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CBSE Class 12 · Physics

Current Electricity

Official NCERT chapter from Physics Part I–II (book code leph1). ExamMaster notes are original teaching at CBSE Class 12 depth.

This lesson follows the official chapter “Current Electricity” in Physics Part I–II. The words below are ExamMaster’s teaching, not a paste from the book. Use the NCERT chapter for the classroom sequence; use these notes to hold the idea without copying exercises or figures.

  • CBSE Class 12
  • Medium level
  • 12 concepts

1Electric Current

Electric current is charge-flow per time as taught: I = Q/t. Ampere is the unit. Current is a through-rate, not a leftover static charge. Direction as the lesson framed (conventional +).

A charge sitting still as “current” is a miss.

Figure. Current is charge crossing one marked cut per second. The cut is area A. A pile of charge sitting still is not a current.

How it works

  1. Name Q through in time tThe given.
  2. Form I=Q/tCurrent.
  3. Keep the ampereThe unit.

2Electric Currents in Conductors

Currents in conductors: charges drift in a field as taught. A metal has free electrons as the school carriers. Current-in-a-wire is this drift-story, not a pipe of plus sitting still.

A vacuum as the default conductor here is a skip.

Figure. A metal has free electrons. The field drives a slow drift from the negative end toward the positive end. The current arrow is the conventional + to − name of that same flow.

How it works

  1. Name free charges in the metalThe conductor.
  2. Say a field makes them driftThe current.
  3. Keep drift as a later heading tooThe link.

3Ohm’s law

Ohm’s law: V = I R as taught, for the ohmic range. R is the ratio V/I. A resistor that keeps R constant is ohmic. Ohm is a ratio-test, not “any device”.

A bulb that changes R called ohmic is a miss if the lesson said so.

Figure. Ohm's law is a straight V–I line through the origin. Slope is R. A line that misses O or bends is not this law.

How it works

  1. Form V/IR.
  2. Ask whether R stays putOhmic?
  3. Keep V=IR as the writeThe law.

Ohm size

V=6 V, I=2 A. Find R.

  • V/I6/2
  • R3 Ω
  • Readohmic write

Pro tip. R is the ratio.

4Drift of Electrons and the Origin of Resistivity

Drift and resistivity: I = n e A vd as taught; ρ sits in R = ρ ℓ / A. Drift is a slow average; resistivity is a material-number. Origin of resistivity is scatter as framed.

Using speed-of-light as vd is a miss.

Figure. Electrons scatter. Superposed on that zig-zag is a slow drift v_d along E. Resistivity belongs to the substance; resistance belongs to this length and this cut A.

How it works

  1. Name n, A, vd as the current-writeDrift.
  2. Write R=ρ ℓ / AResistivity sits here.
  3. Keep vd small as the school cautionAn average.

5Limitations of Ohm’s Law

Limitations of Ohm: some devices are non-linear as taught (a diode mention later). Limitation is a when-not, not a second law. One I–V curve that is not a line is enough.

Forcing V=IR on every graph is a steal.

Figure. Both curves start at the origin. The ohmic line keeps one slope. A hot filament steepens: the same extra current now needs more V, so R is no longer one number.

How it works

  1. Name a non-linear I–V as taughtThe limit.
  2. Keep Ohm for the straight-rangeHonest.
  3. Refuse one law for every deviceA when-not.

6Resistivity of Various Materials

Resistivity of materials: metals, alloys, semiconductors as taught — different ρ and different temperature-looks. A table-look is enough. Resistivity is a material, not a length.

Calling R the same as ρ is a miss.

Figure. Resistivity spans decades, so the axis is log. Copper is a short bar, silicon sits mid-scale, glass is the long insulator bar. A linear axis would hide the metal.

How it works

  1. Name the material-classMetal / alloy / semiconductor.
  2. Read ρ as the material-numberThe heading.
  3. Keep R = ρ ℓ / A as the linkTwo words.

7Temperature Dependence of Resistivity

Temperature dependence: metals typically rise in ρ as T rises as taught; semiconductors often fall. A coefficient α if named. Temperature-look is a trend, not a second Ohm.

Every material rising is a steal.

Figure. For a metal, resistivity rises with temperature from a non-zero ρ0. The line must not be forced through the plot origin — that would invent ρ = 0 at T = 0.

How it works

  1. Name metal-rise or semiconductor-fall as taughtThe trend.
  2. Use R=R0(1+αΔT) if listedThe write.
  3. Keep the class honestNot one slogan.

8Electrical Energy, Power

Electrical energy and power: P = V I = I^2 R = V^2/R as taught. Power is a rate of energy. A 6 V × 2 A = 12 W case is enough. Energy = P t.

Using V/I as power is a miss.

Figure. Power is the product: 4 V times 3 A is 12 W. Energy is that power times time. Do not add V and I.

How it works

  1. Form VI or I^2 RPower.
  2. Multiply by t for energyThe joule-story.
  3. Keep the wattThe unit.

9Cells, emf, Internal Resistance

Cells: emf is the open-write as taught; internal r sits so V = ε − I r on a load. A cell is emf plus r, not an ideal V always. Terminal voltage drops when I rises.

Using ε as V on a heavy load without r is a miss.

Figure. E is the open-circuit job of the cell. On load, internal r drops Ir, so the terminal voltage is E minus Ir. r is inside the cell, not a second battery.

How it works

  1. Name ε and rThe cell.
  2. Write V=ε−Ir on a loadThe terminal.
  3. Keep open I=0 so V=εThe extra.

10Cells in Series and in Parallel

Cells in series add emf (and r) as taught; parallel needs matching caution as framed. Series is a chain; parallel is a side-by-side. One series pair is enough.

Adding emf in parallel the same way as series without the lesson’s caution is a miss.

Figure. Series stacks the cells on one loop, so emfs add and so do internal r. Parallel shares the two ends: emf stays one cell's E and the internals sit side by side.

How it works

  1. Name series or parallelThe join.
  2. Add ε and r as taught for that joinThe result.
  3. Keep the matching-caution on parallelHonest.

11Kirchhoff’s Rules

Kirchhoff: junction current-sum 0; loop voltage-sum 0 as taught. Kirchhoff is a bookkeeping pair for a network, not a third Ohm. One two-loop mention is enough.

Ignoring a junction because “it looks small” is a miss.

Figure. At a junction, charge is not stored: currents in equal currents out. Around a loop, the signed IR drops close on the emfs you walk through.

How it works

  1. Write ΣI=0 at a junctionCurrent rule.
  2. Write ΣV=0 around a loopVoltage rule.
  3. Keep bothThe pair.

12Wheatstone Bridge

Wheatstone: a four-resistor bridge balanced when P/Q = R/S as taught. Balance means the galvanometer current is 0. Wheatstone is a ratio-test, not a power-formula.

Using V=IR on the galvanometer as “must be huge” at balance is a miss — it is zero.

Figure. Four resistors sit on a diamond. G is the bridge from B to C. Balance is P over Q equals R over S — then G carries no current and can be ignored.

How it works

  1. Write the four armsThe bridge.
  2. Set P/Q=R/S for balanceThe condition.
  3. Keep Ig=0 as the lookBalanced.
V=6 V, I=2 A gives R
  1. 3 Ω
  2. 12 Ω
  3. 8 Ω

V/I.

Notes

  • Mapped to the official NCERT chapter “Current Electricity”. Original teaching only — no textbook sentences.
  • Science here is Physics, Chemistry and Biology ideas at this class, never a language or social-science chapter.

Formulas

  • I=Q/t
  • V=IR
  • R=ρ ℓ/A
  • P=VI
  • V=ε−Ir
  • Wheatstone: P/Q=R/S

Recap

Hold these pegs from the official chapter “Current Electricity”. The wording is ExamMaster’s teaching, not a textbook recap.

Electric Current
Electric current is charge-flow per time as taught: I = Q/t.
Electric Currents in Conductors
Currents in conductors: charges drift in a field as taught.
Ohm’s law
Ohm’s law: V = I R as taught, for the ohmic range.
Drift of Electrons and the Origin of Resistivity
Drift and resistivity: I = n e A vd as taught; ρ sits in R = ρ ℓ / A.
Limitations of Ohm’s Law
Limitations of Ohm: some devices are non-linear as taught (a diode mention later).
Resistivity of Various Materials
Resistivity of materials: metals, alloys, semiconductors as taught — different ρ and different temperature-looks.

Practise Current Electricity

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