E=mc^2

呢條公式其實係愛恩斯坦相對論入面最出名的一條公式。
簡單黎講相對論入面話質量(mass)同能量(energy)其實係同一樣野，而佢地的關系就係E=mc^2。即係話能量可以轉成質量，而質量可以變成能量。而我地用緊的核電技術，都係根據呢個理論而得出既。
係核電入面大家都知係將鈾235的原子核，將佢分裂成兩個較細的兩個原子核，從而得出能量。不過點解會有能量，就要用呢條公式。因為原來兩個分裂出黎的原子核的總質量係細過原本的鈾235，而少左的mass就變成左能量。
而係老師教heat既時候因為你地未有能量及質量守衡(conservation of energy and mass)的概念，所以就簡單咁話佢冇晒能量。不過佢所指的只是物質的動能及內能，並不包括質量。
希望幫到你...

2008-02-22 21:15:10 補充：
而質量同能量的分別只係佢地是唔同的單位，就好似長度我地有"米"同"寸"一樣，容量有"升"同"加淪"，而能量就係"公斤(kg)"同"焦耳(J)"。

太深了

他們是不同的energy
By Albert, Mass = Energy.
They are the same in related by the equation E=mc^2.
That is 1 kg mass is equal to 9*10^16J Energy.
What does it mean? If 1kg mass disappear, 9*10^16J energy will be produced.
The principle of Nuclear energy is according th this equation.
In Nuclear Fission, one big nucleus will split into two smaller nucleus. However, the total mass of the two daughter nucleus is smaller than the mass of the parent nucleus. Where have the mass gone? The mass have changed into energy by the famous equation. This energy is called rest energy.
For a substance at 0K(absolute zero), at 200K or even at 10000K, it's mass will not change. Therefore its rest energy is constant.

Your teacher said at 0K the substance has no energy. It is also correct.
A substance contains millions of millions of millions of particles. However, the particles are not at rest. In gas and liquid, particles are moving randomly. In solid, particles are not moving but they are fixed at a certain position. However, they are not at rest. They are vibrating. Therefore, no matter what the state of the substance is, the particles inside do have some KE. The total KE of all the particles in the substance is the energy told by your teacher. However, when the substance is at 0K, (it should be a solid), the particles will stop vibrating. They have no KE so the substance has no energy. But as the substance still has MASS, its rest energy is not zero.
NOTE the internal Energy of a substance is the sum of the KE and PE of all the particles. PE is zero for ideal gas and minimum(negative value) for solid.

NOTE 2: the equation stated by the above one should be
E = m*c*dT while
E = change in energy
m = mass
c = specific heat capacity
dt = change in temperature

I found some information in Wikipedia encylopedia. Hope this help.
http://en.wikipedia.org/wiki/E=mc%C2%B2