2009 notes on resistance

7/28/2019 2009 Notes on Resistance

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Aminiya School Physics Grade 8

RESISTANCE

Students should be able to...

To make a current flow through a conductor, there must be a potential

difference or voltage across it.

Copper connecting wire is a good conductor and a current passes through

it easily where as a similar piece of nichrome wire is not so good and lesscurrent flows for the same potential difference

i.e., nichrome wire has more resistance than the copper wire. Resistance is the opposition to the flow of electrons.

SI unit of resistance is Ohm ()

Larger units are Kilo ohm (k) and Mega ohm (M)

1 k = 1000

1M = 106

The resistance of a wire depends on its dimensions as well as on theconducting ability of the material from which it is made.

Page 1 Notes/Term 2/2009

State that resistance = p.d/current and use the equationresistance= voltage/current in calculations

describe an experiment to measure the resistance of a metallicconductor using a voltmeter and an ammeter and make thenecessary calculationsdiscuss the temperature limitation on Ohms lawuse quantitatively the proportionality between resistance and thelength and the cross-sectional area of a wire.Calculate the net effect of a number of resistors in series and inparallelDescribe the effect of temperature increase on the resistance ofa resistor and a filament lamp and draw the respective sketchgraph of current/voltageDraw circuit diagrams with power sources (cell, battery or a.cmains), switches (closed and open), resistors (fixed and variable),lamps, ammeters, and voltmeters

State that the current from the source is the sum of the currentsin the separate branches of parallel circuitDo calculations on the whole circuit, recalling and using formulaincluding

R = V/I and those for p.d in series, resistors in series andresistors in parallel

Learning

outcome


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Factors affecting resistance

Length: Doubling the length of a wire doubles its resistance, R LCross-sectional area: Doubling the end on area of a wire halves it

resistance RA1

RA

L

The filament of the bulb is made of very thin tungsten wire becausetungsten has a higher melting point

Material: A nichrome wire has more resistance than a copper wire. Soheating elements are normally made of nichrome. Higherresistance offers higher heating effect

Whenever a current flows through a resistance, heating effect occurs. Thisprinciple is used in heating elements and also in the filaments of bulbs.

The heating effect occurs because electrons collide with atoms as they passthrough a conductor. The electrons lose energy and atoms gain energy andvibrate faster. Faster vibrations mean higher temperature.

Connecting wires used in circuits have a low resistance as possible so thatenergy wasted as heat is kept to a minimum

Temperature: For metal conductors, resistance increases with

temperatureFor semiconductors, resistance decreases withtemperature

In metals conductors, as Temperature increases atoms gain energy and

vibrate faster. This opposes the free movement of electrons. Soresistance increases

Constantan and manganin, both copper-based alloys, are often used instandard resistors because their resistances change very little unlessthey are heated strongly.

Semiconductors like silicon and germanium are insulators at room

temperature. As temperature increases more and more electronsbecome free to move. So it conducts as its resistance decreases

Thermistors have a high resistance when cold but a much lowerresistance when hot. They contain semi conducting materials. Someelectrical thermometers use a thermistor to detect a temperaturechange.Carbon is not classed as a semi conductor, but its resistance decreases

with temperature



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RESISTORS

Devices specially made to provide resistance are called resistors.

In a simple circuit, they can reduce the current flow.

In more complex circuits found in radios and TVs, they are used to keepcurrents and p.d at the levels for other circuit components to functionproperly.

A length of thin nichrome wire makes a simple resistor.

In some, the resistance is provided by a thin layer of carbon, while

other contains a long this alloy wire coiled to take up less space.

Circuit symbols

Variable resistors (Rheostats)

It is used for varying current.In hi-fi equipment, rotary (circular) variable resistors are used as volume

controls.

Above figure shows a variable resistor used to control the brightness of abulbIf the connections are between A and C it acts as a fixed resistor andIf connected between A and B, resistance can be varied

Moving the sliding control of the variable resistor to the right increases

the length of resistance wire in the circuit. This reduces the current anddims the bulb



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OHMS LAW

Increase the p.d across the ends of a conductor and a greater currentflows.

But the current depends on the conducting ability of the conductor aswell as the p.d across it. Some conductors offer more resistance to acurrent flow than others.

In 1826, Georg Ohm carried out experiments with different metal wires todiscover how the current through each depended on the potentialdifference applied across its ends. These can be summed up in a law knownas Ohms law which states

The current flowing through a metallic conductor is directly proportionalto the potential difference across its terminals, provided temperature

and other physical conditions remain constant.

Materials which obeys Ohms law is called Ohmic conductor.

Resistance is calculated using the equation

Resistance (R) =)(

)(

Itoroughconduccurrentthr

Vctorcrossconduifferenceapotentiald

R =I

V

Resistance of a metal conductor is defined as the ratio between thepotential difference across the conductor to the current through the

conductor

If potential difference is expressed in Volt(V) and current in Ampere(A),then resistance is expressed in Ohm ()

i.e., Ohm () =)(

)(

AAmpere

VVolt

A conductor has a resistance of 1 if a current of 1A flows through itwhen a potential difference of 1V is applied across its ends.

EXPERIMENT TO MEASURE RESISTANCE



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Resistances up to 50 can be measured using the circuit shown below.

The current through the resistor of unknown resistance R is set to any

convenient value by adjusting the rheostat and note the ammeter reading

(I) and the voltmeter reading (V) then R is calculated using the formula

R =dingammeterrea

eadingvoltmeterr

For greater accuracy, a range of corresponding voltmeter and ammeter

readings can be obtained and a graph of current against potential difference

is plotted as shown below. The value of x/y gives the unknown resistance R

The circuit shown is not suitable for measuring high resistances. If the

resistance is high, the current through the resistor is small, and the small

current drawn by the voltmeter adds its effect to the reading on the meter

EXPERIMENT TO SHOW THAT FILAMENT LAMP DOES NOT OBEYS OHMS LAW



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The current through the filament of the light bulb is varied by adjusting therheostat. As the current is increased from a low value, the temperature ofthe filament rises. When white hot, the filament is at a temperature ofmore than 30000C

Typical potential difference and current values are given in the figure (a)The graph in figure (b) shows that current is not proportional to thepotential difference

The calculations of p.d/current show that the resistance of the filamentbecomes greater as the temperature rises.

Voltage-time graph



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(Filament lamp) (Fixed resistor) (Thermistor)

Resistors in series and parallel

Combined Resistances of resistors in series

If two or more resistors are connected in series, they give a higher

resistance than any of the resistors by itself.

The effect is the same as joining several lengths of resistance wire to form a

longer length

If resistors R1 and R2 are in series, their combined resistance R is given bythe equation

For three of more resistors, the above equation can be extended by addingR3.... and so on.

Combined Resistances of resistors in parallel


Resistanceincreases astemperatureincreases

Obeys Ohms law-ResistanceremainingConstant

Resistancedecreases astemperatureincreases


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If two or more resistors are connected in parallel, they give a lower

resistance than any of the resistors by itself.

The effect is the same as using a thick piece of resistance wire instead of athin one. There is a wider conducting path than before.

If two resistors R1 and R2 are in parallel, their combined resistance R is givenby the equation

For three or more resistors, the equation can be extended by adding1/R3, ...and so on.

If the above equation for two resistors is rearranged, it becomes

Bulbs in Series and Parallel



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In seriesn

In parallel

Circuits and Switches


Thebulbsare

connectedin

series.

Thebulbs

sharethe

potential

difference

(voltage)from

thebattery,

soeachglows

dimly

Ifonebulbis

removed,

the

othergoesout

becausethe

circuitis

broken

The

bulbsare

connectedinparallel.

EachgetsthefullPD

from

thebattery

becauseeachis

connecteddirectlyto

it.

Soeachglows

brightly

Ifon

ebulbisremoved,

the

otherkeeps

workingbecauseitis

still

partofan

unbrokencircuit


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If two or more bulbs are to be powered by one battery, as in a car lightingsystem, they are normally connected in parallel. Each bulb gets the fullbattery p.d. Also, each can be switched ON and OFF independently

Basic circuit rules

There are some basic rules for all series and parallel circuits. The particularcurrent values depend on the resistances and potential differences.The equation V = IR always applies to every resistor.

Resistor colour codes


These two show two alternative ways of drawingexactly the same circuit as that shown in the first

figure

When resistors or other components are inparallel:

The PD (voltage) across each component

is the same

The total current in the mains circuit is

the sum of the currents in the branches

When resistors or other components arein series:

The current through each of the

components if the same

The total PD (voltage) across all the

components is the sum of the PDsacross each of them


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4. Calculate the effective resistance in each of the following



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e

5. Calculate the ammeter reading in each of the following circuits

(a) In Fig (a) what is the value of current flowing when the switch is open?

What current flows when the switch is closed?

(b) In Fig (b) what current flows through the ammeter when the switch is

closed and the variable resistor set to zero? What current flows when

the switch is closed and the variable resistor set at 20 ohm?

(c) Using fig (c) explain why no current will flow when the switch is closed?

What effect will there be on the circuit if the switch remains open but A

is joined to B? What effect will there be on the circuit with the switch

closed with A joined to B?



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2009 notes on resistance

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