My buddy Francesca is a real environmental activist. She often says:

"We need to conserve energy!"

But wait, isn't energy always conserved? I swear I learned that in school.....

Elderly person confused at a laptop:

The language of physics: what does conservation mean?

In physics, if something is conserved it means that it's not changed in quantity.

The total amount of it stays the same, but it could be a different type.

For example: a footballer kicks a ball which then stops.

The chemical energy of the footballer is transferred to the ball as kinetic energy; then to the air and ground as thermal and sound energy.

You could explore the Phet simulation here to see how the total energy of a skater remains the same even though it can change between kinetic, gravitational potential, and thermal.

Screenshot from the Phet simulation showing skater on a ramp and the types of energy involved


In a closed system (e.g. a well-insulated house), energy cannot be...

So what's the point of conserving energy?

When people talk about the need to conserve energy, what they mean is that we need to transfer less energy from natural resources to electricity or heat. We can do this by:

This helps with reducing the amount of greenhouse gases that are emitted into the atmosphere. Flaticon Icon

Ok, that was conservation of energy. What next?

Every machine transfers energy from one type to another. For example:

  • your smartphone transfers chemical energy from its battery to sound and light

  • a petrol engine transfers the chemical energy stored in the fuel to the kinetic energy of the car

  • a washing machine transfers electrical energy to kinetic

  • a wind turbine generator transfers the kinetic energy of wind to electrical energy

BUT some of the energy doesn't go to where we want it to.....!

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You may have noticed that your smartphone gets hot when charging and when you use it a lot. That means that some of the electrical energy is wasted: it's transferred to thermal (heat) energy.

Because the total energy is conserved, the energy your phone can transfer to light and sound (allowing you to watch TikTok) is less than what it gets from the battery.

This is bad news for both you and planet Earth, as Bill Nye the Science Guy confirms.

Bill Nye saying

The language of physics: what does efficiency mean?

In physics, the efficiency of a machine is the amount of useful energy transferred divided by the total input energy.

Equation: efficiency = useful energy output divided by energy input times 100%

On average, petrol engines in cars are about 30% efficient , meaning that only one-third of the chemical energy from the fuel is transferred usefully to make the car move. In other words: if you paid $100 for fuel, $70 are wasted. Flaticon Icon

On the other hand, electric cars are about 80% efficient, meaning that most of the energy supplied by electricity is actually transferred as useful movement and not wasted as heat.

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A Galaxy smartphone charger has efficiency of about 80%. Which of these is FALSE?

So what's the link between energy conservation and energy efficiency?

A machine that has a high efficiency helps to conserve energy.

This is because it transfers most of the energy it receives into the type of energy we want.

Infographic showing that for the same 800 lumens brightness, an LED uses a lot less energy per second than an incandescent.

For example: a traditional incandescent bulb is only 10% efficient, a CFL (compact fluorescent bulb) is 85% efficient, while a modern LED (light emitting diode) is 90% efficient.

So, to get the same amount of light, the LED needs 80% less electrical energy! In other words, it conserves energy because it wastes a lot less.

actor holding a glass with writing

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This Byte has been authored by


Cecilia Astolfi

Learning specialist