Overview: Electric Resistance and Ohm's Law

If in the flow of 1 C of charge in a circuit, the work done by the cell be 1 J, then the emf of the cell is 1 V.

Current density is defined as the current flowing through unit cross-sectional area drawn through that point perpendicular to the direction of flow of current.

Mathematically,
j = \[\frac {I}{A}\]

SI unit = ampere/metre² (A m^-2), Dimensions = [A L^-2].

The terminal potential difference of a cell is equal to the work done for the flow of a unit charge in the external circuit only.

Mathematically,
V = \[\frac {W_{ext}}{q}\]

If a small change ΔV in the potential difference across a part of a non-ohmic circuit causes a change ΔI in electric current, then the ratio ΔV/ΔI is called the 'dynamic resistance' of that part of the circuit.

Mathematically.
\[\frac {ΔV}{ΔI}\]

Metre bridge is a sensitive device based on the principle of Wheatstone's bridge, for the determination of the resistance of a conductor (wire).

The average distance moved by a free electron between two successive collisions is called 'mean free path' of the electron.

It is an important instrument for measuring the emf of a cell or the potential difference between two points of an electric circuit.

The ratio of the intensity of the electric field E at any point within the conductor and the current-density j at that point is called ‘specific resistance' or ‘electrical resistivity' of the conductor and is represented by ρ.

Mathematically,
ρ = \[\frac {E}{j}\]

Dimensions = [M L³ T^-3 A^-2]

The reciprocal of specific resistance is called 'specific conductance' and is represented by σ.

σ = \[\frac {1}{ρ}\]

SI unit = (ohm-metre)^-1 ⇒ (Ω-m)^-1
Dimension = [M^-1 L^-3 T³ A²]

I = \[\frac{E}{\left(\frac{r}{n}+R\right)}=\frac{nE}{r+nR}\]

I = \[\frac{mnE}{nr+mR}\]

When two or more resistances connected between two points are replaced by a single resistance such that there is no change in the current of the circuit and the potential difference between those two points, the single resistance is called the equivalent resistance.

• Purpose of a rheostat: A rheostat is used to control the current in an electric circuit by changing resistance.
• Construction: It has a Nichrome wire wound on a china-clay cylinder with a sliding contact.
• As a current controller: When connected through A–C or B–C, moving the sliding contact changes the current in the circuit.
• As a potential divider: When connected across A and B, and the circuit is taken from A–C (or B–C), the rheostat provides a variable fraction of the applied potential difference.
• Working principle: Sliding the contact changes the wire's effective length, thereby changing its resistance.

The potential difference between two points in an electric circuit is defined as the work done in carrying a unit charge from one point to the other.

1 kilowatt-hour, or 1 unit, is the quantity of electric-energy which is dissipated in 1 hour in a circuit when the electric power in the circuit is 1 kilowatt.

• Principle: The metre bridge works on the Wheatstone bridge principle, and balance is obtained at the null point where the galvanometer shows no deflection.
• Null point condition: At the null point, points B and D are at the same potential and
  \[\frac {P}{Q}\] = \[\frac {R}{S}\]
• Finding unknown resistance: If the wire is divided into lengths l and 100 − l, the unknown resistance is
  S = R\[\frac {(100−l)}{l}\].
• Reducing errors: Errors are reduced by interchanging the known and unknown resistances and taking the mean value.
• Precautions: Keep the null point near the middle, avoid heating the wire, and press the jockey lightly without rubbing.

1 kW-h = 3.6 x 10⁶ W-s = 3.6 × 10⁶ J

Units = \[\frac {watt × hour}{1000}\]

• Null-deflection method: At balance, no current flows through the galvanometer, making the measurement independent of the cell's internal resistance.
• Uniform wire requirement: The potentiometer wire must have a uniform cross-section and material so that the potential drop along the wire is uniform.
• True emf measurement: The emf is measured in open circuit, ensuring the true value of the emf is obtained without energy loss in the cell.
• Sensitivity dependence: The sensitivity of a potentiometer increases as the potential gradient decreases, using a long wire and low current.
• Experimental precautions: Current should not flow for a long time to avoid heating of the wire, and touch the jockey lightly to prevent wire damage.

Statement

The variation of current with voltage is the macroscopic form of Ohm’s law. When the situation is considered at a point, the law is known as Ohm’s law in microscopic (vector) form.

Explanation/Proof

From, V = \[\frac{m}{ne^2\tau}\frac{l}{A}I\]

or

\[\frac{V}{l}=\left(\frac{m}{ne^{2}\tau}\right)\left(\frac{I}{A}\right)\]

But,

\[\frac {V}{l}\] = E, \[\frac {m}{n e^2 τ}\] = ρ and \[\frac {I}{A}\] = j,

\[\therefore\] E = ρ j

Also, ρ = \[\frac {1}{σ}\]​

Hence,

E = \[\frac {1}{σ}\]j or j = σ E

In vector notation,

\[\vec j\] = σ\[\vec E\]

Conclusion

Therefore, for an isotropic substance,

\[\vec j\] ∝ \[\vec E\]

and Ohm’s law in vector form states that the current density is directly proportional to the applied electric field strength, and the ratio of current density to electric field is a constant σ, independent of the electric field producing the current.

• Ohm’s law does not hold when temperature changes due to current flow, causing resistance to vary (e.g., filament bulb).
• In some materials, current starts flowing only after a minimum applied voltage, so the V–I graph is not linear.
• Devices like diodes, thermistors, and vacuum tubes are non-ohmic because their resistance is not constant

• Carbon resistors use colour codes to indicate resistance value; the first two bands give significant figures and the third band gives the multiplying power of 10.
• The fourth colour band indicates the resistor tolerance: gold (±5%), silver (±10%), and no band (±20%).
• The colour sequence Black to White represents digits 0 to 9, and the same colours in the third band represent multipliers 10⁰ to 10⁹.

• Series combination: Same current flows through all resistances, and the equivalent resistance is
  R = R₁ + R₂ + R₃​
• Series property: In a series, the equivalent resistance is greater than the largest individual resistance, and the voltage divides in the ratio of resistances.
• Parallel combination: Same potential difference exists across all resistances and the equivalent resistance satisfies
  ​\[\frac{1}{R}=\frac{1}{R_1}+\frac{1}{R_2}+\frac{1}{R_3}\]
• Parallel property: In parallel, the equivalent resistance is less than the smallest individual resistance, and current divides inversely with resistance.
• Practical use: Household electrical appliances are connected in parallel, so each works independently at the same voltage.

• Series combination: The net power consumed decreases; for identical bulbs,
  P_consumed = \[\frac {P}{n}\]​
  and it is directly proportional to bulb resistance and inversely proportional to rated power.
• Parallel combination: The net power consumed increases; for identical bulbs,
  P_consumed = n P
  and it is inversely proportional to bulb resistance and directly proportional to rated power.