Voltage is analogous to air pressure. Air molecules repel each other. The
reason why isn't important, but the fact is that if you squeeze a volume of
air into a smaller space, the air molecules will exert a greater pressure on
each other and the inner walls of the container. It's like filling a tire.
If you force air into a tire, you will have some air pressure inside the
tire. Force more air in, and you will get more pressure. Force enough air
into the tire, and you will get enough pressure exerted on the tire walls to
support the car. Once the tire is inflated, if you open the filler valve,
the high-pressure air inside the tire will escape to the atmosphere where
the pressure is lower.
There is an electronic component called a
capacitor. We will discuss capacitors later. However, a capacitor acts much
like a tire. We can put electricity into a capacitor and get electrical
pressure. We call electrical pressure voltage. Put in more electricity, and
we will get more voltage. We can store electricity under pressure (voltage)
in a capacitor just like we can store air under pressure in a tire.
Another way to develop air pressure is to block airflow in a hose. For
example, if we open the valve of the air hose used to fill our tire, the air
will escape, and there will be little or no pressure in the hose. However,
if we block that flow, we will get significant pressure in the hose (like
putting your thumb over a garden hose). Likewise, if you block the flow of
electricity in a wire, you will get voltage in the wire.
Voltage is always a differential
Whenever we measure pressure, we are always comparing one pressure to
another. Voltage is no different. Let’s look at the tire again. Let’s say
you are at the beach. The location is important because it puts you at sea
level. Now let’s say you put a pressure gauge on the tire and see that it
contains a pressure of 30 pounds per square inch (30 psi). The question is,
what is the actual pressure inside the tire? Isn’t it 30 psi? Actually, no.
It’s about 45 psi. How can that be? Why does the gauge say 30 psi if the
actual pressure is 45 psi? The reason is that the air around you already has
a pressure of about 15 psi (Actually about 14.7 psi, but let’s keep the math
simple). The gauge is actually comparing the pressure inside the tire to the
pressure outside the tire and telling you the difference. If you drive your
car to an altitude of 6,000 feet above sea level, the ambient pressure is
about 12 psi. Assuming nothing changed except the altitude, your gauge will
now read about 33 psi. In Tibet, there are two roads that go higher than
18,000 feet above sea level. At this altitude, where the ambient air
pressure is about 7.5 psi your gauge will read about 37.5 psi. Finally, if
you could get your tire into outer space, where the ambient pressure is
zero, you gauge will finally read the actual pressure in your tire, which is
Measuring voltage is like measuring air pressure
Voltage, being a measurement of pressure, works the same way. This is why a
voltmeter has two probes. The red probe will normally go to the higher
voltage and the black probe will normally go to the lower voltage. The meter
will then tell the voltage difference between the two probes. One voltage is
meaningless unless it is compared to another voltage.
A barometer tells us the difference between two air pressures
If pressure is always a differential, where do we get the 15 psi for the
ambient air pressure? In 1643 Evangelista Torricelli wanted to find out why
suction pumps could not pump water higher than about 34 feet. He made a
glass tube with an inside diameter that equaled one square inch. The tube
was about three feet long and sealed at one end. He filled the tube with
mercury and upended it into a bowl of mercury. Mercury flowed out of the tube into the bowl as you would expect,
but stopped when the column of mercury in the tube fell to about 30 inches
barometer. At sea level, the ambient air pressure will push the
mercury about 30 inches up the evacuated tube.
The reason the mercury did not fall below 30 inches was that the air was
pushing down on the surface of the mercury in the bowl. This pressure forced
the mercury up the tube as there was no pressure at the top of the tube to
resist it. No air had been introduced into the tube, so the space above the
column of mercury contained a vacuum. Torricelli then weighed the mercury in
the tube. It weighed about 14.7 pounds. Therefore, the pressure pushing the mercury up the tube was equal to about 14.7 pounds per
The height of the mercury in the tube is the same regardless of the size or
shape of the tube. Putting a ruler next to the tube gives you the ambient
air pressure measured in the convenient units of inches of mercury. This is
not a joke. Air pressure may be stated in pounds per square inch, newtons
per square meter (pascals) or bars, but in the U.S. is commonly stated in
inches of mercury (inHg). Other countries may use millimeters of mercury
(mmHg). At sea level, the standard ambient air pressure is 29.92 inHg.
Torricelli’s barometer measures ambient air pressure by comparing it to a
vacuum. We get one pressure compared to another pressure where that other
pressure is absolute zero.
A voltmeter tells us the difference between two voltages
Voltage, being electrical pressure, is measured much like air pressure. A
voltmeter has two electrical probes. When you use a voltmeter, it is telling
you the difference in voltage between those probes. It tells you how much
higher or lower the voltage at the red probe is compared to the voltage at
the black probe.
AA typical voltmeter with its black
and red probes.
If a voltmeter reads 8.2 volts, it is telling you that the voltage at the red
probe is 8.2 volts higher than the voltage at the black probe. If a
voltmeter reads -12.6 volts, it is telling you that the voltage at the red
probe is 12.6 volts lower than the voltage at the black probe. This, of
course, assumes the standard conventional-current model (discussed later)
where positive is greater than negative.
How do you find zero volts?
What is the actual voltage at the black probe? There is no way of knowing.
All a voltmeter can tell you is the difference between two voltages. That’s
it. There is no “actual” voltage at the black probe to measure. All you can
do is tell that some voltage is higher or lower than the voltage at the
black probe. An absolute zero voltage has never been defined so there is no
electrical equivalent of a Torricelli’s barometer. No device can give you an absolute
voltage reading. We will learn what zero volts means in electronics later when we discuss how to measure voltage.
Göethe's barometer acts more like a voltmeter than Torricelli’s barometer
The renowned German writer and amateur scientist, Johann Wolfgang Von
Göethe, developed a water-based barometer that
tells what the ambient air pressure is compared to an arbitrary air pressure
(rather than an absolute pressure like Torricelli's barometer).
barometer. Here the ambient air pressure is lower than the air
pressure sealed inside the vessel. Get out your umbrella.
The internal pressure of Göethe's
barometer is whatever the ambient pressure was at the time the
barometer was filled. A voltmeter can only tell you what one voltage
is compared to some other arbitrary voltage. Göethe's barometer can
only tell you how the current ambient air pressure compares to whatever
pressure is sealed inside. It is useful in predicting weather because
storms usually come following a drop in air pressure.
Voltage vs. electromotive force
Voltage is often called electromotive force (EMF), and it is often said
that they are the same thing. There is actually a subtle difference.
This sometimes confuses the novice and the expert
alike, particularly where electricity and magnetic fields interract. EMF
is so closely related to voltage that the difference is almost not
worth discussing in electronics technology. However, some popular
websites and videos—some created by people with advanced degrees in
physics—that claim some fundamental laws of electricity are flawed.
Their claims are partly based on misunderstanding the nature of EMF and
how it differs from voltage. Their arguments are convincing if you don't
know the nature of EMF and how to properly apply the laws. Complicating
that is their cadre of followers who will loudly question your
intelligence if you question their assertions. This undermines the trust
of novice students. For example, if a student makes a mistake applying
any law to a circuit, how can he or she know if they made a mistake or
encountered an area where the law doesn't apply?
EMF is a
force that tends to make electricity move. This force exists in
batteries, generators, etc., and cannot be measured with a voltmeter.
EMF can be thought of like the heating and rotation of the earth that cause
wind. The heating and rotation spawn forces that make air move, but these forces
cannot be measured with an air pressure gauge. However, if an obstacle blocks
the wind, a pressure differential will develop between the windward and leeward
sides. This pressure differential can be measured with a sensitive pressure
gauge such as a
liquid column manometer.
The EMF in a battery or generator is similar to the forces that cause wind in
that it can't be measured with a voltmeter. However, at the terminals of a battery or generator, the flow of
electricity is blocked. As explained above, blocking the flow of
electricity causes voltage to develop (much like wind encountering an
obstacle). Therefore, the EMF, which cannot
be measured with a voltmeter, pushes electricity. When the electricity
encounters a blockage, such as the open terminals of a battery or
generator, or a circuit between the terminals, voltage develops, which can be measured with a
voltmeter (the only time a voltage won't develop is if there is a direct
short across the terminals).
The sources of EMF tend to be spread over some distance (such as the length
of a coil in a generator). Therefore, EMF is sometimes measured in volts per
unit of distance, such as volts per centimeter. The voltage at the terminals of
a battery or generator equals the total EMF in the device. For example, if a
generator coil is 10 mm long and produces one volt per mm of EMF, the total EMF
is 10 volts. The voltage at the terminals of said generator will also be 10
volts. Perhaps the reason many conflate voltage and EMF is that EMF can be
measured by blocking the flow of electricity to produce voltage, then measuring
that voltage with a voltmeter. 10 volts of EMF will cause a 10-volt differential
between the terminals.
We will further discuss the nature of EMF as appropriate.
Voltage is like air pressure
Just like measuring air pressure, when we measure voltage, we always measure the difference between two voltages (one voltage is meaningless unless we compare it to another voltage).
A voltmeter tells us the voltage difference between the two probes.
We cannot find an absolute voltage because an absolute zero voltage has never been defined.