# Potential Difference, Ohm's Law and Resistance

Card 1: Electrical Potential

## Electrical Potential

The electric potential V at a point in an electric field is the work done to bring a unit ( 1 Coulomb) positive charge from infinity to the point.

Card 2: Potential Difference (Voltage)

## Potential Difference (Voltage)

The potential difference (p.d.) between two points is defined as the energy converted from electrical to other forms when one coulomb of positive charge passes between the two points.

Card 3: Unit of Potential Difference

## Unit of Potential Difference

The SI unit of potential difference is the same as that for e.m.f., i.e. the volt. 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 converted from electrical to other forms when 1 coulomb of positive charge flows through it.

Card 4: Formula of Potential Difference

## Formula of Potential Difference Example 1
How much energy had been transfer when 5 C charges moved across a potential difference of 10V?

The charge, Q = 5C
Potential difference, V = 10V
Energy, E = ?

            \begin{gathered}
V = \frac{E}
{Q} \hfill \\
(10) = \frac{E}
{{(5)}} \hfill \\
E = 5 \times 10 = 50J \hfill \\
\end{gathered}

Card 5: Ohm's Law

## Ohm's Law

The current flowing in the metallic conductor is directly proportional to the potential difference applied across it’s ends, provided that the physical conditions ( such as temperature ) are constant.

Formula: Any other conductors, other than metallic conductors, which obey Ohm’s Law are described as Ohmic conductors.

Example 2
What is the current through an 8Ω toaster when it is operating on 240V?

(In this question, 2 physical quantities are given, they are the "240V" and "80Ω". The question doesn't tell what quantites they are. However we can recognise these quantities from its unit. Ω is the unit of resistance whereas V is the unit of potential difference.)

Resistance, R = 80Ω
Potential difference, V = 240V
Current, I = ?

V = IR
(240) = I(80)
I = 3A

Card 6: Resistance

## Resistance

The resistance R of a material is defined as the ratio V : I, where V is the potential difference across the material and I is the current flowing in it. Unit:

The SI unit of resistance is the ohm (Ω). One ohm is the resistance of a material through which a current of one ampere flows when a potential difference of one volt is maintained.

Card 7: Resistivity

## 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

temperature
Higher temperature - Higher Resistance

the type of material
copper has resistance lower than iron

Card 8: Superconductor

## Superconductor

Superconductors are materials where their electrical resistance is exactly zero at some relatively low temperature.

Card 9: Application of Superconductor

## Application of Superconductor

1. Magnetic Resonance Imaging (MRI)
2. Magnetic-Levitation Train (MagLev)
3. Electric generators
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