After
reading this section you will be able to do
the following:
Define
EMF and explain how it is measured.
Explain
why EMF is important to the flow of electrical current.
List several
examples of sources of electromotive force.
We also need to know something about
the force that causes the electrons to move in an electrical circuit.
This force is called electromotive force, or EMF.
Sometimes it is convenient to think of EMF as electrical pressure.
In other words, it is the force that makes electrons move in a
certain direction within a conductor.
But how do we create this “electrical
pressure” to generate electron flow? There are many sources
of EMF. Some of the more common ones are: batteries, generators,
and photovoltaic cells, just to name a few.
Batteries are constructed so there
are too many electrons in one material and not enough in another
material. The electrons want to balance the electrostatic charge
by moving from the material with the excess electrons to the material
with the shortage of electrons. However, they cannot because there
is no conductive path for them to travel. However, if these two
unbalanced materials within the battery are connected together
with a conductor, electrical current will flow as the electron
moves from the negatively charged area to the positively charged
area. When you use a battery, you are allowing electrons to flow
from one end of the battery through a conductor and something
like a light bulb to the other end of the battery. The battery
will work until there is a balance of electrons at both ends of
the battery. Caution: you should never connect a conductor to
the two ends of a battery without making the electrons pass through
something like a light bulb which slows the flow of currents.
If the electrons are allowed to flow too fast the conductor will
become very hot, and it and the battery may be damaged.
We will discuss how electrical generators
use magnetism to create EMF in a coming section. Photovoltaic
cells turn light energy from sources like the sun into energy.
To understand the photovoltaic process you need to know about
semiconductors so we will not cover them in this material.
Take this link to learn more about
the volt: What
is a volt?
How
does the amp and the volt work together in electricity?
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To understand how voltage and amperage
are related, it is sometimes useful to make an analogy with water.
Look at the picture here of water flowing in a garden hose. Think
of electricity flowing in a wire in the same way as the water
flowing in the hose. The voltage causing the electrical current
to flow in the wire can be considered the water pressure at the
faucet, which causes the water to flow. If we were to increase
the pressure at the hydrant, more water would flow in the hose.
Similarly, if we increase electrical pressure or voltage, more
electrons would flow in the wire.
Does it also make sense that if we
were to remove the pressure from the hydrant by turning it off,
the water would stop flowing? The same is true with an electrical
circuit. If we remove the voltage source, or EMF, no current will
flow in the wires.
Another way of saying this is: without
EMF, there will be no current. Also, we could say that the free
electrons of the atoms move in random directions unless they are
pushed or pulled in one direction by an outside force, which we
call electromotive force, or EMF.
Review
EMF
is electromotive force. EMF causes the electrons to move in
a particular direction.
