Ways to pass on energy

The idea

Besides mechanical work, energy is transferred as heat from a hotter to a colder object in contact, by radiation without contact, in electric circuits, via sound waves, and in nuclear reactions.

A hot cup of tea

You hold a hot cup of tea in your cold hands. Your hands slowly feel warm. Nobody did any pushing or pulling.

How do you think energy reached your hands?

• By heat, from the hotter cup to the colder hands
• By mechanical work done by the cup
• It did not move

Five ways

Open each tab.

More than one way

Doing work is one way of passing on energy, but not the only way. Energy also flows as heat, travels as radiation, moves in electric circuits and with sound waves, and is transferred in nuclear reactions.

Besides mechanical work, energy is transferred as heat (from a hotter to a colder object in contact), by radiation without contact, in electric circuits, via sound waves, and in nuclear reactions.

Notes

Energy is transferred by mechanical work, as heat, by radiation, in electric circuits, via sound waves and in nuclear reactions.

Check yourself

Two objects at different temperatures are in contact. Energy flows…

How does the Sun's energy reach the Earth?

In which case is energy transferred without direct contact?

Which statement is correct?

• from the hotter object to the colder object — correct. Yes!
• from the colder object to the hotter object. Energy flows as heat from the hotter object to the colder one.
• in no direction, because the objects are different. Energy flows as heat between objects at different temperatures, from the hotter to the colder one.
• Through mechanical work done by a rope. Nothing joins the Sun to the Earth. The energy moves by radiation.
• Through radiation, without direct contact — correct. Yes!
• Through an electric circuit joining them. Electric circuits are another way energy is transferred, but the Sun's energy reaches the Earth through radiation.
• A hot pan warming the cold pan placed on it. The pans are in contact, so energy flows as heat by contact.
• A hand pushing a trolley. The hand touches the trolley and does mechanical work on it.
• The Sun's energy reaching the Earth — correct. Yes!
• Mechanical work is one way of transferring energy, but there are other ways too — correct. Yes!
• Energy can be transferred only by mechanical work. Energy can also be transferred as heat, by radiation, in electric circuits, by sound waves and in nuclear reactions.
• Mechanical work cannot transfer energy. Doing mechanical work is one way of transferring energy from one object to another.