Chapter-13
Magnetic Effect of Electric Current
Magnet is any substance that attracts iron or iron-like substance.
Propertirs of Magnet
| (i) Every magnet has two poles i.e., North and South. |
| (ii) Like poles repel each other. |
| (iii) Unlike poles attract each other. |
| (iv) A freely suspended bar magnet aligns itself in nearly north-south direction, with its north pole towards north direction. |
Magnetic Field :
The area around a magnetic in which its magnetic force can be experienced.
| • Its SI unit is Tesla (T). |
| • Magnetic field has both magnitude and direction. |
| • Magnetic field can be described with help of magnetic compass. |
| • The needle of magnetic compass is a freely suspended bar magnet. |
Characteristics of Field Lines
| (i) Field lines arise from North pole and end into South pole of the magnet. |
| (ii) Field lines are closed curves. |
| (iii) Field lines are closer in stronger magnetic field. |
| (iv) Field lines never intersect each other as for two lines to intersect, there must be two north direction at a point, which is not possible. |
| (v) Direction of field lines inside a magnet is from South to North. |
| (vi) The relative strength fo magnetic field is shown by degree of closeness of field lines. |
Magnetic Field of a Bar Magnet
• H.C. Oersted was the first person to state taht electric current has magnetic field.
Right Hand Thumb Rule
Imagine you are holding a current carrying straight conductor in your right hand such that the thumb is pointing towards the direction of current. Then the fingers wrapped around the conductor give the direction of magnetic field.
Magnetic Field Due to Current Through a Straight Conductor
| • It can be represented by concentric circles at every point on conductor. |
| • Direction can be given by right hand thumb rule to compass. |
| • Circles are closer near the conductor. |
| • Magnetic filed Strength of current |
| • Magnetic field _________1_________ Distance from conductor |
Magnetic Field Due to Current Through a Circular Loop
| • It can be represented by concentric circle at every point. |
| • Circles become larger and larger as we move away. |
| • Every point on wire carrying current would give rise to magnetic field appearing as straight line at centre of the loop. |
| • The direction of magnetic field inside the loop is same. |
Factors affecting magnetic field of a circular current carrying conductor
| • Magnetic field Current passing through the conductor |
| • Magnetic field _________1_________ Distance from conductor |
| • Magnetic field No. of turns in the coil. |
| • Magnetic field is additive in nature i.e., magnetic field of one loop adds up to magnetic field of another loop. This is because the current in each circular turn has some direction. |
Solenoid
A coil of many circular turns of insulated copper wire wrapped closely in a cylindrical form.
| • Magnetic field of a solenoid is similar to that of a bar magnet. |
| • Magnetic field is uniform inside the solenoid and represented by parallel field lines. |
| • Direction of magnetic filed : (i) Outside the solenoid : North to South (ii) Inside the solenoid : South to North |
| • Solenoid can be used to magnetise a magnetic like soft iron. |
| Electromagnet | Permanent Magnet |
|---|---|
| 1. It is a temporary magnet, so, can be easily demagnetised. |
1. Cannot be easily demagnetised |
| 2. Strength can be varied. | 2. Strength is fixed. |
| 3. Polarity can be reversed. | 3. Polarity cannot be reversed. |
| 4. Generally strong magnet. | 4. Generally weak magnet. |
Force on a Current carrying Conductor in a Magnetic Field.
Andre Marie Ampere suggested that magnet also exerts an equal and opposite force on a current carrying conductor.
The displacement in the conductor is the maximum when the direction of current is at right angle to hte direction of magnetic field.
Direction of force is reversed on reversing the direction of current.
Fleming's Left Hand Rule
Stretch the thumb, fore fingers and middle finger of your left hand such that they are mutually perpendicular. If fore finger points in the direction of magnetic field, middle finger in teh direction of current then thumb will point in the direction of motion or force.
Electric Motor
A motor is a device which converts electrical energy into mechanical energy. Electric motor is used in electric fans, washing machines refrigerators, mixer and grinder and other appliances.
Principle of a Motor :
An electric motor utilize the magnetic effect of current. It works on the principle that when a rectangular coil is placed in a magnetic field and current is passed through it a torque acts on the coil which rotates it continuously. When the coil rotates the shaft to it also to rotates and electrical energy supplied to the motor is converted into mechanical energy.
Construction of a Motor :
| 1. Armature Coil : An electric motor consists of an rectangular coil ABCD of insulated copper wire wound on a soft iron core called armature. |
| 2. Strong Field Magnet : The coil (armature) is placed between two pole of a strong magnet such that arm AB and CD are perpendicular to the direction of the magnetic field. |
| 3. Split Ring Type Commutator : It consists of two halves of a metallic ring named as P and Q. The two ends of armature coil are connected to these two halves of ring. The function of commutators is that is reverses the direction of current in armature oil. |
| 4. Brushes : Two carbon brushes X and Y press against the commutator. These brushes act as contact between commutator and terminal battery. |
| 5. Battery : It is connected across the carbon brushes. It supplied current to the armature coil. Current in the coil ABCD enters from the source battery through conducting brush X and flow back to the battery through brush Y. |
Working of a Motor :
| 1. When current flows through coil, arm AB and CD experiences magnetic force. |
| 2. On applying Fleming left and hand rule, the force acting on arm AB pushes it downwards and arm CD experiences force in upward direction. |
| 3. Both these forces are equal and opposite. Two equal and opposite foces acting at different position fo armature constitute a couple and rotate the coil in anti- clockwise direction. |
| 4. At half rotation Q makes contact with brush X and P with brush Y. Now the current in the coil get reversed and flows along the path DCBA. |
| 5. The arm AB of the coil that was earlier pushed down is now pushed up and the arm arm CD previously pushed up is now pushed down. These two equal and opposite forces constitute a couple, this couple now rotate the coil in clockwise direction. |
| 6. The reversing of the current is repeated at each half rotation, giving rise to a continuous rotation of the coil and to the axle. Hence electric energy is converted into mechanical energy. |
Commercial Motor Use :
| (i) An electromagnet of place of permanent magnet. |
| (ii) Large number of turns of the conducting wire in the coil. |
| (iii) A soft iron core on which coil is wound plus the coils, is called the armature. |
| (iv) This enhances the power of the motor. |
| • Heart and brain in the human body have significant magnetic field. |
| • MRI (Magnetic Resonance Imaging) : Image of internal organs of body can be obtained using magnetic field of the organ. |
| • Galvanometer : Instrument that can detect the presence of current in a circuit. It also detects the direction of current. |
Electro Magnetic Induction
When a conductor is placed in a changing magnetic field, some current is induced in it. Such current is called induced current and the phenomenon is called electromagnetic induction.
Activity No.1
| (i) Magnet moved into the coil : Momentary deflection in Gindicating presence of current. |
| (ii) Magnet kept stationary inside the coil : No deflection. |
| (iii) Magnet is withdrawn : Momentary deflection in G but in opposite direction of first case. |
Activity No.2
| (i) Switched on : Momentary deflection in G. |
| (ii) Steady current : No deflection. |
| (iii) Switched off : Momentary deflection in G but in opposite direction of the first case. |
Fleming's Right Hand Rule
Hold the thumb, the force finger and the middle finger of right hand at right angles to each other. If the fore finger is in the direction of magnetic field and the thumb points in the direction of motion of conductor, then the direction of induced current is indicated by middle finger.
• Working principle of electric generator.
• Used to find direction of induced current.
Electric Generator
The electric generator is a machine for producing electric current or electricity.
The electricity generator converts mechanical energy (or kinetic energy) into electrical energy.
Principle of Electric Generator : (AC Generator) In an electric generator mechanical energy is used to rotate a conductor in a magnetic field to produce electricity. Generator works on the principle of electromagnetic induction. When a closed coil is rotated in a uniform magnetic field with its axis perpendicular to the magnetic field, the magnetic field lines passing through the coil change and an induced emf is set-up. The principle behind the electric generator is based on Fleming's right hand rule.
Construction of Generator :
| 1. Field Magnet : It strong horse-shoe shaped permanent with concave poles. |
| 2. Armature : ABCD is a rectangular armature coil. It consists of a large number of turns of insulated copper wire wound on a soft iron cylindrical core. |
| 3. Slip rings : These are two brass rings, R1 and R2 rigidly to the two ends of the armature coil. As coil rotates slip rings also rotates. |
| 4. Brushes : These are two graphite rods B1 and B2 which are kept pressed against the slip rings R1 and R2 Through these brushes, the current induced in the armature coil is sent to the external circuit. |
| 5. Axle : The slip rings are placed on the axle which is made to rotate freely from an external source. |
| 6. Galvanometer : To measure current the outer ends of the brushes are converted to the galvanometer. |
Working of Generator :
| 1. The armature coil ABCD is it horizontal position. |
| 2. Now, the coil is rotated clockwise. |
| 3. The arm AB moves upwards while the arm CD moves downwards. |
| 4. The coil cuts the magnetic lines of force. |
| 5. Accordings to Fleming's right hand rule, the induced current flows from A to B in arm AB and C to D in arm CD i.e. it flows along ABCD. |
| 6. The induced current flows in the circuit through B2 |
| 7. After half the rotation of the armature, the arm CD moves upwards and AB moves downwards. The induced current now flows in reverse direction i.e. along DCBA. The current now flows from B1 to B2 |
| 8. Thus the direction of current in the external circuit changes after every rotation. Such a current which changes its direction after equal intervals of time is called alternating current. |
| 9. This devices is called AC Generator. |
D.C. Generator
DC Generator : It is a device which convert mechanical energy into electrical energy.
DC Generator has split ring commutator instead of slip rings.
Split ring commutator : It consists of two semi cylindrical brass rings R1 and R2 attached to the two ends of the armature coil. As the armature coil rotates, the two split rings also rotate about the same axis of rotation.
Alternate Current (A.C.) : The current which reverse its direction peridically.
• In india, A.C. reverse its direction in every
1
second.
100
Time period = 1 + 1 = 1
100 100 100
Frequency = 1
Time period
1
= 1/50
-> 5O Hz
Advantage
• A. C. can be transmitted over long distance without much loss of energy.
Disadvantage
• A. C. cannot be stored.
Direct Current (D. C.) : The current which does not reverse its direction.
• D. C. can be stored.
• Loss of energy during transmission over long distance is high.
• Sources of D. C. : Cell, Battery, Storage cells.
Domestic Electric Circuits
| • There are three kinds of wires used : (i) Live wire (positive) with red insulation cover. (ii) Neutral wire (negative) with black insulation cover. (iii) Earth wire with green insulation cover. |
| • The potential difference between live and neutral wire in India is 220 V. |
| • Pole → Main supply → Fuse → Electricity meter → Distribution box → To separate circuits |
Earth Wire : Protects us from electric shock in case of leakage of current especially in metallic body appliances. It provides a low resistance path for current in case of leakage of current.
Short Circuit : When live wire comes in direct contact with neutral wire accidently.
• Resistance of circuit becomes low.
• Can result in overloading.
Overloading : When current drawn is more than current carrying capacity of a conductor, it result is overloading.
Causes of overloading :
(i) Accidental hike in voltage supply.
(ii) Use of more than one appliance in a single soket.
Safety devices :
(i) Electric fuse
(ii) Earth wire
(iii) MCB (Miniature Cirucuit Breaker)
