7.4 Electric LivesApril 13, 2023

You are buying a new TV, washing machine and fridge. What are the most important factors to consider

when choosing these household

appliances?

Aims

• Look at machines that generate electrical energy and how energy is transferred. [Level 5]

• Use scientific concepts to explain the relative drying effects of cold and warm moving air. [Level 6]

• Analyse information from Sankey diagrams to evaluate the efficiency of differing forms of electrical generation. [Level 7]

Key words

• Efficiency rating

• European energy label

• Kilowatt (kW)

• Power

• Sankey diagram

• Watt (W)

Energy labels

• All home appliances have a European Energy Label which shows the efficiency of a machine

• The efficiency rating is at the top of the label

• People were shocked when they first saw these labels… why?

Mains supplyWashing

machine

Water

Drum

Surroundings

Case

QUESTION: why could cutting down on the energy transferred as movement and sound reduce the amount of energy used?

Mains supplyWashing

machine

Water

Drum

Surroundings

Case

QUESTION: would reducing the amount of water used for washing reduce the amount of electricity used?

Sankey diagrams

• Sankey diagrams show that energy transfers take place in a machine

• The width of each arrow represents how much energy is being transferred

• Energy is always conserved so the total width of all the arrows must remain the same

Power

• Energy is the ability to do work. Power is how fast the work is being done.

• The unit of power is the watt (W or the J/s) and the unit of energy is the joule (J)

• One kilowatt is 1000 W

• A traditional lightbulb uses 60 J of electrical energy and converts it to heat and light – its power is 60 W

Questions

• Jo has a hairdrier with two settings, hot and cool. The cool setting has a power of 600 W but the hot used 1200 J per second. If it takes five minutes to dry her hair on hot, how long would it take on cool to be more energy efficient?