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प्रश्न
The table below shows the current in three different electrical appliances when connected to the 240 V mains supply:
| Appliance | Current |
| Kettle | 8.5 A |
| Lamp | 0.4 A |
| toaster | 4.8 A |
- Which appliance has the greatest electrical resistance? How does the data show this?
- The lamp is connected to the main supply by using a thin, twin-cored cable consisting of live and natural wires. State two reasons why this cable should not be used for connecting the kettle to the mains supply.
- Calculate the power rating of the kettle when it is operated from the 240 V mains supply.
- A main takes the kettle abroad where the mains supply is 120 V. What is the current is the kettle when it is operated from the 120 V supply?
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उत्तर
(a) The lamp has the greatest electrical resistance because the current drawn by it is the least.
`R=V/I`
and hence resistance varies inversely with the current.
(b) The reasons for this are as follows:
- The kettle draws a large amount of current. So the wire may melt down due to heating.
- For the device to be safe, it needs to be have an earthing system. This is missing here.
(c) The power, P will be:
Given - V = 240 V, I = 8.5 A
P = VI
P = 240 V × 8.5 A
P = 2040 W
(d) V1 = 240, V2 = 120 V, I1 = 8.5, I2 = ?
V = IR
`therefore "V"_1/"V"_2 = "I"_1/"I"_2`
`therefore 240/120 = 8.5/"I"_2`
`therefore 2 = 8.5/"I"_2`
`therefore "I"_2 = 8.5/2`
`therefore "I"_2 = 4.25`
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