7.3 Potential Difference, Ohm's Law and Resistance

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7.3 Potential D

ShahidMukhlisPhysics Form5
http://www.one-school.net/Malaysia/UniversityandCollege/SPM/revisioncard/physics/electricity/voltage.htmlhttp://www.one-school.net/Malaysia/UniversityandCollege/SPM/revisioncard/physics/electricity/voltage.html


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Electrical Potential

The electric potential V at a

point in an electric field isthe work done to bring aunit ( 1 Coulomb) positive

charge from infinity to thepoint.


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Potential Difference

(Voltage)

The potential difference (p.d.)

between two points is definedas the energy converted fromelectrical to other forms whenone coulomb of positivecharge passes between thetwo points.


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Unit of Potential Difference

The SI unit of potential difference isthe same as that for e.m.f., i.e. thevolt. We define the volt as follows:

The potential difference (p.d.)between two points in a conductor is 1

volt if 1 joule of energy is convertedfrom electrical to other forms when 1coulomb of positive charge flowsthrough it.


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Formula of Potential

Difference

E


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Examp e

How much energy had been transfer when

5 C charges moved across a potentialdifference of 10V?

Answer:

The charge, Q = 5C

Potential difference, V = 10V

Energy, E = ?


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Ohm's Law

The current flowing in the

metallic conductor is directlyproportional to the potentialdifference applied across itsends, provided that the physicalconditions ( such as

temperature ) are constant.


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Formula

Any other conductors, other than

metallic conductors, which obey OhmsLaw are described as Ohmicconductors.


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What is the current through an 8 toaster when itis operating on 240V?

Answer:

(In this question, 2 physical quantities are

given, they are the "240V" and "80". Thequestion doesn't tell what quantites they are.However we can recognise these quantitiesfrom its unit. is the unit of resistance

whereas V is the unit of potential difference.)

Resistance, R = 80

Potential difference, V = 240V=


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Resistance

The resistance R of a material is defined as the ratio V: I,

whereV

is the potential difference across thematerial and Iis the current flowing in it.

Unit:The SI unit of resistance is the ohm (). One ohm is the resistance of amaterial through which a current of one ampere flows when a potentialdifference of one volt is maintained.

R i ti it


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Resistivity

The resistance R of a given conductor

depends on the:length l,

Longer wire - Higher Resistance

cross-sectional area A,Thicker wire - Lower Resistance

temperatureHigher temperature - Higher

Resistance

the type of materialcopper has resistance lower than iron

S d t


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Superconductor

Superconductors are materials where

their electrical resistance is exactlyzero at some relatively lowtemperature.

Application of Superconductor

1. Magnetic Resonance Imaging (MRI)

2. Magnetic-Levitation Train (MagLev)3. Electric generators

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7.3 Potential Difference, Ohm's Law and Resistance