What is e equals mc2 exactly?

Answer 1

Introduction.

One of the most extraordinary things

about Einstein's energy-mass equation

is its simplicity. However, we still need to

make sure we are using the correct units

when solving the equation,

and that we understand the answer.

The purpose of this page is to solve

the equation as it is and give some idea of

the huge amount of energy locked up in even the

smallest amount of mass.

The Components of the Equation.If we break the equation E = mc2

into its components and write out the terms fully we get:

• E = energy (measured in joules)
• m = mass (measured in kilograms)
• c = the speed of light (186,000 miles per second, or 3 ´ 108 ms-1)

We will now examine each of the terms in a little more detail.

Energy.

Energy is measured in joules (J).

How much energy is one joule?

Not very much really.

If you pick up a large apple and raise it

above your head you will have

used around one joule of energy in the process.

On the other hand we use up huge

amounts of energy every time we switch on a light.

A 100 watt light bulb uses 100 joules of energy every second,

i.e. one watt is one joule per second.

Mass.

Mass is a measure of a body's resistance to acceleration.

The greater the mass the greater the resistance to acceleration,

as anyone who has ever tried to push a heavy object knows.

For our purposes here, we can also think of mass as the

amount of matter in an object.

Mass is measured in kilograms (kg), 1 kg is

about the same as 2.2 pounds.

Note that we haven't said

what the mass is composed of.

In fact it could be anything. It doesn't matter

if we use iron, plastic, wood, rock or gravy.

The equation tells us that whatever the

mass is it can be turned

into energy

(whether it is practical to actually

do so is another matter and will be dealt with in later pages in this series).

The speed of light.

The speed of light is close to 186,300 miles per second

(300,000 km per second). In order to make the equation

"work" we need to convert these numbers into

units that are more suited to our purposes.

In physics, speeds are measured in metres

per second. This is usually abbreviated to ms-1;

that is "metres times seconds to the minus one".

Don't worry if you don't understand this notation.

We could equally write m/s but using ms-1

makes the mathematics easier in the long run. Likewise,

we could either say that the speed of light is

300,000,000 metres per second, or, as is

more usual, express the same figure in scientific notation: 3 x 108 ms-1.

Solving the Basic Equation.Now that we have everything in order

let's have a go at solving the equation.

We will use a mass of

1kg to keep things simple and I will show all of the workings of the equation.

So, with 1kg of mass (around 2.2 pounds) we get:

Note how the units were dealt with and that

kg m2 s-2 is the same as joules

(a rigorous proof of this is outside the scope of these pages).

So from 1kg of matter, any matter,

we can get out 9 x 1016 joules of energy. Writing that out fully we get:

That is a lot of energy! For example,

if we converted 1 kg of mass into

energy and used it all to power a 100

watt light bulb how long could we keep it lit for?

The first thing to do is divide the result by watts

(remember that 1 watt is 1 joule per second):

How long is that in years?

A year (365.25 days) is 31,557,600 seconds,

so we get:

That is a very long time!

Of course,

converting mass into energy

is not quite that simple, and apart

from with some tiny particles in

experimental situations has never

been carried out with 100% efficiency. Perhaps that's just as well.

Conclusion.We have seen that the equation is very easy to solve

as it is and that for even a small amount of

mass a huge amount of energy can, at least in theory,

be released. Other pages in this series show how the

energy can be released in practical ways

as well as using the equation in a more mathematical and physical way.

E = mc2

A huge amount of energy from a small amount of mass.