mass_and_energy - Mass and Energy Michael Fowler,...

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Mass and Energy Michael Fowler , University of Virginia 3/1/2008 Rest Energy The fact that feeding energy into a body increases its mass suggests that the mass m 0 of a body at rest , multiplied by c 2 , can be considered as a quantity of energy. The truth of this is best seen in interactions between elementary particles. For example, there is a particle called a positron which is exactly like an electron except that it has positive charge. If a positron and an electron collide at low speed (so there is very little kinetic energy) they both disappear in a flash of electromagnetic radiation. This can be detected and its energy measured. It turns out to be 2 m 0 c 2 where m 0 is the mass of the electron (and the positron). Thus particles can “vaporize” into pure energy, that is, electromagnetic radiation. The energy m 0 c 2 of a particle at rest is called its “rest energy”. Note, however, that an electron can only be vaporized by meeting with a positron, and there are very few positrons around normally, for obvious reasons-they just don’t get far. (Although occasionally it has been suggested that some galaxies may be antimatter!) Einstein’s Box An amusing “experiment” on the equivalence of mass and energy is the following: consider a closed box with a flashlight at one end and light-absorbing material at the other end. Imagine the box to be far out in space away from gravitational fields or any disturbances. Suppose the light flashes once, the flash travels down the box and is absorbed at the other end. Now it is known from Maxwell’s theory of electromagnetic waves that a flash of light carrying energy E also carries momentum p = E / c . Thus, as the flash leaves the bulb and goes down the tube, the box recoils, like a gun, to conserve overall momentum. Suppose the whole apparatus has mass M and recoils at velocity v . Of course, v << c .
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mass_and_energy - Mass and Energy Michael Fowler,...

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