An electric bulb rated for 500 W at 100 V is used in a circuit having a 200 V supply. The resistance R that must be put in series with the bulb, so that the bulb draws 500 W, is :
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A wire, when connected to a 220 V mains supply, has power dissipation . Now, the wire is cut into two equal pieces, which are connected in parallel to the same supply. Power dissipation in this case is . Then is :
.
R is reduced by a factor of 4.
So, P is increased by a factor of 4.
The resistance of hot tungsten filament is about 10 times the cold resistance. What will be the resistance of 100 W and 200 V lamp when not in use?
When the filament is hot,
Hence, when cold, the resistance is 40 .
Two electric bulbs A and B are rated 60 W and 100 W, respectively. If they are connected in parallel to the same source, then,
Given . We know that current flowing through the bulb is I = P/V. We also know that as both the bulbs are connected in parallel, potential difference (V) across both the bulbs is the same. Thus, . Since the power of bulb B is greater than that of bulb A, bulb B draws more current than bulb A.
A factory is served by a 220 V supply line. In a circuit protected by a fuse marked 10 A, the maximum number of 100 W lamps in parallel that can be turned on is :
The current required by each bulb is :
If n bulbs are joined in parallel, then,
.
A 100 W bulb designed to operate on 100 V is to be connected across a 500 V source. Find the resistance to be put in series so that bulb consumes 100 W only.
A current wire is hidden in a wall. Its position can be located with the help of a
Magnetic needle deflect in the presence of a magnetic field produced by the wire.
The resistance of an ideal voltmeter is
The resistance of an ideal voltmeter is considered as infinite.
The net resistance of a voltmeter should be large to ensure that :
The resistance of voltmeter is too high, so that it draws negligible current from the circuit, hence potential drop in the external circuit is also negligible.
A voltmeter of resistance 1000 Ω gives full-scale deflection when a current of 100 mA flow through it. The shunt resistance required across it to enable it to be used as an ammeter reading 1 A at full-scale deflection is :
By using
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