SSC, SCIENCE PART I, NEW SYLLABUS, FOR BOARD EXAM 2020,
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. The odd one out is generator. It is an electrical device for producing electricity. Fuse wire, bad conductor and rubber gloves have high resistance and are used for blocking electricity. Thus, they can be used as a safety measure against heavy electricity. b. The odd one out is thermometer. It is an instrument for measuring the temperature of a body. Rest of the three are electrical instruments based on the phenomenon of electromagnetism and are used for measuring some electrical parameters such as current and voltage. c. The odd one out is magnet. Loud speaker, microphone and electric motor are based on the phenomenon of electromagnetism.
Explain the construction and working of the following. Draw a neat diagram and label it.
a. Electric motor
b. Electric Generator (AC)
a. Motor principle:
The basic principles on which the electric motor works is the magnetic effect of current. A current carrying rectangular coil starts rotating when placed in a magnetic field.
Construction: The given figure illustrates the internal parts of a simple electric motor.
A motor consists of a rectangular coil ABCD of insulated copper wire.
The coil is placed between two magnetic poles such that the magnetic field acts normal on lengths AB and CD.
The coil is connected with two carbon brushes at points B1 and B2 respectively.
The inner sides of these carbon brushes are in contact with half rings R1 and R2, which are insulated and in contact with an axle (not shown in the figure).
When a current is allowed to flow through the coil ABCD by closing the switch, the coil starts rotating anti-clockwise.
This happens because a downward force acts on length B1 B2 and at the same time, an upward force acts on length R1 R2.
As a result, the coil rotates anti-clockwise. The current in length B1 B2 flows from B1 to B2, and magnetic field acts from left to right normal to length B1 B2.
Hence, according to Fleming’s left hand rule, a downward force acts on length AB.
Similarly, the current in length CD flows from C to D, and magnetic field acts from left to right normal to its length.
Hence, an upward force acts on length R1 R2. These two forces cause the coil B1 B2 R1 R2 and the axle to rotate anti-clockwise.
After half-rotation, the position of length B1 B2 and R1 R2 get interchanged.
Simultaneously, half ring R2 comes in contact with brush B1 and half ring R1 comes in contact with brush B2 respectively.
Hence, the direction of current in coil ABCD gets reversed and flows through DCBA.
An electric device that reverses the direction of current in a circuit is called a commutator.
Thus, the split ring acts as a commutator of the electric motor.
Now, due to the reverse direction of current in lengths AB and CD, an upward force acts on length AB, which pushes it up and a downward force acts on length CD,
which pushes it down. As a result, the coil ABCD further rotates anti-clockwise.
The reversal of the current through the coil ABCD repeats at each half-rotation, while its anti-clockwise rotation continues.
b. Electric Generator principle : An electric generator is a machine that generates electricity by rotating its rotor in a magnetic field. Thus, it converts mechanical energy into electrical energy.
Construction: A generator consists of a rectangular coil MNST of insulated copper wire placed between two strong magnetic poles.
The two ends of the coil MNST are connected with brushes A and B of rings C and D respectively.
The inner sides of the rings are insulated. They are attached with an axle X, which can be rotated mechanically.
Brushes A and B are connected with a galvanometer that can measure the flow of current in coil MNST.
Working: When the axle is rotated, lengths MN and ST move up and down respectively.
Since lengths MN and ST are moving in a magnetic field, a current is induced in these lengths caused by electromagnetic induction.
The direction of the induced current in both the lengths is given by Fleming’s right hand rule.
Since length MN is moving upwards in the magnetic field that acts from left to right, the direction of the induced current will be from M to N.
Similarly, the direction of the induced current in length ST will be from S to T.
Hence, an induced current will set up in the coil in the direction MNST, which produces deflection in the galvanometer.
After half-rotation, length MN starts moving down, whereas length ST starts moving up.
The direction of the induced current in the coil gets reversed i.e., the induced current will now flow from T to M via S and N i.e., TSNM.
Therefore, we can conclude that after each half-rotation, the direction of the induced current is reversed.
This current is called an alternating current (AC). An AC reverses its direction after equal time intervals.
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.
Electromagnetic induction means generation of a current in a coil due to relative motion between the coil and the magnet.
AC generator is a mechanical device which converts mechanical energy into AC electrical power.
DC generator is a mechanical device which converts mechanical energy into DC electrical power.
Direction of Current
In an AC generator, the electrical current reverses direction periodically.
In a DC generator, the electrical current flows only in one direction.
In an AC generator, the coil through which the current flows is fixed while the magnet moves. The construction is simple and costs are less.
In a DC generator, the coil through which the current flows rotates in a fixed field. The overall design is very simple but construction is complex due commutators and slip rings.
AC generator does not have commutators.
DC generators have commutators to make the current flow in one direction only.
AC generators have slip-rings.
DC generators have split-ring commutators.
Which device is used to produce electricity? Describe with a neat diagram.
a. Electric motor
c. Electric Generator (DC)
The device used for producing electricity is Electric generator (DC). It is based on the phenomenon of electromagnetic induction.
Working: When the axle is rotated, lengths MN and ST move up and down, respectively.
Since lengths MN and ST are moving in a magnetic field, a current gets induced in these lengths caused by an electromagnetic induction.
The direction of the induced current in both the lengths is given by Fleming’s right hand rule.
In this arrangement, brush A always remains in contact with the length moving up, whereas brush B always remains in contact with the length moving down.
Here, split rings C and D act as a commutator. In this case, the direction of the current induced in the coil will be from M to T via N and S for the first half-rotation, and from T to M via S and N for the second half-rotation of coil MNST.
Hence, we get a unidirectional current called direct current (DC).
How does the short circuit form? What is its effect?
Short circuit occurs when naked live and neutral wires touch each other.
In such situations, the resistance of the circuit becomes very less. Now, according to Ohm’s law, current is inversely proportional to resistance.
Thus, the decrease in value of resistance of the circuit raises the current to a significant amount.
As a result, the wires become hot and sparks are caused by Joule’s heating effect of current.
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 joules.
a. Tungsten metal is used to make a solenoid type coil in an electric bulb because its melting point is very high. Thus, when a high amount of current is passed through it, it becomes red hot and emit lights without getting burnt.
b. In the electric equipment producing heat, such as iron, electric heater, boiler, toaster etc., an alloy such as Nichrome is used, not pure metals because of the following reasons:
(i) Resistivity of Nichrome is more compared to pure metal.
(ii) Melting point of Nichrome is high as compared to pure metal.
(iii) Nichrome does not get oxidized when heated in air whereas metal does.
c. For electric power transmission, Copper or Aluminium wire is used because they provide a low resistance path to the flow of current.
Thus, the power loss in the low resistance transmission wire will be less.
d. In practice, the unit kWh is used for the measurement of electrical energy, rather than joules. This is because the joule is a very small unit and the energy consumption in day to day life is very large i.e.
it comes in figures of 106 to 108. Thus, to reduce the complexity of handling such large figures, a bigger unit was required.
This bigger unit used for the measurement of electrical energy is kWh and is related to joule as1 kWh = 3.6 × 106 J
Hence, the energy reading commercially became simpler by using this bigger unit instead of joule.
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 the 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 describing the magnetic field near a long, straight current carrying conductor is :The magnetic lines of force are in concentric circles with the wire as the center, in a plane perpendicular to the conductor
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.
A solenoid is a long straight insulated wire, such as a copper coil, often wrapped around a cylinder-shaped body. The diameter of the solenoid is lesser than its length. It produces a magnetic field when electric current is passed through it.
Magnetic field produced by a solenoid is shown below:
Magnetic field produced by a bar magnet is shown below:
On comparing field lines produced by a solenoid with that produced by a bar magnet, we observe that they are very much identical. Thus, a solenoid acts as a bar magnet when a current is passed through it.
Name the following diagrams and explain the concept behind them.
a. It represents Fleming’s right hand rule used for finding the direction of induced current with respect to the directions of the magnetic field and motion of the conductor.
The direction of current induced in a conductor can be obtained by holding the thumb, index finger, and the middle finger of your right hand mutually perpendicular to each other.
In this situation, the thumb indicates the direction of the motion of the conductor, the index finger points along the magnetic field, and the middle finger points along the current induced in the conductor.
b. It represents Fleming’s left hand rule used for finding the direction of magnetic force when a current carrying conductor is placed in a magnetic field.
This rule states that if you stretch the thumb, index finger, and middle finger of your left hand such that they are mutually perpendicular to each other, then your index finger represents the direction of the field, the middle finger represents the direction of the current, and the thumb represents the direction of the force experienced by the conductor.
Identify the figures and explain their use.
(a) Figure represents a fuse. An electric fuse is a safety device that protects the wiring against excessive heating caused by an excess supply of current. It melts when heavy current flows through the circuit, thereby causing the circuit to become open. (b) Figure represents an MCB. An MCB is a device which functions as a fuse, but does not require replacement. MCB falls down to break the circuit when heavy amount of current flows through it. Once the fault is rectified, the MCB is reset. (c) Figure (c) represents a DC generator. It is a device that generates electricity by rotating its rotor in a magnetic field. Thus, it converts mechanical energy into electrical energy.
Solve the following example.
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?
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?
c. Who will spend more electrical energy? 500 W TV Set in 30 mins, or 600 W heater in 20 mins?
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).