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.

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• 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.

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).

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 (R_s) of a series combination containing resistances R₁, R₂, R₃... is

• 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 (R_p) of a parallel combination containing resistances R₁, R₂, R₃... is given by

• 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

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

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.