MAGNETS- learn electromagnetic force, electric motor, natural and artificial magnets, polarity, north pole, south pole, demagnetization, magnetic force, field, Flemings-left hand rule, magnetic flux, DC motor, induce emf, motional emf

 A magnet can be defined as any substance that can attract substances such as iron, steel, Nickel, etc

               Types of magnet

1. Natural Magnets: These are magnets found as large deposit in the earth crust. They are ores of iron known as Magnetites (Fe3O4)
2. Artificial Magnets: These are magnets produced in the Laboratory or industry from iron or alloys of iron.

     Comparing magnetic properties of                       iron and steel

1. Iron can easily be magnetized but it will take Steel longer time to be magnetize
2. Magnets made of iron can easily loose their magnetic properties while those made of steel can retain their properties over a long period of time.
3. Soft iron are used for making temporary magnets while steel are use for making permanent magnets.
4. Iron produces a stronger magnet than steel when subjected to the same magnetic force.

          Method of making Magnets

Magnets can be made in three ways, they are

• Electrical method: In this method, a soft iron bar is placed in a solenoid and a direct current is made to flow through it. It is the best and quickest method of making magnets.

NB: A solenoid is a long piece of wire which is wound in the shape of a coil.

The region where the attracting power of a magnet is strongest is refer to as the POLE.

The POLARITY of the magnet formed, depends on the DIRECTION OF FLOW OF CURRENT. if the current is flowing clockwisely through one end, then such end will be a SOUTHPOLE, but if anticlockwisely, then such end will be NORTH POLE.

Another method of making magnets is :

• Single touch method: In this method, the pole of a magnet is use for striking the surface of a soft iron bar in a repeated manner.

The end of the bar that is touch last will have opposite polarity to that of it's striking pole. From the diagram above, End A becomes the SOUTH, while End B becomes the NORTH.

• Double or divided touch method: This method involves the use of two magnets with opposite poles for striking the surface of the soft iron bar. The polarity of the magnet formed can be obtained by using the concept of the single touch. In the figure below End A becomes the SOUTH pole, while End B becomes the NORTH pole.

Note: Like poles repel each other while unlike poles attract each other. Below is the diagram

   The diagram above shows two magnets of opposite poles facing each other. Unlike poles attracting each other

    Like poles repelling each other.

                   Demagnetization

This is a process whereby a magnet is made to lose its magnetism. Demagnetization can be achieved by the following

1. Electrical method
2. Heating method
3. Mechanical method (Hammering method)

Electrical method: The best way to demagnetising a magnet is through electrical method by placing it inside a solenoid through which an alternating current is flowing. The solenoid is placed with its axis pointing east to west.

Heating method: This involves heating the magnet until it is red hot and then allowing it to cool while lying in a East- West direction.

Mechanical method ( Hammering): hammering a magnet will cause weakening of the magnetism in the magnet.

A Magnetic keeper( Armature) is a bar made of soft iron or steel which is placed across the pole of a magnet to help preserve the strength of the magnet.

                Electromagnetic Field

When you have two magnets facing each other, they are likely to attract or repel each other. The Force of attraction or repulsion is called MAGNETIC FORCE while the region where the magnetic force is felt is called MAGNETIC FIELD. magnetic field is a vector quantity.

Electromagnetic field is the region where you have a current flowing in a magnetic field.

          Magnetic Flux

Magnetic Flux (ᶲ) can be defined as the product of magnetic flux density (B) and Area magnetic flux (A). I.e ᶲ = BA.

The S.I unit of magnetic flux density is Tesla. Magnetic Flux density (B) is telling you the strength of the magnetic field.

  Force on a current carrying conductor in a magnetic field

A current carrying conductor will experience a force when placed in a magnetic field. The Force on the conductor is as a result of the interaction between the field around the conductor and the field of the magnet. The Force experience by the conductor depends on the following factors

1. Strength of the magnetic flux (B)
2. Magnitude of the current flowing (I)
3. Length of the conductor.(L)
4. Angle between the conductor and the field.

I.e 

Examples:

1) The Force experienced by a current carrying conductor of length 200cm is 3N. Calculate the current in the conductor if it is flowing in a direction 45° with the field of 1.0T.

                   Solution

    

2) A conductor of length 3m carries a current of 0.5A when kept in a magnetic field of magnetic flux density 0.4 Tesla. The maximum force acting on it is?

                        Solution

NB: The Force on the conductor will make the conductor to start rotating in the field. The direction of motion can be obtain using the Flemings left - hand rule which states that 'If the thumb, fore-finger and middle finger of the left hand are placed mutually at right angles to each other, then the thumb points towards the direction of the magnetic force, the forefinger points towards the direction of the magnetic field and the middle finger points towards the direction of the electric current.

Force between two parallel current carrying conductor

When two current carrying conductors are placed parallel to each other, there will be interaction between the magnetic flux around the conductors. These interaction will produce force on the conductors. The Force will be ATTRACTIVE if the current is flowing in the SAME direction but REPULSIVE if the current is flowing in OPPOSITE direction. 

The diagram above shows the force of attraction, when current flow in the same direction.

The diagram above shows Force of Repulsion, when current flow in opposite directions.

The Force experience by the conductors depends on the magnitude of current flowing, the length of the conductors and the distance between the conductors. Generally,

Where Ia and Ib are the magnitudes of current flowing through the conductors, L is the length of the conductor and d is the distance between the conductors. The closer the conductors, the higher the force between them. 

    The Force per unit Length can be express as

Example:

Force on a moving charge in a Magnetic field

A moving charge will experience a force when it enters a magnetic field. The magnitude of the force depends on the following factors:

1. magnitude of the charge (q)
2. velocity or speed (V)
3. strength of the magnetic field (B)
4. direction of motion.

The charge will experience NO force when it is moving PARALLEL to the field or in the direction of the field. The charge will experience MAXIMUM force when it is moving PERPENDICULAR to the field.

Example:

                  Electromagnet

These are materials made of soft iron that behaves as magnet only when current is flowing through them. Immediately current stop flowing, they loose all their magnetic properties.

        Applications of Electromagnet

1. They are use in the construction of electric bell.
2. They are use in the construction of telephone eyepiece.
3. They can be used to separate iron from mixtures containing non magnetic elements.
4. They can be used for lifting heavy equipment made from iron.

                       Electric Motor

This is a device use for converting electrical energy into mechanical energy. It is also called a D.C motor. The principle of operation of an electric motor is based, that a current carrying coil will experience a force in a Magnetic field. The direction of force, magnetic field and electric current can be determined using Flemings left-hand rule which have been stated above. Electric motor is found in fans, CD player, cassette player and other devices where current production bring about motion.

          Electromagnetic Induction

This concept explain how electric current can be produced as a result of relative motion between the magnet and the coil. When a magnet is plunged into a coil, current will be produced in the coil, when the magnet is also withdrawn from the coil, current is still observe to be flowing in the coil. When the magnet is held stationary and the coil is now move towards the magnet or away from the magnet, current is still observe to be flowing in the coil. This current is called Induced Current and it is due to Induced E.M.F

      Factors affecting Induced E.M.F

1. Strength of the magnets (B): The stronger the magnet the higher the induce E.M.F
2. Area of coil( A): The larger the area of coil, the greater the induce E.M.F
3. No of turns in the coil (N): Induce E.M.F increases with number of turns in the coil.
4. Relative Velocity or speed of the magnet and the coil(V): There will be no current in the coil of the magnet is kept stationary in the coil. The greater the relative velocity, the higher the Induce E.M.F

Induce E.M.F = BANV

Also, Induce E.M.F = BANW

Where 'w' is the angular velocity or speed.

When a conductor is withdrawn from a magnetic field, emf will be induced in the conductor. This emf is called motional emf

Motional emf = BLV

           

                        Questions

1)

I Iron retains it's magnetic much longer than steel.

II Iron is more easily magnetize than steel.

III iron is more easily demagnetize than steel.

IV Iron produces a stronger magnet than steel.

Which combination of the above makes iron preferable to steel in the making of electromagnet.

A I and II only

B II and III only

C I, III and IV only

D II, III and IV only

2)In storing magnets, keepers are used to (a)reduce self- demagnetization (b) cancel the effect of the Earth's magnetic field (c) protect the magnet from stray electric fields (d)increase the strength of the magnet.

3) The Force on a current carrying conductor in a magnetic field is greatest when (a) the conductor makes an angle of 60° with the field (b) Force is independent of the angle between the field and the conductor (c) conductor is parallel with the field (d) conductor is at right angles with the field.

4) A straight wire 10cm long, carrying a current of 5.0A is in a uniform field of  0.50T. What is the Force on the wire when it is at right angle to the field?

5) A conductor of length 3m carries a current of 0.6A while kept in a magnetic field of magnetic flux density 0.5T. The maximum force acting on it is?

6) 

If current flows in the direction of the arrows in the solenoid above, the north pole is at __________

a) C.         (b) D.    (c)  P.      (d) Q

7)The South Pole of a magnet can never be separated from the North Pole because of a property known as (a) Magnetic octopole (b) magnetic dipole (c) magnetic quadrupole (d) Magnetic monopole

8) In the diagram below, If the south poles of two magnets stroke a steel bar, the polarities at X and Z are?

(a) North and South

(b) South and South

(c) North and North

(d) South and North

9)  

  

   

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