Power: How Fast Energy Flows

Lesson 7 of 10 · 7 min · Free, no account needed

What you will learn

Power is how fast energy is moved: P = W ÷ t, energy in joules divided by time in seconds.
Its unit is the watt: 1 W = 1 J every second. A 100 W charger moves 100 J each second.
The same job done in half the time needs twice the power. Time = energy ÷ power.

A 20 W charger and a 100 W charger fill the same battery. What exactly is different?
The 20 W one takes 50 minutes. Drag the marker to your guess for 100 W.
Two runners, same weight, same stairs. Press Play.
Watch the work bars and the stopwatches.
Pick a charger that fills this 60 kJ battery in 20 minutes.
Turn the dial, watch the clock, then Check.
How fast energy flows is called power.
Power is energy moved per second: P = W ÷ t. W is the work or energy in joules, t the time in seconds.
The unit is the watt: 1 W = 1 J/s. 1 kW = 1000 W.
Turn it around and you get the charging rule: t = W ÷ P. Same energy, more power, less time.
Put each device on the power scale.
Game: Human generator. Keep five 20 W lamps lit for 10 seconds.
Every tap of Pedal is 50 J. Stay in the green band.
Code: build a function that works out charging time in minutes, then call it for 20 W and 100 W.
Tap the lines in order. The battery holds 60000 J.
Crane A lifts 1000 J in 2 s. Crane B lifts 3000 J in 10 s. Which is more powerful?
Tap a crane, then Check.
What could your watts run? Tap your bike.

Lessons are interactive and need JavaScript. Turn it on to play them.