What points should you remember from this? If you can put potential
energy into a charge (as in a battery, for example) then whatever energy the
charge acquires in the process is transferred totally to whatever elements
the charge passes through as it wends its way back to the point where it has
no potential energy.
Electrical engineers would say this slightly differently. Here's how they
would say it.
If a charge is raised from zero
voltage (zero potential energy) to a higher voltage (as in a battery, for
example), then when that charge moves back to the point of zero potential energy
it passes through voltages that sum to whatever the voltage was that it passed
through to acquire the energy.
This is
really a statement of conservation of energy, and you can realize that once
you remember that voltage is really potential energy per unit of charge.
We also
need to be more precise in our discussion of voltage. Engineers
communicate with symbols, and they use special symbols to show voltages.
Let's look at the circuit we used earlier.
We
have added symbols to define all of the voltages in the circuit. For
example, we have defined a symbol, VB, that represents the
voltage across the battery. For the voltage, VB , as we
have defined it, we can compute the energy added to a charge, Q, when it
moves from the bottom of the battery (at the "-" sign) to the top of the
battery (at the "+" sign) as Q*VB. Let's get a complete set
of statements about what happens as charge moves around this circuit.
As a charge, Q, moves from the
bottom of the battery (at the "-" sign) to the top of the battery (at the "+"
sign), the charge gains
an amount of energy, Q*VB. The units are coulombs for charge,
volts for voltage and joules for energy.
As a charge, Q, moves from the
top of element #1 (at the "+" sign) to the bottom of element #1 (at the "-"
sign), the charge loses
an amount of energy, Q*V1.
As a charge, Q, moves from the
top of element #2 (at the "+" sign) to the bottom of element #2 (at the "-"
sign), the charge loses
an amount of energy, Q*V2.
We can
generalize these statements.
If we have used the convention
introduced above - with "+" and "-" signs - for voltage, then when a charge, Q,
moves from the end of the element labelled with a "-" sign to the end of the
element labelled with a "+" sign, the charge gains QV joules, where V is the
voltage across the element.
Now, it's
time for you to answer a few questions.
Questions
Here is a more complex
circuit.
Imagine that you have a charge,
Q, which is moved between various points in the circuit. The points we
will consider are marked with little green squares, and have alphabetical
labels (A through F). Answer the following Questions.
Q1
.
If a charge moves from point B to point C, how much energy does the charge lose?
Be careful with your signs.
Q2
.
If a charge moves from point C to point D, how much energy does the charge lose?
Be careful with your signs.
Q3
.
If a charge moves from point D to point E, how much energy does the charge lose?
Be careful with your signs.
We'll repeat the diagram so
you don't have to scroll to answer the last few questions.
Q4
.
If a charge moves from point E to point F, how much energy does the charge lose?
Be careful with your signs.
Q5
.
If a charge moves from point F to point A, how much energy does the charge lose?
Be careful with your signs.
Finally, you may have noticed a funny little symbol connected to point E in
the drawing in the questions. That symbol is a ground symbol, and it
has some importance. Ground is the reference voltage from which all
other voltages in a circuit can be measured.
Let us
consider a battery connected in a piece of electronic equipment. Very
often there is some obvious reference from which you can measure voltage.
In homes and buildings that reference is the ground. Interestingly, ground level
is often used as a reference when you compute potential energy of a weight that
has been raised, so that's another little thing that electrical and mechanical
systems share.
The
"electrical community" has come to agreement that the potential of the earth
itself is the reference from which voltages are to be measured. In
many pieces of electrical and electronic instrumentation there is a terminal
connected directly to ground. Those terminals look like the following.
The black connector on an electronic instrument will be the ground
connection. (And, the British will refer to it as the "earth"
connection.)
Like mechanical potential energy, electrical potential energy and voltage are
measured from a reference. For mechanical energy, that might be ground
level. It's just that some reference needs to be chosen. (And, it is
chosen, not pre-ordained by nature!)
Electrical systems need a reference and the reference usually chosen is
ground. That means the reference is the earth itself. (In
America, we usually refer to that as "ground" while the English refer to it
as "earth".) In any event, in any piece of electrical or electronic
equipment, "ground" voltage is always available. It's the voltage at
the third prong of the plug you put into the wall socket.
In
any event, the voltage level of the ground in your vicinity is chosen as a
reference voltage, and often voltages are measured from that reference.
Since we must always talk about voltage differences, we should realize that
if we say that some electrical terminal (a point in space) is at a voltage
level of 120 volts, we mean that the voltage difference between that point
and ground is 120 volts.
Rule to Remember:
Whenever you talk about a voltage, you are always talking about a voltage
difference! - and you should specify the two points in space where that
difference exists.
Shown
below is a picture of a wall plug. The small circular hole at the
bottom is connected directly to ground. If you trace out the wiring in
your home, you should find that the wire that makes the connection to the
circular hole is connected (behind the walls) to a water pipe, or something
else that makes good contact with the ground on which the house is built.
What's
more, if you measure the voltage between the other connections and ground you
will usually find that one of them is at a voltage of 120 volts. We would
say that that voltage is 120 volts measured with respect to ground. We
take advantage of that connection in electronic instrumentation and many
instruments can measure voltage with respect to ground, or can generate a
voltage relative to ground. This is a source of voltage that is very
common.
Finally, the last important concept. Don't try to plug a plug into the
picture above. Use a real wall plug.
At
this point, you've started to get acquainted with voltage. If you have
to use circuits with live voltage you'll need to know how to measure
voltage, and a few other things. That's the next section.
Measuring Voltage
If you
deal with circuits you will need to be able to measure voltages in circuits.
That's the one skill you absolutely must have if you want to check that a
circuit is operating properly. You know that Murphy's law prevails.
If anything can go wrong, it will.
You
will always need to check a circuit's operation to see if it is working
correctly. Actually, you'll probably need to check it to find out why
it isn't working. Many times you will do that using a voltmeter.
In this section we'll discuss how to use a voltmeter to measure voltages in
an operating circuit.
There are many other situations in which you would want to be able to
measure voltage. For example, you might have an LM35 temperature
sensor. Then the output of the sensor is a voltage that is
proportional to the temperature of the sensor.
We
will give you a choice here. You can continue in this lesson, or you
can read the lesson devoted entirely to voltage measurements. Click
here to go to that lesson which covers numerous laboratory measurements and
gives you several experiments to perform.
Using a Voltmeter
Here's a representation of a voltmeter. For our introduction to the
voltmeter, we need to be aware of three items on the voltmeter.
The display. This is
where the result of the measurement is displayed. Your meter might be
either analog or digital. If it's analog you need to read a reading off a
scale. If it's digital, it will usually have an LED or LCD display panel
where you can see what the voltage measurement is.
The positive input terminal,
and it's almost always red.
The negative input terminal,
and it's almost always black.
Next, you
need to be aware of what the voltmeter measures. Here it is in a
nutshell.
A voltmeter measures the
voltage difference between the positive input terminal of the voltmeter and the
negative input terminal.
That's it. That's what it measures. Nothing more, nothing less -
just that voltage difference. That means you can measure voltage
differences in a circuit by connecting the positive input terminal and the
negative input terminal to locations in a circuit.
Next,
we'll look at a circuit diagram. We'll show a voltmeter connected to
the circuit diagram - a mixed metaphor approach. Forgive us for that,
but let's look at it. It's right below.
Here's
a voltmeter shown connected to a circuit. This shows where you would place
the leads if you wanted to measure the voltage across element #4.
Notice that the voltmeter
measures the voltage across element #4, +V4.
Notice the polarity definitions
for V4, and notice how the red terminal is connected to the "+" end
of element #4. If you reversed the leads, you would be measuring -V4.
Here's a
portion of a circuit board. You want to measure the voltage across
R27. Click on where you should put the voltmeter leads.
What If ?
At this
point, you're starting to become comfortable with voltage. Don't become
too comfortable. Always respect two things about voltage.
It can give you a shock, and a
large enough voltage can be lethal.
It's more complicated than
we've really indicated so far.
You Need To Know More About
Voltage
So far,
we've just examined voltage as though it were across one device. However,
if we look at the example circuit we used before we realize that there are lots
of voltages in this circuit. If we measure them, how do we know our
measurements make sense? There are laws that voltage obeys. The most
important one is Kirchhoff's Voltage Law, (KVL) and it's the subject of another lesson.
It is an important relationship that voltage obeys, and it is the starting point
for analysis of circuits of any complexity. That's it for this lesson. You
can exit this lesson and start another lesson by clicking the up-pointing arrow
below. Or you can go directly to several places from this page. You
can use any of these hotwordsto take you to a lesson of your choice.
