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Magnetic Effects of Electric Current
1.0Magnetism
It was known to the Greeks, more than 2000 years ago, that certain stones called 'lodestones' from Magnesia attract pieces of iron. Lodestone is now called 'magnetite ore'. The word 'magnetic' comes from 'Magnesia', the island where lodestone (magnetite ore) was found. The island of Magnesia is situated in west of the present day Turkey. Chinese used magnetic needles for navigation on ships as early in 400 B.C. In 1820, Hans Christian Oersted accidentally discovered that a compass needle got deflected when an electric current passed through a metallic wire placed nearby. Through this observation, Oersted showed that electricity and magnetism were related phenomena.
Now, it is established that there are two sources of magnetism namely, electric currents and permanent magnets.
The magnetic repulsion between like poles can cause one magnet to float over another. The magnetic train (Maglev train) floats several centimetres above the guideway, providing a smooth and almost frictionless ride.
2.0Permanent magnets
A solid of any shape or size which can attract pieces of materials like iron, cobalt, nickel is called magnet. A magnet in the shape of a rod or a bar is called a bar magnet (see figure).
Magnetic poles
The places where the attracting power of a bar magnet is maximum are called 'poles'. Poles are regions or small areas not the points. Poles are situated near the ends of bar magnet, not exactly at the ends.
Properties of a bar magnet
(1) Attractive nature: When iron filings are put near a bar magnet, the magnet attracts iron filings towards it. The attracting power is maximum near the ends (poles) and minimum at the centre (neutral region). (2) Directive property : When a magnet is suspended freely, it aligns itself to north-south direction (see figure).
- The pole of the bar magnet pointing towards north direction when suspended freely is called 'north pole (or north seeking pole)'. The pole of the bar magnet pointing towards south direction when suspended freely is called 'south pole (or south seeking pole)'.
A magnet is always dipole (3) Poles exist in pairs : In a bar magnet there are always two poles which are equal in strength and opposite in nature. This means that 'magnetic mono poles do not exist' (see figure). If a magnet is broken into number of pieces, each piece becomes a magnetic dipole. (4) Like poles repel and unlike poles attract. This means, south-south or north-north repel while south-north attract (see figure).Unlike poles attractLike poles repel (5) Inductive nature : When certain substances like soft iron, steel, cobalt, nickel are placed near a bar magnet, they acquire magnetisation called 'induced magnetisation'. The phenomenon is called 'magnetic induction'. It involves inducing opposite pole in a magnetic material like iron on the side facing the magnetic pole.
Natural magnets
Natural occurring minerals or ores having magnetic properties are called 'natural magnets'. Due to their irregular shapes and weak attracting power, natural magnets are nowadays, rarely used. e.g. Lodestone.
Artificial magnets
Nowadays, pieces of iron and many other materials of suitable shapes and sizes are made as magnets by passing current through a wire wound around them. Such magnets are called 'artificial magnets' (see figure). e.g. Bar magnet, U-shaped magnet, magnetic needle, etc.
Building Concepts 1 One of the two identical bars is magnetised. How will you find out, without using any aid, which one of them is magnetised? Explanation: First, we will take one of the bars in our hand and touch its end with the centre of the other bar. If there is a significant attraction between them, then the bar in hand is magnetised. If there is a negligible attraction between the two bars, then the bar in hand is unmagnetised, this means the other one is magnetised. This is because the central part of a magnet is its neutral region (the region where the attractive power is minimum). If the magnetised bar is in our hand, this means we are touching its pole to the unmagnetised bar, there will be a significant attraction between the two bars. If the unmagnetised bar is in our hand, this means we are touching its end with the neutral region of the magnetised bar, thus, the attraction between them will be negligible.
- Repulsion is the sure test for magnetism because it takes place only between two like poles of a magnet whereas attraction takes place between two unlike poles of a magnet and also between a magnet and a magnetic material.
3.0Magnetic field
A three-dimensional region of influence surrounding a magnet, in which other magnets or materials like iron are affected by magnetic forces is called 'magnetic field'. The space surrounding a magnetic dipole (or magnet) in which magnetic effects can be experienced is called 'magnetic field'.
Active Physics 1
- Fix a sheet of white paper on a drawing board using some adhesive material. Place a bar magnet in the centre of it. Sprinkle some iron filings uniformly around the bar magnet. You can use salt-sprinkler for this purpose.
- Now tap the board gently. You will observe that iron filings align themselves in a specific curved manner around the magnet. Now try to trace a particular curve around the magnet. This particular curve represents a magnetic field line.
(a) Bar magnet(b) Horseshoe magnet
Magnetic field lines (or magnetic lines of force)
A Magnetic field line is an imaginary curve such that the tangent to the curve at any point on it gives the direction of magnetic field at that point. Direction of magnetic field at A (a tangent at A)
Properties of magnetic lines of force
(1) In a magnet, magnetic field lines arise from north pole in the surrounding space and enter the south pole.
Building Concepts 2 North pole of a bar magnet is placed near an iron bar. (a) If the iron bar is attracted towards the north pole of the bar magnet, does it mean that iron bar is a magnet? (b) If the iron bar is repelled by the north pole of the bar magnet, does it mean that iron bar is a magnet? Explanation: (a) No, the iron bar may or may not be a magnet. This is because an unmagnetised iron bar is also attracted towards a bar magnet. (b) Yes, the iron bar is a magnet. This is because only like poles of two magnets repel ; a magnet and an unmagnetised iron can never repel each other. Here, we can say that 'true test of magnetisation on materials is the repulsion, not the attraction'.
Active Physics 2
- Fix a sheet of white paper on a drawing board using some adhesive material. Take a small magnetic compass needle and a bar magnet. Place the bar magnet at the centre of paper.
- Move the compass near the north pole of the bar magnet. The needle will deflect in such a way that south pole points towards the north pole of the bar magnet. Mark the positions of its two ends using a sharp pencil. Now, move the needle to a new position such that its south pole occupies the position previously occupied by its north pole. Repeat this process till you reach the south pole of the magnet.
- Join the points marked on the paper by a smooth curve. This curve represents a magnetic field line. If you repeat the above procedure to draw as many lines as you can, you will get a complete pattern of magnetic field lines of a bar magnet. Also, try to observe the deflection in the compass needle as you move it along a magnetic field line. You will find that the deflection increases as the needle is moved towards the poles.
How to make a permanent magnet?
(1) A magnet can be made from steel rod holding it in north-south direction and repeatedly hammering it. Once it becomes magnet, it retains this property unless it is heated to a high temperature. (2) A magnet can be made from steel rod if we strike the steel rod with one end of a bar magnet a large number of times, always with same pole. (3) A magnet can be made of steel rod if we place it inside a solenoid and run an electric current. The magnetic field produced by the solenoid magnetises the rod. When the electric current is stopped, the rod retains the magnetism. Permanent magnets are usually made of hard steel, carbon steel, chromium steel, cobalt steel, tungsten steel, alnico (an alloy of ), etc. Such materials require strong magnetic field for their magnetisation. But, once they get magnetised, they have a residual magnetism for a long period of time.
- The materials like iron, nickel, cobalt when placed in a magnetic field get strongly magnetised in the direction of field i.e., the field inside them is greatly enhanced. Such materials are called 'ferromagnetic materials'.
Uses of magnets
(1) They are used in radio and stereo speakers. (2) They are used in almirah and refrigerator doors to keep them in closed position. (3) They are used on video and audio cassette tapes. (4) They are used on the hard discs and floppies for computers. (5) They are used in different children's toys. (6) In medicine, they are used in magnetic resonance imaging (MRI) scanners to examine the inner body parts of human beings. (7) Electromagnets are frequently used nowadays for various purposes. e.g. lifting heavy iron pieces, electromagnetic separation in metallurgy, etc. (8) Magnetic compass needle is used to find the approximately north south direction.
- The magnetisation of a magnet can be removed by heating it to very high temperatures or putting the magnet in a current-carrying solenoid in such a way that magnetisation of the magnet is opposite to the applied magnetic field.
Building Concepts 3 In each of the following magnetic fields shown in figure, predict the nature of magnetic field. Also, compare the strength (magnitude) of magnetic fields at points and shown in each case.
4.0Electric current as a source of magnetism
Apart from permanent magnets, electric current is also a source of magnetism. Oersted, by performing experiments, concluded that 'moving charges or electric currents produce a magnetic field in the surrounding space'.
- East, West, North and South are in Horizontal Plane
Active Physics 3
- Take a magnetic needle (NS) which can rotate freely about a vertical axis in a horizontal plane. Place this needle below a straight conducting wire such that needle and the wire are parallel to each other.
- When the current flows in the circuit to [see figure(a)], north pole of needle gets deflected towards the west (Keyword : 'SNOW' i.e., South to North, deflection to West). If the direction of current is now reversed i.e., from B to A [see figure(b)], the north pole gets deflected towards east. Since the magnetic needle can be deflected by an another magnet, thus, the current in the wire must be producing a magnetic field.
Magnetic field due to a current-carrying straight conductor
The magnetic field lines around a straight current-carrying conductor are concentric circles with the conductor located at their centre. The plane of these concentric field lines is perpendicular to the conductor. A simple experiment carried out by Oersted in 1820, clearly demonstrates that a current-carrying conductor produces a magnetic field. In this experiment, several compass needles are placed in a horizontal plane on a circle with a long vertical wire at its centre. When there is no current in the wire [see figure(a)], all needles point in the same direction i.e., north - south direction. However, when the wire carries a steady current, the needles all deflect in directions tangent to the circle [see figure(b)].
Right hand thumb rule
'Imagine that you are holding a current-carrying straight conductor in your right hand and the thumb is stretched along the direction of current, then, your fingers will wrap around the conductor in the direction of the field lines of the magnetic field' (figure). Magnetic field produced by a straight current-carrying conductor is directly proportional to the current flowing through it and inversely proportional to the distance from the conductor. That is, more the current, more will be the strength of magnetic field at a given point and vice-versa. More the distance from the conductor, less will be the strength of the magnetic field and vice-versa.
Active Physics 4
- Take a piece of cardboard and insert a straight conducting wire to pass through its centre, perpendicular to the plane of cardboard. The carboard should be fixed in a position. Sprinkle some iron filings on the cardboard. You may use a salt sprinkler for this purpose.
- Now, pass a steady electric current through the wire using the setup shown in figure. Gently tap the cardboard a few times. Observe the pattern of the iron filings. You would find that the iron filings align themselves showing a pattern of concentric circles around the wire. These concentric circles represent the field lines of the magnetic field produced by the wire.
- If we place a magnetic needle at any point on a particular circle, the needle will align tangentially to the circle, showing the direction of the magnetic field at that point. If the direction of current is reversed, the needle will align tangentially but pointing in opposite direction. This shows that direction of magnetic field is reversed on the reversing the direction of the electric current.
Numerical Ability 1
- A vertical wire carries current upwards. In which direction the magnetic field be at the north of the wire.
Solution:
Decode the problem
Identify the direction of current,
Apply the rule, Use right hand thumb rule to find the direction of magnetic field.
Direction of magnetic fieldDirection of Electric current Applying right hand thumb rule, the magnetic field at the north of the wire will be directed Westward.
- The current carrying vertical wire in the plane of the paper from downward to upward as shown in figure. On which point strength of magnetic field will be maximum.
Solution: Decode the problem To find the strength of magnetic field at a point, Apply the relation, Magnetic field is inversely proportional to distance. Magnetic field produced by a straight current-carrying conductor is inversely proportional to the distance from the conductor. Point A is closest the wire, so strength of magnetic field is maximum at A .
Magnetic field due to a current-carrying circular loop
Let us take a conducting wire in the form of circular loop and an electric current is flowing through it. At every point of the loop, the magnetic field lines are in the form of concentric circles surrounding the loop.
The size of these circles would become larger and larger as we move away from the wire. At the centre of the circular loop, the arcs of these big circles would appear as straight lines. Every point on the wire carrying current would give rise to the magnetic field appearing as straight lines at the center of the loop. Every point of the loop contributes to the magnetic field lines and the magnetic field at any point near the loop is the resultant of the individual contributions of all the points of the loop.
- The right hand thumb rule is also called Maxwell's corkscrew rule (or right hand screw rule). If a right-handed screw moves forward in the direction of the conventional current then the direction of rotation of the screw gives the direction of the magnetic field lines.
Right hand rule for a current-carrying circular loop.Maxwell's corkscrew rule
Building Concepts 4 (a) A current through a horizontal power line flows in west to east direction. What is the direction of magnetic field at a point directly below it and at a point directly above it? (b) A vertical wire carries an electric current in upward direction. What is the direction of magnetic field at point to the north of it and a point to the east of it? Explanation: (a) The current is in the west-east direction [see figure(a)]. Applying the right-hand thumb rule, we get that the direction of magnetic field at a point above the wire is from north to south (towards south). The direction of magnetic field at a point directly below the wire is from south to north (towards north). (b) The current is in the vertically upward direction [see figure(b)]. Applying the right-hand thumb rule, we get that the direction of magnetic field at a point to the north of it is east to west (towards west). The direction of magnetic field at a point to the east of the wire is from south to north (towards north).
Active physics 5 (1) Take a rectangular cardboard having two holes. Insert a circular coil having large number of turns through the two holes perpendicular to the plane of the cardboard. (2) Now, pass a steady electric current through the wire using the setup shown in figure. Gently tap the cardboard a few times. Observe the pattern of the iron filings. This pattern represents the field lines due to the current-carrying circular wire. You will find that field lines near the centre are almost straight and the field lines near the wire are curved.
5.0Magnetic field of a solenoid
If a long, straight conducting wire is bent into a coil of several closely spaced loops, the resulting device is a solenoid. This device acts as a magnet only when it carries a current. A solenoid is a long insulated wire wound in the form of a circular helix where neighbouring turns are closely spaced.
- Imagine grasping the solenoid in your right hand with your fingers curled in the direction of the current flow (from positive to negative). Your thumb will then point in the direction of the magnetic north pole of the solenoid.
Electromagnets
A strong magnetic field produced inside a solenoid can be used to magnetise a piece of magnetic material like soft iron, when placed inside the current-carrying solenoid. The magnet so formed is called an electromagnet. A magnet consisting of a soft iron core with a coil of insulated wire wound around it is called electromagnet. When a current flows through the wire, the core becomes magnetised and when the current ceases to flow the core loses its magnetisation. Electromagnets are used in switches, electric bells, metal-lifting cranes, and many other applications.
Usually, soft iron is used to make electromagnets because it easily gets magnetised and easily gets demagnetised. Mumetal an alloy of nickel, iron and copper is also used to make electromagnets. (7) Strength of an electromagnet increases with the current and is directly proportional to the number of coils per unit length. The more tightly the turns are wound, stronger will be the magnetic field produced by an electromagnet.
6.0Force on a current-carrying conductor in a magnetic field
An electric current flowing through a conductor produces a magnetic field in the surrounding space and exerts a force on a magnet placed near it. French scientist Andre Marie Ampere suggested that the magnet must also exert an equal and opposite force on the current-carrying conductor.
Fleming's left-hand rule
The direction of force on a current-carrying conductor is given by Fleming's left-hand rule. According to this rule, 'stretch the thumb, forefinger and central finger of your left hand such that they are mutually perpendicular. If the fore finger points in the direction of magnetic field and the central finger in the direction of current, then the thumb will point in the direction of motion or the force acting on the conductor.'
Active Physics 6 (1) Take a small aluminium rod PQ and suspend it horizontally using two wires and a stand (see figure). Place a horseshoe magnet in such a way that rod lies in between the poles of magnet and magnetic field is directed upwards.
- Force on a current-carrying straight rod (or wire) is largest when the direction of current is at right angles to the direction of the magnetic field and it is zero when the rod and field are parallel to each other.
- The direction of force on a current-carrying conductor placed in a magnetic field is always perpendicular to the plane containing current and magnetic field.
- Force on a charge at rest placed in a magnetic field is zero. Thus, just as moving charges produce a magnetic field, so too only moving charges are affected by a magnetic field.
Building Concepts 5 (a) An electron enters a magnetic field at right angles to it, as shown in figure. What will be the direction of force acting on the electron? (b) Predict the motion of electron in the magnetic field. Explanation: Magnetic Electron Building concepts 5 (a) We know that the direction of force is perpendicular to the direction of magnetic field and current as given by Fleming's left hand rule. Now, the direction of current is taken opposite to the direction of motion of electrons. Thus, the direction of current is vertically upward, magnetic field is to the right. By applying Fleming's left hand rule, we find that the force is directed into the page. (b) Since force is always perpendicular to motion (velocity), such a motion will be a circular motion with its plane perpendicular to B . (see figure)
Numerical Ability 2
- Find the direction of magnetic force on the current carrying conductor shown in figure.
Solution: Decode the problem Identify the direction of current and magnetic field. Apply the rule, Use Fleming's left hand rule to find the direction of force. Applying Flemming's left hand rule, the direction of magnetic force is upwards.
- A wire is suspended vertically between the poles of a magnet as shown in the figure. The magnetic field is directed into the page. When there is no current in the wire, it remains vertical [see figure (a)]. When the current is upward [see figure (b)], in which direction will the wire move?
Solution: Decode the problem Identify the direction of current and magnetic field. Apply the rule, Use Fleming's left hand rule to find the direction of force. Applying Fleming's left-hand rule, the direction of magnetic force is towards left.
7.0Alternating current (AC) and direct current (DC)
The electric current, whose magnitude varies with time and direction reverses periodically, provided its amplitude is constant, is called 'alternating current' (see figure). In India, the frequency of AC is 50 Hz i.e., 50 cycles per second. In one cycle, the direction of AC changes twice, thus, in one second, the direction of AC changes times. In other words, AC changes direction after every second. The electric current, whose magnitude and direction do not vary with time is called 'direct current'. Usually, DC is produced by a cell or a battery (see figure).
Advantages of AC over DC (1) AC voltages can be easily increased (step up) or decreased (step down) with the help of transformers. (2) Long distance transmission takes place at high voltage (i.e., less current) to minimise heat losses. This is done easily by using AC voltage because an AC voltage can easily be increased by using a transformer. (3) The cost of generation of is less than that of . (4) AC devices are simple, robust and require less care as compared to DC devices.
Disadvantages of AC over DC (1) is more dangerous than . (2) A device operating at 220 V AC has to sustain a peak value of approximately 310 V . (3) For processes like electrolysis or electroplating, AC cannot be used, only DC can be used. (4) An alternating current flows mainly on the surface of conductor (called skin effect). Thus, instead of a single thick wire, for AC , we have to use several thin wires twisted to form a main wire, which increases its cost of manufacturing.
8.0Domestic electric circuits
(1) The power supply to houses are given through overhead electric poles having aluminium wires or through underground cables. One wire of power supply is called live wire (or positive) which has usually a red insulation cover. Another wire, with black insulation cover is called neutral wire (or negative). In our country, the potential difference between these two wires is 220 V . (2) These wires pass into an electricity meter through a main fuse at the meter-board in the house. The main fuse is joined in series with the live wire. Through the main switch, they are connected to the line wires in the house. The line wires supply electricity to separate circuits within the house. Mostly, two separate circuits are used, one of 15 A current rating for appliances with higher power ratings such as geysers, air conditioners, air coolers, refrigerators, etc. The other circuit is of 5 A current rating for bulbs, tubelights, CFLs, fans, etc. (3) There is an earth wire, which has insulation of green colour, is usually connected to a metal plate deep in the earth near the house. This is used as a safety measure, particularly for the devices with a metallic body like electric press, toaster, table fan, refrigerator, etc. The metallic body is connected to the earth wire, which provides a lowresistance conducting path for the current. Thus, any leakage of current to the metallic body of the device will go into the earth through the earth wire. Thus, the user will not get a severe electric shock. (4) In each separate circuit, different devices can be connected across the live and neutral wires. Each device has a separate switch to 'on' or 'off' the flow of current through it. All the devices in domestic electric circuits are joined in parallel in order that each device has equal potential difference.
- Earthing (or grounding) is the process of transferring charge to the Earth. This is done by using a conducting wire or a conducting rod.
Overloading
We know that electric fuse in a circuit prevents damage to the appliances and the circuit due to overloading.
- Overloading is a condition in which excessively high current flows through a circuit. Overloading can occur in many ways : (1) When the live wire and the neutral wire come into direct contact, the resistance in the circuit becomes very low and the current in the circuit abruptly increases. This is called short-circuiting. This usually occurs when the insulation of wires is damaged or there is a fault in the appliance. (2) Overloading can also occur due to an accidental hike in the supply voltage. (3) Sometimes, overloading is caused by connecting too many devices to a single socket.
- An electric fuse prevents the electric circuit and the appliance from a possible damage by stopping the flow of unduly high electric current. The Joule's heating that takes place in the fuse melts it to break the electric circuit.
- For lines used for bulbs, tubelights, fans, a 5 A fuse is used and for lines used to feed power to high current appliances like geyser, electric heater usually 15 A fuse is used.
- Right hand thumb rule : It is used to determine the direction of the magnetic field around a current-carrying wire or a solenoid.
- Fleming's left-hand rule : This rule helps of determine the direction of force experienced by a current-carrying conductor in a magnetic field.
- Magnetic field due to a solenoid : Inside a solenoid, the magnetic field is uniform and parallel to the axis of the solenoid, while outside the solenoid, the field resembles that of a bar magnet.
9.0Concept Map
10.0Some Basic Terms
- Lodestones :- Lodestones are naturally magnetized pieces of the mineral magnetite. They are naturally occurring magnets, which can attract iron.
- Phenomena :- Something that exists and can be seen, felt, etc.
- Magnetic Dipole :- A magnet is called a "magnetic dipole" because it has two poles (north and south).
- Acquire :- To obtain something.
- Induced :- To cause or produce
- Tangent :- A straight line that touches a curve but does not cross it
- Intersect :- To meet and cross at a point.
- Isolated :- Not connected with others.
- Adhesive :- That can stick, or can cause two things to stick together.
- Retains :- To keep or continue to have something.
- Magnetic Resonance Imaging (MRI) :- Magnetic Resonance Imaging (MRI) is a noninvasive imaging technology that produces three dimensional detailed anatomical images.
- Concentric Circles :- A concentric circles is defined as two or more circles with a common center.
- Mu-metal :- An alloy containing nickel, iron, and copper, characterized by high magnetic permeability and low hysteresis losses.
- Perpendicular :- A straight line that makes the right angle ( 90 degrees) with the other line.