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NEET Physics Notes Electrostatics-Electrical Resistance

NEET Physics Notes Electrostatics-Electrical Resistance

Electrical Resistance

Electrical Resistance

The obstraction offered by any conductor in the path of flow of current is called electrical resistance (R).
NEET Physics Notes Electrostatics-Electrical Resistance 1
Its SI unit is Ohm Omega
In terms of free electron density n and relaxation time tau the resistance of a conductor is given by
NEET Physics Notes Electrostatics-Electrical Resistance 2
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.
NEET Physics Notes Electrostatics-Electrical Resistance 3

Electrical Resistivity

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 tau
Resistivity, NEET Physics Notes Electrostatics-Electrical Resistance 4
SI unit of resistivity is Omega 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.
NCERT Solutions for Class 8 Maths Chapter 14 Factorisation Ex 14.1 13
NEET Physics Notes Electrostatics-Electrical Resistance 6
Series and Parallel Combinations of Resistors

NEET Physics Notes Electrostatics-Electrical Resistance 7

  • 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.
    Thus,
    Rs = R1 + R2 + R3 +…
  • Parallel Grouping In parallel grouping same potential difference V appears across each resistance.NEET Physics Notes Electrostatics-Electrical Resistance 8
  • The current is distributed among various resistors in the inverse ratio of their resistances. Thus,
    NEET Physics Notes Electrostatics-Electrical Resistance 9
  •  Equivalent resistance in parallel grouping Rp is given by
    NEET Physics Notes Electrostatics-Electrical Resistance 10
  •  If n identical resistances are first connected in series and then in parallel, the ratio of the equivalent resistance
    NEET Physics Notes Electrostatics-Electrical Resistance 11 or NEET Physics Notes Electrostatics-Electrical Resistance 12
  •  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 NEET Physics Notes Electrostatics-Electrical Resistance 13
  • The diagonal of a face NEET Physics Notes Electrostatics-Electrical Resistance 14
  • Alomg a side NEET Physics Notes Electrostatics-Electrical Resistance 15

Temperature Dependence of Resistance

Resistance and resistivity of metallic conductors increases with increase in temperature. The relation is written as
NEET Physics Notes Electrostatics-Electrical Resistance 16
Where, R0 and NEET Physics Notes Electrostatics-Electrical Resistance 17 are values of resistance and resistivity at 0° C and NEET Physics Notes Electrostatics-Electrical Resistance 18 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.
NEET Physics Notes Electrostatics-Electrical Resistance 19
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
P= VI
where, V is the potential difference across the conductor, I is the current flowing through the conductor.
Other expressions for power
NEET Physics Notes Electrostatics-Electrical Resistance 20
SI unit of electric power is Watt.
Another important unit is Horse Power (HP), 1HP = 746 W.

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