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eV. kilograms. UNITS!. Joules. m/s. Most physical quantities have units. You are probably already aware of the FUNDAMENTAL units for length, distance and time: meters (m), kilograms (kg), and seconds (s) You are also probably aware of the unit for energy: the joule (J).

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**eV**kilograms UNITS! Joules m/s**Most physical quantities have units.**You are probably already aware of the FUNDAMENTAL units for length, distance and time: meters (m), kilograms (kg), and seconds (s) You are also probably aware of the unit for energy: the joule (J). This unit is derived from the FUNDAMENTAL units for length, distance and time.**To see how this works, consider the formula for the energy**of motion (kinetic energy): kinetic energy = ½mv2 A 2 kg mass moving at 1 m/s has kinetic energy = ½(2 kg)×(1 m/s)2 = 1 kg m2/s2 = 1 joule We say that the unit joule is derived from the units meters, kilograms and seconds: 1 J = 1 kg m2/s2**To get an idea of how much energy a joule represents,**100 J of energy would…**To get an idea of how much energy a joule represents,**100 J of energy would… …raise the temperature of 10 ml of water by about 2.4°C**To get an idea of how much energy a joule represents,**100 J of energy would… …raise the temperature of 10 ml of water by about 2.4°C …be the energy of motion (kinetic energy) of a hockey puck travelling at about 125 km/h**To get an idea of how much energy a joule represents,**100 J of energy would… …raise the temperature of 10 ml of water by about 2.4°C …be the energy of motion (kinetic energy) of a hockey puck travelling at about 125 km/h …be the energy you would get from the complete combustion of about 2 mg of gasoline**Although the joule is fine for measuring the amounts of**energy we encounter in everyday experience, it is too big a unit to measure the energy of tiny particles. Instead, physicists use another unit called: The electron volt (eV).**To get an idea of how much energy an electron volt**represents, if you were to take a 1.5 volt AAA battery out of your calculator (and were clever enough to make a small particle accelerator with it)…**To get an idea of how much energy an electron volt**represents, if you were to take a 1.5 volt AAA battery out of your calculator (and were clever enough to make a small particle accelerator with it)…**To get an idea of how much energy an electron volt**represents, if you were to take a 1.5 volt AAA battery out of your calculator (and were clever enough to make a small particle accelerator with it)… …you could give an electron 1.5 electron volts of energy. With a 9 volt battery, 9 electron volts, and so on.**If you want to convert from joules to electron volts,**you must divide by 1.60×10-19.* So 1 joule of energy is equivalent to a whopping 6.25×1018 eV. Or, to put another way, 1 eV = 1.60×10-19 J. *This conversion factor is the charge of an electron, measured in another unit called the coulomb.**When it comes to mass, the kilogram**is also much too big a unit to use for particles. Physicists often use units of energy (eV) to express mass, since you have seen that mass and energy are related by the formula E = mc2.**To see how this works, we can perform a short calculation:**The mass of the electron is 9.1×10-31 kg. If we put the mass of the electron into the formula E = mc2, we get: E = (9.1×10-31 kg) ×(3×108 m/s)2 = 8.2×10-14 J ≈ 510000 eV = 0.51 MeV (Note: For 1.0 kg we would obtain 9.0×1016 Joules = 5.6×1035 eV !)**Now, if we do the same calculation**for twice the mass of the electron: 2 × (9.1×10-31 kg) = 1.8×10-30 kg (≈ 10-30 kg) The energy of the two electrons would be: 2 × 0.51 MeV = 1.02 MeV (≈ 1 MeV) This gives us a handy approximate conversion factor: 1 MeV of energy for every 10-30 kg of mass.

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