NEET Physics Notes Electrostatics-Electrical Resistance
The obstraction offered by any conductor in the path of flow of current is called electrical resistance (R).
Its SI unit is Ohm
In terms of free electron density n and relaxation time the resistance of a conductor is given by
Resistance of Different Materials
Resistance offered by the conductors is minimum while resistance offered by an insulator is maximum. Semiconductors have resistance which is intermediate to conductor and insulator
V-I Characteristics of Ohmic and Non-ohmic Conductors
Substances obeying Ohm’s law are called ohmic resistors, e.g. metals and their alloys. V-I graph for an ohmic resistor is an inclined straight line. Substances which do not obey Ohm’s law are called non-ohmic resistors, e.g. electrolytes, gases, thermionic tubes, transistors, rectifiers, etc., V-I graph for them is non-linear.
Resistivity is a characteristic of material, i.e. it does not depend upon dimensions of the material. Although it depends on temperature.
In terms of free electron density n and relaxation time
SI unit of resistivity is m and its dimensional formula is [ML3T–3A-2].
Colour Code for Resistors
The value of resistance used in electric and electronic circuit vary over a wide range.
Such high resistances used are usually carbon resistances and the values of such resistances are marked on them according to a colour code.
Series and Parallel Combinations of Resistors
- Series Grouping In series grouping of resistances same current I flows through all the resistances.
The potential difference applied across the combination is distributed across various resistors in the direct ratio of their resistances,
i.e. V = V1+ V2 + V3 + …
and V1 :V2 :V3 … = R1 : R2 : R3 …
- Total equivalent resistance in series grouping is equal to the sum of individual resistances.
Rs = R1 + R2 + R3 +…
- Parallel Grouping In parallel grouping same potential difference V appears across each resistance.
- The current is distributed among various resistors in the inverse ratio of their resistances. Thus,
- Equivalent resistance in parallel grouping Rp is given by
- If n identical resistances are first connected in series and then in parallel, the ratio of the equivalent resistance
- If there are n resistors of different values, then we can have 2″ different possible combinations.
- If a skeleton cube is made with 12 equal resistances, each having a resistance R, then the net resistance across (a) the
- diagonal of
- The diagonal of a face
- Alomg a side
Temperature Dependence of Resistance
Resistance and resistivity of metallic conductors increases with increase in temperature. The relation is written as
Where, R0 and are values of resistance and resistivity at 0° C and are two constants whose value vary from metal to metal.
Electric Energy and Power
Whenever the electric current is passed through a conductor, it becomes hot after short time. This indicates that the electric energy is being converted into heat energy. This effect is known as heating effect of current or Joule heating effect.
SI unit of electric energy is joule but another unit is watt-hour.
The rate at which work is done by the source of emf in maintaining the effect of current in a circuit is called electric power of the circuit
where, V is the potential difference across the conductor, I is the current flowing through the conductor.
Other expressions for power
SI unit of electric power is Watt.
Another important unit is Horse Power (HP), 1HP = 746 W.