1. Tell the odd one out. Give proper explanation.
(a) Fuse wire, bad conductor, rubber gloves, generator.
(b) Voltmeter, Ammeter, galvanometer, thermometer.
(c) Loud speaker, microphone, electric motor, magnet.
(a) Generator. It converts mechanical energy into electric energy, the remaining three do not.
(b) Thermometer. It measures temperature, the remaining three measure electrical quantities.
(c) Magnet. It exerts a force on a magnetic material, the remaining three convert one form of energy into another.
2. Explain the construction and working of the following. Draw a neat diagram and label it.
(a) Electric motor
(b) Electric Generator (AC)
A device which converts electrical energy into mechanical energy is called electric motor.
Principle of working:
Electric motor works on the principle that a current carrying conductor is placed in a magnetic field experiences force.
Construction:
(i) Armature coil: Rectangular loop of copper wirehaving resistive coating.
(ii) Strong Magnet: Armature coil is placed betweenthe north pole and south pole of magnet (such ashorse shoe magnet).
(iii) Split ring: The two halves of the ring (x and y)have a resistive coating on the inner side andconducting surface on the outer side.
(iv) Carbon Brushes: Two stationary carbon brushes (E and F) are connected to the outer side of the split ring.
(v) Battery: The battery supplies the electric current to the armature coil.
Working of the electric motor:
(i) When the circuit is completed, the current flows in the branch AB of the loop from A to B through the carbon brushes E and F.
(ii) Since the direction of the magnetic field is from north pole to south pole, according to the Fleming’s left hand rule, a force is exerted on the branch AB in downward direction and CD in upward direction.
(iii) Thus, the loop and the axle start rotating in an anticlockwise direction.
(iv) After half rotation, the current in a loop starts flowing in the direction DCBA.
(v) Therefore, a force is exerted on the branch CD in downward direction and on the branch AB in the upward direction, and the loop continues to rotate in the anticlockwise direction.
(vi) Thus, the current in the loop is reversed after each half rotation and the loop and the axle continue to rotate in the anticlockwise direction.
A generator which converts mechanical energy into electrical energy in the form of alternating current is called AC Generator.
Principle of working:
(i) Electric generators works on the principle of electromagnetic induction.
Construction:
(i) Armature coil: Rectangular loop of copper wire having resistive coating.
(ii) Strong Magnet: Armature coil is placed between the north pole and south pole of magnet (such as horse shoe magnet).
(iii) Slip rings: The ends of the armature are connected to conducting rings R1 and R2 which are fixed to the axle with the resistive coating in between the ring and the axle.
(iv) Carbon Brushes: The stationary carbon brushes B1 and B2 are connected to a galvanometer.
(v) Galvanometer: It detects the presence of current in the circuit.
Working:
(i) When the armature coil ABCD is rotated by external force in the magnetic field provided by strong magnets, it cuts the magnetic lines of forces. Thus, the changing magnetic field produces induced current in the coil.
(ii) After rotation of axle, arm AB moves upwards, while arm CD moves downwards. Therefore, ABCD rotates in the clockwise direction. As per the Fleming’s right hand rule, the
current is induced in the Coil and moves from A → B and C → D. Current flows in the direction ABCD.
(iii) Current flows from B2 to B1 in the first half of the revolution in the external circuit.
(iv) After half revolution, arm AB comes to the position of arm CD and arm CD takes the position of arm AB. Due to this, the induced current flows in the direction DCBA. But, arm AB is in contact with B1, through slip ring and arm CD is in contact with B2.
(v) In the second half, in the external circuit, electric current flows from B1 to B2, i.e. opposite to the previous half rotation.
(vi) This repeats after every half rotation, and alternating current is produced. This is what is called AC Generator.
3. Electromagnetic induction means
(a) Charging of an electric conductor.
(b) Production of magnetic field due to a current flowing through a coil.
(c) Generation of a current in a coil due to relative motion between the coil and the magnet.
(d) Motion of the coil around the axle in an electric motor.
The correct statement is (c), Generation of a current in a coil due to relative motion between the coil and the magnet.
4. Explain the difference :
AC generator and DC generator.
5. Which device is used to produce electricity? Describe with a neat diagram.
(a) Electric motor
(b) Galvanometer
(c) Electric Generator (DC)
(d) Voltmeter
(c) Electric generator (DC) – The device that converts mechanical energy into the electrical energy in the form of direct current is called DC generator.
Principle of working:
(i) DC generator works on the principle of electromagnetic induction.
(ii) When the coil rotates in a magnetic field, the magnetic field induces a current in this coil.
The induced current then flows into the circuit connected to the coil.
Construction:
(i) Armature coil: Rectangular loop of copper wire having resistive coating.
(ii) Strong Magnet: Armature coil is placed between the north pole and south pole of magnet (such as horse shoe magnet).
(iii) Split ring: The two halves of the ring (X and Y) have a resistive coating on the inner side and conducting surface on the outer side.
(iv) Carbon Brushes: Two stationary carbon brushes (E and F) are connected to the outer side of the split ring.
(v) Electric bulb or Galvanometer : (a) The output is shown by the glowing bulb connected across the carbon brushes. (b) Galvanometer is connected in the circuit to detect the presence of electric current in the circuit.
Working:
(i) When the coil of DC generator rotates in the Magnetic field, potential difference is produced in the coil.
(ii) As the coil rotates in the magnetic field, the induced current produces according to Flemming’s right hand rule.
(iii) The presence of current can be detected either by electric bulb or galvanometer.
(iv) In DC generator, the flow of current in the external circuit is in one direction as long as the coil rotates in the magnetic field. This is because one brush is always in contact with the branch moving up and other brush is in contact with the branch moving downward in the magnetic field.
(v) In this way this generator is called DC generator.
Short circuiting:
If a live wire (phase wire) and a neutral wire come in direct contact or touch each other, short circuiting takes place.
Effects of short circuiting:
(a) During a short circuit, the resistance of the circuit becomes very small and hence huge amount of current flows through it.
(b) This produces a large amount of heat and raises the temperature. As a result, the circuit catches fire and can cause damage.
7. Give Scientific reasons.
(a) Tungsten metal is used to make a solenoid type coil in an electric bulb.
(b) In the electric equipment producing heat e.g. iron, electric heater, boiler, toaster etc, an alloy such as Nichrome is used, not pure metals.
(c) For electric power transmission, copper or aluminium wire is used.
(d) In practice the unit kWh is used for the measurement of electrical energy rather than joule.
(a) Tungsten metal is used to make a solenoid type coil in an electric bulb.
Ans. (i) The electric bulb works on the principle of heating effect of electric current.
(ii) The intensity of light emitted by the filament of an electric bulb depends on the temperature of the filament.
(iii) When electric current is passed through the filament, a large amount of heat is generated and filament becomes hot, then emits light.
(iv) The melting point of Tungsten is very high (3380°C), so that the filament can be heated to a high temperature without melting it.
(v) Hence, the filament of an electric bulb is made up of Tungsten.
(b) In the eclectic equipment producing heat e.g. iron, electric heater, boiler, toaster etc, an alloy such as Nichrome is used, not pure metals.
Ans. (i) The appliances like iron, electric toaster etc. works on the principle of heating effect of an electric current.
(ii) At high temperature pure metal can get oxidised and also pure metals have low resistivity.
(iii) An alloy such as Nichrome, has high resistivity and can be heated to a high temperature without undergoing oxidation.
(iv) Thus, Nichrome is preferred over other pure metals and are used for making coil for devices working on heating effect of electric current.
(c) For electric power transmission, copper or aluminium wire is used.
Ans. (i) Copper and Aluminium are good conductors of electricity.
(ii) They have less resistivity, due to less resistivity, these metals have high conductivity.
(iii) Copper and aluminium have free electrons and are highly ductile metals.
(iv) Hence, connecting wires in the electric circuit are generally made of copper and aluminium.
(d) In practice the unit kWh is used for the measurement of electrical energy rather than joule.
Ans. (i) The unit of electric power 1W is a very small unit, hence 1000W or 1kW is used in practice.
(ii) Now if 1kW power is used for 1 hour, it means 1kW × 1h i.e. 1kWh energy is consumed.
(iii) joule is the SI unit for energy which is very small, hence on commercial scale kWh is used to measure electrical energy.
8. Which of the statement given below correctly describes the magnetic field near a long, straight current carrying conductor?
(a) The magnetic lines of force are in a plane, perpendicular to the conductor in the form of straight lines.
(b) The magnetic lines of force are parallel to the conductor on all the sides of conductor.
(c) The magnetic lines of force are perpendicular to the conductor going radially outward.
(d) The magnetic lines of force are in concentric circles with the wire as the center, in a plane perpendicular to the conductor.
The correct statement is, (d) The magnetic lines of force are in concentric circles with the wire as the centre, in a plane perpendicular to the conductor.
9. What is a solenoid? Compare the magnetic field produced by a solenoid with the magnetic field of a bar magnet. Draw neat figures and name various components.
Solenoid:
(i) When a copper wire with a resistive coating is wound in a chain of loops (like a spring) it is called solenoid.
(ii) When an electric current is passed through a solenoid, magnetic field is produced in it.
(iii) The magnetic lines of force are similar to that of the lines of a bar magnet.
(iv) Solenoid has all properties of the field produced by a bar magnet.
(v) One end of the coil acts as a south pole, while the other end as north pole.
Various components are:
(a) Copper wire with Resistive coating,
(b) Battery
(c) Plug key
(i) Fleming’s right hand rule
(ii) This rule is used to find the direction of the induced current.
(iii) Stretch the thumb, the index finger and the middle finger in such a way that they will be perpendicular to each other. In this position, the thumb indicates the direction of motion of the conductor, the index finger the direction of the magnetic field, and the middle finger shows the direction of the induced current. This rule is known as Fleming’s right hand rule.
(i) Fleming’s left hand Rule.
(ii) This rule is used to find the direction of the force on the conductor.
(iii) According to this rule, the left hand thumb, index finger, and the middle finger are stretched so as to be perpendicular to each other. If the index finger is in the direction of the magnetic field, and the middle finger points in the direction of the current, then the direction of the thumb is the direction of the force on the conductor.
11. Identify the figures and explain their use.
(i) Cartridge Fuse
(ii) Fuse is a device which protects the appliances from excess current.
(i) Miniature circuit breaker (MCB)
(ii) When the current in the circuit suddenly increases this switch opens and current stops.
(iii) Different types of MCBs are in use.
(i) DC generator
(ii) DC Generator is used to generate direct current.
(a) Heat energy is being produced in a resistance in a circuit at the rate of 100 W. The current of 3 A is flowing in the circuit. What must be the value of the resistance? (Ans : 11 W)
(b) Two tungsten bulbs of wattage 100 W and 60 W power work on 220 V potential difference. If they are connected in parallel, how much current will flow in the main conductor? (Ans : 0.72A)
(c) Who will spend more electrical energy? 500 W TV Set in 30 mins, or 600 W heater in 20 mins? (Ans : TV Set)
(d) An electric iron of 1100 W is operated for 2 hrs daily. What will be the electrical consumption expenses for that in the month of April? (The electric company charges Rs 5 per unit of energy). (Ans : Rs 330)
