Resistance

Presented By: Mazhar Iqbal

BSME-12213

What other words can we use to explain resistance?

Resistance

Impede Obstruct

Oppose

Slow down

Concept of Resistance

Resistance (R)

This is the ratio of the p.d. across a conductor to the current flowing through it. i.e. R = V

I

An ohm (Ω) is the SI derived unit of electrical resistance.

A conductor has an electrical resistance of one ohm when a constant potential difference of one volt applied between its ends produces in this conductor a current of one ampere.

ohm (Ω)

Abohm:An abohm (abΩ) is the basic unit of electrical resistance in the emu-cgs system of units (electromagnetic centimeter-gram-second system of units). A potential difference of one abvolt will drive a current of one abampere through a resistance of one abohm.One abohm is equal to 10⁻⁹ ohms = 1 nanoohm in the SI system of units.

Statohm: A statohm (statΩ) is the basic unit of electrical

resistance in the esu-cgs system of units (electrostatic centimeter-gram-second system of units).

A potential difference of one statV will drive a current of one statampere through a resistance of one statohm. The statohm is an extremely large unit; 1 statΩ ≈ 900 GΩ in the SI system of units.

Units

Ohm’s Law

Water AnalogyLow pressure

PumpWater

High pressure

ValveWater Flow

Constriction

Source of EMF

Resistor

High potential

Low potential

Switch

E

RI

+ -

The source of emf (pump) provides the voltage (pressure) to force electrons (water) through electric resistance (narrow constriction).

Factors affecting Resistance of a conductor

Resistance depends on Temperature Material of conductor Length Cross-sectional area

TemperatureThe resistance of a metallic conductor increases as the temperature increases e.g. copperAt very low temperatures resistance can become zero: superconductivity.

LengthResistance of a uniform conductor is directly proportional to its length.

i.e. R L

Factors affecting Resistance of a conductor

Cross-sectional areaResistance of a uniform conductor is inversely proportional to its cross-sectional area.

i.e. R 1 A

Factors affecting Resistance of a conductor

Material The material also affects the resistance of a

conductor by a fixed amount for different materials. This is known as resistivity ().

R = L = constant of proportionality A Unit: ohm meter m

= Rd 2 (For a wire with circular cross-sectional area) ___4L

Example . When a 3-V battery is connected to a light, a current of 6 mA is observed. What is the

resistance of the light filament?

Source of EMF

RI

+ -

V = 3 V 6 mA

3.0 V

0.006 A

VR

I

R = 500 WR = 500 W

1 V1

1 A

The reciprocal of resistance is conductance (G), measured in siemens (S)

G = 1/R (Siemens, S)

A resistance of 1 M is equivalent to a conductance of 10-6 S and a resistance of 10 is equivalent to a conductance of 10-1 S.

Conductance

An Ohmmeter may be used to perform the following tasks: Measure the resistance of individual or

combined elements Detect open-circuit (high-resistance) and

short-circuit (low-resistance) situations Check continuity of network connections and

identify wires of a multilead cable Resistance is measured by simply connecting

the two leads of the meter across the resistor.

Ohmmeters

http://micro.magnet.fsu.edu/electromag/electricity/resistance.html

http://homepages.nildram.co.uk/~vwlowen/radio/alarm/how2.htm

www.regentsprep.org/Regents/physics/phys03/bresit/default.htm

www.sciencejoywagon.com www.sciencejoywagon.com• www.radiodaze.com/rescarbcomp.htm

References

circuits

The end

Thank You!