Physics Standard 12 Vol I>Chapter -1 - Current Electricity>EVALUATION>Q 6

MEDIUM

12th Tamil Nadu Board

IMPORTANT

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What are ohmic and non-ohmic devices?

Important Points to Remember in Chapter -1 - Current Electricity from Tamil Nadu Board Physics Standard 12 Vol I Solutions

1. Current:

(i) Current through a given area of a conductor is the net charge passing per unit time through the area.

(ii) SI unit of current is ampere and it is a scalar quantity.

2. Electromotive force:

The work done per unit charge by the source in taking the charge from lower to higher potential energy is called the electromotive force, or emf, of the source.

3. Ohm's law:

(i) Ohm’s law: The electric current $I$ flowing through a substance is proportional to the voltage $V$ across its ends, i.e., $V \propto I$ or $V = R I$ , where $R$ is called the resistance of the substance. The SI unit of resistance is ohm(Ω).

(ii) The resistance $R$ of a conductor depends on its length $l$ and cross-sectional area $A$ through the relation, $R = \frac{ρ l}{A}$ where $ρ$ is called resistivity of the material, which depends on the temperature.

(iii) Electrical resistivity of substances varies over a very wide range. Metals have low resistivity, in the range of $10^{- 8} Ω m$ to $10^{- 6} Ω m$ . Insulators like glass and rubber have $10^{22}$ to $10^{24}$ times greater resistivity. Semiconductors like Si and Ge lie roughly in the middle range of resistivity on a logarithmic scale.

4. Current density and drift velocity:

(i) Current density $j$ gives the amount of charge flowing per second per unit area normal to the flow, $j = n q v_{d}$ where $n$ is the number density (number per unit volume) of charge carriers, q is the charge, and $v_{d}$ is the drift velocity of the charge carriers. For electrons $q = - e$ .

(ii) Drift velocity, $v_{d} = e E τ / m$ where $e$ is the charge of the electron, E is the electric field, m is the mass of the electron and $τ$ is the relaxation time.

(iii) Resistivity, $ρ = \frac{m}{n e^{2} τ}$ where $e$ is the charge of the electron, $n$ is the number of electrons per unit volume, m is the mass of the electron and $τ$ is the relaxation time.

5. Temperature coefficient of resistivity:

In the temperature range in which resistivity increases linearly with temperature, the temperature coefficient of resistivity is defined as the fractional increase in resistivity per unit increase in temperature.

6. Combination of resistors:

(i) Total resistance $R$ of $n$ resistors connected in series is given by: $R = R_{1} + R_{2} + \dots + R_{n}$

(ii) Total resistance $R$ of $n$ resistors connected in parallel is given by: $\frac{1}{R} = \frac{1}{R_{1}} + \frac{1}{R_{2}} + \dots \dots + \frac{1}{R_{n}}$

7. Kirchhoff's law:

(i) Kirchhoff’s junction Rule: At any junction of circuit elements, the sum of currents entering the junction must be equal the sum of currents leaving it.

(ii) Kirchhoff’s Loop Rule: The algebraic sum of changes in potential around any closed loop must be zero.

8. Wheatstone bridge:

The Wheatstone bridge is an arrangement of four resistances $- R_{1}, R_{2}, R_{3} a n d R_{4}$ . The null-point condition is given by $\frac{R_{1}}{R_{2}} = \frac{R_{3}}{R_{4}}$

9. Potentiometer:

The potentiometer is a device to compare potential differences. The device can be used to measure potential difference; internal resistance of a cell and compare emf’s of two sources.

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