Length, Cross-Sectional Area, Kind of Material, Resistivity andTemperature: Their Relationships

FACTORS AFFECTING FACTORS AFFECTING

RESISTANCERESISTANCE

Conductors will always allow the flow of current.

Electrical cords or wires are the best and common

examples of conductors used in our house.

Are all electrical wires used in our house of same size, length,

and kind of material?How do length, size, kind of

material, resistivity and temperature affect the conductivity of a wire?

FACTORS AFFECTING RESISTANCE…

RESISTANCE OF A MATERIAL… Resistance of the conducting

wires used in electric circuits varies with the dimension of the length and area, and depends on the material the wire is made of.Using different lengths of wire will affect resistance. It can be illustrated by the following…

Case 2

Case 1

LENGTH OF THE WIRE …In the previous illustration, it is noticeably visible that the case 1-bulb is brighter than the case

2-bulb.This can be summarized as “the longer the conducting

wire, the higher is its resistance”. This means that resistance is always affected

by the length of wire; or L α R.In a particle approach, it can be explained as “the longer wire, the more energy is needed to push the electrons to move

along the circuit.0

LENGTH OF MATERIAL …Variable resistors (wire-wound)

are best examples of conductors made of a long

wire looped around insulator bases, used to regulate

current. The volume control of a radio is a variable resistor.

“As the volume knob is turned, the effective length of the resistance wire changes.

Resistance and the amount of current in the circuit change, making the sound from the

speaker loud or soft.”

RESISTANCE AND WIRE’s DIAMETER …How does a wire’s diameter

affect its resistance?Consider this illustrations…

The illustration simply shows that “the bigger & thicker the

wire, the lesser is its resistance”.

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A wire with smaller

diameter.A wire with larger

diameter.

WIRE’s DIAMETER …

It is very important that the loads in a circuit be limited to the amount of current that the wire can safely carry; otherwise

it can result in heat build-up causing a fire.

WIRE WIRE GAUGEGAUGE

WIRE’s CURRENT WIRE’s CURRENT CARRYING CAPACITYCARRYING CAPACITY

1414 15 A15 A1212 20 A20 A1010 30 A30 A

*WIRE GAUGE & *WIRE GAUGE & CORRESPONDING CURRENT CORRESPONDING CURRENT

RATING.RATING.

RESISTANCE IN DIFFERENT KIND OF MATERIALS …Resistance of an ohmic conductor also depends on the kind of material it is made of. The composition of materials greatly affect their ability to

conduct. This means that electric current varies more

than almost any other physical property of a material.

Conducting wires maybe made up of any metallic materials. To

consider metals, there are different examples of metallic

substances in the periodic table.

RESISTANCE OF DIFFERENT MATERIALS …Resistivities (at 20oC) of

different materials.MATERIALSMATERIALS RESISTIVITY (RESISTIVITY (.m).m)SilverSilver 1.59 x 101.59 x 10-8-8

CopperCopper 1.70 x 101.70 x 10-8-8

GoldGold 2.44 x 102.44 x 10-8-8

AluminumAluminum 2.82 x 102.82 x 10-8-8

TungstenTungsten 5.60 x 105.60 x 10-8-8

IronIron 1.00 x 101.00 x 10-7-7

Stainless SteelStainless Steel 7.20 x 107.20 x 10-7-7

PlatinumPlatinum 1.10 x 101.10 x 10-7-7

Continuation of Materials’ Resistivity …Resistivities (at 20oC) of

different materials.MATERIALSMATERIALS RESISTIVITY (RESISTIVITY (.m).m)LeadLead 2.20 x 102.20 x 10-7-7

NichromeNichrome 1.50 x 101.50 x 10-7-7

CarbonCarbon 3.50 x 103.50 x 10-5-5

GermaniumGermanium 4.60 x 104.60 x 10-1-1

SiliconSilicon 6.40 x 106.40 x 10+2+2

GlassGlass 101010 10 –– 10101414

Hard RubberHard Rubber approx. 10approx. 101616

SulfurSulfur 10101515

MATERIAL’s RESISTIVITY …Resistivity () is the reciprocal

of conductivity. The list of resistivities of materials clearly

show that resistance varies depending on the what kind of material a conductor is made

of.The temperature of a wire also

affects the resistance of the wire. For most metals, resistivity increases with the temperature.Can you explain why? How can it

be clearly explained?

TEMPERATURE AFFECTING RESISTANCE …As the temperature increases,

the atoms in the wire vibrate more vigorously. The

conduction of electrons would make more collisions with the

vibrating atoms. The resistance of the wire would

increase. In the event that the wire is hot, it will have a

greater resistance than a cool wire.

Resistance, resistivity, length and cross-sectional area of a conductor can be expressed as…

R A

L

LENGTH OF MATERIAL … where:= resistivity of the conductor in .m.L = length of the conducting wire used.A = cross-sectional area of the conductor (m2)

R A

L