Notes: Magnets | Science & Pedagogy Paper 2 for CTET & TET Exams - CTET & State TET PDF Download

Magnets

A piece of iron or other material which exhibits properties of magnetism, such as attracting other iron-containing objects is known as a Magnet. The ancient Greeks were the discoverers of magnetite, a mineral that contains magnetic properties.
Magnets have different properties some of them are listed below:

• A magnet is always dipolar i.e. it has two poles.
• Like poles always repel while unlike poles attract each other.
• A freely suspended bar magnet will always point in the north-south direction.

Magnetic and Non-Magnetic Material

Those materials which are attracted by a magnet are magnetic materials. Iron, Nickel, and Cobalt.

The substances which are not attracted by a magnet are called non-magnetic substances. For example, wood, paper

Natural Magnets
A natural magnet is a magnet that is found in nature. An example of a natural magnet is the lodestone, which is also called magnetite.

Artificial Magnets
Artificial magnets are those magnets which are made by humans. These magnets can further be classified into two parts

• Temporary Magnets
• Permanent Magnets

Temporary Magnets
Temporary magnets are made up of those materials that work like permanent magnets in the presence of a magnetic field but lose their magnetism in absence of that magnetic field. Examples are pyrrhotite, ferrite, and columbite.

Permanent Magnets
These magnets don’t lose their magnetic properties. These have a wide range of application in electrical appliances. Examples of these magnets are magnets on our refrigerator, headphones, microphones, etc.

Magnetic Compass
It is an instrument which is used to find directions. It works on the third property that a freely suspended magnet will always point in the north-south direction.

Magnetic Field

It is the space surrounding a magnet within which magnetic effects can be felt. Pictorially it can be depicted with the help of magnetic field lines.

Properties of Magnetic Field Lines:

• Field lines always start from the north pole and end at the south pole.
• Field lines are denser near the pole and expand everywhere else.
• Field lines never intersect with each other.
• Field lines are continuous loops.
• Field strength is proportional to the line density.
• At a point, the field is tangent to the magnetic field line.

Magnetic Field due to a current-carrying conductor

• Oersted observed that the magnetic needle gets deflected in the presence of a current-carrying conductor.
• This was proof that the current-carrying conductor produces a magnetic field around the conductor.
• Magnetic Field is directly proportional to the current flowing in the conductor and inversely proportional to the distance from the wire/conductor.

Right-hand thumb rule

• It states that when a person holds a current-carrying wire in his right hand and if the thumb points along the direction of current then the direction in which the fingers incircle gives the direction of Magnetic field lines.

Electromagnet

• An electromagnet is a type of magnet in which the magnetic field is produced by an electric current. Electromagnets usually consist of wire wound into a coil.

How does an Electric bell work?
An electric bell has an electromagnet in it. It consists of a coil of wire wound(wrapped) on an iron piece. The coil acts as an electromagnet. An iron strip with a hammer at one end is kept close to the electromagnet.

There is a contact screw near the iron strip. When the iron strip is in contact with the screw, the current flows through the coil making it an electromagnet. This attracts the hammer. In the process, the hammer at the end of the strip strikes the gong of the bell to produce a sound. However, when the electromagnet pulls the iron strip, it also breaks the circuit. The current through the coil stops flowing. The coil is no longer an electromagnet. It no longer attracts the iron strip. The iron strip comes back to its original position and touches the contact screw again. This completes the circuit. The current flows in the coil and the hammer strikes the gong again. This process is repeated in quick succession. The hammer strikes the gong every time the circuit is completed. This is how the bell rings.

Fleming's Left-hand rule

It states that if we stretch the thumb, forefinger and middle finger of our left hand such that they are mutually perpendicular and point the forefinger in the direction of the magnetic field and the centre finger in the direction of the current, then the thumb gives the direction of force acting on the conductor.

Electric motor

It is a device that is used to convert electrical energy into mechanical energy. It is used in fans, automobiles, etc.

Principle of Electric motor
Electric Motor works on the principle of electromagnetism. When a rectangular coil is placed in a magnetic field and a current is passed through it, a force acts on the coil, which compels the coil to rotate continuously. As the coil relates to the shaft, the rotation of the coil leads to rotation of the shaft.

Electromagnetic induction
It is the process of production of electricity through magnetism.
Reason for electromagnetic induction whenever there is a change in the magnetic field in a region induced current flows through that region.

Fleming's Right-hand rule

It states that if we stretch the thumb, forefinger and middle finger of right hand such that they are mutually perpendicular and the forefinger points in the direction of the magnetic field and centre finger in the direction of induced current, then the thumb gives the direction of motion of the conductor.

Electric generator

It is a device that converts mechanical energy into electrical energy by using principles of electromagnetism.
There are two types of electric current:
(i) A.C generator                                            
(ii) D. C generator

Electric Generators work based on Electromagnetic Induction.
A.C. stands for alternating current and the generator which produces alternating current is called A.C. Generator similarly D.C. stands for direct current and the generator which produces direct current is called D.C. Generator

Domestic electric circuits

In our houses, we get A.C. supply of 50Hz frequency and 220V electric power.

There are three wires in a domestic circuit which are as follows:

(i) Live wire: These are insulated by red plastic covering and are positive in nature.

(ii)  Neutral wire: These are insulated by Black plastic covering and are negative in nature.

(iii) Earth wire: These are insulated by green plastic covering. These wires are installed for safety measures to ensure that any leakage of current to a metallic body does not give any serious shock to a user.

Some important devices which are part of a domestic electric circuit are Fuse and M.C.B.(Main Circuit Break) these devices are used to regulate the current supply and protect the user from damage like short-circuiting and overloading which are the result of excessive supply.