Physics · Class 12 Science
Ch 3Current Electricity — Class 12 Physics, concept-first.
So far, electric charges have been studied mainly at rest -- electrostatics. This chapter turns to charges in motion: an electric current is nothing but charge flowing steadily through a conductor, and understanding how and why it flows is the foundation for every practical circuit, from a torch bulb to a power grid.
Key concepts
Hover a concept to preview it and jump to its most relevant Q&A.
Electric Current
Electric current is defined as the net rate of flow of electric charge across a chosen cross-section of a conductor.
Most relevant Q&A
Chapter contents
The NCERT structure, section by section. Open a section to see its questions, then read the concept-first solution.
Introduction
So far, electric charges have been studied mainly at rest -- electrostatics. This chapter turns to charges in motion: an electric current is nothing but charge flowing steadily through a conductor, an…
Electric Current and Flow of Charge in a Metallic Conductor
A metallic conductor is held together by a lattice of positive ions, but a large number of its outermost (valence) electrons are not bound to any one ion -- they are free to move throughout the body o…
Drift Velocity of Free Electrons
Consider a free electron in a metal at some instant just after it has suffered a collision with a lattice ion.
Mobility and its Relation with Electric Current
Mobility of a charge carrier is defined as the magnitude of its drift velocity per unit applied electric field:
Ohm's Law, Resistance, Resistivity and Conductivity
Ohm's law states that, provided the physical conditions of a conductor (in particular, its temperature) remain unchanged, the current flowing through it is directly proportional to the potential diffe…
V-I Characteristics: Ohmic and Non-ohmic Conductors
The - characteristic of a conducting device is simply a graph of the potential difference applied across it against the resulting current , usually obtained by varying (using a rheostat or a variable…
Temperature Dependence of Resistance
For most conductors, over a reasonably small temperature range, the change in resistance with temperature is found to follow a simple linear law:
Grouping of Resistances: Series, Parallel and Mixed
Series combination. When resistors are connected end-to-end in a single chain, so that exactly the same current must flow through each one in turn (there being no other path available), the combinatio…
EMF, Terminal Potential Difference and Internal Resistance of a Cell
The electromotive force (EMF) of a cell, denoted , is defined as the potential difference between its two terminals when NO current is being drawn from it (an open circuit) -- equivalently, it is the…
Grouping of Cells: Series, Parallel and Mixed
Cells in series. When identical cells, each of EMF and internal resistance , are connected in series (the positive terminal of each joined to the negative terminal of the next, so they all "aid" each…
n Cells of Unequal EMF in Series
Consider cells of EMFs and internal resistances , all connected in series so as to aid one another (each cell's positive terminal joined to the next cell's negative terminal, all driving current the s…
Two Cells of Unequal EMF in Parallel
Consider two cells of EMFs and internal resistances , connected in parallel between two common nodes and (like terminals joined together), with an external resistance connected across and .
Kirchhoff's Laws and Their Application to Multi-loop Circuits
Many practical networks -- containing several cells and several resistors wired together, with more than one closed loop -- cannot be reduced to a single equivalent resistance by the series-parallel r…
Wheatstone Bridge Principle
The Wheatstone bridge is an arrangement of four resistances , connected to form the four arms of a quadrilateral (conventionally drawn as a diamond), with a battery connected across one diagonal (thro…
Metre Bridge
The metre bridge (also called a slide-wire bridge) realises the Wheatstone bridge principle (Section 3.12) using a one-metre length of uniform resistance wire, mounted on a metre scale, in place of tw…
Potentiometer: Principle
A potentiometer consists of a long (often several metres), uniform resistance wire, through which a steady current is maintained by a driver cell (of EMF higher than any PD to be measured) connected i…
Measurement of Potential Difference Using a Potentiometer
To measure an unknown potential difference (for instance, the PD across a resistor carrying current in some external circuit) using a potentiometer, the wire's potential gradient must first be establi…
Comparison of EMFs of Two Cells Using a Potentiometer
To compare the EMFs and of two cells using a potentiometer, each cell is connected in turn into the secondary circuit (usually via a two-way key that allows quickly switching between them without dist…
Measurement of Internal Resistance of a Cell Using a Potentiometer
To measure the internal resistance of a cell using a potentiometer, the cell (of EMF ) is first connected into the secondary circuit ALONE, with its own circuit left open (no current drawn from the ce…
Summary
This chapter built up WBCHSE Unit 2's account of current electricity in the order the syllabus lists it.
Sample & Board Papers
Sample papers and previous-year board questions for this subject.
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- Q1a) Establish Ohm's law from the concept of drift velocity of free electrons. b) Define mobility of free electrons. c) A potential difference…Preview
- Q2Two cells each of emf e but internal resistances r1 and r2 are connected in series through an external resistance R. If the potential differ…Preview
- Q3A carbon resistor is coloured with four different bands — red, green, orange and silver respectively. Find the range of its probable resista…Preview
- Q4a) In a potentiometer experiment why is it necessary to use a long wire? Length and resistance of a potentiometer wire are 4 m and 10 Ω resp…Preview
- Q5State Kirchhoff's rules of electric circuits. **OR** A battery of e.m.f. 1.5 V and internal resistance 0.2 Ω is being charged with a current…Preview
- Q6(a) Define relaxation time of the free electron drifting in a conductor. (b) Derive an expression for the resistivity of a good conductor, i…Preview
- Q7In a current carrying conductor the ratio of the current density to the electric field at a point is called (a) Resistivity (b) Conductivity…Preview
- Q8A current of 3 A flows through a 3 Ω resistor when connected across a battery. The same battery supplies a current 0.75 A when connected acr…Preview
- Q9Explain with the help of graph, the variation of conductivity with temperature for a metallic conductor. (1+1) **OR** How would you arrange…Preview
- Q10(a) Explain the term 'drift velocity' of electrons in a conductor. Hence obtain the expression for current through a conductor in terms of d…Preview
- Q11The resistance of a bulb-filament is 100 Ω at a temperature of 100°C. If the temperature coefficient of the resistance be 0.005/°C, its resi…Preview
- Q12Name the materials used for making standard resistance. Give reasons for this choice. [1+1] **OR** What will be the change in drift velocity…Preview
- Q13(a) Define current density and relaxation time. [1] (b) Derive an expression for resistivity of a conductor in terms of number density of ch…Preview
- Q14In the circuit, it is given that AB = 6 Ω, BC = 3 Ω, CD = 6 Ω, DA = 12 Ω and G = 10 Ω. Current through the galvanometer will be (a) 8.7 mA (…Preview
- Q15A carbon resistor is coloured with four different rings having colours brown, orange, green and silver respectively. Find the range of its p…Preview
- Q16(a) A battery of emf E and internal resistance r is connected to an external resistance R. Show that power in the external circuit will be m…Preview
- Q17In the figure two electric cells A and B are connected in parallel. Their emfs are 3 volt and 2 volt respectively and internal resistances a…Preview
- Q18Given figure shows I – V curve of a metal wire at three different temperatures T₁, T₂ and T₃. In this case, we can come to a conclusion that…Preview
- Q1918 similar cells are connected in mixed combination to get maximum current through an external resistance. If 6 cells are connected in serie…Preview
- Q20A potentiometer wire of length 100 cm has resistance 10 Ω. It is connected in series with R and an accumulator of emf 2V and negligible inte…Preview
- Q21In the circuit given below, the equivalent resistance between the two points A and B will be (a) 4 Ω (b) 1 Ω (c) 3 Ω (d) 2 Ω ![A resistor ne…Preview
- Q22Which of the following statement(s) is/are true ? A potential difference of V is applied at the two ends of a conductor of length l and area…Preview
- Q23A wire of length l and resistance R is stretched till its length becomes x times its original length. Its new resistance will be (a) x²R (b)…Preview
- Q24In which case will the null condition of a Wheatstone bridge change ? (a) If the resistances in different arms are changed (b) If the positi…Preview
More questions
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- Example 1Define electric current. Write the relation between current and the charge flowing through a cross-section of a conductor, and state the SI…Free
- Example 2A conductor carries a steady current of $2\ \text{A}$. Find the charge that flows through any cross-section of the conductor in $5$ minutes.Free
- Example 3A copper wire has a free-electron number density $n = 8.5 \times 10^{28}\ \text{m}^{-3}$ and a cross-sectional area $A = 1 \times 10^{-6}\ \…Free
- Example 4A conductor of length $2\ \text{m}$ has a potential difference of $4\ \text{V}$ applied across it, which produces a drift velocity of $2.5 \…Preview
- Example 5A wire of resistance $10\ \Omega$, length $2\ \text{m}$ and uniform cross-sectional area $0.5\ \text{mm}^2$ is given. Calculate the resistiv…Preview
- Example 6Sketch and describe the general shape of the $V$-$I$ characteristic for (a) an ohmic resistor and (b) a p-n junction diode (a non-ohmic devi…Preview
- Example 7A resistor has a resistance of $100\ \Omega$ at $20^\circ\text{C}$. If the temperature coefficient of resistance of its material is $\alpha…Preview
- Example 8Three resistors $R_1 = 2\ \Omega$, $R_2 = 3\ \Omega$ and $R_3 = 6\ \Omega$ are available. Find the equivalent resistance when they are conne…Preview
- Example 9A cell of EMF $2\ \text{V}$ and internal resistance $0.5\ \Omega$ is connected to an external resistor of $4.5\ \Omega$. Find the current dr…Preview
- Example 10Two cells of EMF $\varepsilon_1 = 1.5\ \text{V}$ (internal resistance $r_1 = 0.5\ \Omega$) and $\varepsilon_2 = 2.0\ \text{V}$ (internal res…Preview
- Example 11Two cells of EMF $\varepsilon_1 = 2\ \text{V}$ (internal resistance $r_1 = 1\ \Omega$) and $\varepsilon_2 = 1.5\ \text{V}$ (internal resista…Preview
- Example 12Two cells of EMF $8\ \text{V}$ and $10\ \text{V}$ (each branch, including the cell's own internal resistance, has a net resistance of $2\ \O…Preview
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- Q13Write the microscopic expression for the resistivity of a conductor, $\rho = m/(ne^2\tau)$, in terms of the relaxation time $\tau$ of the fr…Free
- Q14Distinguish between the drift velocity and the random (thermal) velocity of the free electrons in a metallic conductor carrying a current, s…Free
- Q15Using the microscopic (free-electron) picture of conduction, explain why the resistance of a metallic conductor increases with a rise in tem…Free
- Q16Two resistors of $6\ \Omega$ and $3\ \Omega$ are connected in parallel. Find their equivalent resistance.Preview
- Q17State the difference between the EMF and the terminal potential difference of a cell. Under what condition are the two equal in magnitude?Preview
- Q18State Kirchhoff's junction rule and Kirchhoff's loop rule for an electrical network. Name the physical conservation principle that each rule…Preview
- Q19State the principle of the Wheatstone bridge and write down its balance condition, defining each symbol used.Preview
- Q20State two advantages of using a potentiometer, rather than a voltmeter, to measure the potential difference or the EMF of a cell.Preview
- Q21A cell of EMF $1.5\ \text{V}$ and internal resistance $1\ \Omega$ is connected to an external resistance of $2\ \Omega$. Find the terminal p…Preview
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- Q22In the network shown, $R_1 = 4\ \Omega$ and $R_2 = 4\ \Omega$ are connected in parallel; this combination is joined in series with $R_3 = 3\…Free
- Q23Four identical cells, each of EMF $1.5\ \text{V}$ and internal resistance $0.5\ \Omega$, are connected in series. This series combination is…Free
- Q24Five identical cells, each of EMF $2\ \text{V}$ and internal resistance $1\ \Omega$, are connected in parallel. This combination is connecte…Free
- Q25A battery is made by joining $n = 3$ identical cells (each of EMF $2\ \text{V}$ and internal resistance $0.5\ \Omega$) in series to form one…Preview
- Q26In a metre bridge, a resistance box is connected in the left gap and an unknown resistance $X$ in the right gap. When the resistance box is…Preview
- Q27In a Wheatstone bridge, the four arms carry resistances $P = 4\ \Omega$, $Q = 6\ \Omega$ and $S = 9\ \Omega$ (the standard/known arm), with…Preview
- Q28On a potentiometer, a cell of unknown internal resistance $r$ gives a balance point at $300\ \text{cm}$ when its circuit is open. When a res…Preview
- Q29A potentiometer wire is $4\ \text{m}$ long. A standard cell of EMF $2\ \text{V}$, connected across the whole wire's resistance in the primar…Preview
- Q30A copper wire of cross-sectional area $2 \times 10^{-6}\ \text{m}^2$ carries a current of $3.2\ \text{A}$. If the free-electron number densi…Preview
- Q31A wire of length $5\ \text{m}$ and uniform cross-sectional area $1 \times 10^{-6}\ \text{m}^2$ has a resistance of $2\ \Omega$. Calculate (a…Preview