Ques 1: Define a solenoid. Compare the magnetic field produced by a solenoid with that of a bar magnet?
Ans: A coil of many circular turns of wire wrapped in the shape of a cylinder, is called a solenoid. The magnetic field lines in a solenoid, through which current is passed, is very similar to that of a bar magnet. One end of the coil acts like a magnetic north pole, while the other acts like a south pole. The magnetic field produced by a long solenoid has all the properties of the field produced by a bar magnet.
Ques 2: Give one important advantage of AC over DC
Ans: A.C can be stepped up and stepped down which means that the voltage can be increased or decreased. Hence it can be transmitted to long distances without much loss of energy. So A.C is preferred over D.C.
Ques 3: Give the circuit symbol for a fuse. Explain its importance in a circuit.
A fuse is a very important device used for protecting electric circuits. It is a wire made out of a metal like tin or tin alloy having a very low melting point. When a high current flows through a circuit, the fuse wire gets heated or melts due to short circuiting or overloading. Hence the circuit is broken and the current stops flowing. This saves all the appliances of the circuit.
Fuse wires are of various capacities. A fuse with 5 ampere capacity will be thinner than a fuse with 15 ampere capacity. A fuse of 5 amps is used in circuits where lights and fans are connected whereas a fuse of 15 amps is used in power circuits where appliances like electric heater, geyser, electric iron and air conditioner are connected.
Ques 4: Give a note on Magnetism in Human beings.
Ans: Whenever there is an electric current, there is a magnetic field. Even the extremely weak ion currents that travel along the nerve cells in our body produce magnetic fields. When we try to touch something, our nerves carry an electric impulse to the muscles we need to use. This impulse creates a temporary magnetic field. These fields are about one billionth as weak as the Earth’s field. Two main organs in the human body where the magnetic field produced is significant are heart and brain.
Ques 5: PQ is conductor × represents magnetic is ⊥ to the paper field and into the plane of the paper.
Ans: Fleming’s left hand rule gives the direction of force experienced by a current carrying conductor kept in a magnetic field. According to it, when the thumb, first finger and second finger of the left hand are kept perpendicular to each other such that the first finger points towards the direction of magnetic field, the central finger is along the direction of current, then the thumb shows the direction of the force acting on the conductor.
Ques 6: Define Electromotive force.
Ans: The motion of a magnet, with respect to the coil, produces an induced potential difference. This induced potential difference is called electromotive force which sets up an induced electric current in the circuit. The motion of a magnet, with respect to the coil, produces an induced potential difference.
Ques 7: What is meant by earthing? Why should electrical appliances be earthed?
Ans: The metal body of appliances like fridge, cooler, mixer etc. are connected to a an earth wire so that any leakage of current to the body of the appliance goes to the earth and does not give electric shock. This is called earthing. It is used as a safety measure in order to prevent electric shocks to the users.
Ques 8: What is a solenoid?
Ans: A solenoid is a long cylindrical conductor coil, having a large number of turns of insulated copper wire.
Ques 9: State Fleming’s right Hand Rule. Give the principle, construction and working of the AC generator with a simple diagram. What modification will you suggest so that the output is DC
Ans: According to Fleming’s right hand rule, when the thumb and the central finger of right hand are kept perpendicular to each other, the thumb shows the direction of motion of the conductor, the first finger the direction of magnetic field when the current induced is in the direction of central finger.
Principle: It works on the principle of electromagnetic induction. Induced current is produced, whenever current is produced.
Construction: A generator consists of mainly four parts, namely coil, magnets, slip rings and brushes just like an electric motor.
Coil: A large number of insulated copper wires wound on a rectangular frame.
Magnets: A large permanent magnet to provide a strong a magnetic field.
Slip rings: Two solid rings connected the two ends of the coil used to convey the current produced to outside circuit.
Brushes: Two carbon brushes remain in sliding contact with slip rings.
Working: The coil of the generator is rotated with the help of an axel. When coil rotates, it cuts through the magnetic filed of the magnet. So a current is induced in the coil by electromagnetic induction. The direction of this current is given by Fleming’s right hand rule.
As the coil turns clockwise, arm AB moves up and arm CD goes down. The direction of the current is from A to B and C to D. When after half rotation CD starts going up and AB starts coming down, the direction of the current in the coil also reverses. Now it is from D to C and from B to A. This alternating current with the help of slip rings which are in sliding contact with brushes B1 and B2 is given out to the circuit. Hence the current produced by the generator is alternating and such a generator is called AC generator.
To get direct current in place of slip rings, split rings are used so that one brush is always in contact with the arm that goes downward. Then the current given out to the outer circuit is in the same direction. This type of generator is called DC generator.
Ques 10: How can you convert an A.C. into a D.C. generator?
Ans: An A.C. generator can be converted into a D.C. generator by replacing the solid ring arrangement with split ring arrangement.
Ques 11: What is a magnetic field?
Ans: The region around a magnet, in which the magnetic force of attraction and repulsion is felt, is called a magnetic field.
Ques 12: Distinguish between a solenoid and a bar magnet. Draw the magnetic lines for both
The solenoid is a long coil containing a large number of close turns of insulated copper wire. The magnetic field produced by the current carrying solenoid is similar to the magnetic field produced by a bar magnet. A solenoid is used for making electromagnets.Differences between a bar magnet and solenoid:
It is a permanent magnet.
The strength of a bar magnet cannot be changed.
The polarity (North – South) of a bar magnet cannot be changed.
It is a temporary magnet. It acts as a magnet only as long as the current passes through it.
The strength of a solenoid can be changed by changing the number of turns in its coil or by changing the current passing through it.
The polarity of a solenoid can be changed by changing the direction of current in its coil.
Ques 13: What is electromagnetic induction? Explain how the movement of a magnet towards or away from a coil carrying a galvanometer produce current? Write the rule to find the direction of current in this above coil.
Ans: Whenever the magnetic field through a conductor changes, and induced current and e. m. f. is set up in the conductor. This is known as electromagnetic induction.
OrElectromagnetic induction is a process where a conductor placed in a changing magnetic field causes the production of a voltage across the conductor.
The rule being applied is Fleming's right-hand rule. Fleming’s right-hand rule is used to determine the direction of current induced in a coil due to its rotation in a magnetic field.
When the magnet is moved towards the coil, there is a change in the magnetic field which induces a current in the coil. The same thing will happen when the magnet is moved away.
Ques 14: Which effect of electric current is utilized in the working of an electric fuse?
Ans: An electric fuse works on the heating effect of current.
Ques 15: What will you do if you see a person coming in contact with a live wire?
Ans: Such a person should be provided with an insulated support of wood, plastic or rubber.
Ques 16: Name an instrument in which the directive property of a magnet is used.
Ans: Compass needle makes use of the directive property of a magnet.
Ques 17: Name the elements of Earth’s magnetic field.
Ans: The elements of Earth’s magnetic field are angle of dip, declination and horizontal component of earth’s magnetic field.
Ques 18: Explain why, two magnetic lines of force do not intersect.
Ans: The magnetic lines of force do not intersect one another due to the fact that the resultant force on a north pole at any point can be only in one direction. But if the two magnetic lines of force intersect one another, then the resultant force on a north pole placed at the point of intersection will be along directions, which is not possible.
Ques 19: State the right hand thumb rule.
If you hold the thumb, the forefinger and the centre finger of your right hand at right angles to one another. Adjust you hand in such a way that forefinger points in the direction of magnetic field, and the thumb points in the direction of motion of conductor, then the direction in which centre finger points, gives the direction of induced current in the conductor.
Ques 20: What is the cause of earth’s magnetism?
Ans: Earth’s magnetism is due to the magnetic effect of current which is flowing in the liquid core at the center of the earth.