CBSE Class 12 Physics (2026–27)
Chapter 3: Current Electricity
20 Important Questions & Answers
Current Electricity includes topics such as electric current, drift velocity, Ohm’s law, resistivity, electrical power, combination of resistors, cells, Kirchhoff’s laws, Wheatstone bridge, meter bridge, and potentiometer. These topics are part of the CBSE Class 12 Physics syllabus.
Q1. What is electric current? Define its SI unit.
Answer:
Electric current is the rate of flow of electric charge through a conductor. If a charge (Q) passes through a cross-section of a conductor in time (t), then the current is given by (I = Q/t). The SI unit of electric current is ampere (A). One ampere is defined as the current when one coulomb of charge flows through a conductor in one second. Current is a scalar quantity and is measured using an ammeter connected in series with the circuit. Conventional current flows from higher potential to lower potential, whereas electrons move in the opposite direction. Electric current forms the basis of all electrical circuits and devices.
Q2. Explain drift velocity of electrons.
Answer:
The random motion of free electrons in a conductor becomes directed when an electric field is applied. The average velocity acquired by electrons due to the electric field is called drift velocity. Although electrons move randomly at high speeds, the drift velocity is quite small, generally of the order of (10^{-4}) m/s. Drift velocity is directly proportional to the electric field applied across the conductor. Electric current in a conductor arises because of this directed motion of electrons. The relationship between current and drift velocity is given by (I = nAe v_d), where (n) is the number density of electrons, (A) is the area of cross-section, and (v_d) is the drift velocity.
Q3. State Ohm’s Law and mention its limitations.
Answer:
Ohm’s Law states that the current flowing through a conductor is directly proportional to the potential difference across it, provided temperature and other physical conditions remain constant. Mathematically, (V = IR), where (R) is the resistance of the conductor. This law is valid only for ohmic conductors such as metallic wires. It does not apply to semiconductor devices like diodes and transistors, where the current-voltage relationship is nonlinear. Ohm’s Law also fails when temperature changes significantly because resistance varies with temperature. Thus, it is applicable only under controlled physical conditions and for materials exhibiting linear V-I characteristics.
Q4. Define resistivity and mention the factors affecting it.
Answer:
Resistivity is the intrinsic property of a material that opposes the flow of electric current. It is defined as the resistance of a conductor having unit length and unit cross-sectional area. Its SI unit is ohm-metre (Ωm). Resistivity is represented by the symbol (ρ) and is given by (ρ = RA/L). It depends on the nature of the material and temperature but does not depend on the dimensions of the conductor. Metals generally have low resistivity, while insulators have very high resistivity. As temperature increases, the resistivity of metallic conductors also increases because electron collisions become more frequent.
Q5. Differentiate between emf and potential difference.
Answer:
Electromotive force (emf) is the total energy supplied by a source per unit charge to move charge through the entire circuit. Potential difference is the energy consumed per unit charge between two points of a circuit. Emf is measured when no current flows through the circuit, whereas potential difference is measured when the circuit is operating. Emf is represented by (E), while potential difference is represented by (V). The SI unit of both quantities is volt (V). Due to internal resistance, the terminal potential difference of a cell is usually less than its emf when current is drawn from the cell.
Q6. What is internal resistance of a cell?
Answer:
The resistance offered to the flow of current inside a cell by its electrolyte and electrodes is called internal resistance. It is represented by (r). When current flows through a cell, a part of the emf is used in overcoming this internal resistance. Therefore, the terminal voltage becomes less than the emf. Internal resistance depends on the nature and concentration of the electrolyte, distance between electrodes, area of electrodes immersed, and temperature. A good cell should have low internal resistance to deliver maximum current efficiently. Internal resistance plays an important role in determining the performance and efficiency of electrical cells.
Q7. Explain the series combination of resistors.
Answer:
In a series combination, resistors are connected end-to-end so that the same current flows through each resistor. The equivalent resistance of the combination is equal to the sum of all individual resistances. Mathematically,
(R = R_1 + R_2 + R_3 + …)
The total potential difference across the combination equals the sum of potential differences across each resistor. Since equivalent resistance increases in a series arrangement, the current flowing through the circuit decreases. Series combinations are commonly used where the same current is required through multiple components. However, if one resistor breaks, the entire circuit becomes open and current stops flowing.
Q8. Explain the parallel combination of resistors.
Answer:
In a parallel combination, all resistors are connected across the same two points. Therefore, the potential difference across each resistor remains the same. The reciprocal of equivalent resistance is equal to the sum of reciprocals of individual resistances:
(1/R = 1/R_1 + 1/R_2 + 1/R_3)
The equivalent resistance of a parallel combination is always less than the smallest resistor in the circuit. The total current supplied by the source is divided among different branches according to their resistances. Parallel circuits are widely used in household wiring because each appliance receives the same voltage and functions independently of others.
Q9. Define electrical power and write its formulas.
Answer:
Electrical power is the rate at which electrical energy is consumed or converted into other forms of energy. It is represented by (P). The SI unit of power is watt (W). One watt is the power consumed when one joule of energy is used in one second. Electrical power can be calculated using different formulas:
(P = VI),
(P = I^2R),
and (P = V^2/R).
The choice of formula depends on the quantities known. Electrical power determines the energy consumption of electrical devices and is important for calculating electricity bills and designing electrical circuits safely.
Q10. State Kirchhoff’s Current Law (KCL).
Answer:
Kirchhoff’s Current Law states that the algebraic sum of currents entering and leaving a junction in an electrical circuit is zero. In simple words, the total current entering a junction is equal to the total current leaving it. This law is based on the principle of conservation of charge. Since charge cannot accumulate at a junction, whatever charge enters must leave. Mathematically,
(\sum I = 0).
KCL is extremely useful for analyzing complex electrical circuits involving multiple branches. It helps in determining unknown currents and forms one of the fundamental laws used in network analysis.
Q11. State Kirchhoff’s Voltage Law (KVL).
Answer:
Kirchhoff’s Voltage Law states that the algebraic sum of all potential differences and electromotive forces around any closed loop in an electrical circuit is zero. This law is based on the principle of conservation of energy. While moving around a closed loop, the total energy gained from sources must equal the total energy lost across circuit elements. Mathematically,
(\sum V = 0).
KVL is widely used in solving complex electrical networks and determining unknown currents and voltages. Together with Kirchhoff’s Current Law, it forms the foundation of circuit analysis in electrical engineering and physics.
Q12. What is a Wheatstone bridge?
Answer:
A Wheatstone bridge is an electrical arrangement used for accurately measuring an unknown resistance. It consists of four resistances connected in the form of a bridge and a galvanometer connected between two junctions. When no current flows through the galvanometer, the bridge is said to be balanced. Under balanced conditions,
(P/Q = R/S),
where (S) is the unknown resistance. The Wheatstone bridge is highly accurate and sensitive for resistance measurements. It is commonly used in scientific laboratories and measuring instruments. The principle of the Wheatstone bridge is also used in devices such as strain gauges and sensor circuits.
Q13. What is a meter bridge?
Answer:
A meter bridge is a practical application of the Wheatstone bridge used to determine an unknown resistance. It consists of a one-meter-long uniform wire stretched on a wooden board. The bridge works on the principle that resistance is proportional to the length of the wire. By adjusting the jockey on the wire, a balance point is obtained where no current flows through the galvanometer. The unknown resistance can then be calculated using the balance condition. Meter bridges are simple, accurate, and commonly used in school and college laboratories for resistance measurements and verification of resistance laws.
Q14. State the principle of a potentiometer.
Answer:
A potentiometer works on the principle that the potential drop across a uniform wire carrying a constant current is directly proportional to its length. Thus,
(V \propto l).
The device consists of a long resistance wire connected to a battery. By measuring the balancing length, unknown potential differences or emf values can be determined accurately without drawing current from the source. Because of this feature, a potentiometer is more accurate than a voltmeter. It is commonly used to compare the emf of cells, measure potential difference, and determine the internal resistance of a cell.
Q15. Why is a potentiometer preferred over a voltmeter?
Answer:
A potentiometer is preferred over a voltmeter because it measures potential difference using a null-deflection method. During measurement, no current is drawn from the source under investigation. Therefore, the measured value remains unaffected by the internal resistance of the source. In contrast, a voltmeter draws a small amount of current, causing slight errors in measurement. Potentiometers provide highly accurate results and can measure small potential differences precisely. They are also used for comparing emf values and determining internal resistance of cells, tasks that cannot be performed accurately using an ordinary voltmeter.
Q16. What is conductivity?
Answer:
Conductivity is the ability of a material to allow electric current to pass through it. It is represented by the symbol (σ). Conductivity is the reciprocal of resistivity:
(σ = 1/ρ).
Its SI unit is siemens per metre (S/m). Materials with high conductivity, such as copper and silver, allow current to flow easily and are used for making electrical wires. Conductivity depends on the number of free electrons available in the material. As the resistivity of a material increases, its conductivity decreases. Conductivity is an important property for selecting materials in electrical and electronic applications.
Q17. Explain the temperature dependence of resistance.
Answer:
The resistance of metallic conductors increases with temperature. When temperature rises, atoms in the conductor vibrate more vigorously, causing frequent collisions with moving electrons. These collisions oppose electron flow and increase resistance. The relationship is expressed as:
(R_t = R_0(1 + αΔT)),
where (α) is the temperature coefficient of resistance. In metals, resistance increases with temperature, while in semiconductors it decreases. This property is important in designing electrical equipment and temperature-sensing devices. Understanding temperature dependence helps engineers predict circuit behavior under varying environmental conditions.
Q18. What are cells connected in series? Mention one advantage.
Answer:
Cells are connected in series when the positive terminal of one cell is connected to the negative terminal of the next cell. In this arrangement, the total emf of the combination equals the sum of the individual emfs. Series connection is useful when a higher voltage is required. The effective internal resistance also increases because internal resistances add together. Such combinations are commonly used in flashlights and battery packs where greater potential difference is needed. The main advantage of series connection is that it provides a larger emf, enabling the circuit to operate devices requiring higher voltages.
Q19. What are cells connected in parallel? Mention one advantage.
Answer:
Cells are connected in parallel when all positive terminals are joined together and all negative terminals are joined together. In this arrangement, the emf remains equal to that of a single cell, but the effective internal resistance decreases. As a result, the combination can supply a larger current for a longer time. Parallel combinations are useful when high current output is required without increasing voltage. They are commonly used in backup power systems and battery banks. The major advantage of parallel connection is improved current-delivering capacity and longer battery life.
Q20. Write any four applications of current electricity in daily life.
Answer:
Current electricity has numerous applications in everyday life. First, it is used in household appliances such as fans, refrigerators, televisions, and washing machines. Second, it powers lighting systems in homes, offices, and streets. Third, it is essential for operating communication devices such as mobile phones, computers, and internet equipment. Fourth, current electricity drives industrial machines and manufacturing processes. It is also used in medical equipment, transportation systems, and scientific instruments. Modern civilization depends heavily on the controlled flow of electric current for comfort, communication, healthcare, and technological development.