# AP Inter 2nd Year Physics Notes Chapter 6 Current Electricity

Students can go through AP Inter 2nd Year Physics Notes 6th Lesson Current Electricity will help students in revising the entire concepts quickly.

## AP Inter 2nd Year Physics Notes 6th Lesson Current Electricity

→ The rate of flow of charge through any section of the conductor is called strength of the electric current and is measured in ampere i = $$\frac{\mathrm{Q}}{\mathrm{t}}$$

→ Ohms law: A constant temperature, the current passing through a conductor is proportional to the potential difference between its ends. V ∝ I or V = IR where R is resistance of a conductor or resistor.

→ Resistor is a device that opposes the flow of current through it.

→ The direction of conventional current is from high to low potential and is opposite to the direction of electron flow.

→ The resistors which obey Ohm’s law are called Ohmic resistors.

→ The resistors which does not obey ohm’s law are called non-ohmic resistors.

→ Specific resistance is the resistance of a wire of unit length and unit area of cross-section. ρ = $$\frac{\mathrm{RA}}{l}$$. Unit of specific resistance is Ω m.

→ Current per unit area (I/A) is called current density (j). It’s unit is A/m2.

→ Reciprocal of resistance is called conductance and measured in Siemen.

→ Reciprocal of resistivity is called conductivity and measured in Simen/meter σ = $$\frac{1}{\rho}$$

→ The ratio of increase in resistance for 1°C rise of temperature to its resistance at 0°C is called temperature coefficient of resistance (α).

→ The fractional change in resistivity per unit change in temperature is called temperature coefficient of resistivity.

→ The work done per unit charge is called emf of a cell E = $$\frac{\mathrm{W}}{\mathrm{q}}$$ and measured in volts.

→ The resistance offered by the electrolyte in the cell for the flow of charges is known as internal resistance.

→ emf developed inside the cell opposes the flow of charges in the circuit is called back emf.

→ The speed with which an electron gets drifted in a metallic conductor under the application of an external electric field is called the drift velocity (Vd).

→ The average drift velocity resulting, from the application of unit electric field strength is called mobility (μ). μ = $$\frac{V_d}{E}$$ It’s unit is m2s-1 volt-1.

→ The flow of electricity in liquids is called electrolysis and constituted by the motion of ions.

→ The flow of electricity in gases is called discharge and constituted by the motion of ions.

→ When a source of emf E is connected to an external resistance R, the voltage Vext across R is given by Vext = I(R + r) where r is internal resistance of the source.

→ In series connection of resistors, total resistance Rs = R1 + R2 + …………. + Rn

→ In parallel connection of resistors, total resistance is given by
$$\frac{1}{\mathrm{R}_{\mathrm{P}}}=\frac{1}{\mathrm{R}_{\mathrm{I}}}+\frac{1}{\mathrm{R}_2}+\ldots \ldots \frac{1}{\mathrm{R}_{\mathrm{n}}}$$

→ Kirchhoffs junction rule: At any junction of circuit elements, the sum of currents entering the junction must equal the sum of currents leaving it.

→ Kirchhoffs loop rule: The algebraic sum of changes in potential around any closed loop must be zero.

→ Principle of wheat stones bridge is R4 = R3 × $$\frac{\mathrm{R}_2}{\mathrm{R}_1}$$

→ Meter bridge works on the principle of Wheat stone bridge.

→ Using meter bridge unknown resistance can be measured accurately.

→ Potentiometer is more sensitive and measures accurate potential differences.

Formulae

→ Current I = $$\frac{\mathrm{q}}{\mathrm{t}}=\frac{\mathrm{ne}}{\mathrm{t}}$$

→ Drift velocity, Vd = $$\frac{\mathrm{eE} \tau}{\mathrm{m}}$$

→ I = neA Vd also I = $$\frac{\mathrm{nAe}^2 \tau}{\mathrm{m}} \mathrm{E}$$

→ Electron mobility, m = $$\frac{e \tau}{m}$$

→ Current density, j = $$\frac{\mathrm{I}}{\mathrm{A}}$$
Also j = $$\frac{\mathrm{ne}^2 \tau}{\mathrm{m}} \mathrm{E}$$

→ Ohm’s law, V = IR

→ Resistance, R = $$\frac{\mathrm{V}}{\mathrm{I}}$$ Also R = $$\frac{\rho \mathrm{l}}{\mathrm{A}}$$
Also R = $$\frac{\mathrm{m}}{\mathrm{ne}^2 \tau} \cdot \frac{1}{\mathrm{~A}}$$
And ρ = $$\frac{m}{n e^2 \tau}$$

→ Conductance G = $$\frac{1}{\mathrm{R}}$$
Resistivity σ = $$\frac{1}{\rho}$$

→ Temperature coefficient of resistivity
α = $$\frac{\mathrm{R}-\mathrm{R}_0}{\mathrm{R}_0 \theta}$$

→ Colour code of carbon resistance

→ For series connection,
Rs =R1 + R2 + R3 + ………….

→ For parallel connection,
$$\frac{1}{R_p}=\frac{1}{R_1}+\frac{1}{R_2} \ldots \ldots . .$$

→ Terminal PD or terminal voltage
V = E – Ir = $$\left[\frac{E-V}{V}\right] R$$

→ From Kirchhoff’s first law, εi = 0

→ Kirchhoff’s second law. εE = εIR

→ Wheat stone bridge principle,
$$R_4=R_3 \times \frac{R_2}{R_1}$$