Ohm’s law:

Ohm’s law states that electric current flowing through a conductor is directly proportional to the potential difference applied across it (i.e., conductor). This law can also be stated in the form of an expression as given below :

V = I R

where V = potential difference applied across the conductor, I = current flowing through conductor, and R = resistance of conductor. V is measured in volt or V, I is measured in ampere or A, and resistance is measured in ohm or Ω.

Conductance:

Reciprocal of resistance is called conductance. Its SIU is siemens or S.

Drift velocity:

When electric field is applied to conductor then there is definite drift of electrons and hence net flow of electrons in the direction opposite to electric field. Though drift velocity is small (about 10^-3 m/s) because of large number of free electrons, we get good current.

Specific resistance or resistivity (ρ):

The resistance of wire per unit length and per unit cross sectional area gives us the specific resistance or resistivity (ρ) of the material of wire. SIU of specific resistance or resistivity is Ωm. Depending upon resistivity, materials are classified as (i) conductors, (ii) insulators, and (iii) semiconductors.

Conductivity (σ):

Reciprocal of resistivity is called conductivity (σ) of the material. SIU of conductivity is S/m or siemens/metre.

Temperature coefficient of resistance (α):

The temperature coefficient of resistance (α) is defined as the increase in resistance per unit original resistance at 0^oC, per degree rise in temperature. It is given by the following expression :

α = (R_t – R₀) / (R₀ t)

where Rt = resistance of wire at t^oC, R₀ = resistance of wire at 0^oC, t = temperature in ^oC, α = temperature coefficient of resistance of material of wire.

Conductor:

A material whose resistivity is negligibly small is called a conductor.

Insulator:

A material whose resistivity is very high is called an insulator.

Semiconductor:

A material whose resistivity lies between that of conductor and insulator is called a semiconductor.

Thermistor:

A semiconductor whose resistivity is temperature dependent is called a thermistor. Thermistor is abbreviation of THERMal resISTOR.

Positive temperature coefficient (PTC):

Resistance of thermistor having positive temperature coefficient increases with temperature.

Negative temperature coefficient (NTC):

Resistance of thermistor having negative temperature coefficient decreases with temperature.

Superconductivity:

Superconductivity is a phenomenon in which resistivity of some materials becomes zero at particular low temperature. That material remains in superconducting state at all temperatures below that particular temperature. For example, mercury becomes superconductor at 4.2 K.

Critical temperature (T_C):

The particular low temperature at which a material becomes a superconductor is called a critical temperature (T_C). That material remains in superconducting state at all temperatures below the critical temperature.

Internal resistance of a cell:

A cell has internal resistance and certain amount is spent in overcoming internal resistance. Thus, total energy spent in moving a unit charge round the complete circuit is used in two parts: (a) a part is spent in overcoming internal resistance, and (b) remaining part is spent in overcoming external resistance. Potential difference of a cell : Energy spent by source of emf in circulating per unit charge through external resistance is called potential difference of the cell.

Electromotive force (EMF) of a cell:

The energy supplied by a cell to circulate a unit charge once round the complete circuit, is called EMF of the cell. EMF is abbreviation of Electro Motice Force, it is not a force but energy spent per unit charge.

Battery:

The combinations of cells is called a battery.

Cells in series:

When cells in a battery are in series then negative terminal of first cell is connected to positive terminal of second cell, negative terminal of second cell is connected to positive terminal of third cell, and so on. Load is connected between positive terminal of first cell and negative termi- nal of last cell. If V is p.d. of each cell and there are n cells in battery then p.d. of battery is now n.V. However current capacity of battery is same as that of a single cell. Thus there is addition of voltages.

Cells in parallel:

When cells in a battery are in parallel then positive terminals of all cells are connected together to create positive terminal of battery. Similarly, negative terminals of all cells are connected together to create negative terminal of battery. Load is connected between positive and negative terminals of battery. If I is current supplied by each cell and there are n cells i the battery then current supplied by battery is now n.I. However, voltage output of battery is same as that of a single cell. Thus there is addition of currents.

Work done by electric current:

Assuming that electric current I is used to generate heat across a conductor having resistance R, work done by this electric current is given by the following expres- sion:

W = I² R t

where W = work done by current, I = current, R = resistance, t = time. W is in J or joule, I is in A or ampere, R is in Ω or ohm, t is in s or second.

Power in electric circuit:

Assuming a simple electric circuit that consists of only a cell and a resistance, power in this electric circuit (P) is given by the product of potential difference across and resistance (V) and current flowing through circuit (I). The expression for power is given below:

P = V. I

SIUnit of power is W or watt. Thus 1 watt = 1 volt × 1 ampere. Power is scalar quantity. FPS unit of power is horse power. 1 hp = 745.7 watt.

Relation between watt and horse power:

Relation between watt (W) and horse power (hp) is given below: 1 hp = 745.7 W

Units Displayed in Electric Meter:

An electric meter shows how much electric energy you have consumed in terms of “units.” This “unit” is nothing but amount of energy consumed when an electric appliance of 1000 W is used for 1 hour or 3600 second; i.e., i unit = 1 kWh = 1 kilo.watt.hour. Thus : 1 “unit” of energy = 1000 W × 3600 s = 360000 J = 3.6 × 10⁶ J