Class 12 Physics · Original EZT practice
Current Electricity (Class 12 Chapter 3): summary and practice
This is where circuits get quantitative: Ohm's law links voltage, current and resistance, real batteries lose voltage to internal resistance, and Kirchhoff's two rules untangle any network. All questions are original EZT tutor practice following the chapter's own section order.
8 original worked examples, 8 fresh self-check questions and 4 mixed questions, with explained answers. These are not copied textbook exercises, an official chapter index or a full answer key. No affiliation with NCERT is claimed.
Official sources for topic and edition context, checked 21 October 2026:
1. Ohm's law and resistance
For many conductors V = IR, and the resistance itself is R = rho L / A where rho is the material's resistivity. Ohm's law has limits - some materials and devices do not follow it.
2. Drift of electrons
Electrons drift slowly through a conductor under the applied field, and that drift velocity is what makes the current; resistivity comes from collisions along the way. Resistivity also changes with temperature.
3. Electrical energy and power
Power dissipated in a resistor is P = VI = I^2 R = V^2 / R. These three forms are the same statement in different variables.
4. Cells, emf and internal resistance
A real cell has emf E and internal resistance r. With external resistance R the current is I = E/(R + r) and the terminal voltage drops to V = E - Ir. Cells in series add emf and internal resistance.
5. Kirchhoff's rules
Junction rule: currents entering a junction equal currents leaving. Loop rule: the sum of voltage changes around any closed loop is zero. Together they solve any circuit network.
6. Wheatstone bridge
Four resistors P, Q, R, S form a balanced bridge when P/Q = R/S; then the middle arm carries no current and unknown resistances can be measured precisely.
Choose, solve, check, then try a fresh question.
Before calculating, read the method prompt and name the condition that makes it useful. Write your own attempt. Open the steps only when you need them, and use the check to catch a wrong sign, unit or interpretation. Finish with the fresh question before reading its answer.
Compute resistance from resistivity
Choose the method: R = rho L / A.
EZT practice question
A wire of resistivity 1.6 x 10^-8 ohm m is 10 m long with cross-section 10^-6 m^2. Find its resistance.
Show the worked solution
- R = rho L / A.
- = (1.6 x 10^-8 x 10) / 10^-6 = 1.6 x 10^-7 / 10^-6.
Answer: R = 0.16 ohm.
Check: Longer wire or thinner wire means more resistance - the formula shows both effects.
Watch for this: Do not invert the formula: area sits in the denominator.
Now try it yourself
Same wire, double the length - new resistance?
Check your answer
0.32 ohm.
R is proportional to L.
Use emf and internal resistance
Choose the method: I = E/(R + r), then V = E - Ir.
EZT practice question
A cell of emf 10 V and internal resistance 2 ohm drives an 8 ohm resistor. Find the current and the terminal voltage.
Show the worked solution
- I = E/(R + r) = 10/(8 + 2) = 1 A.
- V = E - Ir = 10 - 1 x 2.
Answer: I = 1 A and V = 8 V.
Check: The terminal voltage (8 V) is below the emf (10 V) - the missing 2 V drops across the internal resistance.
Watch for this: Do not put E into V = IR for the external resistor; use the terminal voltage there.
Now try it yourself
E = 12 V, r = 1 ohm, R = 5 ohm - find I and V.
Check your answer
I = 2 A, V = 10 V.
I = 12/6 = 2 A; V = 12 - 2 = 10 V.
Combine cells in series
Choose the method: Emfs add; internal resistances add.
EZT practice question
Three cells, each of emf 2 V and internal resistance 0.5 ohm, are joined in series. Find the total emf and internal resistance.
Show the worked solution
- Series emf = 2 + 2 + 2 = 6 V.
- Series internal resistance = 0.5 + 0.5 + 0.5.
Answer: Total emf 6 V and internal resistance 1.5 ohm.
Check: Series cells behave like one bigger cell with summed emf and summed internal resistance.
Watch for this: Do not apply parallel formulas to series cells; the combination rules differ.
Now try it yourself
Two such cells in series - emf and internal resistance?
Check your answer
4 V and 1 ohm.
Both quantities simply add in series.
Work an electrical power numerical
Choose the method: P = VI, then R = V / I.
EZT practice question
A 920 W heater runs on 230 V mains. Find the current it draws and its resistance.
Show the worked solution
- I = P / V = 920 / 230 = 4 A.
- R = V / I = 230 / 4.
Answer: I = 4 A and R = 57.5 ohm.
Check: Cross-check with P = I^2 R: 16 x 57.5 = 920 W - consistent.
Watch for this: Pick the power form that matches the knowns; here P = VI is the direct route.
Now try it yourself
A 460 W appliance on 230 V - current?
Check your answer
2 A.
I = 460 / 230 = 2 A.
Separate drift velocity from signal speed
Choose the method: The electrons drift slowly; the effect travels fast.
EZT practice question
The chapter distinguishes how fast electrons drift from how fast the electric effect travels. What is the difference?
Show the worked solution
- Individual electrons drift through the conductor at a very small average velocity under the field.
- The electric field and its effect, however, propagate through the circuit at nearly the speed of light - which is why a bulb glows instantly.
Answer: Drift velocity is the slow average motion of electrons; the signal is the field's effect, which establishes almost instantly.
Check: This explains why switches respond immediately even though any one electron barely moves.
Watch for this: Do not say electrons race to the bulb; the chapter's own example contrasts the two speeds.
Now try it yourself
Which quantity is linked to current: drift speed or signal speed?
Check your answer
Drift speed.
Current depends on how much charge drifts through a section per second.
Apply Kirchhoff's two rules
Choose the method: Junction rule for currents; loop rule for voltages.
EZT practice question
State Kirchhoff's junction rule and loop rule.
Show the worked solution
- Junction rule: the sum of currents entering a junction equals the sum leaving it - charge does not pile up.
- Loop rule: the algebraic sum of all voltage changes around a closed loop is zero - energy is conserved.
Answer: Currents balance at every junction; voltage changes sum to zero around every loop.
Check: The two rules together give enough equations to solve any resistor network.
Watch for this: Sign discipline matters: fix a loop direction and keep voltage rises and drops consistent.
Now try it yourself
At a junction, 5 A enters and 2 A leaves by one wire. What leaves by the other wire?
Check your answer
3 A.
5 in, 2 + 3 out.
Balance a Wheatstone bridge
Choose the method: P/Q = R/S at balance.
EZT practice question
In a Wheatstone bridge, P = 10 ohm, Q = 20 ohm and R = 15 ohm. What S balances the bridge?
Show the worked solution
- Balance condition: P/Q = R/S.
- S = R x Q / P = 15 x 20 / 10.
Answer: S = 30 ohm.
Check: Ratios match: 10/20 = 15/30 = 1/2, so the middle arm carries no current.
Watch for this: Pair the arms correctly: P over Q equals R over S, not P over R.
Now try it yourself
With P = Q, what value of S balances if R = 15 ohm?
Check your answer
15 ohm.
Equal left arms need equal right arms.
Handle temperature dependence
Choose the method: Resistivity of metals rises with temperature.
EZT practice question
How does the resistivity of a metal change when it is heated?
Show the worked solution
- The chapter shows resistivity of metals increasing with temperature.
- A hotter metal offers more resistance to the same current.
Answer: It increases - metal resistivity rises as temperature rises.
Check: That is why a heating element's resistance measured hot differs from its cold value.
Watch for this: Do not assume resistance stays constant across temperatures in numericals that mention heating.
Now try it yourself
A filament measured hot has higher or lower resistance than cold?
Check your answer
Higher.
Metal resistivity grows with temperature.
Try a mixed set
Choose a method or explanation before opening each answer.
1. Current with a real cell?
Show answer and reason
I = E/(R + r).
The internal resistance r shares the circuit.
2. Terminal voltage of a cell?
Show answer and reason
V = E - Ir.
Some emf is lost across the internal resistance.
3. Three forms of electrical power?
Show answer and reason
P = VI = I^2 R = V^2/R.
Ohm's law converts between them.
4. Wheatstone balance condition?
Show answer and reason
P/Q = R/S.
Then the middle arm carries no current.
Try it yourself
Work these on paper before checking any notes. Answers are deliberately not shown here.
- E = 12 V, r = 1 ohm, R = 5 ohm: find the current and terminal voltage.
- A 460 W appliance runs on 230 V. What current does it draw?
- P = 10, Q = 20, R = 15: what S balances the Wheatstone bridge?
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Questions about this practice page
- What is internal resistance?
- The resistance inside the cell itself. It makes the terminal voltage drop below the emf when current flows: V = E - Ir.
- How is drift velocity different from the speed of the electric signal?
- Electrons drift very slowly on average, but the field's effect establishes through the circuit almost instantly - that is why devices respond the moment you switch on.
- When is a Wheatstone bridge balanced?
- When P/Q = R/S. At balance the middle arm carries no current, which lets the bridge measure unknown resistances precisely.
- What do Kirchhoff's two rules say?
- The junction rule balances currents at every junction; the loop rule says voltage changes sum to zero around any closed loop.
- Are these the NCERT exercise questions for Current Electricity (Class 12)?
- No. EZT pages are original practice following the chapter's section order - use them to prepare for the textbook exercises, not as a substitute for your book.