levo

Energy – Forces Doing Work

The watt, and calculating efficiency

  • 3 things to know
  • Paper 2

Overview

One watt as one joule per second, energy from power and time, and efficiency from useful and total energy or power, as a decimal or a percentage.

What you need to know

Answer each one before you reveal it.

Your recallNot started
Key term

State the energy transfer represented by a power of one watt.

Key term

Express the watt in terms of joules and seconds.

Numbers

Calculate the energy transferred by a 60 W bulb left on for 30 s.

Questions from this subtopic

Each one with the model answer underneath.

  1. 1

    State the energy transfer represented by a power of one watt.

    Model answer

    One joule transferred each second.

  2. 2

    Express the watt in terms of joules and seconds.

    Model answer

    One watt is one joule per second, J/s.

  3. 3

    A lamp is marked 12 W. State the energy it transfers in one second.

    Model answer

    12 J.

  4. 4

    Calculate the energy transferred by a 60 W bulb left on for 30 s.

    Model answer

    Energy = 60 × 30 = 1800 J.

  5. 5

    A machine transfers 250 J of energy each second. State its power.

    Model answer

    250 W.

  6. 6

    A motor does 4500 J of useful work while 6000 J is supplied to it. Calculate its efficiency.

    Model answer

    Efficiency = 4500 ÷ 6000 = 0.75.

Other ways to work this

The same content, four other ways.

What is inside Levo

Six ways to work the same material.

1Closed book
2Open book
3Closed book

The 3-step method, run for you

Answer cold, learn it, then prove it stuck. Levo tracks the jump between your first attempt and your last.

Key Concepts of Physics
Motion and Forces
Conservation of Energy
Waves

Cram a whole paper in one sitting

Sort every topic into what you know and what you do not, then drill only the weak ones.

2.5 × = 9000 s.
360027001000

Blank Blitz makes facts automatic

Rapid timed gap-fills on the terms and numbers you need to recall without thinking.

ALength is measured in metres, with the symbol m.
BMass is measured in kilograms, symbol kg.
CTime is measured in seconds, symbol s.

Build It gets the exam shape right

Assemble a full-mark answer from its marking points, in the order an examiner wants them.

Watt means the unit of power, symbol W.
TrueFalse

Test It catches misconceptions

Spot the fault in a statement. The traps are the mistakes students actually make.

721941163128510

Brain Breaks between sessions

Short focus games to reset attention, so the next set lands better than the last.

Every question in Energy – Forces Doing Work, on a schedule.

Attempt a question from memory, write the correct version, then attempt it again. Levo records the jump between your first attempt and your last, and brings each question back on the day you are due to forget it.

Or £46 a year, about £3.83 a month. Cancel anytime.