resistors ohm’s law and combinations of resistors (see chapter 13 in the a+ certification book)
TRANSCRIPT
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 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) 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
Ie-
e-
e-
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
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
R = V
I
Conductors and Insulators
It is easy to produce a current in a material with low resistance; such materials are called conductors E.g. copper, gold, silver
It is difficult to produce a current in a material with high resistance; such materials are called insulators E.g. glass, rubber, plastic
Semiconductor
A substance having a resistivity that falls between that of conductors and that of insulators E.g. silicon, germanium
A process called doping can make them more like conductors or more like insulators 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 Doubling the voltage, doubles the current Resistance is independent of voltage or current
V
I Slope=I/V=1/R
Ohmic
Ohm’s law is an empirical observation 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 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 = I R 120 V = I (240 ) I = 0.5 V/ = 0.5 A
Resistors in series
Each resistor obeys Ohm’s law V1 = I1 R1 and V2 = I2 R2
The current through the resistors is the same I1 = I2 = I
R1 R2
I1 I2
V1 V2
a b
Equivalent resistance (series)
The equivalent resistance is the value of a single resistor that takes place of a combination 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 V1 = V2 = Vab
The current is split between the resistors I = I1 + I2
R1
R2
Equivalent resistance (parallel)
I = I1 + I2
Vab = V1+
V2
Req R1 R2
1 = 1+
1
Req R1 R2
Series/Parallel Recap
Series Resistors in series have the same current Their voltages add up to the total voltage
Parallel 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,
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 One of the differences between Baby AT and
ATX cases is in the fan