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