Short note SSC JE part 2

Q. 1. Why are domestic appliances connected in parallel?

Ans. The domestic appliances are connected in parallel due to the following reasons :
(i) The domestic appliances are rated for the same voltage (i.e., 230 V) but with different powers. When connected in parallel, the performance of each appliance becomes independent of the other. Thus a 230 V, 250 W TV receiver can be operated independently in parallel with a 230 V, 100 W lamp.
(ii) If a break occurs in any one of the branch circuits, it will have no effect on the other branch circuits.

Q. 2. Why does a cell possess internal resistance?

Ans. When current flows through a cell, it meets opposition from the electrodes and the electrolyte. This opposition to currently offered by the cell is called its internal resistance.

Q. 3. Why is Wheatstone bridge method preferred to a voltmeter-ammeter method for measuring resistance?

Ans. The Wheatstone bridge method is preferred to a voltmeter-ammeter method for measuring resistance due to the following reasons:
(i) The Wheatstone bridge method is independent of the fluctuations and variations in the supply voltage.
(ii) It removes the objectionable feature of the voltmeter-ammeter method where the accuracy of measurement is limited by the accuracy of the calibration of instruments.

Q. 4. Why is the Wheatstone circuit called a bridge?

Ans. The circuit is called a bridge because the galvanometer bridges the opposite junctions of the circuit.

Q. 5. What is the meaning of algebraic sum?

Ans. The algebraic sum is one in which the sign of the quantity is taken into account. For example, if a current flowing towards point O is assigned a ‘positive’ sign, then the current flowing away from point O will be assigned a negative sign. Similarly, if a rise in voltage is assigned a positive sign, then a fall in voltage will be assigned a negative sign.

Q. 6. Which type of grouping of cells will be suitable to obtain higher current and voltage?

Ans. Series-parallel grouping of cells. To produce a large voltage, a number of cells are connected in series, and to produce a large current, a number of sets of such series-connected cells are connected in parallel. Such a connection is called series-parallel grouping.

Q. 7. When we require a d.c. supply, we look for a dc. source other than a batten. Why?

Ans. Because batteries are costly and require frequent replacement.

Q.8 what is the objection to having lamps in a house lighting circuit connected in series?

Ans. The lamps in a house-lighting circuit are not connected in series due to the following reasons :
(i) If a break occurs in any part of the circuit, no current will flow and the entire circuit becomes useless.
(ii) A high supply voltage is required if the lamps (or other devices) are to be connected in series. For example, if five 230 V lamps are to be connected in series, then the supply voltage would have to be 5 x 230 = 1150 V. Therefore, the series connection is not practicable for lighting circuits.
(iii) For efficient operation, only those lamps or devices should be connected in series that have the same current rating. However, electrical devices (e.g., heater, toaster, lamp, etc.) have different current ratings. Obviously, they cannot be connected in series for efficient operation.

Q. 9. Can you measure the e.m.f of a cell with a voltmeter?

Ans. Not accurately. It is because when the voltmeter is connected across the cell, current starts flowing through it. This causes a voltage drop across the internal resistance of the cell. Consequently, the voltmeter will not indicate the e.m.f. of the cell. Remember e.m.f. of a cell is the voltage across the open-circuit terminals of the cell i.e., voltage across the terminals of the cell when it carries no current.

Q. 10. When the resistance connected in series with a cell is halved, the current is not exactly doubled but is slightly less. Why?

Ans. This is owing to the internal resistance r of the cell. The current delivered by the cell is
I =E/R+ r
When external resistance R is made R/2, the current will be slightly less than 2 I.

Q.11. Why is it easier to start a car engine on a warm day than on a chilly day?

Ans. It is because the internal resistance of a car battery on a chilly day is much more than on a warm day.

Q. 12. When resistances are connected in parallel, the total resistance decreases. Why?

Ans. When resistances are connected in parallel, the effective area of the cross-section of the conductor increases. For this reason, the total resistance decreases (R ac 1/A).

Q. 13. It is a general belief that a person touching a high-power line gets stuck to the line. Is it true?

Ans. This is not true. The fact is that a current of even a few mA is enough to make the nervous system ineffective for a moment. As a result, the affected person has difficulty removing his hand off the ‘live wire. We think that the person has got stuck to the line.

Q. 14. Why does the glow of lamps become weaker when a heavy current appliance is switched on in the house?

Ans. Domestic appliances are connected in parallel across the supply. When the heavy current appliance is switched, the total resistance of the system in the room is decreased as the appliance is connected in parallel with other loads. This increases the line current. This in turn causes a greater voltage drop in the line. Therefore, the voltage available across the lamps decreases and so is their glow.

Q. 15. Is current a scalar or vector?

Ans. Current is a scalar. It is because currents at a junction in an electric circuit do not obey the laws of vector algebra.

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