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 = 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 I e-e- e-e- 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 V 1 = I 1 R 1 and V 2 = I 2 R 2  The current through the resistors is the same I 1 = I 2 = I R1R1 R2R2 I 1  I 2  V1V1 V2V2 ab

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  V ab = V 1 + V 2 (the voltages add up to the total)  V ab = I 1 R 1 + I 2 R 2  V ab = I (R 1 + R 2 )  V ab = I R eq  R eq = R 1 + R 2

Resistors in parallel  The voltage across the resistors is the same V 1 = V 2 = V ab  The current is split between the resistors I = I 1 + I 2 R1R1 R2R2

Equivalent resistance (parallel)  I = I 1 + I 2 V ab =V1V1 + V2V2 R eq R1R1 R2R2 1=1 + 1 R1R1 R2R2

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