Q30 of 72 Page 12

An electric bulb of 40W is connected to a source of 2 20v. What will be the current drawn and the resistance of the bulb?

In this case we have been given power P and voltage V, so the formula to be used for calculating the current will be: P = V × I Here, Power, P = 40 watts Voltage, V = 220 volts And, Current, I =? (To be calculated) Now, putting these values in the above formula, we get: 40 = 220 × I I = = Thus, Current, I = 0.18 amperes.

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28 Potential difference across the end of a wire is 0.6V which carries a current of 8 amperes. Find the resistance of the wire. 29 In the given circuit, calculate the current shown by the ammeter and total resistance of the circuit. 31 Read the following statements. Write true or false against each:
(i)   The quantity of charge flowing past a point multiplied by the time gives the current.
(ii)  The flow of charge through a conducting wire, connected to a cell, is due to the chemical reaction inside the cell.
(iii) The resistivity of all pure metals increases with rise in temperature.
(iv) Ohm's law is a relation between the power used, the current and the potential difference in a circuit.
(v)  A series circuit has only one conducting path for the electrons that move through it; a parallel circuit has multiple conducting paths.
(vi) A conducting wire offers resistance to the flow of electrons because electrons repel each other in the wire. 32 Explain the following:
(i)  Why is tungsten used almost exclusively as filament for incandescent lamps?
(ii) Why are the conductors of electric heating devices, such as toasters and electric irons, made of an alloy rather than a pure metal?
(iii) Why is the series arrangement not used for domestic circuits?
(iv) How does the resistance of a wire vary with its cross-sectional area?