

Clock Face Rule:

A current carrying loop works like a disc magnet. The polarity of this magnet can be understood with the help of clock face rule. If the current is flowing in anti-clockwise direction, then the face of the loop shows North Pole. On the other hand, if the current is flowing in clockwise direction, then the face of the loop shows South Pole.

Magnetic field and number of turns of coil:

Magnitude of magnetic field gets summed up with increase in the number of turns of coil. If there are ‘n’ turns of coil, magnitude of magnetic field will be ‘n’ times of magnetic field in case of a single turn of coil.

Magnetic Field due to a Current in a Solenoid:

Solenoid is defined as the coil with many circular turns of insulated copper wire and they are wrapped closely in the shape of cylinder.

A current carrying solenoid produces same pattern as of magnetic field as a bar magnet. One end of solenoid behaves as the North Pole and another end behaves as the South Pole. Magnetic field lines are parallel inside the solenoid; similar to a bar magnet. It shows that inside a solenoid, magnetic field is same at all points.

Magnetic materials can be magnetized, by producing a strong magnetic field inside the solenoid. Such a magnet formed by producing magnetic field inside a solenoid is called electromagnet.

Force on a Current-Carrying Conductor in a Magnetic Field

A current carrying conductor exerts a force when a magnet is placed in its vicinity. Also a magnet exerts equal and opposite force on the current carrying conductor. This was illustrated by Marie Ampere, a French Physicist and thus considered as founder of science of electromagnetism.

The direction of force over the conductor gets reversed with the change in direction of flow of electric current. Also it is noted that the magnitude of force is the highest when the direction of current is at right angles (900) to the magnetic field.

Fleming’s Left Hand Rule:

If the left hand is stretched in such a way that the index finger, middle finger and thumb are in mutually perpendicular direction, then the index finger shows the direction of magnetic field and the middle finger shows the direction of electric current.

Devices such as electric motor, electric generator, etc works on this principle.

Electric Motor

Electric motor is a device which converts electrical energy into mechanical energy. It works on Fleming’s left hand rule.

A rectangular coil is suspended between the two poles of a magnet and the electric supply is connected with the help of a commutator. Commutator is a device which reverses the direction of flow of electric current through a circuit

When electric current is supplied to the coil of electric motor, the coil gets deflected because of magnetic field. As it reaches the half way, the split ring acts as commutator and reverses the direction of flow of electric current. The direction of forces acting on the coil also reverses because of the reversal of direction of current. The change in direction of force pushes the coil; and it moves another half turn. Thus, the coil completes one rotation around the axle. This process keeps on continuing to keep the motor in rotation.

Electromagnetic Induction

The phenomenon in which electric current is generated by varying magnetic fields is appropriately called electromagnetic induction.

This phenomenon is studied by Michael Faraday.

Let us explain in the following way. Let us see that a conductor is moved inside a magnetic field and hence electric current is induced in the conductor. A potential difference is induced in conductor when it is brought into relative motion with a magnetic field. This is called as electromagnetic induction.

Electromagnetic induction can be explained by Fleming’s right hand rule. If the right hand is stretched in such a way that the index finger, middle finger and thumb are in mutually perpendicular direction, then the thumb shows the direction of movement of the conductor, the index finger shows the direction of magnetic field and the middle finger shows the direction of induced electric current in the conductor.

Electromagnetic induction is used in the conversion of kinetic energy into electrical energy.

Electric generator:

Electric generator is a device which converts mechanical energy into electrical energy.

A rectangular armature is placed within the magnetic field of a permanent magnet. Armature is a soft iron core with large number of conducting wire turns over it. The armature is attached to wire and is placed in such a way that it can move around an axle. An electric current is induced when the armature moves within the magnetic field. The direction of induced current starts changing when the armature crosses the halfway mark of its rotation. Thus, the direction of current changes once in every rotation. Due to this, the electric generator usually produces alternate current, i.e. AC. A split ring commutator is used to convert an AC generator into a DC generator. This produces direct current.

AC – Alternating current:

Current in which direction is changed periodically is called Alternating Current. In India, most of the power stations generate alternating current. The direction of current changes after every 1/100 second in India.That is frequency of AC in India is 50 Hz.

DC – Direct current:

Current that flows in one direction only is called Direct current. Electrochemical cells produce direct current.

Domestic Electric Circuits

In our homes, we receive supply of electric power through a main supply (also called mains), either supported through overhead electric poles or by underground cables. The wires with red insulation cover is called live wire which is positive. And the other wire with black insulation is called neutral wire which is negative. The Potential difference between the two is 220 V, 50 Hz in our country. These wires pass into a meter through a main fuse. They are connected to the line wires in house through the main switch. These wires supply electricity to separate circuits in the house. The earth wire having insulation in green color is connected to a metal plate deep in the earth near the house. This is used as a safety measure. The metallic body is connected to the earth wire and it provides a low-resistance conducting path for the current. It ensures that any leakage of current to the metallic body of the appliance keeps its potential to that of the earth, and the user may not get a severe electric shock.

Electric fuse is an important component of all domestic circuits. A fuse in a circuit prevents damage to the appliances and the circuit due to overloading. Overloading can occur when the live wire and the neutral wire come into direct contact. This occurs when the insulation of wires is damaged or some fault in any appliance. In such a situation, the current in the circuit suddenly increases. This is called short- circuiting. This is prevented by electric fuse.