Question

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

1. Which appliance has the greatest electrical resistance? How does the data show this?
2. 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.
3. Calculate the power rating of the kettle when it is operated from the 240 V mains supply.
4. 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?

Answer 1

(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:

1. The kettle draws a large amount of current. So the wire may melt down due to heating.
2. 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`