SUB SCIENCE AND TECHNOLOGY NAME OF TEACHER BHALERAO
• • • SUB : -SCIENCE AND TECHNOLOGY NAME OF TEACHER: - BHALERAO NANDKUMAR KHANDERAO ASSTT. TEACHER SCHOOL : - NARSINHA VIDYALAYA RANJANI TAL-AMBEGAON, - DIST -PUNE.
All about Electromagnetism.
Sub-Unit Magnetic lines of force. Magnetic field due to a current-carrying conductor. Force on a current-carrying conductor in a magnetic field. Electric motor. Electromagnetic induction. Electric generator. Domestic electric circuit.
Learning points Electromagnetism. Magnetic field. Interaction between magnetic pole. Magnetic lines of force. orsted's discovery. Magnetic field due to a current-carrying conductor. Right hand thumb rule. Magnetic field due to a current through a circular loop. Electromagnet. Solenoid. Magnetic field due to a current in a solenoid. Permanent magnets. Force on a current-carrying conductor in a magnetic field. Fleming’s left hand rule. Use of magnetism in medical science. Electric motor. Use of a DC motor. Magnetic levitation train. Electromagnetic induction. Fleming’s right hand rule. Galvanometer. Direct current [D. C. ] & Alternating current [A. C. ]. Electric generator. Domestic electric circuit. Safety measures in using electricity. Short circuiting & overloading. .
New-Syllabus • • • • Magnetic field due to a current through a circular loop. Electromagnet. Solenoid. Magnetic field due to a current in a solenoid. Permanent magnets. Force on a current-carrying conductor in a magnetic field. Fleming’s left hand rule. Use of magnetism in medical science. Electric motor. Use of a DC motor. Magnetic levitation train. Electromagnetic induction. Galvanometer. D. C. & A. C. current. Electric generator. Domestic electric circuit.
Electromagnetism. In this picture sharp heavy iron scrap materials attached to a big disc. This process is used in loading & transporting scrap &loose iron material in a steel mill. It is not feasible to create a permanent magnet of such big size & store such a big magnet. Therefore magnetism is induced in the dice with help of electricity. Such a magnet is called an electromagnet.
Magnetic field. Activity 5. 1 Place a white paper on a drawing board place a bar magnet in the middle of the paper. Sprinkle some iron filing on it. Tap the board gently. What do you observe? The iron filing are arranged around the magnet in definite curved lines forming a symmetric pattern. They get close to each other near the poles. The lines are less crowded in the middle region around the magnet. These lines are called magnetic field lines or magnetic lines of force. A field of force that exists in the space around the magnet or a current carrying conductor is called magnetic field. Inter action between magnetic poles like poles repel each other and unlike magnetic poles attract each other.
Magnetic lines of force. Activity 5. 2 Take a drawing board & fix a white sheet of paper on it. Place a bar magnet at the center &draw its out line with help of a pencil place the magnetic needle close to any pole. Wait till the needle comes to rest. Draw the dots with pencil. Properties of magnetic lines of force. 1. Magnetic lines of force are closed continuous curves. They start from north pole and end on south pole. 2. The tangent at any point on the magnetic lines of force gives the direction of the magnetic field at that point. 3. No two magnetic lines of force can intersect each other. 4. Magnetic lines of force are crowded where the magnetic field is strong and far from each other where the field is weak.
Activity 5. 3 Make a circuit by connecting the two terminals of the electric cell to the key as shown in the fig. Place the compass needle close to it. Note the direction in which compass needle is pointing. While watching the compass needle carefully, close the key for few seconds. Replace the key & watch again. Repeat the step again by changing its polarity. It is observed that the needle get deflected when we press the key. If the polarity is reversed , the needle gets deflected in the opposite direction. Thus if a current is passed through a wire , electric field is produced near it. This was discovered by the scientist Han Oersted.
Activity Take a battery (12 v) a variable resistance , an ammeter (0 -5 A), a plug key , needle & a long straight thick copper wire. Take a rectangular cardboard, pass the wire through the center. Take care that cardboard is fixed. Make the arrangement as shown in the fig. Sprinkle iron filing uniformly on the cardboard. Close the key &tap the cardboard gently. If we increase the current through the wire , the needle will deflect more & if we move the compass away from the wire, the deflection in the needle will start decreasing. Thus 1) magnitude of the field produced at a given point is directly proportional to the magnitude of the current passing in a wire. 2)magnetic field produced by a given current in the wire decreases as the distance from the wire increases.
Right hand Rule & Activity 5. 5 Magnetic field due to current carrying conductor Right hand thumb rule –You are holding a current carrying straight conductor in your right hand such that the thumb points towards the direction of current then the curled fingers around the conductor will give the direction of the magnetic field. Activity 5. 5 1. Take a rectangular cardboard with two holes. Take a coil of large number of turns. Insert the coil through the holes such that the turns are normal to the plane of the cardboard. Connect two ends of the coil in series with a battery, a key and a rheostat as shown in the figure. 2. Sprinkle iron filings uniformly on the cardboard. Plug the key, tap the cardboard gently.
Magnetic field due to a current through a circular loop In figure it is seen that at every point of the loop, the lines are in the form of circles. At the centre of the loop, the areas of these circles would appear as straight lines. The magnetic field produced by a current carrying wire at a given point depends directly on the current passing through it. A coil has ‘n’ turns, the field produced is ‘n times’ larger than that produced by a single turn. Activity 5. 6 Connect the free ends of the wire to the two terminals of the cell for few seconds. Bring small iron pins near the tip of the nail and observe. Disconnect the wire ends and observe.
Magnetic field due to current in a solenoid. Solenoid- A coil of many turns of insulated copper wire wrapped (wound) in the shape of a cylinder is called a solenoid. Magnetic field due to a current in a solenoid are similar to those in the case of a bar magnet. A current carrying solenoid behaves like a bar magnet & with it we can magnetism iron & some alloys. Permanent magnet – The materials used to prepare permanent magnet are usually alloys such as Alnico , nipermag, carbon steel chromium steel & cobalt & tungsten steel. Permanent magnets are used in a microphone, an electric clock, an ammeter, a voltmeter, a speedometer, etc.
Force on a current carrying conductor in a magnetic field. Activity 5. 7 - Take a small aluminum rod AB suspend it horizontally by means of two connecting wires from a stand as shown in fig. Now place a strong horse shoe magnet in such a way that the rod is between the two poles with the field directed upwards. If a current is now passed through the rod from B to A , you will observe that the rod gets displaced. This caused by the force acting on the current carrying rod. The magnet exert a force on the rod directed towards the right, with the result that the rod will get deflected to the right. On reversing the current or interchanging the poles of the magnet, the direction of the rod will reverse. The above activity indicates that there is a relationship between the direction of the current, the field & the motion of the conductor. Electric current flowing through a conductor produces a magnetic field. In the above activity it is seen that the rod is displaced which suggest that force is exerted on the rod carrying the current when placed in a magnetic field.
Fleming left hand rule. Stretch the forefinger, the center finger & the thumb of your left hand mutually perpendicular to each other. If the forefinger shows the direction of the field & the central finger shows the direction of the current, then the thumb will point towards the direction of the motion of the conductor.
Use of magnetism in medical science. Electric current produced magnetic field even if the current is very weak. Ion current which travels along the nerve cells in our body produces magnetic field. This forms the basis of obtaining images of heart and brain or images of different part of body. This is done using a technique called Magnetic Resonance Imaging (MRI) Analysis of these images help doctors to diagnose disorders of the brain.
Activity Take three ring shaped magnets , insulated copper wire (1. 5 m) , battery & paper clips cut the wire in to three pieces. One piece should be about 1 m long & the other two 0. 25 m long each. Take 1 m piece of wire and wrap this wire around two fingers to form a coil. Take the loose end of this wire and wrap it once along the diameter of the coil. The two arms of the coil should be directly opposite to each other. Bend the paper clip and prepare a holder for a coil. Pile three magnets on the table. Place the coil above on the paper clip holder. Take free ends of the wire and connect them to the battery. Give a gentle spin to the coil. Observation-The coil continues to rotate for long time. (As long as the electric cell provide the energy needed as the kinetic energy for rotational motion of the coil.
Electric Motor. A device which converts electric energy into mechanical energy is called an electric motor. Principle: Electric motor works on the principle that a current carrying conductor placed in a magnetic field experiences a force. 1. Armature coil : A large number of turns of insulated copper wire wound on iron core in rectangular shape forms an armature coil ABCD. 2. Strong magnet : The armature coil is placed in between two pole pieces of strong magnet. This provides a strong magnetic field. 3. Split ring commutator : It consists of a metallic ring. The ends of the armature coil are connected to these rings. Commutator reverses the direction of current in the armature coil. 4. Brushes : Two carbon brushes B 1 and B 2 are used to press the commutator. 5. Battery : the battery supplies the current to the armature coil.
Magnetic levitation train. Magnetic levitation trains do not run on rails but “float” above them. A current passes through the electromagnets in the track and on the train. The magnetism produced lifts train upwards.
Activity Take a coil AB having 10 -15 turns. Connect the two end of coil to the galvanometer as shown in fig. Take strong bar magnet. Move the north pole of the magnet towards the B end of the coil. Observe the deflection in the galvanometer. Note the direction of the deflection (i. e. right or left ). Now this with the south pole. Again observe the deflection. Note its direction. What will happen if instead of magnet a coil is moved? If both coil and magnet are kept stationary, do you observe any deflection?
Activity Take two coils of about 50 turns. Insert them over anon -conducting cylindrical roll, as shown in fig. Connect coil 2 to a galvanometer. Plug the key and observe the deflection in the galvanometer. Unplug the key and again observe the deflection.
Electromagnetic induction The process by which a changing magnetic field in a conductor induces a current in another conductor is called electromagnetic induction. A current can be induced in a conductor either by moving it in a magnetic field or by changing the magnetic field around the conductor. When a current carrying conductor is placed in a magnetic field such that the direction of current is perpendicular to the magnetic field, it experiences a force. But if a conductor is moving inside a magnetic field or magnetic field is changing around a fixed conductor, electric current is generated. This was first studied by Michael Faraday.
Michael Faraday. Michael Faraday in 1821 found that electric city could produced rotary motion. Today’s electric motor are based on this principle. In 1831 he showed that relative movement between a magnet and a coil of wire could induce electricity in the coil an idea which gave birth to modern generators.
Fleming’s right hand rule. Stretch the thumb, and middle finger of the right hand so that they are perpendicular to each other, as shown in the fig. If the forefinger indicates the direction of the magnetic field and the thumb shows the direction of the motion of conductor, the middle finger will show the direction of induced current. Galvanometer- is an instrument which is used to detect the presence of current in a circuit If the current in a circuit is zero, the galvanometer will show zero deflection or no deflection. The deflection is either side of the zero mark depending on the direction of the current.
Direct and Alternating current. Direct Current Alternating Current 1. The magnitude and direction of the current is constant. 1. 2. This type of current cannot be used on large scale of electricity for household purpose. 2. This type of current is used in electrical household appliances such as electric heater, iron, refrigerator, etc. 3. The frequency of direct current is zero. 3. The magnitude and direction of the current is reverses periodically. Frequency of alternative current in India is about 50 Hz.
Electric Generator. An electric device which converts mechanical energy into electrical energy is called an electrical generator. Principal-Electric generator works on the principal of electromagnetic induction. When the coil of electric generator rotates in a magnetic field induces a current in this coil. This induced current then flows into the circuit connected to the coil. Type of electric generator 1. AC Generator 2. DC Generator 1. AC Generator: A generator which converts mechanical energy into electrical energy in the form of alternating current is called AC Generator. 2. DC Generator : A generator which converts mechanical energy into electrical energy in the form of direct current or DC is called as DC generator.
A C Generators. Construction : The main components of AC generator are shown in fig. 1)Armature 2)strong magnet 3)split ring 4)brushes. Armature, strong magnets & brushes are the same as used for electric motor. Split ring: The two ends of the armature coil are connected to two brass slit rings R 1 & R 2. These ring rotate along with the armature coil. Working : When the armature coil ABCD rotates in the magnetic field provided By the strong magnets , it cuts the magnetic lines of forces. Thus the changing magnetic field produces induced current in the coil. The direction of induced current is determined by the Fleming’s right hand rule. The current flows out through the brush B 1 in one direction in the first half of the revolution and through the brush B 2 in the next half revolution. In the reverse direction, this process is repeated. Therefore, the induced current produced is of alternating nature. Such a current is called as alternating current.
DC Generator. DC generator or Dynamo. Construction-The main components of DC Generator are 1) Armature coil 2)Strong magnets 3) Split rings or commutator 4)Brushes 5)Bulb. The components such as armature coil, strong magnets, brushes are same which are used for AC generators. Split ring or commutator- is the same as used in electric motor. Bulb- The output is shown by the glowing bulb connected across the carbon brushes. Working- When the coil of DC generator rotates in the magnetic field, potential difference is produced in the coil. This gives rise to the flow of current.
Domestic electric circuit. A domestic electric circuit involves a phase wire (live wire), a neutral wire, an earth wire, the Electricity Board’s fuse, the electricity meter, the distribution box containing the main switch and fuses for each circuit with various appliances such as a tube light , a bulb, a fan, a refrigerator, etc. In India, the potential difference between the live wire and the neutral wire is the range 220 V 250 V and the frequency of alternating current is 50 Hz.
Safety measures in using electricity. Electric fuse and earthing are necessary for safety of electrical appliances and their users.
Short circuiting & overloading Short circuiting If a live wire and a neutral wire come in direct contact or touch each other, short circuiting takes place. During a short circuit, the resistance of the circuit becomes very small and hence huge amount of current flows through it. This produces a large amount of heat and raises the temperature. As a result, the circuit catches fire. Overloading means the flow of large amount of current in the circuit beyond the permissible value of current, if many electrical appliances of high power rating such as geyser, heater, motor, oven are switched on simultaneously, overloading occurs. This causes fire. This can be avoided by not connecting many appliances at a time in the circuit.
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