Here you will get all important topics of class 10 science chapter 13 magnetic effect of current such as : types of conductor, electromagnet, Solenoid, right hand rule, left hand hand rule, one tesla and much more
LIST OF TOPICS
- INTRODUCTION OF MAGNET
- MAGNET AND MAGNETIC FIELD
- PARMANANT AND TEMPORARY MAGNET
- TYPES OF CUNDUCTOR(3)
- FLEMING LEFT AND RIGHT HAND RULE
- KICKING WIRE EXPERIMENT
- FARADAY LAW OF ELECTROLYSIS
- A.C. GENERATOR & D.C. GENERATOR
- PRINCIPLE OF MOTOR AND GENERATOR
The science of magnetism developed from the observation that certain ore could attract bits of iron and pointed in a particular direction when freely suspended. This ore was originally found in the district of Magnesia in Asia Minor (magnesia in the modern town of manias in western Turkey) and was, therefore named Magnetite.
- Substances like iron, steel, nickel and cobalt get attracted by a magnet. Such substances are called magnetic substances. On the other hand, substances like wood, rubber, glass, plastic etc. which are not attracted by a magnet are called non-magnetic substances.
- The property of attracting small pieces iron was referred to as magnetism. A magnet is a substance which has both attractive and directive properties. Magnet consists of a number of oxides of iron with Fe3O4 Dr. William Gilbert confirmed that the earth itself behaved as natural magnet.
- The branch of physics that deals with magnetism due to electric current is called Electromagnetism.
- One Tesla = The magnetic field induction at a point is said to be one Tesla if a charge of 1 coulomb while moving at right angles to a magnetic field with a velocity of 1m/sec experiences a force of 1N at that point.
- Magnets are available in different shapes as well. For example, bar magnets, horse-shoe magnets, magnetic needles, ring magnets, disc magnets etc.
Magnetic field and magnetic field lines
The space around a magnet in which the force of attraction and repulsion due to which the magnet can be detected is called the magnetic field (OR Magnetic field is a region near a magnetized body where magnetic forces can be detected.
- Magnetic field lines are the lines of force that generated from North Pole of a bar magnet to South Pole of that magnet.
Properties of magnetic field lines
- A magnetic field line is directed from North Pole to south-pole outside the Magnet.
- A magnetic field line is a closed and continuous curve.
- The magnetic field lines are strong near the poles of magnet and weak in middle of the magnet.
- The magnetic field lines never intersect each other.
A natural magnet is a piece of the iron ore, magnetite (Fe3O4) which has the property of attraction towards certain substances and pointing the north – south direction when suspending freely. A natural magnet is also called Lodestone.
MAGNETIC FIELD DUE TO A CURRENT CARRYING STRAIGHT CONDUCTOR
There exists a magnetic field around a straight current-carrying conductor and the magnetic field in the form of concentric circular field lines in respect of the centre.
- North Pole of the compass points in the direction of field at any point P.
- Strength of field line may increased by increasing the passage of current per unit time. Thus field lines directly proportional to the amount of current that pass through the conductor. B α I
- Strength of magnetic field line is strong near the conductor body but weak at far point from conductor. Thus the field line strength is inversely proportional to the distance of point at which we need to find field strength.
B α 1/r
Direction of magnetic field: right-hand Thumb Rule
Imagine the straight conductor in your right hand such that the thumb points in the direction of current, the direction of curling of fingers of the right hand gives the direction of magnetic field lines.
1 T =10 4 G.
MAGNETIC FIELD DUE TO CURRENT CARRYING CIRCULAR COIL
Soon after the Oersted’s discovery of magnetic effect of current carrying wire, Ampere found that a loop of wire also had a magnetic field. The direction of magnetic field changes when the direction of flow of current is reversed.
- The strength of the magnetic field increases with increase in current
- The strength of the magnetic field at the centre of a circular coil carrying current is proportional to the number of turns in the coil (n). As we increase the turns, the magnetic field strength also increases. As we go away from the wire, the strength of the magnetic field decreases, and the concentric circles grow larger in size.
- The strength of the magnetic field is inversely proportional to the radius of the circular coil i.e. the field strength reduces as the radius of the coil increases.
- The magnetic field lines near the coil are nearly circular and concentric.
- The field lines are in the same direction in the space enclosed by the coil.
- The magnetic field at the centre of the coil can be taken to be uniform.
- The direction of the magnetic field at the centre is perpendicular to the Plane of the coil.
- Magnetic field is maximum at its centre.
Clock Rule (direction of magnetic field)
Looking at the face of the coil, if the current around that face is in clockwise Direction, the face is the south – pole, while if the current around that face is in the anticlockwise direction, face is the North Pole.
- Current in clockwise direction generate south polarity.
- Current in anticlockwise direction generate north polarity.
Magnitude (B) of Magnetic field
The magnitude of the magnetic field (B) at the centre of the coil is:
- Directly proportional to the current(I) flowing through it, i.e.
B α I
- Inversely proportional to the radius (r) of the coil, i.e.
B α 1/r
- Directly proportional to the total number of turns (N) in the coil, i.e. B α N
This is due to the reason that the current in all the circular turns of the coil is in the same direction. As such, the resultant magnetic field due to the coil is equal to the sum of the fields due to all these turns.
B α NI/r
MAGNETIC FIELD DUE TO A CURRENT-CARRYING SOLENOID
An insulated copper wire wound on a cylindrical cardboard (or plastic) tube Such that its length is greater than its diameter is called a solenoid.
- The magnetic field lines inside the solenoid are nearly straight and parallel to its axis. Thus, the magnetic field inside a solenoid is uniform.
- A current-carrying solenoid behaves like a bar magnet with fixed polarities at its end.
- The polarity of the solenoid can be changed by reversing the direction of the current.
- The strength of the magnetic field produced by a current carrying solenoid depends on:
- The number of turns – larger the number of turns, greater is the magnetism produced
- The strength of the current – when current increases, magnetism also increases
- Nature of ‘core – material’ used in making the solenoid – if we use soft – iron as a core for the solenoid, then it produces the strongest magnetism.
Direction of magnetic field
The end of the current-carrying solenoid at which the current flows anticlockwise behaves as a north pole while that end at which the direction of current is clockwise behaves as a south pole.
An electromagnet is a temporary strong magnet and is just a solenoid with its winding on a soft iron core.
The strength of the electromagnet depends upon:
- The number of turns per unit length of the solenoid
- the current through The solenoid
An electromagnet is made in the u- shape Magnet and is called a horse – shoe magnet
Uses of electromagnets
- For lifting and transporting large masses of iron in the form girders.
- In medical practice for removing pieces of iron from wounds.
A Permanent magnet is made from steel. As steel has more retentively than iron, it does not lose its magnetism easily.
Apart from different varieties of steel (carbon steel, chromium steel, cobalt and tungsten steel) some alloys like Alnico (aluminium, nickel, cobalt alloy of iron) and Nipermag (an alloy of iron, nickel, aluminium and titanium) are used to make every strong permanent magnets.
Uses of permanent magnets
Permanent magnets are used in
- Electric meters (Galvanometers, Voltmeters, Ammeters and Speedometers).
- Microphones and Loudspeakers.
- Electric clocks.
Difference between an electromagnet and a permanent magnet
- An electromagnet is a temporary magnet as it can readily be demagnetized by stopping the current through the solenoid.
- An electromagnet produces a strong magnetic field whose strength can be changed by changing the current through the solenoid.
- The polarity of an electromagnet can easily be reversed by changing the Direction of current through the solenoid.
- A permanent magnet cannot be readily demagnetized.
- The magnetic field of a permanent magnet is comparatively weak and its Strength cannot be changed.
- The polarity of a permanent magnet is fixed and cannot be easily reversed.
NOTE: An electromagnet is a temporary magnet and it consists of a core of soft iron with insulated copper wire wound around it. A permanent, on the other hand is made from steel or other alloys (Alnico and Nipermag).
Force acting on a current-carrying conductor in a magnetic field
When a current carrying conductor is placed in a magnetic field, it experiences, a force except when it is placed to the magnet field.
The force acting on a current –carrying conductor in a magnetic field is due to Interaction between:
- Magnetic field due to current – carrying conductor.
- External magnetic field in which the conductor is placed.
The resultant of these two magnetic fields is not uniform. It is weaker on one Side of the conductor then on its other side. The conductor, therefore, experiences a resultant force in the direction of the weaker Magnetic field.
Kicking wire experiment
This experiment show that the relation b/w
- direction of force on wire carrying current
- Direction of current flowing through it.
- Direction of the magnetic field in which the wire is placed.
Fleming left – Hand rule (motor rule)
This left hand rule was suggested by J.A Fleming so it became Fleming’s left hand rule. Stretch the thumb, the first finger and the central of the left hand so that they are mutually perpendicular to each other. If the first (fore) finger points in the Direction of the magnetic field, the central finger points in the direction of current, then the thumb points in the direction of motion of the conductor.
Characteristics of magnetic force
- Magnetic force act only on moving charges and not on stationary charges.
- No magnetic force acts on a charge if it is moving along the direction of the Magnetic field.
- The direction of magnetic force is perpendicular to (i) the direction velocity of the charge and (ii) the direction of the magnetic field.
Difference b/w electric and magnetic force
- Magnetic force is always perpendicular to the magnetic field whereas electric force is always collinear with the electric field.
- Magnetic force is velocity dependent that’s mean it act only when the charge particle is in motion whereas electric force is independent of the state of rest or of motion of charge particle.
ELECTRIC MOTOR (DC MOTOR)
An electric motor is a device for converting electric energy into mechanical energy. Thus an electric motor is the reverse of an electric generator. There are two types of electric motors (i) AC motor and (ii) DC motor.
The principle of a DC motor is very much different from that of an AC motor. It is important to remember that all the electric appliances like fan, air-conditioner, coolers, washing machines, mixers and blenders work on AC (house-hold power supply) and as such have AC motors Installed in them.
When a coil carrying current is placed in a magnetic field, it experiences a torque. As a result of this torque, the coil begins to rotate.
Construction : It consists of the following five parts:
- Armature: it consists of turns of insulated copper wire wound over a soft iron core.
- Field Magnet: magnetic field is supplied by a permanent magnet NS.
- Split-ring or commutator: these are two halves of the same metallic ring. The ends of the armature coil are connected to these halves which also rotate with the armature.
- Brushes or sliding contacts: these are two flexible metal plates or carbon rods B1 and B2 which are so fixed that they constantly touch the revolving rings.
- Battery: it consists of few cells and is connected across the brushes. The brushes pass the current to the rings from where it is carried to the armature.
Speed of rotation of the motor can be increased by
- Increasing the strength of the current through the armature.
- Increasing the number of turns in the coil of the armature.
- Increasing the area of the coil, and
- Increasing the strength of the magnetic field.
Uses of DC Motor
- These are used in electric fans (exhaust, ceiling or table) for cooling and Ventilation.
- These are used for pumping water.
- Big DC motor is used for running tram-cars.
- Small DC motor is used in various toys.
FARADY’S LAWS OF ELECTROMAGNETIC INDUCTION
- Whenever there is a change in magnetic flux linked with a coil, an electric Current is induced. This induced Pd last goes long as there is a change in the magnetic flux linked with the coil.
- The magnitude of the induced current is directly proportional to the rate of change of magnetic flux linked with the coil.
This Consists of coil, C of a few turns of wire. A sensitive galvanometer, G is included in series with the coil. When magnet is stationary, there is no deflection in the galvanometer.
Fleming’s right-Hand Rule (Dynamo Rule)
Stretch the thumb, the first finger and the central finger of the right hand so That they are mutually, perpendicular to each other. If the first (fore) finger Points in the direction of magnetic field, the thumb points in the direction of motion of the conductor, then the central finger points in the direction of Induced current.
ELECTRIC GENERATORS, DIRECT CURRENT AND ALTERNATING CURRENT
A generator or dynamo is a machine used for generating electric current by Converting mechanical energy into electrical energy.
When the current produced by a dynamo changes continuously in magnitude and periodically in direction several times in a second, the current is known as Alternating current (written in short as AC) and the machine that produces it is called an AC generator. But when the current produced by a dynamo does not Change in direction and magnitude, it is called the direct current (written in short as DC) and the machine that produces it is called a DC generator.
- An electric current which change its direction (i.e. polarity) after a certain fixed Interval of time is called alternating current.
Frequency of AC
The number of cycles completed by the AC in one second is called the frequency of AC.
- An electric current which always flows in the same direction is called direct Current.
Advantages of AC over DC
- With the help of a transformer, AC at any desired voltage can be obtained.
- The power wastage in AC transmission is almost negligible and as such the Cost of transmission is low.
- When requited, AC can be changed to DC.
- AC machines are very stout and durable and do not need much maintenance.
Disadvantages of AC over DC
- AC is more dangerous than DC as it attracts a person towards it whereas DC repels.AC gives a serious shock to a person as compared to DC.
- AC cannot be used for electroplating, electrotyping and other such electrolytic Processes. In such cases DC has to be used.
ALTERNATING CURRENT (AC) GENERATOR
An AC generator converts mechanical energy into electric energy.
Whenever in a closed circuit (I.e. a coil), the magnetic field lines change, an induced current is produced.
- Armature: armature is also called the coil, consists of a large number of turns of insulated copper wire wound over a soft iron core. It revolves around an axle b/w the two poles of a strong magnet.
- Field Magnet: the magnetic field supplied by a permanent magnet in a small dynamo and by an electromagnet in case of a big commercial dyanmo . The poles of magnet are N-S.
- Slip Rings: R1 and R2 are the two hollow metal rings held at different heights. End of the armature coil connected to ring R1
- Brushes or sliding Contacts: B1 and B2 are flexible metal plates or carbon rods. These are brushes.
Magnitude of induced E.M.F. in a generator can be increased by : increasing the number of turns of its armature, increasing area of the armature, increasing speed of rotation of the armature and increasing the strength of the magnetic field.
Principle, construction and working of a DC generator is the same as that of an AC generator except that in place of slip – rings as sliding contacts, we have a split – ring or a commutator. In a split-ring, R1 and R2 are the two halves of the same ring. The ends of the Armature coil are connected to these rings and these rings rotate with the armature.