PERMANENT AND TEMPORARY MAGNETS :
The degree to which magnetism is retained by a given piece of iron depends entirely upon its constitution. Steel retains the largest amount while soft iron retains the least. Therefore pieces of steel are employed to prepare permanent magnets, whereas soft iron is used for preparing temporary magnets, i.e., magnets that retain their magnetism only as long as the current flows in the magnetising coil. They lose their magnetism as soon as the current is switched off. Such magnets are known as electromagnets.
11.2 (a) Electromagnet :
An electric current can be used for making temporary magnets known as electromagnets. As electromagnet works on the magnetic effect of current. When current is passed through a long coil called solenoid, a magnetic field is produced. It has been found that if a soft iron rod called core, is placed inside a solenoid then the strength of magnetic field becomes very large because the iron core gets magnetised by induction. This combination of a solenoid and a soft iron core is called an electromagnet.
Electromagnets can be made in different shapes and sizes depending on the purpose for which they are to be used.
Factors affecting the strength of an electromagnet are :
(i) The number of turns in the coil : If we increase the number of turns in the coil, the strength of electromagnet increases.
(ii) The current flowing in the coil : If the current in the coil is increased, the strength of electromagnet increases.
(iii) The length of air between its poles : if we reduce the length of air gap between the poles of an electromagnet, then its strength increases.
For example, the air gap between the poles of straight bar type electromagnet is quite large, so a bar type electromagnet is not very strong. One the motherland the air gap between the poles of a U-shaped electromagnet is small, so it is a very strong electromagnet.
Electromagnets are used in electric bells, telegraphs, telephones and several other instruments. Since the magnetisation depends on the current flowing through the coil, it is possible to obtain very powerful electromagnets by increasing the current.
Soft iron can be easily magnetised every by a weak magnetic field, whereas steel can be magnetised only by strong magnetic field.
Less energy is required for magnetising soft iron. Soft iron loses its magnetism immediately, whereas steel retains it magnetism.
11.2 (b) Difference between a Bar Magnet (or Permanent Magnet) and an Electromagnet :
| S.No. | Bar magnet (or permanent magnet) | Electromagnet |
(1) (2) (3) (4) | The bar magnet is a permanent magnet. A permanent magnet produces a comparatively weak force of attraction. The strength of a permanent magnet cannot be changed. The (north-south) polarity of permanent of manget is fixes and cannot be changed. | An electromagnet is a temporary magnet. Its magnetism is only for the duration for which current passes through it, so the magnetism of an electromagnet can be switched on or switched off as desired. An electromagnet can produce very strong magnetic force. The strength of an electromagnet can be changed by changing the number of turns in its coil or by changing the current passing through it. The polatiry of an electromagnet can be changed by changing the direction of current in its coil. |
Permanent magnets are usually made of alloys such as carbon-steel, chromium-steel, cobalt-steel, tungsten-steel, nipermag and alonico. Nipermag is an alloy of iron, nickel, aluminum and titanium whereas ALNICO is an alloy of aluminum, nickel and cobalt. Permanent magnets of these alloyws are much more stronger than those made of ordinary steel, such strong permanent magnets are used in microphones, loudspeakers, electric clocks, ammeters, voltmeters, speedometers and many other devices.
11.2 (c) Methods of Demagnetising a Permanent Magnet :
(i) Magnet can be demagnetised by :
(A) Self – demagnetisation, if the magnet is strode without using magnetic keepers.
(B) Dropping it from a height or by rough handling.
(C) Heating or hammering the magnet.
(ii) Magnet can be demagnetised by placing it within a solenoid and passing high frequency AC through it.
11.3 USED OF MAGNETISM IN MEDICINE :
An electric current always produces a magnetic field. Even weak ion currents that travel along the nerve cells in our body produce magnetic fields.
When we touch something, our nerves carry an electric impulse to the muscles we need to use. The impulse produces a temporary magnetic field. These field are very weak and are one billionth of the earth’s magnetic field. Heart and brain are the two main organs in the human body where the magnetic field produced is significant. The magnetic field inside the body forms the base of obtaining the images of different body parts. This is done by using a technique called Magnetic Resonance Images (MRI). Analysis of these images helps in medical diagnosis. Magnetism has thus, got important uses in medicine.
11.4 MAGINETIC FORCE :
11.4 (a) Force on a Current-Carrying Conductor in a Magnetic Field :
Immediately after Oersted’s discovery of electric currents producing magnetic fields and exerting forces on magnets, Ampere suggested that magnet must also exert equal and opposite force on a current-carrying conductor. When a current carrying conductor is kept in a magnetic field (not parallel to it), a force acts on it. This force is created due to the interaction of magnetic field of the current in the conductor and the external magnetic field on the conductor. As a result of this superposition, the resultant magnetic field on one side of conductor is weaker than on the other side. hence the conductor experience a resultant force in one direction.
Take a small aluminum rod AB. Suspend it horizontally by means of two connecting wires from a stand. Now, place a strong horseshoe magnet in such a way that the rod is between the two poles with the field directed upwards. If a current is now passed in the road from B to A, we will observe that the rod gets displaced. This displacement is caused by the force acting on the current-carrying rod. The magnet exerts a force on the rod directed towards the right, with the result the rod will get deflected to the right. If we reverse the current or interchange the poles of the magnet, the deflection of the rod will reverse, indicating thereby that the direction of the force acting on it gets reversed. This shows that there is a relationship among the directions of the current, the field and the motion of the conductor.
11.4 (b) Direction of Force on Current Carrying Conductor :
The direction of force obtained by the Fleming’s left hand rule.
Fleming left hand rule :
Stretch the forefinger, middle finger and the thumb of you lef hand mutually perpendicular to each other as shown in figure. It the forefinger indicates the direction of the magnetic field and the middle finger indicates the direction of current, then the thumb will indicated the direction of motion (i.e., force) on the conductor.
11.4 (c) Magnitude of Force :
Experimentally it is found that the magnitude of the force acting on a current carrying conductor kept in a magnetic field in direction perpendicular to it, depends on the following factors :
(i) The force F is directly proportional to the current flowing in the conductor, i.e. F 
(ii) The force F is directly proportional to the intensity of magnetic field, i.e. F 
(iii) The force F is directly proportional to the length of the conductor (inside the magnetic field), i.e. F
Combining these we get, F 
Where K is constant whose value depends on the choice of units. In S.I. units K = 1 and the unit of magnetic field is tesla (T). 1 tesla is equal to 1 Newton ampere-1 metre-1 or 1 Weber metre-2.
Force is directly proportional to sin 
Combining all we have F = BI
Special cases :
(i) When 
Force on a current – carrying conductor placed parallel or ant parallel to field is zero.
(ii) If 
maximum when placed perpendicular to magnetic field.
(iii) I of B = 0, F = 0 i.e. the coil placed in field free area doesn’t experience any force.
A moving charge in a magnetic field (direction of motion not parallel to the field direction) experiences a force called Lorentz force. Since current is due to flow of charge, therefore a conductor carrying current will experience a force.
The force acting on a current – carrying conductor placed in a magnetic field is :
F = B
Now, if a charge Q flows in time t then the current 
we get :
Suppose the particle carrying the charge Q travels a length 
particle will be equal to 
Force on moving charge, F = B × q × v
Where B = Magnitude of magnetic field, Q = Charge on the moving particle and v = Velocity of the charged
particle (in metre per second). In vector notation
DAILY PRACTICE PROBLEMS # 11
OBJECTIVE DPP – 11.1
1. The intensity of a magnetic field is defined as the force experienced by a :
(A) standard compass (B) unit positive charge
(C) unit negative charge (D) unit north pole
2. A wire carrying a current of 5A is placed perpendicular to a magnetic induction of 2T. The force on each centimeter of the wire is :
(A) 1N (B) 100 (C) 0.1 N (D) 10 N
3. If a soft iron piece is buried under the surface of earth in the north and south direction, then
(A) it will acquire the properties of a magnet (B) its properties will not change
(C) it will behave like an insulator (D) can’t say with surity
4. Force acting on a stationary charge Q in the magnetic field B is –
(A) B Q V (B) BV/Q (C) Zero (D) BQ/V
5. A proton is moving with velocity 104 m/s parallel to the magnetic field of intensity 5 tesla. The force on the proton is –
(A) 8 × 10-15 N (B) 104 N (C) 1.6 × 10-19 N (D) Zero
6. A wire of length 
(A) 
7. The permanent magnets are kept with soft iron pieces at ends and keepers :
(A) to magnetise the soft iron pieces (B) to increase the strength of the magnets
(C) to avoid self demagnetisation (D) for physical safety of the magnets
SUBJECTIVE DPP-11.2
1. How can we find the direction of magnetic force on current carrying conductor ?
2. What is an electromagnet ? How dose it differ from a permanent magnet ? State three factor on which the strength of an electromagnet depends.
3. State Fleming’s left hand rule.
4. The pole faces of a permanent magnet are shown in the figure. A wire of length 4 cm, carrying a current of 10A is placed in the central region, where the magnetic field is 0.2T. Calculate the magnitude of the force on the wire ?
