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Physics · 04 / 12

Electrical power and energy

Middle school · Worked example and 4 exercises with solutions, at your pace.

Need a foundation?Units: numbers have dimensionsEquations: keep the balance level
UnderstandSee an exampleGuided exercise3 progressive challenges

The principle, simply

Power is energy transfer per unit time. In direct current P=UI, using watts, volts and amperes. Constant power gives E=Pt. Watts × seconds gives joules; kilowatts × hours gives kWh. One kWh is 3.6 million joules.

P=U*I
THE PICTURE TO REMEMBERElectrical power and energy
  1. 01Voltage and current
  2. 02Power
  3. 03Compatible duration
  4. 04Transferred energy

WORKED EXAMPLE

From principle to reasoning

100 W for 2 h uses 0.2 kWh or 720,000 J. A resistor at 12 V and 0.5 A dissipates 6 W.

Solve a problem in this subject

  1. Draw the system

    Define the studied object, reference frame and positive axis. A force diagram shows external interactions acting on the system.

  2. Sort quantities

    Separate data and unknowns; convert units before substitution. 36 km/h = 10 m/s, so never insert 36 into a formula expecting SI speed.

  3. Write the model

    Choose a law matching assumptions: constant acceleration, thin lens, ohmic resistor. Isolate the unknown symbolically before rounding.

  4. Interpret

    Check dimensions and signs. Negative velocity gives direction on the chosen axis. Discuss scale and model limitations.

YOUR TURN

01 · Try before checking

A 20 W lamp runs for 3 h. Find energy in Wh.

One hint

Use watts and hours.

Reveal and understand the solution

60 Wh = 0.06 kWh = 216,000 J.

Compare method, conditions and answer; identify the exact part to revisit.

IMPROVE THROUGH PRACTICE

Three additional challenges with solutions

Start with the first challenge. Write your method before opening a hint; use the solution to find the first step that needs work.

Reflect after practising
02 · Recognise and understandLink to this exercise

A device receives 12 V and 2 A. Find electrical power.

A hint to get started

P = UI.

Step-by-step solution
  1. P = 12 × 2.
  2. P = 24 W.
  3. A watt measures energy per unit time.

Which step did you miss? Retry without help, then explain why the method works.

03 · Apply the methodLink to this exercise

A 20 W lamp runs for 3 h. Find energy in Wh.

A hint to get started

For Wh, use watts and hours.

Step-by-step solution
  1. E = Pt = 20 × 3.
  2. E = 60 Wh = 0.06 kWh.
  3. In joules this is 60 × 3,600 = 216,000 J.

Which step did you miss? Retry without help, then explain why the method works.

04 · Go furtherLink to this exercise

A converter receives 100 J and provides 80 J useful energy. Find efficiency.

A hint to get started

Compare useful energy to input energy.

Step-by-step solution
  1. η = useful/input = 80/100.
  2. η = 0.8, or 80%.
  3. The other 20 J are transferred in other forms, not destroyed.

Which step did you miss? Retry without help, then explain why the method works.

The trap to avoid

Power differs from energy; kW and kWh are not interchangeable.

Without notes, can you…

  • Explain the principle in your own words?
  • Rebuild the diagram and its connections?
  • Solve the exercise and justify the method?

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