Lesson 13 - Magnetic Effect of Electric Current-Class 10 Science Notes

- May 19, 2021

Magnetic Effect of Electric Current

magnetic effect of electric current

Magnet:- Magnet is substance which attracts iron, cobalt and nickel or iron like substance.

Or An object which is capable of producing magnetic field and attracting unlike poles and repelling like poles.

Types of magnets:- There are following types of magnets-

1. Permanent magnet- These magnets do not lose their magnetic property once they are magnetized.

2. Temporary magnet- Temporary magnets can be magnetized in the presence of a magnetic field. When the magnetic field is removed, these materials lose their magnetic property.

3. Electromagnet- Electromagnets consist of a coil of wire wrapped around the metal core made from iron. When this material is exposed to an electric current, the magnetic field is generated making the material behave like a magnet.

Various shapes of magnets:- Various shapes of magnet available for use-

1. Bar magnet

2. Horseshoe magnet

3. Disc magnet

4. Sphere magnet

5. Cylindrical magnet

6. Ring magnet

We will study about bar magnet

Properties of magnet:- Properties of magnet are as follows-

1. A freely suspended magnet always points towards the north and south direction.

2. The pole of a magnet which points toward the north direction is called north pole or north seeking.

3. The pole of a magnet which point toward south direction is called south pole or south seeking.

4. Like poles of magnets repel each other while unlike poles of magnets attract each other.

5. Magnet always exist as dipole.

6. Two poles can never be separated: if we try to cut them then also, both the poles will exist even in a small piece of magnet. It automatically develops the lost polarity.

7. A magnet always develops certain area around it where its effect can be felt i.e. magnetic field.

Uses of magnets:- Magnets are used

1. in refrigerators, electric generators, electric motors

2. in speakers, electric bells, electric cranes

3. in radio and stereo speakers

4. in audio and video cassette players.

5. in children’s toys.

6. on hard discs and floppies of computers.

7. for constructing magnetic needles and mariner’s compass.

8. for the separation of iron filling from other solid mixture.

Magnetic Field:- The area around a magnet where a magnet force is experienced is called magnetic field. It is vector quantity i.e. it has both magnitude and direction.

The force exerted by a magnet can be detected using a magnetic compass or any other magnet. The magnetic field is represented by magnetic field lines.

Magnetic Field Lines:- The imaginary lines of magnetic field around a magnet are called field lines or magnetic field lines.

When iron fillings are allowed to settle around a bar magnet, they get arranged in a pattern which mimics the magnetic field lines.

Direction of field lines:- Outside the magnet, the direction of magnetic field lines is taken from North pole to South pole.

Inside the magnet, the direction of magnetic field lines is taken from South pole to North pole.

Strength of magnetic field:- The closeness of field lines shows the relative strength of magnetic field. Closer field lines near the poles of a magnet shows the more strength. Magnetic field is weak far from poles comparatively.

Characteristic of magnetic field lines:- Characteristic of magnetic field lines are as-

1. Field lines arise from North pole and end into South pole of magnet.

2. Field lines are closed curves.

3. Field lines are closer in stronger magnetic field.

4. Magnetic field lines give the direction of magnetic force.

5. Field lines never intersect each other as for two lines to intersect, there must be two north direction at a point, which is not possible.

6. Direction of field lines outside the magnet is from North pole to South pole.

7. Direction of field lines inside the magnet is from South pole to North pole.

Oersted’s experiment:- Oersted conducted an experiment and proved that a wire carrying an electric current produces a magnetic field around it. The direction of magnetic field was confirmed by the deflection of magnetic needle kept near the current carrying conductor or near the circuit.

Ampere swimming rule:- Suppose a person is swimming in the direction of current such that current enters the man through his feet and leaves his body through his head then the magnetic needle kept below it will get deflected towards it.

Snow rule:- According to this rule, when the current flows from the south to the north, the needle deflects towards the west.

Magnetic field due a current carrying straight conductor:- A current carrying straight conductor has magnetic field in the form of concentric circles around it. Magnetic field of a current carrying straight conductor can be shown by magnetic field lines.

Direction of magnetic field of a current carrying straight conductor:- Direction of magnetic field through a current carrying straight conductor depends upon direction of flow of electric current.

Let a current carrying conductor be suspended vertically and the electric current is flowing from south to north. In this case, the direction of magnetic field will be anticlockwise. If the current is flowing from north to south, the direction of magnetic field will be clockwise.

Right-Hand-Thumb rule:- This rule is used to find direction of magnetic field through a current carrying straight conductor.

According to this rule, If a current carrying conductor is held by right hand, keeping the thumb straight and if the direction of electric current is in the direction of thumb, then the direction of wrapping of other fingers will show the direction of magnetic field. It is also known as maxwell’s Corkscrew rule.

Maxwell’s corkscrew rule:- According to this rule, if the direction of forward movement of screw shows the direction of the current, then the direction of rotation of screw shows the direction of magnetic field.

Properties of magnetic field:- Properties of magnetic field produced by a current carrying straight conductor are as-

1. Magnetic lines are concentric circles.

2. The direction of magnetic lines reverse as we reverse the direction of the current.

When current flows upward, the direction of magnetic lines is anticlockwise.
When current flows downward, the direction of magnetic lines is clockwise.

3. The magnitude of magnetic field increases with increase in electric current and decreases with decrease in electric current.

4. The magnitude of magnetic field produced by electric current decreases with increase in distance and increases with decrease in distance.

5. Magnetic field lines are always parallel to each other.

6. No two field lines cross each other.

Magnetic field due to a current carrying circular loop:- The magnetic field produced around a circular loop is also circular. As we move away from the loop, the concentric circle becomes bigger. At the center, the magnetic lines are parallel.

Characteristics:-

1. The magnetic lines are circular at the points from where the current enters or leaves the coil.

2. Within the space enclosed by the coil, the field are in the same direction.

3.Near the center of the coil, the magnetic lines are almost parallel to each other.

4..At the center of the coil, the plane of magnetic field lines is at right angles to the plane of circular coil.

5.Magnetic field produced is directly proportional to the current and inversely proportional to the distance from the conductor.

6.The part from where the magnetic lines enter the coil facing us is considered as South Pole and the other is North Pole.

Current flowing clockwise=south

Current flowing anti-clockwise=north

Magnetic field in number of turns of coil:- Magnetic field gets summed up with increase in the number of turns of coil.

If there are ‘n’ turns in a coil, magnitude of magnetic field will be ‘n’ times of magnetic field in case of a single turn of coil.

Strength of magnetic field at center of loop(coil):- Strength of magnetic field at center of loop depends on-

1. The radius of coil:- The strength of the magnetic field is inversely proportional to the radius of the coil. If the radius increases, the magnetic strength at the center decreases.

2. The number of turns in the coil:- If the number of turns in the coil increases, the strength of magnetic field increases at the center of loop, because the current in each circular turn is having the same direction.

3. The current flowing in the coil:- If the current in the coil increases, the strength of the magnetic field increases.

Solenoid:- Solenoid is the coil with many circular turns of insulated copper wire wrapped closely in the shape of a cylinder.

Magnetic field due to a current in a solenoid:- A current carrying solenoid produces similar pattern of magnetic field as a bar magnet. One end of solenoid behaves as the north pole and another end behaves as the south pole.

Properties of field lines inside a solenoid:-

1. The field lines inside the solenoid are in the form of parallel straight lines.

2. This indicates that the magnetic field is the same at all points inside the solenoid. Therefore, the field is uniform inside the solenoid.

3. The field lines inside the solenoid are in the form of parallel straight lines. This property is used to make an electromagnet.

4. A strong magnetic field is produced inside the solenoid.

Electromagnet:- A magnet is made by magnetic field produced inside a solenoid using magnetic materials like soft iron is called an electromagnet.

An electromagnet consist of a long coil of insulated copper wire wrapped on a soft iron.

Factors affecting the strength of an electromagnet:- Factors affecting the strength of an electromagnet are as follows-

1. Number of turns in the coil

2. Current flowing in the coil

3. Length of gap between the poles.

Steel is not used for making electromagnet because it does not lose its magnetism even if the current is switched off. i. e., it forms a permanent magnet.

Properties of electromagnet:-

1. The magnetic field produced is generally very strong.

2. The strength of the magnetic field can be controlled by controlling various factors such as the current and the number of turns in the solenoid.

3. The polarity of the magnet can be changed by reversing the direction of current while magnetic field is being produced by solenoid.

Difference between Electromagnet and permanent magnet:-

Electromagnet	Permanent magnet
1.The magnetic field produced is generally very strong.	1.Generally the magnetic field produced low and moderate.
2.The strength of the magnetic field can be controlled by controlling various factors such as the current and the number of turns in the solenoid.	2.The strength of the magnetic field of permanent magnet is permanent but changes(decreases) with the temperature.
3.The polarity of the magnet can be changed by reversing by reversing the direction of current.	3.The polarity of the permanent magnet can not changed.
4.It is a temporary magnet, so it can be easily demagnetized.	4. It cannot be easily demagnetized.
5.Normally soft iron is used for making electromagnets.	5.Cobalt, Steel etc., are used for making permanent magnets.

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. Similarly, a magnet also exerts equal and opposite force on the current carrying conductor. This was suggested by Marie Ampere, a French physicist and 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. It is observed that the magnitude of force is highest when the direction of current is at right angles to the magnetic field.

Kicking wire experiment:- When a strong horse shoe magnet is placed in such a way that the rod lies between the two poles with the magnetic field directed upwards. For this the north pole of the magnet is kept vertically above the aluminum rod from end B to end A. It is observed that the rod is displaced. It is also observed when the direction of current flowing is reversed through rod the direction of displacement is also reversed.

Conclusion:-

1. A magnetic field exerts a force on a magnet placed in the vicinity of the conductor.

2.A force is exerted on the current carrying aluminum rod when it is placed in a magnetic field.

3. The direction of force is also reversed when the direction of current through the conductor is reversed.

4.The direction of force acting on the current carrying rod gets reversed when the direction of current is reversed.

5. The force on the conductor depends upon the direction of current and the direction of the magnetic field.

The force on the conductor:- The force on the conductor depends upon the following two things-

1. The direction of current and

2.The direction of the magnetic field.

Fleming’s left hand rule:- According to this rule, stretch the thumb, forefinger and middle finger of your left hand such that they are mutually perpendicular. If the first finger points in the direction of magnetic field and the second finger in the direction of current, then the thumb will point in the direction of motion or the force acting on the conductor.

Many devices like electric motor, electric generator, loudspeaker etc. works on Fleming’s left hand rule.

Devices that use current carrying conductor and magnetic field:- Electric motor, electric generator, loudspeakers, microphones and measuring instruments etc.

MRI:- The magnetic inside the body forms the basis of obtaining the images of different body parts. This is done using a technique called Magnetic Resonance Imaging (MRI).

Properties of magnetic field inside the body:-

1. Magnetic field is produced in our body which travels along the nerve cells.

2. This nerve impulse produces a temporary magnetic field.

3. These fields are very weak and are about one-billionth of the earth’s magnetic field.

4. Human heart and brain are two organs which produce magnetic field.

Electric motor:- An electric motor is a device which converts electric energy into mechanical energy. It is of two types- AC motor and DC motor.

Principle: An electric motor works on the principle that a current carrying conductor experiences a force when placed in a magnetic field. If the direction of the magnetic field and that of the direction of current which pass through the magnetic field are mutually perpendicular then the direction of the force is given by Fleming’s left hand rule.

Construction: It consists of the following parts-

1. Armature: It is rectangular coil ABCD which is suspended between the two poles of a magnetic field.

Role of armature –

i. The armature operates by rotating along with a magnetic field.

ii. The armature will rest when the resultant field is aligned with the stator (or static) field.

2. Commutator or Split ring: These are cylindrical shape metallic rings which are divided into two halves. The inner sides of these halves are insulated and attached to an axle.

Role of split ring – It works as commutator in electric motor that reverses the direction of the flowing of electric current.

Commutator: Commutator is a device which is used to reverse the direction of the flowing of electric current.

3. Magnet: Magnetic field is supplied by a permanent magnet NS.

4. Brushes: Sliding contacts or brushes X and Y which are made of graphite and fixed on split ring. These are connected to battery and armature with connecting wires.

5. Axle: Axle is a rotating rod like structure which is situated in the center of armature and split rings are fitted on it.

6. Battery: These are consists of few cells.

Working:- When an electric current is supplied to the coil of the electric motor, it gets deflected because of magnetic field. As it reaches the halfway, the split ring which acts as commutator reverses the direction of flow of electric current. Reversal of direction of the current, reverses the direction of forces acting on the coil. The change in direction of force pushes the coil, and it moves another half turn. Thus the coil completes one rotation around the axle. Continuation of this process keeps the motor in rotation.

Uses of motors:- These are used in many devices-

i. Electric fans

ii. Pumping water

iii. Mixers

iv. Grinders

v. Blenders

vi. Cutters

vii. DVD players

viii. Computers

ix. Washing machines

x. Various toys etc.

The properties of commercial motors:- Commercial motors are powerful motors. Due to following properties these are powerful

i. Electrical magnet is used in commercial motor in spite of permanent magnet.

ii. Number of turns of insulated copper wire is more in current carrying coil.

iii. Soft iron core is used to wrap the coil, by which power is increased.

Electro Magnetic Induction:- The process by which a changing magnetic field I a conductor induces a current in another conductor is called electromagnetic induction.

The discovery of electromagnetic induction was made by Michael Faraday.

Induced Electric current: When we placer a moving magnet inside a coil, there produces electric current in the coil’s circuit which can be shown in Galvanometer by deflection of it’s needle. The motion of magnet with respect to coil induces a induced potential difference, due to which flows induced electric current in circuit.

The induced current is found to be highest when the direction of motion of the coil is at right angles to the magnetic field.

Methods to induce electric current in a coil:- Two way by which we can induce electric current in a coil-

1. 1.By moving coil in a magnetic field.

2. 2.By changing the magnetic field around it.

In most situations, it is convenient to move the coil in a magnetic field.

Fleming’s Right-hand Rule:- This is used to know the direction of induced current in a coil placed in a variable magnetic field.

According to this rule, “Stretch the thumb, forefinger and middle finger of right hand so that they are perpendicular to each other. If the forefinger indicates the direction of the magnetic field and the thumb shows the direction of motion of conductor, then the middle finger will show the direction of induced current.

Electric Generator:- An electric device which converts mechanical energy into electrical energy is called an electric generator.

Principle: Electric generator works on the principle of electromagnetic induction. When the coil of electric generator rotates in a magnetic field, the magnetic field induces a current in this coil. This induced current then flows into the circuit connected to the coil.

Types of generator: There are two types of generators-

1.AC Generator

2. DC Generator

1.AC Generator:- A generator which converts mechanical energy into electrical energy in the form of alternating current is called AC Generator.

Construction:- The main components of AC generator are as shown in fugure-

i. Armature: It is rectangular coil ABCD which is suspended between the two poles of a magnetic field.

ii. Strong Magnet: Magnetic field is supplied by a permanent magnet NS.

iii. Slip rings: The two ends of the armature coil are connected to two brass slip rings R1 and R2. These rings rotate along with the armature coil.

iv. Brushes: Sliding contacts or brushes B1 and B2 which are made of graphite and fixed on split ring. These are connected to battery and armature with connecting wires.

v. Axle: Axle is a rotating rod like structure which is situated in the center of armature and split rings are fitted on it.

Working:-

i. When the armature coil ABCD rotates in the magnetic field provided by the strong magnets. It cuts the magnetic lines of forces.

ii. Thus the changing magnetic field produces induced current in the coil. The direction of induced current is determined by the Fleming’s right hand rule.

iii. The current flows out through the brush B1 in one direction in the first half of the revolution and through the brush B2 in the next half revolution in the reverse direction, this process is repeated.

iv. Therefore, the induced current produced is of alternating nature. Such a current is called as alternating current.

2.DC Generator:- A generator which converts mechanical energy in the form of direct current is called as DC generator.

Construction:- The main components of a DC generator are-

i. Armature: It is rectangular coil ABCD which is suspended between the two poles of a magnetic field.

ii. Strong Magnet: Magnetic field is supplied by a permanent magnet NS.

iii. Commutator or Split ring: These are cylindrical shape metallic rings which are divided into two halves. The inner sides of these halves are insulated and attached to an axle.

Commutator: Commutator is a device which is used to reverse the direction of the flowing of electric current

iv. Brushes: Sliding contacts or brushes B1 and B2 which are made of graphite and fixed on split ring. These are connected to battery and armature with connecting wires.

v. Axle: Axle is a rotating rod like structure which is situated in the center of armature and split rings are fitted on it.

Working:-

i. When the coil of DC generator rotates in the magnetic field, potential difference is produced in the coil. This gives rise to the flow of current.

ii. In DC generator, the flow of current in the circuit is in the same direction as long as the coil rotates in the magnetic field.

iii. This is because one brush is always in contact with the arm of the armature moving up and other brush is in contact with the arm of the armature moving downward in the magnetic field.

AC and DC current

AC (Atlernating Current): Current in which direction is changed periodically is called Alternating current.

In India, most of the power stations generate alternate current. The direction of current changes after every 1/100 second i.e. the 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.

Advantage of AC over DC:

i. AC can be transmitted over long distances without much loss of energy.

ii. AC can be easily converted to DC.

iii. AC can be controlled by the use of choke which involves less loss of power whereas, DC can be controlled using resistances which involves high energy loss.

iv. Cost of generator of AC is much less than that of DC.

v. AC machines are stout and durable and do not need much maintenance.

Disadvantage of AC:

i. AC cannot be used for the electrolysis process or showing electromagnetism as it reverses its polarity.

ii. AC is more dangerous than DC.

Domestic Electric Circuits:- We receive electric supply through mains supported through the poles or cables. In our houses, we receive AC electric power of 220V with a frequency of 50 Hz.

The three wires are as follows-

1. Live wire – (Red insulated, Positive)

2. Neutral wire – (Black insulated, Negative)

3. Earth wire – (Green insulated)

Short Circuit:-Short circuiting is caused by the touching of live wires and neutral wire and sudden a large amount of current flows.

It happens due to

i. Damage of insulation in power lines.

ii. A fault in an electrical appliances.

Overloading:- The overloading of electrical wire in any circuit die to the flow of large amount of current through it is called overloading of the electrical circuit.

Cause:

i. Accidental hike in voltage supply

ii. Use of more than one appliance in single socket

A sudden large amount of current flows through the wire, which causes overheating of wire and may cause fire also.

Safety devices:-

1. Electric Fuse

2. Earth Wire

3. MCB (Miniature Circuit Breaker)

1.Electric Fuse:- It is a protective device used for protecting the circuit from short-circuiting and overloading. It is a piece of thin wire of material having a low melting point and high resistance.

Fuse is always connected to live wire
Fuse is always connected in series to the electric circuit.
Fuse is always connected to the beginning of an electric circuit.
Fuse is always connected to the beginning of an electric circuit.
Fuse works on the heating effect

2.Earth wire:- Protects us from electric shock in case of leakage of current especially in metallic body appliances. It provides a low resistance path for current in case of leakage of current.

Earthing:- There can be a chance that the insulation breaks or cracks and the bare wire touches the metal case and then the person touching them might get a shock. To avoid such incidents, the metal cases are earthed.

By earthing we mean that the metallic body is connected to a thick copper wire which is buried deep under the earth. And at the end, the copper plate is surrounded by a mixture of charcoal and common salt.

3.MCB (Miniature Circuit Breaker): MCB is an automatically operated electrical switch used to protect electrical circuits from damage caused by excess current from an overload or short circuit.

MCB typically rated up to a current up to 125A, do not have adjustable trip characteristics, and can be thermal or thermal magnetic in operation.