Chapter- 12
Electricity
Charge
Protons and electrons possess some charge. Proton has positive charge. Electron has negative charge. Neutron does not possess any charge. Like charges repel each other and unlike charges attract each other.
Conductors and Insulators
Conductors are those materials in which electrons can move freely. All metals are conductors. Insulators do not have any free electrons to move. For example, wood and plastic.
Electric Current
The flow of electric charge is known as Electric Current. It is expressed in terms of rate of flow of charges.
The SI unit of electric current is Ampere (A).
Direction of electric current is the same as the direction of positive charges and opposite to the direction of flow of negative charges.
Potential Difference
Work done per unit charge in taking charge from one point to another is known as Potential Difference. The unit of potential difference is volt (V). 1V is defined as the potential difference between two points if 1 Joule of work is done to move 1 coulomb charge from one point to another.
Ohm’s law
Potential difference between two points is directly proportional to the current, provided the temperature is constant.
V ∝ l
⇒ V = lR
R is a constant known as Resistance. The SI unit of resistance is the ohm (Ω)
Factors on which resistance of a conductor depends-
It is directly proportional to length of conductor.
Inversely proportional to the area of cross-section.
Directly proportional to the temperature.
Depends on the nature of material.
Resistivity
Resistivity is the property of the material. The SI unit of resistivity is ohm-metre.
Resistivity of metals varies from 10-8 to 10-6.
Resistivity of insulators varies from 1012 to 1017
Copper and aluminium are used in electrical transmission due to their low resistivity.
Resistance = Resistivity * Length of Conductor/Cross Sectional Area
Resistors in series
When two or more resistors are joined in series, then their total resistance is given by the formula-
RS = R1 + R2 + R3
The current will remain the same through all resistors. Total voltage is given by-
V = V1 + V2 + V3
Voltage across each resistor is given as –
V1 = lR1
V2 = lR2 [V1 + V2 + V3 = V]
V3 = lR3V = lR
⇒ V = lR1 + lR2 + lR3
lR = l(R1+ R2 + R3)
R = R1 + R2 + R3
Resistors in parallel
In this case, voltage is the same across each resistor and is equal to the applied voltage. Total current is given as-
V/R = V/R1 + V/R2 + V/R3
1/Rp = 1/R1 + 1/R2 + 1/R3
Advantages of Parallel Combination over Series Combination
If one component fails in series combination, then complete circuit is broken and no component can work properly. Different appliances need different current, this can be met through parallel.
Heating effects of Electric Current
When charge Q moves against the potential difference V in time t, the amount of work is given by-
Joule’s Law of Heating
Heat produced in a resistor is directly proportional to the square root of current.
It is also directly proportional to resistance for a given current.
Also, directly proportional to time
H = l2 Rt
Filament of electric bulb is made up of tungsten because it has a very high melting point and also does not oxidize readily at a high temperature.
Electric fuse is a safety device to protect electrical appliances from short circuit.
Electric Power
The rate at which electric energy is dissipated or consumed in an electric current. The SI unit of power is Watt.
P = Vl
⇒ P = l2 R = V2/R
The commercial unit of electric energy is kilowatt hour (KWh).