Current Electricity — Class 12 ISC Physics revision notes
A 7-page visual revision sheet for Current Electricity, written for Class 12 ISC Physics. Formulas, diagrams and the traps that cost marks, laid out to be read the night before rather than the term before. Free, and it opens inside CapCompass.
Page 1 as PDF
Current Electricity in one look
drift, Kirchhoff, and the two null instruments
Current, drift and the microscopic picture
- current I = q/t, and conventionally it flows in the direction a POSITIVE charge would move, which is opposite to the electron flow
- it is a scalar, despite having a direction, because currents add arithmetically at a junction and not by the parallelogram law
- free electrons move randomly at high speed; a field adds a small drift velocity v_d on top, of the order of a millimetre per second
- I = n A e v_d, with n the free electron density, and v_d = eEτ/m where τ is the relaxation time
- mobility μ = v_d / E, and the signal travels at nearly the speed of light even though the electrons crawl, because the FIELD is established almost instantly all along the wire
Ohm's law, resistance and resistivity
- Ohm's law
V = IR, at constant temperature - resistance from geometry
R = ρ l / A - resistivity, microscopically
ρ = m / (n e^2 τ) - current density
J = I/A = σE, with σ = 1/ρ - with temperature
R_t = R_0 (1 + α t), α positive for a metal - power
P = VI = I^2 R = V^2 / R
Page 1 of 7. The full sheet, all 7 pages, opens free in the app.
What this Current Electricity sheet covers
- Current, drift and the microscopic picture
- Ohm's law, resistance and resistivity
- Ohm's law
- resistance from geometry
- resistivity, microscopically
- current density
- with temperature
- power
- What temperature does, and to whom
- Where Ohm's law fails
- Cells, EMF and internal resistance
- terminal voltage on discharge
- terminal voltage on charging
- current in a simple circuit
- cells in series
- n identical cells in parallel
- maximum power transfer
- Series and parallel, and what each preserves
- Kirchhoff's rules
- The Wheatstone bridge
- In the lab: the metre bridge
- The potentiometer
- Galvanometer, ammeter and voltmeter
- Heating and power
- Combinations of cells, chosen sensibly
- Why a null method beats a deflection method
- Charging and discharging a capacitor
- Endless ladders and symmetric networks
- Traps that cost marks
- Tricks that save time
How CapCompass uses this sheet
These notes are not a standalone PDF. CapCompass plans Physics chapter by chapter against your real exam dates and free hours, and when the plan reaches Current Electricity the sheet opens inside the study block itself — at the page covering the sub-topic that block is on, with the timer still running. Revision arrives at the moment the plan says revise, instead of having to be gone looking for.