POTENTIAL DIFFERENCE, CURRENT, RESISTANCE, RESISTOR AND ITS COLOR CODING

 POTENTIAL DIFFERENCE, CURRENT, RESISTANCE, RESISTOR AND ITS COLOR CODING

The energy transferred due to the movement of unit charge between 2 points in a circuit is termed as potential difference. It is observed that potential difference has the same effect as EMF

 

Take a battery source ,a resistor, a voltmeter and connecting wire. Now to check the open circuit voltage of the battery, connect the battery terminals to voltmeter. It gives EMF of battery.

Now, let connect battery and resistor in series connection. The voltage across battery represents the potential difference across the battery. The current leaves the battery source from the positive terminal and enters the resistor( also known as load) at the positive terminal. The direction of current and EMF are same. The voltage across the resistor represents potential difference.

So potential difference is defined as The energy transferred due to the movement of unit charge between two points in a circuit

 

As long as  the cell is not connected to an external load circuit, the voltage measured across its terminals is the EMF. When the cell is connected to an external loading circuit like resistance, a current flow and the voltage measured now across the cell or the load is potential difference.

There will be a difference between the emf measured first and p.d measured later. The p.d will be lower than the emf. This is due to the internal drop in the cell due to the current flowing through the internal resistance of the cell.

 

Unit of PD

The Volt(V) is the standard International (SI) unit of potential difference or Electromotive Force. A potential of one volt appears across a resistance of one ohm when a current of one ampere flows through that resistance.

Current

Current is the rate of flow of electrons and that current flows whenever a movement of charges exists in a substance. It is measured in terms of numbers of holes or numbers of electrons passing a single point in one second. Unit of current is Ampere (Amp).

Power

Power is the rate of working. The SI unit of power (P) is joule per second or watt(W) In practice kilowatt (KW) is frequently used.

P = W/t = F x (L/t) = F x u

Energy and power in terms of electrical parameters :

Power consumed by the circuit is , P = V.I = I².R = V²/R watts

Energy consumed by the circuit is  E = P x t watt hours

 

1 kilowatt = 1000 watts

1 kilowatt-hour   3,600,000 joules or 3.6MJ and is 1 unit of electric power

 

Energy is the capacity to do work. One joule is the work done when a force of 1N act through a distance of 1 metre in the direction of force.

Workdone = F x L

 

Resistance

Resistance is the property of material to resist (oppose) the flow of current . unit of resistance is ohm. A conductor is said to have a resistance of one ohm if it permits one ampere current to flow through it when one volt is impressed across its terminals.

 

Resistivity

Resistivity of a material is defined as the resistance of a wire of length one metre and area of cross section of one square metre.

Unit of resistivity is ohm-m. (Ω-m)

R = pl/A

R- resistance - Ω

p (rho) – resistivity – Ω-m

l – length – m

A – area of cross-section – m²

Resistivity of different material has to be studied when choosing a conductor. For example

Silver = 1.62 x 10^-8 Ω-m

Copper = 1.76 x 10^-8 Ω-m

Aluminium = 2.83 x 10^-8 Ω-m

Gold = 2.40 x 10^-8 Ω-m

Based on the values of the resistivity, materials can be classified as conductors , semiconductors and insulators

Conductor – Resistivity =10^-8 Ω-m   example  : all metals and their alloys

Semiconductors - Resistivity =10^-5 to 10⁰Ω-m   example : pure germanium and silicon  

Insulators - Resistivity =10¹¹ to 10¹⁵Ω-m   example : Glass , mica, wood etc

 

 

 

Colour coding of resistor

The resistor value is easily identified by colour code method. The colours are printed on the top of the resistor.

Colour                                              Number               Multiplier            tolerance

Black      -              B             -              0              -              1              -             

Brown   -              B             -              1              -              101         -             

Red        -              R             -              2              -              102         -             

Orange -              O             -              3              -              103         -

Yellow   -              Y              -              4              -              104         -

Green   -              G             -              5              -              105         -

Blue       -              B             -              6              -              106         -

Violet    -              V             -              7              -              107         -

Grey      -              G             -              8              -              108         -

White    -              W            -              9              -              109         -             

Gold      -                              -                              -              10-1       -              5

Silver     -                              -                              -              10-2       -              10

No colour                            -                              -                              -              20

On most resistors of 5% tolerance or greater, there are usually four bands. The first three bands are aligned towards one end of the resistor. The fourth band is on the opposite end.

Start reading from the colour band closest to the edge. The first two bands denote the first and second significant digits of the resistor value.

Example first colour is yellow and 2^nd is violet then first two digit is 47

Now the third band represents the Multiplier. 10⁵ Ω

47 x 10⁵ Ω

And the fourth band represents the resistor tolerance .

So the value of resistor is 4.7M  Ω +/-10%