Saturday 26 January 2019
Friday 25 January 2019
Q. Why are the electrovalent compounds dissociated when dissolved in water?
answer. The electrovalent compounds remain bound together by electrostatic force. This force becomes weaker in the water medium.
Q. Write two characteristics of fuse wire.
Answer. Two characteristics of fuse wire are given below
(a) Fuse wire should have high resistance.
(b) Melting point of fuse wire should be low.
Q. Why is a resistance called the starting resistance connected in series with the armature coil of a motor?
Answer. The back emf produced is minimum at the start and thus current through the armature is maximum at the beginning and this current at the start may be so large that armature coil may got burnt. So a resistance called 'the starting resistance' is connected in series with the armature coil.
Q. Can a DC dynamo act as a DC motor?
Answer. A DC motor is exactly similar in construction to a DC dynamo, having a field magnet, a commutator and an armature. Thus a D.C dynamo may act as a DC motor when the current from an external DC source is led to the commutator through carbon brushes, from where the current flows to the armature coil and the DC motor starts working.
Q. Why is the core of a transformer laminated?
Answer. The core is laminated to reduce the effect of eddy currents.
Q. Why does a spark appear at the switch when a circuit containing a coil is switched off.
Answer. When the circuit is switched off, the current begins to fall off very quickly and therefore, a corresponding large back emf is induced which tends to maintain the current. The induced emf is usually large enough to break the insulation of the air gap of the switch and so a spark is produced.
Q. A bird sitting on a naked high tension wire carrying current is quite safe. If the wire between his feet he cut and joined by a long loop of wire say about 100 m long, will the bird be safe still now?
Answer. In this case wire would touch the ground and circuit would be completed through the earth and the body of the bird and thus current would flow and it would no longer safe.
Q. Define 1 Ohm.
Answer. If 1 ampere current is flowing through a conductor and the potential difference between two ends of the conductor is 1 volt, then the resistance of the conductor will be considered as 1 ohm.
Q. Why is a silver voltameter more useful in measuring current accurately?
Answer. In a silver voltamerer, measurable quantity of silver is deposited on the cathode in comparatively less time. For this a silver voltameter is mroe useful in measuring current accurately.
Q. What is the difference between a voltmeter and a voltameter?
Answer. The device used to measure the potential difference between any two parts of an electrical circuit is known as voltmeter. But a voltameter is an arrangement consisting a vessel, electrolyte, the electrodes etc. by which electrolysis is carried out.
Q. Mercury is conductor of electricity, still why is not considered as an electrolyte?
Answer. Mercury is not ionised when electric current passes through it. In mercury free electrons are the charge carriers. For this mercury is not considered as an electrolyte.
Q. What is thomson coefficient?
Answer. The emf produced in a conductor between its two points maintained at 1 degree temperature difference is called Thomson coefficient.
Q What is Thomson effect?
Answer. When a current flows through unequally heated conductors, heat energy is evolved or absorbed not only at the junctions but also throughout the metals forming the thermocouple. This phenomenon is known as Thomson effect.
Q. State the principle difference between an ammeter and voltmeter?
Answer. An ammeter is a low resistance galvanometer connected with a shunt in parallel. A voltmeter is a high resistance galvanometer having a high resistance in series.
Q. What are the differences between the electric and magnetic field energies?
Answer. A magnetic field is produced by a moving charge, but an electric field is produced even by a stationary electric charge. The energy stored in an electric field is potential while the energy stored in a magnetic field is of kinetic energy type.
Q. Why is a little sulphuric acid added to water for electrolysis of water?
Answer. Pure water is not a conductor of electricity. On adding a little sulphuric acid in water, the water becomes conductor of electricity and electrolysis of water can be carried out.
Q. Is electrolysis possible with alternating current?
Answer. No, when direct current is passed the electrolyte is decomposed in positive and negative ions which move towards cathode and anode respectively. But in the case of alternating current, the direction of current changing continuously the ions cannot move in a particular direction and thus electrolysis is not possible.
Thursday 24 January 2019
Q. What is Peltier effect?
Answer. If a current be sent through a circuit made of two dissimilar metals, heat is evolved at one of the junctions which thereby gets heated while heat is absorbed at the other junction producing a cooling. The rates of heat generation and absorption are proportional to the current. This phenomenon is known as Peltier effect.
Q. What is neutral temperature?
Answer. The temperature of the hot junction of a thermocouple at which the thermo-emf assumes a maximum value, the other junction being kept cold, is a constant for a given thermocouple and is known as neutral temperate of the couple.
Q. What is Seeback effect?
Answer. When two wires of dissimilar metals are joined together at their ends to form a complete circuit and if the two junctions be maintained at different temperatures, an electric current flows in the circuit. This phenomenon is known as Seeback effect.
Q. What is inversion temperature?
Answer. The temperature of the hot junction at which the emf becomes zero and reverses in direction is called the inversion temperature.
Q. What is the law of intermediate metals?
Answer. The law of intermediate metals states that the thermoemf produced by two different metals having junctions at different temperatures is independent of a third metal introduced in the circuit; provided the junctions formed by this metal are at the same temperature.
Q. What is thermo electric power?
Answer. At a particular temperature, the change in thermoemf of a thermocouple to increase the temperature of its hot junction by one degree is known as the thermo-electric power of the thermocouple.
Q. Why is tungsten not used in a fuse wire?
Answer. In a fuse wire such a metal should be sued where resistance is high but melting point is low, so that when accidentally high current passes through the circuit the fuse wire is very much heated due to its high resistance and easily melts due to its low melting point and thereby breaks the circuit. As the melting point of the tungsten is very high, it is not suitable to be used as a fuse wire?
Q. why is tungsten used in the filament of an electric bulb?
Answer. Tungsten is metal with high melting point. So, tungsten does not melt even when it glows with very high current passing through it. This is why tungsten is used in the filament of an electric bulb.
Q. why is nichrome used in the heating coil of an electric heater?
Answer. The heating coil of an electric hater must be made of a metal whose resistance and melting point are high. As nichrome has both very high resistance and high melting point, so it is used in the heating coil of an electric heater.
Can two likely charged balls be attracted to each other? If yes, explain why?
Answer. Yes, two likely charged balls can be attracted to each other. If the charge on one ball be much greater than that on the other and larger charge be bought suddenly near the smaller one, then the induced opposite charge on the smaller one would be greater than the charge existing on the smaller one. Thus larger charged ball would attract the smaller charged ball containing remaining opposite charge.
Why should circuits containing capacitor be handled cautiously even when there is no current?
Answer. When the circuit containing capacitor is connected to a source of high voltage, the capacitor gets charged to a high potential on removing the source of voltage. The capacitor is not discharged of its own. Thus charged capacitor may cause a severe shock to the person working with it.
The radius of earth is 6400 km. Find its capacitance.
Answer. The capacitance of an isolated spherical conductor of radius r= 
electrons tend to move towards the region of a higher potential. Is it true or false?
Answer. The positive charge moves from higher to lower potential. The electrons have negative charge, so they move from lower to higher potential. Thus the statement is true.
What is atomic polarsiability?
Answer. Atomic polarsiability is the electric dipole moment induced in the atom by an electric field of unit strength.
What is atomic dipole moment?
Answer. Atomic dipole moment is the product of the positive charge on the atom and the distance between the center of positive and negative charges in the direction of the applied electric field.
Wednesday 23 January 2019
Is the electromotive force a real force?
Answer. No, it is the amount of energy converted in a source from other forms of energy to electrical energy when unit charge passes through the source.
Is 'dry cell' really dry?
Answer. A dry cell is not really dry. It is essentially a Leclanche cell using very small amount of water.
Can the emf of a cell be increased by increasing the amount of the active liquid and the size of the electrodes?
Answer. The emf of a cell does not depend upon the amount of the active liquid and the size of the electrodes. So in this way emf cannot be increased.
What happens to the electrons in AC and DC?
Answer. In D.C., there is a steady flow of electrons in one direction. In A.C. the electrons change direction every half a cycle and also there are fluctuations in the speed of the electrons.
In what way does the passage of electric current through electrolyte differ from conduction of current through metal?
Answer.
In metals the charge carriers are streams of free electrons, whereas in electrolytes charge carriers are streams of both positive and negative ions.
In metals negative charge flows from lower to higher potential ends. In electrolytes positive and negative charges flow from higher to lower and lower to higher potential ends respectively constituting the same current.
Drift velocity of electrons in a conductor is very low, even then why do the lights turn so quickly after the switch is on?
Answer. It depends on the speed of setting up of electric field in the conductor which in turn sets electrons to drift, creating current through each cross section of the conductor. The field is set up in a conductor with speed of light. It does not depend upon the drift velocity.
A potential difference is applied to a wire. How is the drift velocity of the electrons affected if (a) applied potential difference be doubled, (b) length of the wire be doubled and (c) diameter of the wire be doubled?
Answer.
(a) Drift velocity is doubled.
(b) Drift velocity is halved
(c) No change in the drift velocity.
Tuesday 22 January 2019
Name a substance in which resistance decreases under pressure.
Graphite and Carbon powder.
Applying this principle micro-phone is made.
TV remote also works on this principle.
How resistance of a conductor depends on radius of conductor?
Answer. Resistance of a conductor is inversely proportional to the square of radius of the conductor.
For example: If radius of two conductors be 3x and 4x then ratio of the resistance of the conductors be 16:9
Monday 21 January 2019
How resistance depends on length of wire?
Resistance is directional proportional to the length of the wire.
If length of the wire increases resistance of the wire will also be increased.
Resistance
What is resistance?
There are two definition of resistance....
Conceptual definition:
The property of a conductor by virtue of which it opposes the flow of electric current.
Definition of resistance from Ohm’s law:
We know that, mathematical form of ohm’s law,
V=RI
R=V/I
The resistance of a conductor is the ratio of potential difference across two ends of a conductor to the current flowing to the conductor.
What is the SI unit of resistance?
Ohm
There are two definition of resistance....
Conceptual definition:
The property of a conductor by virtue of which it opposes the flow of electric current.
Definition of resistance from Ohm’s law:
We know that, mathematical form of ohm’s law,
V=RI
R=V/I
The resistance of a conductor is the ratio of potential difference across two ends of a conductor to the current flowing to the conductor.
What is the SI unit of resistance?
Ohm
Subscribe to:
Posts (Atom)