1. eV
kilograms
UNITS!
Joules
m/s

2. 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.

3. A 2 kg mass moving at 1 m/s has
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

4. To get an idea of how much energy a joule represents,
100 J of energy would…

5. 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

6. 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

7. 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

8. Instead, physicists use another unit called:
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).

9. 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)…

10. 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)…

11. 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.

12. 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.

13. 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.

14. 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
≈ eV = 0.51 MeV
(Note: For 1.0 kg we would obtain
9.0×1016 Joules = 5.6×1035 eV !)

15. 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 (≈ 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 kg of mass.