Physics · Current Electricity · Chapter notes
Current Electricity · Class 12 NEET Notes
Class 12 notes on Current Electricity for NEET: drift, Ohm's law and resistivity, combinations, cells and EMF, Kirchhoff's laws and the Wheatstone bridge, updated for the reduced syllabus.
In short
Current Electricity is one of the most reliably scoring chapters in NEET Physics, and nearly every question comes from four ideas: drift velocity behind the current, Ohm's law with resistivity, series and parallel combinations, and the balanced Wheatstone bridge. Learn those four properly and the chapter stops producing surprises.
Contents
- ·What Is In the NEET Syllabus
- 1Current and Drift the crawl inside the signal
- 2Ohm's Law and Resistivity and when Ohm's law fails
- 3Combinations series and parallel
- 4Cells and EMF internal resistance and grouping
- 5Kirchhoff's Laws for anything the shortcuts cannot do
- 6The Wheatstone Bridge and the metre bridge
- ★Current Electricity · Fact Sheet
What Is In the NEET Syllabus
- Potentiometer is NOT in the NEET syllabus, and neither are carbon resistor colour codes.
- Both were removed, so there is nothing on them here.
- The Wheatstone bridge and metre bridge remain, and so do Kirchhoff's laws.
- If you are also preparing JEE, note that potentiometer is very much in the JEE syllabus. The two exams differ here.
Current and Drift
- Current is the rate of flow of charge: I = q / t, measured in ampere.
- Conventional current flows opposite to the electron flow.
- Inside a wire the electrons move at random. A field gives them a slow net drift.
- I = nAev(d), where n is free electron density and v(d) the drift speed.
- Mobility μ = v(d)/E, the drift speed per unit field.
- Current density J = I/A, and J = σE where σ is conductivity.
- Doubling the wire radius quarters the drift speed for the same current, since A goes as r².
Ohm's Law and Resistivity
- V = IR, provided the physical conditions stay constant.
- Resistance R = ρL/A. It depends on the shape of the conductor.
- Resistivity ρ is a property of the material alone, and does not depend on shape.
- Stretching a wire to n times its length makes R become n²R, because the area shrinks too.
- With temperature: R(T) = R(0)[1 + αΔT]. Metals have positive α, semiconductors negative.
- It fails when V and I are not proportional: diodes, thyristors, electrolytes.
- It also fails if the current heats the conductor enough to change its resistance.
- So Ohm is describing a property, not a law of nature. Some materials simply have it.
- Those that do are called ohmic conductors.
Combinations
- Series: R = R1 + R2 + ... The total is larger than the largest one.
- Parallel: 1/R = 1/R1 + 1/R2 + ... The total is smaller than the smallest one.
- Use that as a check. If your parallel answer exceeds the smallest resistor, it is wrong.
- n equal resistors R: series gives nR, parallel gives R/n. So the ratio is n².
Cells and EMF
- EMF is the potential difference across a cell on open circuit, when no current flows.
- Terminal voltage while discharging is V = E − Ir, which is always less than E.
- While being CHARGED, V = E + Ir, which is greater than E. That reversal is often examined.
- Cells in series: EMFs add, and internal resistances add. Use when R is large.
- Cells in parallel: EMF stays E, internal resistance becomes r/n. Use when R is small.
- Maximum power reaches the external resistor when R = r, and then the efficiency is only 50 percent.
Kirchhoff's Laws
- Junction rule: the total current into a junction equals the total out. This is conservation of charge.
- Loop rule: the sum of potential changes around any closed loop is zero. This is conservation of energy.
- Assume a current direction. If the answer comes out negative, the real direction is the opposite.
- Going through a resistor with the current, the potential drops by IR. Against it, the potential rises.
The Wheatstone Bridge
- Balance condition: P / Q = R / S. Then the galvanometer reads zero.
- P and Q lie in series along one branch, R and S along the other.
- At balance the middle branch carries no current, so it can be removed entirely, or replaced by any resistance, without changing anything.
- The metre bridge is a Wheatstone bridge whose lower two arms are lengths of uniform wire. So R/S = l/(100 − l).
- Many complicated-looking networks are secretly balanced bridges.
- Check P/Q against R/S first. It takes five seconds.
- If they match, delete the middle branch and what remains is a simple series-parallel network.
- Attempting the full reduction without checking wastes minutes.
★ Current Electricity · Fact Sheet
Every rule for revision day.
CURRENT AND DRIFT
I = q/t = nAev(d)Drift is about 10⁻⁴ m/s
The field travels near light speed.
RESISTIVITY
R = ρL/Aρ depends on material only
Stretch to n times: R becomes n²R.
TEMPERATURE
R(T) = R(0)[1 + αΔT]Metals: α positive
Semiconductors: α negative.
COMBINATIONS
Series: R = R1 + R2, current sharedParallel: 1/R = 1/R1 + 1/R2
Parallel total < smallest.
CELLS
Discharging: V = E − IrCHARGING: V = E + Ir
Max power when R = r.
KIRCHHOFF
Junction: charge conservedLoop: energy conserved
Negative answer means wrong guess.
WHEATSTONE
P / Q = R / S at balanceMiddle branch carries no current
so it can be deleted.
METRE BRIDGE
R / S = l / (100 − l)Pair each resistance with
the length on its own side.
NOT IN NEET
Potentiometer is removed.So are resistor colour codes.
Both remain in JEE.
Before the exam
What the paper actually asks from this chapter
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