Electricity Blog 3

ELECTRICITY - LESSON 3

ELECTRICITY

Good morning ,

It is a pleasure to know that you all are logging in and doing your work

Today we will make an effort to --

Evolve Ohm's Law and express it mathematically.

Verify Ohm's Law experimentally.

Develop meaning of resistance using Ohm's Law.

List the factors which affect resistance.

Explain resistivity, conductors, resistors and insulators.

Experimentally determine the factors affecting resistance.

But first of all let us try to answer yesterday's uestion :

The amount off charge moved  = 5 C.
Thus, the potential difference = V2-V1 = 230-240 = 10 V
Therefore, Work done = Potential difference * Charge moved.
That is, 10 * 5 = 50 Joules.
Hence, the  work done in moving a charge of 5 coulombs from a point at 230 volts to another point at 240  volts is 50 J.

A German physicist found the relationship between the current I, flowing in a wire and the potential difference across its terminals.

Do watch the following Video - (Ohm's Law )

https://youtu.be/ldNPl67x-E8

Ohm's Law

Ohm’s Law states that the electric current is directly proportional to potential difference between two points.



Potential difference between two points is directly 

proportional to the current, provided temperature is 

constant.

V ∝ l

⇒ V = lR



R is a constant known as Resistance. The SI unit of resistance is ohm (Ω)




Resistance is a measure of the opposition to current flow in 

an electrical circuit. 

Current through different materials are different.

(you did study about conductors and insulators)

Explanation:

An object may offer varying degrees of electrical conductance. It depends on its elemental and molecular properties. 

Some elements like silver, copper have high conductivity while others like carbon, rubber do not conduct electricity. We call this property as resistance.

Factors on which resistance of a conductor depends-

CLICK HERE - Factors that effect resistance 

Length of conductor: Long electric wires create more resistance to the electric current.

                                   R ∝ l   (i) 

  

Area of cross section: More area of conductor facilitates the flow of electric current through more area and thus decreases the resistance. This is the cause that thick copper wire creates less resistance to the electric current.

                                  R ∝ 1/A   (ii)

Nature of material: resistivity (). It is the property of material.

Resistance R is directly proportional to resistivity of material.

R ∝ p… (iii)

From equation (i), (ii) and (iii)

R ∝ p l / A

Temperature: Resistance also varies with temperature. Resistance increases with increase in temperature and decreases with decrease in temperature. 

During the holidays study about Resistivity - Page 207

We will discuss about it after the holidays

Rheostat is the instrument used to change the resistance of the circuit.   

WHY IS VOLTMETER CONNECTED IN PARALLEL ?

Votmeter has a high resistance and it will minimise the current flowing through the circuit if it is connected in series.

Hence it is connected in parallel so that the potential difference remains constant.

WHY IS AMMETER CONNECTED IN SERIES ?

Ammeter has a low resistance and in series, the current remains constant.

If we connect an Ammeter in parallel there will be higher flow of current through the Ammeter, it would cause a short circuit, resulting in damaging the Ammeter and the circuit.

Formulas done today : 


To find the Voltage, ( V ) [ V = I x R ] 
                           V (volts) = I (amps) x R (Ω)

To find the Current, ( I ) [ I = V ÷ R ] 
                           I (amps) = V (volts) ÷ R (Ω)

To find the Resistance, ( R ) [ R = V ÷ I ] 
                           R (Ω) = V (volts) ÷ I (amps)

Home Assignment

1) Try Numericals based on the above formula 

2) Draw the labeled diagram of an electric circuit comprising of a cell, a resistor, an ammeter, a voltmeter and a switch (or plug key).