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# N°3 – Energy quantities training (A level)

Energy allows movement.

The International unit for energy is the joule which corresponds to the raising of a weight of 1 Newton (a weight ≈ 100 grammes) by a height of 1 [m].
Energy, “displacement of a weight (a force) by a height”, is a physical value composed of length, mass and time.
The Joule is the name given to [kg.m2/s2]

All energy can be converted into heat.
If an object whose weight is 1 newton, placed within a perfectly isolated volume of 1 [m3] of air at 20 [°C] (68°F), is gently raised by 1 [m], then dropped, the temperature will rise by 0.00083 [°C]… (0.00083 [K]).
In fact, when everything comes back to rest, the air will have received 1 joule of energy which will be converted into heat.

If an object whose weight is 1 newton, placed within a perfectly isolated volume of 1 [m3] of water at 20 [°C] (68°F), is gently raised by 1[m], and then dropped, the temperature will rise by 0.000,000,24 [°C]… (0.000,000,24 [K]).
In fact when everything comes back to rest the water will have received 1 joule of energy which will be converted into heat.

The joule can thus be used to quantify quantities of heat, but is a unit which is extremely small for our branch of activity. We will therefore tend to use:

• The kilojoule: [kJ] = 1,000 [J] (0.95 Btu)
•  The megajoule: [MJ] = 1,000,000 [J] (948 Btu)

The symbol for heat is the letter: Q