Transferring Electrical Energy
Overview
Somewhere in your home there is a small box with a row of digits on it, and those digits have been climbing since the day the building was wired. Nobody watches them. They tick up a little while the kettle boils, faster while the shower runs, imperceptibly while a phone charger sits plugged in and forgotten. Once every three months somebody writes the number down, takes away the number written down last time, multiplies the difference by a price, and posts a bill. That is the whole of this topic in one sentence, and every step of it is examinable.
What you will build here is the bridge between physics and money. You will meet power as a rate rather than an amount, and see why a machine rated at three kilowatts can cost less to run than a lamp rated at a tenth of that. You will learn the one equation that ties power to the current and the potential difference an appliance works at, and then use it for the job the specification names outright: working out the current an appliance actually draws and choosing a fuse that will protect it without cutting it off in normal use. Finally you will find out why electricity is sold in a unit invented for the purpose, the kilowatt-hour, and how to turn two meter readings and a price into a total.
Objectives
- Electrical appliances are designed to transfer energy. Students should be able to give examples of such devices and identify the energy transfers.
- The rate at which energy is transferred by an appliance is called the power. The relationship between power, P, energy transferred, E, and time, t, is: P = E / t
- The power transfer, P, in any device is related to the current, I, flowing through it and potential difference, V, across it: P = I ×V Students should be able to calculate the current through an appliance from its power and the potential difference of the supply and from this determine the size of fuse needed.
- The relationship between energy transferred, E, potential difference, V, and charge, Q, is: E =V × Q
- The amount of energy an appliance transfers depends on how long the appliance is switched on for and its power rating. It is often more convenient to measure energy transfers in domestic appliances in kWh instead of J due to the small size of the latter.
- The relationship between energy transferred, E, from the mains, power, P, and time, t, is: E(kW h) = P(kW)×t(h) Students will not be required to convert between kilowatt-hours and joules. Students should be able to calculate the cost of mains electricity given the cost per kilowatt-hour and interpret and use electricity meter readings to calculate total cost over a period of time.
Mind map
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Lesson Note
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Lesson Evaluation
Congratulations on completing the lesson on Transferring Electrical Energy. Now that youve explored the key concepts and ideas, its time to put your knowledge to the test. This section offers a variety of practice questions designed to reinforce your understanding and help you gauge your grasp of the material.
You will encounter a mix of question types, including multiple-choice questions, short answer questions, and essay questions. Each question is thoughtfully crafted to assess different aspects of your knowledge and critical thinking skills.
Use this evaluation section as an opportunity to reinforce your understanding of the topic and to identify any areas where you may need additional study. Don't be discouraged by any challenges you encounter; instead, view them as opportunities for growth and improvement.
- Which of these is the unit of power? A. Joule B. Watt C. Coulomb D. Volt Answer: B
- A 60 W lamp is switched on for 5.0 minutes. How much energy does it transfer? A. 12 J B. 300 J C. 3600 J D. 18 000 J Answer: D
- An appliance is marked 230 V, 920 W. Which is the smallest suitable fuse for its plug? A. 3 A B. 5 A C. 13 A D. 30 A Answer: B
- A 2 kW heater is switched on for 3 hours. How much energy does it transfer, in kilowatt-hours? A. 0.67 kWh B. 1.5 kWh C. 6 kWh D. 6000 kWh Answer: C
- Why is domestic electricity measured in kilowatt-hours rather than in joules? A. The joule is not a unit of energy B. A joule is so small that household totals would be enormous numbers C. The kilowatt-hour measures power rather than energy D. Meters are unable to respond quickly enough to count joules Answer: B
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