Current electricity (Class 12): Ohm's law, Kirchhoff and bridge circuits
Current electricity explains how charge flows through a wire and a network. From Ohm's law to the potentiometer, these labs let you connect the circuit, read the meters and plot the graph just as in the school practical.
NCERT Class 12 Physics, Chapter 3 (Current Electricity). Every lab below runs in 3D in your browser or on a phone, in English and Hindi.
Try the labs
Ohm's law: V–I characteristics · free · Chapter 3
To verify Ohm's law for a metallic wire, find its resistance and resistivity, and to compare ohmic and non-ohmic conductors.
Kirchhoff's laws on circuits · plan · Chapter 3
To analyse a two-loop circuit using Kirchhoff's junction rule and loop rule.
Metre bridge and Wheatstone bridge · plan · Chapter 3
To determine the resistance of an unknown wire/coil using a metre bridge (a practical form of the Wheatstone bridge).
Potentiometer: emf comparison, internal resistance · plan · Chapter 3
To compare the emfs of two cells and determine the internal resistance of a cell using a potentiometer.
Internal resistance of a cell, combination of cells · plan · Chapter 3
To study the internal resistance of a cell, the terminal potential difference and the grouping of cells.
Drift velocity of electrons in a conductor · plan · Chapter 3
To understand the drift of free electrons in a conductor, the microscopic form of Ohm's law and the meaning of drift velocity, relaxation time and mobility.
Galvanometer: resistance and figure of merit (half-deflection) · plan · Chapter 3
To find the resistance G and the figure of merit k of a moving-coil galvanometer by the half-deflection method.
Resistivity and temperature dependence of a wire · plan · Chapter 3
To study the dependence of resistance on length, area and temperature and to find the temperature coefficient of resistance.
Resistor colour code and series/parallel combinations · free · Chapter 3
To read the value of a resistor from its colour code and verify the equivalent resistance, current and power of series and parallel combinations.
Resistance versus temperature: metal, platinum RTD and NTC thermistor · plan · Chapter 3
To study the variation of resistance with temperature for a metal, a platinum RTD and an NTC thermistor and to find the temperature coefficient of resistance.
Thermocouple: Seebeck effect, neutral and inversion temperature · plan · Chapter 3
To study the Seebeck effect in a thermocouple: thermo-emf vs temperature, neutral temperature and temperature of inversion.
Wheatstone bridge: balance condition and galvanometer current · plan · Chapter 3
To study the Wheatstone bridge, find the balance condition and measure an unknown resistance.
Ohm's law and resistivity
For a metallic conductor at constant temperature the current is proportional to the potential difference, V = IR. The resistance depends on the shape through R = ρl/A, where the resistivity ρ is a property of the material and the temperature.
The drift velocity of electrons is tiny, about a millimetre per second, even though the electric field travels at nearly the speed of light.
V = IRR = ρ l / AI = n e A vdRₜ = R₀(1 + αΔT)
Kirchhoff's rules
The junction rule says the sum of currents entering a node is zero (charge conservation). The loop rule says the sum of potential changes around a closed loop is zero (energy conservation). Together they solve any network.
ΣI = 0 at a junctionΣV = 0 around a loop
Bridges and the potentiometer
A Wheatstone bridge is balanced when P/Q = R/S, and the metre bridge uses a uniform wire for this. A potentiometer compares emf by finding the balance length where no current flows through the galvanometer, so it measures the true emf, not the terminal voltage.
The internal resistance of a cell follows from the balance lengths with and without a shunt: r = R(l₁ − l₂)/l₂.
P/Q = R/SE₁/E₂ = l₁/l₂r = R (l₁ − l₂) / l₂
Frequently asked questions
Why is a potentiometer more accurate than a voltmeter?
At the balance point it draws no current from the cell, so the cell's internal resistance causes no drop and the reading equals the emf.
Why does resistance of a metal increase with temperature?
The lattice ions vibrate more and the electrons collide more often, so the relaxation time falls.
What is a null method?
A method where the reading is taken when a detector shows zero deflection, as in the metre bridge, which avoids errors of the detector itself.
Is Ohm's law always valid?
No. Diodes, thermistors and filament lamps do not have a straight V–I graph; it holds for metals at constant temperature.