VOLTAGE
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?
----------------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
1. EMF is electromotive force. EMF causes the electrons to move in a particular
direction.
2. EMF is measured in units called volts.
RESISTANCE
There is another important property that can be measured in electrical systems. This is
resistance, which is measured in units called ohms. Resistance is a term that describes the
forces that oppose the flow of electron current in a conductor. All materials naturally contain
some resistance to the flow of electron current. We have not found a way to make conductors
that do not have some resistance.
If we use our water analogy to help picture resistance, think of a hose that is partially plugged
with sand. The sand will slow the flow of water in the hose. We can say that the plugged hose
has more resistance to water flow than does an unplugged hose. If we want to get more water
out of the hose, we would need to turn up the water pressure at the hydrant. The same is true
with electricity. Materials with low resistance let electricity flow easily. Materials with higher
resistance require more voltage (EMF) to make the electricity flow.
The scientific definition of one ohm is the amount of electrical resistance that exists in an
electrical circuit when one amp of current is flowing with one volt being applied to the circuit.
Is resistance good or bad?
Resistance can be both good and bad. If we are trying to transmit electricity from one place to
another through a conductor, resistance is undesirable in the conductor. Resistance causes
some of the electrical energy to turn into heat so some electrical energy is lost along the way.
However, it is resistance that allows us to use electricity for heat and light. The heat that is
generated from electric heaters or the light that we get from light bulbs is due to resistance. In
a light bulb, the electricity flowing through the filament, or the tiny wires inside the bulb,
cause them to glow white hot. If all the oxygen were not removed from inside the bulb, the
wires would burn up.
An important point to mention here is that the resistance is higher in smaller wires. Therefore,
if the voltage or EMF is high, too much current will follow through small wires and make
them hot. In some cases hot enough to cause a fire or even explode. Therefore, it is sometimes
useful to add components called resistors into an electrical circuit to slow the flow of
electricity and protect of the components in the circuit.
Resistance is also good because it gives us a way to shield ourselves from the harmful energy
of electricity. We will talk more about this on the next page.
Review
1. Resistance is the opposition to electrical current.
2. Resistance is measured in units called ohms.
3. Resistance is sometimes desirable and sometimes undesirable.
AMPERAGE
It is very important to have a way to measure and quantify the flow of electrical current.
When current flow is controlled it can be used to do useful work. Electricity can be very
dangerous and it is important to know something about it in order to work with it safely. The
flow of electrons is measured in units called amperes. The term amps is often used for short.
An amp is the amount of electrical current that exists when a number of electrons, having one
coulomb (ku`-lum) of charge, move past a given point in one second. A coulomb is the
charge carried by 6.25 x 10^18 electrons. 6.25 x 10^18 is scientific notation for
6,250,000,000,000,000,000. That is a lot of electrons moving past a given point in one
second!
Since we cannot count this fast and we cannot even see the electrons, we need an instrument
to measure the flow of electrons. An ammeter is this instrument and it is used to indicate how
many amps of current are flowing in an electrical circuit.
Review
1. Amperage is a term used to describe the number of electrons moving past a fixed
point in a conductor in one second.
2. Current is measured in units called amperes or amps.
Voltage is the Cause, Current is the Effect
Voltage attempts to make a current flow, and current will flow if the circuit is complete. Voltage is
sometimes described as the 'push' or 'force' of the electricity, it isn't really a force but this may help
you to imagine what is happening. It is possible to have voltage without current, but current cannot
flow without voltage.
Voltage and Current
Voltage but No Current
No Voltage and No Current
The switch is closed making a
complete circuit so current can flow.
The switch is open so the circuit is
broken and current cannot flow.
Without the cell there is no source
of voltage so current cannot flow.
Voltage, V
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Voltage is a measure of the energy carried by the charge.
Strictly: voltage is the "energy per unit charge".
The proper name for voltage is potential difference or p.d.
for short, but this term is rarely used in electronics.
Voltage is supplied by the battery (or power supply).
Voltage is used up in components, but not in wires.
We say voltage across a component.
Voltage is measured in volts, V.
Voltage is measured with a voltmeter, connected in parallel.
The symbol V is used for voltage in equations.
!!! Connecting a voltmeter in parallel
Voltage at a point and 0V (zero volts)
Voltage is a difference between two points, but in electronics we
often refer to voltage at a point meaning the voltage difference
between that point and a reference point of 0V (zero volts).
Zero volts could be any point in the circuit, but to be consistent
it is normally the negative terminal of the battery or power
supply. You will often see circuit diagrams labelled with 0V as
a reminder.
Zero volts for circuits with a dual supply
Some circuits require a dual supply with three supply connections as
shown in the diagram. For these circuits the zero volts reference point is the
middle terminal between the two parts of the supply.
On complex circuit diagrams using a dual supply the earth symbol is
often used to indicate a connection to 0V, this helps to reduce the
number of wires drawn on the diagram.
The diagram shows a ±9V dual supply, the positive terminal is +9V,
the negative terminal is -9V and the middle terminal is 0V.
Current, I
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Current is the rate of flow of charge.
Current is not used up, what flows into a component must flow out.
We say current through a component.
Current is measured in amps (amperes), A.
Current is measured with an ammeter, connected in series.
To connect in series you must break the circuit and put the ammeter acoss the gap, as shown in the
diagram.
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The symbol I is used for current in equations.
Connecting an ammeter in series
1A (1 amp) is quite a large current for electronics, so mA (milliamps) are often used. m (milli) means
"thousandth":
1mA = 0.001A, or 1000mA = 1A
Electrical resistance and conductance
The electrical resistance of an electrical element measures its opposition to the passage of an
electric current; the inverse quantity is electrical conductance, measuring how easily
electricity flows along a certain path. Electrical resistance shares some conceptual parallels
with the mechanical notion of friction. The SI unit of electrical resistance is the ohm (Ω),
while electrical conductance is measured in siemens (S).
An object of uniform cross section has a resistance proportional to its resistivity and length
and inversely proportional to its cross-sectional area. All materials show some resistance,
except for superconductors, which have a resistance of zero.
The resistance of an object is defined as the ratio of voltage across it to current through it:
For a wide variety of materials and conditions, the electrical resistance R is constant for a
given temperature; it does not depend on the amount of current through or the potential
difference (voltage) across the object. Such materials are called Ohmic materials. For objects
made of ohmic materials the definition of the resistance, with R being a constant for that
resistor, is known as Ohm's law.
In the case of a nonlinear conductor (not obeying Ohm's law), this ratio can change as current
or voltage changes; the inverse slope of a chord to an I–V curve is sometimes referred to as a
"chordal resistance" or "static resistance".
Conductors and resistors
A 65-Ω resistor, as identified by its electronic color code (blue–green–black).
An ohmmeter could be used to verify this value.
Objects such as wires that are designed to have low resistance so that they transfer current
with the least loss of electrical energy are called conductors. Objects that are designed to have
a specific resistance so that they can dissipate electrical energy or otherwise modify how a
circuit behaves are called resistors. Conductors are made of highly conductive materials such
as metals, in particular copper and aluminium. Resistors, on the other hand, are made of a
wide variety of materials depending on factors such as the desired resistance, amount of
energy that it needs to dissipate, precision, and cost.
DC resistance
The resistance of a given resistor or conductor grows with the length of conductor and
decreases for larger cross-sectional area. The resistance R and conductance G of a conductor
of uniform cross section, therefore, can be computed as
where is the length of the conductor, measured in metres [m], A is the cross-sectional area
of the conductor measured in square metres [m²], and ρ (rho) is the electrical resistivity (also
called specific electrical resistance) of the material, measured in ohm-metres (Ωm).
Resistivity is a measure of the material's ability to oppose electric current. For purely resistive
circuits conductance is related to resistance R by:
For practical reasons, any connections to a real conductor will almost certainly mean the
current density is not totally uniform. However, this formula still provides a good
approximation for long thin conductors such as wires.
AC resistance
If a wire conducts high-frequency alternating current, then the effective cross sectional area of
the wire is reduced because of the skin effect. If several conductors are together, then due to
proximity effect, the effective resistance of each is higher than if that conductor were alone.
These effects are so small for low frequency of ordinary household AC that they should
ordinarily be treated as if it were DC resistance.
When an alternating current flows through the circuit, its flow is not opposed only by the
circuit resistance, but also by the opposition of electric and magnetic fields to the current
change. That effect is measured by electrical reactance. The combined effects of reactance
and resistance are expressed by electrical impedance.
Measuring resistance
An instrument for measuring resistance is called an ohmmeter. Simple ohmmeters cannot
measure low resistances accurately because the resistance of their measuring leads causes a
voltage drop that interferes with the measurement, so more accurate devices use four-terminal
sensing.
Resistor
A resistor is a two-terminal electronic component which implements electrical resistance as a
circuit element. When a voltage V is applied across the terminals of a resistor, a current I will
flow through the resistor in direct proportion to that voltage. The reciprocal of the constant of
proportionality is known as the resistance R, since, with a given voltage V, a larger value of R
further "resists" the flow of current I as given by Ohm's law:
Resistors are common elements of electrical networks and electronic circuits and are
ubiquitous in most electronic equipment. Practical resistors can be made of various
compounds and films, as well as resistance wire (wire made of a high-resistivity alloy, such as
nickel-chrome). Resistors are also implemented within integrated circuits, particularly analog
devices, and can also be integrated into hybrid and printed circuits.
The electrical functionality of a resistor is specified by its resistance: common commercial
resistors are manufactured over a range of more than 9 orders of magnitude. When specifying
that resistance in an electronic design, the required precision of the resistance may require
attention to the manufacturing tolerance of the chosen resistor, according to its specific
application. The temperature coefficient of the resistance may also be of concern in some
precision applications. Practical resistors are also specified as having a maximum power
rating which must exceed the anticipated power dissipation of that resistor in a particular
circuit: this is mainly of concern in power electronics applications. Resistors with higher
power ratings are physically larger and may require heat sinking. In a high voltage circuit,
attention must sometimes be paid to the rated maximum working voltage of the resistor.
The series inductance of a practical resistor causes its behavior to depart from ohms law; this
specification can be important in some high-frequency applications for smaller values of
resistance. In a low-noise amplifier or pre-amp the noise characteristics of a resistor may be
an issue. The unwanted inductance, excess noise, and temperature coefficient are mainly
dependent on the technology used in manufacturing the resistor. They are not normally
specified individually for a particular family of resistors manufactured using a particular
technology. A family of discrete resistors is also characterized according to its form factor,
that is, the size of the device and position of its leads (or terminals) which is relevent in the
practical manufacturing of circuits using them.
Units
The ohm (symbol: Ω) is the SI unit of electrical resistance, named after Georg Simon Ohm.
An ohm is equivalent to a volt per ampere. Since resistors are specified and manufactured
over a very large range of values, the derived units of milliohm (1 mΩ = 10−3 Ω),
kilohm (1 kΩ = 103 Ω), and megohm (1 MΩ = 106 Ω) are also in common usage.
The reciprocal of resistance R is called conductance G = 1/R and is measured in Siemens (SI
unit), sometimes referred to as a mho. Thus a Siemens is the reciprocal of an ohm: S = Ω − 1.
Although the concept of conductance is often used in circuit analysis, practical resistors are
always specified in terms of their resistance (ohms) rather than conductance.
Potentiometer
A potentiometer (colloquially known as a "pot") is a three-terminal resistor with a sliding
contact that forms an adjustable voltage divider. If only two terminals are used (one side and
the wiper), it acts as a variable resistor or rheostat. Potentiometers are commonly used to
control electrical devices such as volume controls on audio equipment. Potentiometers
operated by a mechanism can be used as position transducers, for example, in a joystick.
Potentiometers are rarely used to directly control significant power (more than a watt), since
the power dissipated in the potentiometer would be comparable to the power in the controlled
load (see infinite switch). Instead they are used to adjust the level of analog signals (e.g.
volume controls on audio equipment), and as control inputs for electronic circuits. For
example, a light dimmer uses a potentiometer to control the switching of a TRIAC and so
indirectly control the brightness of lamps.
Electric current
Electric current means, depending on the context, a flow of electric charge (a phenomenon)
or the rate of flow of electric charge (a quantity). This flowing electric charge is typically
carried by moving electrons, in a conductor such as wire; in an electrolyte, it is instead carried
by ions, and, in a plasma, by both.
The SI unit for measuring the rate of flow of electric charge is the ampere, which is charge
flowing through some surface at the rate of one coulomb per second. Electric current is
measured using an ammeter.
The SI unit of charge, the coulomb, "is the quantity of electricity carried in 1 second by a
current of 1 ampere." Conversely, a current of one ampere is one coulomb of charge going
past a given point per second:
That is, in general, charge Q is determined by steady current I flowing for a time t as Q = It.
Voltage
The voltage between two points is a short name for the electrical driving force (the concept of
driving force is not a force measured in newtons) that could determine an electric current
between those points. It is used interchangeably with electric potențial difference and electric
tension. Specifically, voltage is equal to energy per unit charge. In the case of static electric
fields, the voltage between two points is equal to the electrical potential difference between
those points. In the more general case with electric and magnetic fields that vary with time,
the terms are no longer synonymous.
Electric potential is the energy required to move a unit of electric charge to a particular place
in a static electric field.
Voltage can be measured by a voltmeter. The unit of measurement is the volt.