Electricity | Revision Notes

Electricity | Revision Notes

Revision Notes

Electric Current

  • Electric current is expressed as the amount of charge flowing through a particular area in unit time.
  • Quantitatively, electric current is defined as the rate of flow of electric charge.

​​​​​Current, I =\frac{Q}{t}

  • The S.I. unit of current is ampere (A), where 1 ampere = 1 coulomb/second.
  • 1 mA = 10−3 A, 1 μA = 10−6 A
  • The conventional direction of electric current is the one in which positive charges move orderly.

Electric Potential Different 

  • A battery provides the driving force required to move the charges along the wire from one terminal to another.
  • The chemical reaction within a cell generates the potential difference across the terminals of the cell.
    This constant potential difference between the two terminals of the cell maintains a constant electric current through the circuit. Thus, in order to maintain a constant electric current, the cell has to spend the chemical energy stored in it.
  • Electric potential difference between two points in an electric circuit, carrying some current, is the amount of work done to move a unit charge from one point to another.

V = \frac{W}{Q}

where V is potential difference, W = workdone and Q = amount of charge moved.

  • The S.I. unit of potential difference is volt (V), where 1 volt = 1 joule/coulomb.
  • One volt is the potential difference between two points in a current carrying conductor when one joule of work is done to move a charge of one coulomb from one point to another.
  • Potential difference is measured in a circuit by an instrument called the Voltmeter. It is always connected in parallel across the points components where it is to be measured.

Electric Circuit

  • A continuous conducting path between the terminals of a source of electricity is called an electric circuit.
  • A drawing showing the way various electric devices are connected in a circuit is called a circuit diagram.
  • Some commonly used circuit elements are given below:

Ohm’s law

  • According to Ohm’s law, the current (I) flowing through a conductor is directly proportional to the potential difference (V) across its ends, provided its physical conditions remain the same.
    V ∝ I
    V / I = Constant
    V / I = R
    V = IR
    where R is a constant of proportionality called resistance of the conductor.
  • Resistance is the property of a conductor to resist the flow of charges through it.
  • The S.I. unit of resistance is ohm ( Ω ).
  • If we plot the V-I graph for a conductor, then it shows a linear nature.

  • The slope of the graph gives the resistance of the conductor.
  • If the potential difference across two ends of conductor is 1 V and the current through it is 1 A, then the resistance of the conductor is 1 Ω.
  • From R = V / I , 1 ohm = 1 volt/ampere
  • In practice, the current needs to be increased or decreased. This is achieved by a variable resistor called a rheostat.
  • A conductor offering less resistance is called a good conductor, and a conductor offering high resistance is called a poor conductor. An insulator offers a very high resistance.

Factors on which the Resistance of a Conductor depends

The resistance of a conductor is

  • Directly proportional to the length (l) of the conductor.
  • Inversely proportional to the area of cross-section (A) of the conductor.
  • Depends on the nature of the material of the conductor.
  • Depends on the temperature.

Resistivity

  • The resistance of a conductor is directly proportional to its length (l) and inversely proportional to its area of cross section (A).
R ∝ l / A
R = ρ l / A

where ρ is a constant of proportionality called specific resistance or resistivity of the material of the conductor.

  • The S.I. unit of resistivity is ohm metre (Ω m).

Combination of Resistances

Resistances in Series

  • When two or more resistors are joined from end to end, the resistances are said to be connected in series.

  • The current flowing through each resistance is the same.
  • The potential difference is the sum of potential differences across all the individual resistors.

i.e. V = V1 + V2 + V3                       ...(1)

were V1, V2, V2 are the potential difference across R1, R2 and R3 respectively.

 

  • The equivalent resistance (Rs) of a series combination containing resistances R1, R2, R3... is
Rs = R1 + R2 + R3 + ...
  • The equivalent resistance is greater than the greatest resistance in the combination.

Resistances in Parallel

  • The potential difference across each resistance is the same and is equal to the potential difference across the combination.
  • The main current divides itself, and a different current flows through each resistance.
  • The equivalent resistance (Rp) of a parallel combination containing resistances R1, R2, R3... is given by
1 / Rp = 1 / R1 + 1 / R2 + 1 / R3 + ...
  • The equivalent resistance is lesser than the least of all the resistances in the combination

Heating Effect of Electric Current

  • The effect of electric current due to which heat is produced in a conductor, when current passes through it, is called the heating effect of electric current.
  • The total work (W) done by the current in an electric circuit is called electric energy and is given as
W = VIt = i2Rt

This energy is exhibited as heat. Thus, we have H = VIt = i2Rt.
This is called Joule’s Law of Heating, which states that

the heat produced in a resistor is directly proportional to the

  • Square of the current in the resistor
  • Resistance of the resistor
  • Time for which the current flows through the resistance

Practical Applications of the Heating Effects of Electric Current

  • Electrical appliances like laundry iron, toaster, oven, kettle and heater are some devices based on Joule’s Law of Heating.
  • The concept of electric heating is also used to produce light, as in an electric bulb.
  • Another application of Joule’s Law of Heating is the fuse used in electric circuits.

Electric Power

  • Electric power is the rate at which electrical energy is produced or consumed in an electric circuit
P = Vi = i2R
P = V2/ R

The S.I. unit of power is watt (W).

  • One watt of power is consumed when 1 A of current flows at a potential difference of 1 V.
  • The commercial unit of electric energy is kilowatt hour (kWh), commonly known as a unit.
1 kWh = 3.6 MJ