Topic Menu

Cyber-chess a chess instruction site
Questions

The cost of electricity - Higher Level Questions

Q1. Sally loves playing her CD Player. She has it switched on (sometimes very loudly!) all of the time she is in the house. Her sister, Lucy, gets very annoyed at this and appeals to her mother’s sense of thrift by accusing her of costing the family a lot of money in electricity bills. Lucy suggests that Sally should only be allowed to play her CD Player for one hour a day (to save money).

Sally argues that her sister’s blow-dry hairstyle costs too much and she should get a simpler style!

Data from Appliances

Hairdrier

1600W;230V;50Hz

CD-Player

14W; 230V; 50Hz

  1. What unit does the electricity board measure electrical energy in?What is the equation linking this unit to time and power of appliance?How many Units does it take for Sally to play her CD player for one hour?How many Units does it take for Lucy to have her hair-drier on for an hour?If they were both allowed to use 1 Unit of electricity a week, how many hours of use would each girl have?
  2. If a Unit of electricity costs 21.52p how much would it cost per week for
    1. Lucy to dry her hair 10 minutes each day.
    2. Sally to play her CDs for 10 hours a day

     

Q2. Lucy is moving into her own place in September. She will be sharing a house in the city with four other girls. The house is in a rather dimly lit side street and the girls want to leave the porch light on during the hours of darkness for security reasons. They would then be able to come and go as they pleased.

However, they are very concerned about having to pay the electricity bill.

Power of bulb in the porch light: 100W

Cost of having a motion sensor installed: £55-00

A Unit of electricity costs 21.52p

In summer there are less hours of darkness than in the winter but the average number of hours of darkness over a year long period is 12h each night

There are 365 days in a year and 366 in a leap year making an average of 365.25 days a year.

 

  1. Using the information above, and your knowledge of electricity costing, find out how much it would cost for the girls to have the porch light automatically switch on at dusk and off at dawn for a year.
  2. How much would that cost Lucy each month?
  3. Would you advise the girls to leave the porch light on all of the time or should they invest in a motion sensor device to switch the light on just when someone approaches it?

ANSWERS