### Summary of important points

• Thales discovered that rubbed amber could attract silk.
• When charges appear on objects after rubbing, it is called frictional electricity. When two objects are rubbed, opposite charges appear on both the objects.
• Charges are of two types : positive and negative. When glass is rubbed with silk, positive charge appears on glass and negative on silk. This convention was introduced by American Scientist Benjamin-Franklin.
• Like charges repel and unlike charges attract. Charges are additive in nature.
• Charge on a body is always integral multiple of the minimum charge called electronic charge. This is called quantisation of electric charge.
• According to the Coulomb’s law of electrostatics, force between two point charges is directly proportional to the product of charges, and inversely proportional to the square of distance between them and acts along the line joining the charges. Coulomb’s law hold true only for point charges.
• Conventionally, current electricity is flow of positive charge, that is why it flows from positive terminal to negative terminal of the cell in the external circuit.
• Substances which allow passage (flow) of current through them, are called conductors. All metals are good conductor of electricity, silver is the best conductor. Conductors have large number of free electrons, that is why flow of current takes place easily.
• Substances which do not allow passage (flow) of current through them, are called insulators. Plastic, wood, rubber, etc. are insulators as these do not have large number of free electrons.
• The branch of physics which deals with charges in motion is called current electricity.
• Flow of charge per unit time is called electric current. It is a scalar quantity and its SI unit is ampere (A).
• According to the Ohm’s law the current (I) flowing through a conductor is directly proportional to the potential difference (V) across the ends of conductor, if physical conditions are kept constant.
• Substances which obey Ohm’s law are called ohmic substances. All metals are ohmic i.e. graph between V and I will be linear.
• Substances which do not obey Ohm’s law are called non-ohmic substances. Liquid electrolytes, vacuum tubes, semiconductors are non-ohmic i.e. the graph between V and I is non-linear.
• Resistance of a conductor is the obstruction caused by the conductor to the flow of current through it. SI unit of resistance is ohm and its symbol is Ω.
• Resistors are said to be connected in series between two points if there is only one path for current to flow between two points or when resistors are connected end to end consecutively.
• In series combination, the following points are to be noted:
a) current through each resistor is the same.
b) equivalent resistance is equal to the sum of individual resistors.
c) the total voltage across the combination is always equal to the sum of voltages across individual resistors.
d) the current in the circuit is independent of relative positions of various resistors in the circuit.
e) the equivalent resistance is greater than the greatest individual resistor connected in the circuit.
• Resistors are said to be connected in parallel if the potential difference across each of them is the same and is equal to the applied potential difference. Or when one end of all resistors are connected to one common point and the other end is connected to another common point then they are said to be connected in parallel.
• In parallel combination, the following points are to be noted:
a) the potential difference across each resistor is the same and is equal to the potential difference applied.
b) the reciprocal of equivalent resistance is equal to the sum of the reciprocals of individual resistances.
c) the total current in the circuit is equal to the sum of individual currents through the individual resistors.
d) the current through each resistor is inversely proportional to the resistance of the resistor.
e) the equivalent resistance is less than the least resistance used in the circuit.
• When electric current is passed through a wire, it gets heated, due to conversion of electric energy into heat energy.
• This heating is basically, due to the work done against resistance to maintain current in the wire. The role of resistance in electrical circuits is very similar to the role of friction in mechanics.
• According to the Joule’s law of heating, the heat produced (H) is directly proportional to the square of current passing through the wire (I2), resistance of the wire (R), and time (t) for which current passes through the wire.
• Electric power is defined as the rate at which work is done by electric current. Its symbol is P, SI unit is watt or volt-ampere.
• If am appliance consumes one joule of electrical energy in one second, its power will be 1 watt.
• When appliances are connected in parallel, the total power consumed is equal to the sum of individual powers of appliances.
• When appliances are connected in series, then the reciprocal of total power is equal to the sum of reciprocal of individual powers.
• Electric energy is defined as the total work done by an electric current in a given time. SI unit of energy is joule and commercial unit is kWh.

#### Important Formulae    