Transcript Resistors

Resistors
Ohm’s Law and Combinations of
resistors
(see Chapter 13 in the A+ Certification book)
Electric Charge
 Electric charge is a fundamental property of
some of the particles that make up matter,
especially (but not only) electrons and
protons
 It comes in two varieties
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Positive (protons have positive charge)
Negative (electrons have negative charge)
Current
 If charges are moving, there is a current
 Current is rate of charge flowing by, that is,
the amount of charge going by a point each
second
 It is measured in units called amperes (amps)
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The currents in computers are usually measured
in milliamps (1 mA = 0.001 A)
 Currents are measured by ammeters
Current Convention
 Current has a direction
 By convention the direction of the current is
the direction in which positive charge flows
 If negative charges are flowing (which is
often the case), the current’s direction is
opposite to the particle’s direction
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Potential Energy and Work
 Potential energy is the ability to due work,
such as lifting a weight
 Certain arrangements of charges, like that in
a battery, have potential energy
 What’s important is the difference in
potential energy between one arrangement
and another
Voltage
 With charge arrangements, the bigger the charges,
the greater the energy
 It is convenient to define the potential energy per
charge, known as the electric potential (or just
potential)
 The potential difference (a.k.a. the voltage) is the
difference in potential energy per charge between
two charge arrangements
 Comes in volts
 Measured by a voltmeter
Resistance
 The ratio of voltage to current
R = V
I
 Indicates whether it takes a lot of work (high
resistance) or a little bit of work (low resistance) to
move charges
 Comes in ohms ()
 Measured by ohmmeter
Conductors and Insulators
 It is easy to produce a current in a material
with low resistance; such materials are called
conductors
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E.g. copper, gold, silver
 It is difficult to produce a current in a
material with high resistance; such materials
are called insulators
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E.g. glass, rubber, plastic
Semiconductor
 A substance having a resistivity that falls
between that of conductors and that of
insulators
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E.g. silicon, germanium
 A process called doping can make them more
like conductors or more like insulators
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This control plays a role in making diodes,
transistors, etc.
Ohm’s Law
 Ohm’s law says that the current produced by
a voltage is directly proportional to that
voltage
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Doubling the voltage, doubles the current
Resistance is independent of voltage or current
I
Slope=I/V=1/R
V
Ohmic
 Ohm’s law is an empirical observation
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Meaning that it is something we notice tends to
be true, rather than something that must be true
Ohm’s law is not always obeyed. For example, it
is not true for diodes or transistors
A device which obeys Ohm’s law is said to
“ohmic”
Resistor
 A ohmic device, that purpose of which is to
provide resistance in a circuit
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By providing resistance, they lower voltage or
limit current
Example
 A light bulb has a resistance of 240  when
lit. How much current will flow through it
when it is connected across 120 V, its normal
operating voltage?
V=IR
 120 V = I (240 )
 I = 0.5 V/ = 0.5 A
Resistors in series
 Each resistor obeys Ohm’s law
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V1 = I1 R1
and
V2 = I2 R2
 The current through the resistors is the same
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I1 = I2 = I
V1
a
R1
I1 
V2
R2
I2 
b
Equivalent resistance (series)
 The equivalent resistance is the value of a single
resistor that takes place of a combination
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Has same current and voltage drop as combo
Vab = V1 + V2 (the voltages add up to the total)
Vab = I1R1 + I2R2
Vab = I (R1 + R2)
Vab = I Req
Req = R1 + R2
Resistors in parallel
 The voltage across the resistors is the same
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V1 = V2 = Vab
 The current is split between the resistors
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I = I 1 + I2
R1
R2
Equivalent resistance (parallel)
 I = I1 + I 2
Vab
Req
1
Req
=
=
V1
R1
1
R1
+
+
V2
R2
1
R2
Series/Parallel Recap
 Series
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Resistors in series have the same current
Their voltages add up to the total voltage
 Parallel
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Resistors in parallel have the same voltage
Their currents add up to the total current
Multimeter
 A multimeter can serve as a voltmeter, ammeter or
ohmmeter depending on its setting
 To measure the voltage across a resistor, the
voltmeter is placed in parallel with it
 To measure the current through a resistor, the
ammeter is placed in series with it
 To measure the resistance of a resistor, the resistor
is removed from the circuit and each end is
connected to an end of the ohmmeter
Checking continuity
 A wire or cable is metal on the inside and
thus has a low resistance
 A broken cable has a high resistance
 To check a cable,
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remove the cable,
set the multimeter to ohmmeter
Check each wire for “continuity”
Heat
 A basic principle of physics is that energy is
conserved, that is, energy is never lost or
gained but only rearranged and put in
different forms
 When we have a simple resistor circuit, the
potential energy that was in the battery
becomes heat which is another form of
energy
Cooling off
 When you run a computer, heat is constantly
being generated because current is passing
through circuits that have resistance
 Too much heat can damage the circuits
 The heat sink and the fan are used to reduce
the amount of heat
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One of the differences between Baby AT and
ATX cases is in the fan