SAMPLE PROBLEM 17B How many units of electricity are used by a 2 500 W dishwasher during its two-hour cleaning cycle?
SAMPLE ANSWER 17B
Energy power time 2.5 kW 2 h 5 kW h. 5 kilowatt-hours of electrical energy are used.
EXERCISE 17.1 JOULE’S LAW AND KILOWATT-HOURS
Q1 How much heat is produced in a 5 resistor by a constant current of 6 A flowing for two minutes? Q2 A heating coil gives out 3.6 kJ of heat when a current of 5 A flows for three minutes.What is the resistance of the coil?
Q3 80 g of water is heated from 15.5 to 24.8 using a heating coil.A steady current of 1.5 A was allowed to flow for 150 s. Calculate the resistance of the coil. (Specific heat capacity of water 4 180 J kg1 K1.) Hint: Q mc.
Q4 What is the resistance of the 60 W filament of a light bulb when it is connected to a 230 V power supply? Hint: I
P V.
Q5 How many units of electricity are saved by replacing six 100 W bulbs with six 18 W energy-efficient bulbs over a period of 200 hours of usage?
Q6 An electric toaster uses 900 W during the 80 s it takes to make toast. If it is used every day, how many units of electricity does it use over the course of a year?
‘SECTION A’ EXPERIMENT QUESTION
Q1 In an experiment to verify Joule’s law, a heating coil was placed in a fixed mass of water. A current I was allowed to flow through the coil for a fixed length of time and the rise in temperature was recorded.This was repeated for different values of I.
The table shows the data recorded. Table 17.2
I /A / C
1.0 1.2
1.8 4.5
2.6 8.5
3.4 14.0 4.2 20.3 5 29.4 5.8 40.0 (i) Describe, with the aid of a labelled diagram, how the apparatus was arranged in this experiment.
(ii) Using the given data, draw a suitable graph on graph paper and explain how your graph verifies Joule’s law.
(iii) Why was a fixed mass of water used throughout the experiment? (iv) Apart from using insulation, give one other way of reducing heat losses in the experiment.
