magnetic effect of electric current, class 10

 

Magnet:  Magnet is an object that attracts objects made of iron, cobalt and nickel. Magnet comes to rest in North – South direction, when suspended freely.

Use of Magnets: Magnets are used

·         in refrigerators.

·         in radio and stereo speakers.

·         in audio and video cassette players.

·         in children’s toys and;

·         on hard discs and floppies of computers.

Properties of Magnet

·         A free suspended magnet always points towards the north and south direction.

·         The pole of a magnet which points toward north direction is called north pole or north-seeking.

·         The pole of a magnet which points toward south direction is called south pole or south seeking.

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

 

Hans Christian Oersted (1777-1851)

Oersted showed that electricity and magnetism are related to each other. His research later used in radio, television etc. The unit of magnetic field strength is name Oersted in his honour.

Oersted Experiment

On passing the current through the copper wire XY in the circuit, the compass needle which is placed near the conductor gets deflected. If we reverse the direction of current, the compass needle deflects in reverse direction. If we stop the flow of current, the needle comes at rest.

Hence, it concludes that electricity and magnetism are linked to each other. It shows that whenever the current will flow through the conductor, then magnetic field around will develop.

               
Magnetic field: The area around a magnet where a magnetic force is experienced is called the magnetic field. It is a quantity that has both direction and magnitude, (i.e., Vector quantity).

Magnetic field and field lines: The influence of force surrounding a magnet is called magnetic field. In the magnetic field, the force exerted by a magnet can be detected using a compass or any other magnet. The magnetic field is represented by magnetic field lines.

The imaginary lines of magnetic field around a magnet are called field line or field line of magnet. When iron fillings are allowed to settle around a bar magnet, they get arranged in a pattern which mimics the magnetic field lines. Magnetic field is a vector quantity, i.e. it has both direction and magnitude.

Direction of field line: Outside the magnet, the direction of magnetic field line is taken from North pole to South Pole. Inside the magnet, the direction of magnetic field line is taken from South pole to North pole.

Strength of magnetic field: The closeness of field lines shows the relative strength of magnetic field, i.e. closer lines show stronger magnetic field and vice – versa. Crowded field lines near the poles of magnet show more strength.

 Properties of magnetic field

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

·         The magnitude of magnetic field produced by electric current decreases with increase in distance and vice – versa. The size of concentric circles of magnetic field lines increases with distance from the conductor, which shows that magnetic field decreases with distance.

·         Magnetic field lines are always parallel to each other.

·         No two field lines cross each other.

It is taken by convention that magnetic field lines emerge from North pole and merge at the South pole. Inside the magnet, their direction is from South pole to North pole. Therefore magnetic field lines are closed curves.

Magnetic field lines due to current a current carrying straight conductor
A current carrying straight conductor has magnetic field in the form of concentric circles, around it. Magnetic field of current carrying straight conductor can be shown by magnetic field lines.
The direction of magnetic field through a current carrying conductor depends upon the direction of flow 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.

The direction of magnetic field, in relation to direction of electric
current through a straight conductor can be depicted by using the Right Hand Thumb Rule. It is also known as Maxwell’s Corkscrew Rule.

 Right-Hand Thumb 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.

Maxwell’s Corkscrew rule: As per Maxwell’s Corkscrew 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.

Magnetic field lines due to a 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. Since, magnetic field lines tend to remain closer when near to the conductor, so the magnetic field would be stronger near the periphery of the loop. On the other hand, the magnetic field lines would be distant from each other when we move towards the centre of the current carrying loop. Finally, at the centre, the arcs of big circles would appear as a straight line.

A circular loop behaves like a disk magnate whose one face act as North Pole and other South.

The direction of the magnetic field can be identified using Right Hand Thumb’s Rule. Let us assume that the current is moving in anti-clockwise direction in the loop. In that case, the magnetic field would be in clockwise direction, at the top of the loop. Moreover, it would be in an anti-clockwise direction at the bottom of the loop.

Clock Face Rule: A current carrying loop works like a disc magnet. The polarity of this magnet can be easily 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 due to a current in a Solenoid: Solenoid is the coil with many circular turns of insulated copper wire wrapped closely in the shape of a cylinder. 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.

Magnetic field lines are parallel inside the solenoid, similar to a bar magnet, which shows that magnetic field is same at all points inside the solenoid.

Magnetic field produced by a solenoid is similar to a bar magnet.
The strength of magnetic field is proportional to the number of turns and magnitude of current.
By producing a strong magnetic field inside the solenoid, magnetic materials can be magnetized.

 

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.

The strength of the magnetic field at the centre of the loop(coil) depends on :
(i) Increasing cross-section area/The radius of the 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 centre decreases
(ii) The number of turns in the coil : As the number of turns in the coil increase, the magnetic strength at the centre increases, because the current in each circular turn is having the same direction, thus, the field due to each turn adds up.
(iii) The strength of the current flowing in the coil: As the strength of the current increases, the strength of three magnetic fields also increases.

(iv) Inserting a soft iron core: when core is inserted it get magnetized due to magnetic effect of coil due to which it also form magnetic lines thus magnetic field intensity increase.

 

Electromagnet: An electromagnet consists of a long coil of insulated copper wire wrapped on a soft iron. Magnet formed by producing magnetic field inside a solenoid is called electromagnet.

Electromagnets are very widely used in electric and electromechanical devices, including:

• Motors and generators.
• Electric bells and buzzers.
• Loudspeakers and headphones.
• Magnetic recording and data storage equipment: tape recorders, VCRs, hard disks.
• MRI machines.

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.

 

Fleming’s Left-Hand Rule: 

If we stretch the thumb, middle finger and the index finger of the left hand mutually Perpendicular to each other in such a way that Index Finger represents the direction of the magnetic field (B), Middle Finger represents the direction of the current (I) than Thumb points towards the direction of force (F) act on the conductor placed in magnetic field.

 

Domestic Electric Circuits: We receive electric supply through mains supported through the poles or cables. In our houses, we receive AC electric power of 220 V with a frequency of 50 Hz.
The 3 wires are as follows

·         Live wire – (Red insulated, Positive)

·         Neutral wire – (Black insulated, Negative)

·         Earth wire – (Green insulated) for safety measure to ensure that any leakage of current to a metallic body does not give any serious shock to a user.

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

It happens due to

·         Damage of insulation in power lines.

·         a fault in an electrical appliance.

·         overloading

 Overloading of an Electric Circuit: The overheating of electrical wire in any circuit due to the flow of a large current through it is called overloading of the electrical circuit.

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

Overloading can occur when the live wire and the neutral wire come into direct contact.

 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 works on the heating effect.

·         )Electric Fuse current rating defines the maximum value of safe current allowed to flow through fuse without melting it

·         Tin-lead alloy is general used to make fuse wire

Earthing:

Earthing is the process of transferring and immediate discharge of electrical energy to the earth directly through a low resistance wire.

·         The earth wire, which has insulation of green colour, is usually connected to a metal plate deep in the earth near the house.

·         This is used as a safety measure, especially for those appliances that have a metallic body, for example, electric press, toaster, table fan, refrigerator, etc.

·         The metallic body is connected to the earth wire, which provides a low-resistance conducting path for the current. Thus, 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.