CBSE Class 10 Science (Chapter 11 : Electricity)
20 Important Questions & Answers
(As per CBSE 2026–27 syllabus)
Electricity chapter covers electric current, potential difference, Ohm’s law, resistance, combination of resistors, heating effect of current, electric power, and electrical energy.
Q1. What is electric current? State its SI unit.
Answer:
Electric current is the rate of flow of electric charge through a conductor. It is represented by the symbol I. Mathematically,
[I=\frac{Q}{t}]
where Q is the charge flowing through a conductor and t is the time taken. 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 measured using an ammeter, which is connected in series in a circuit. Electric current is essential for operating electrical appliances and devices used in daily life.
Q2. Define electric potential difference and write its formula.
Answer:
Electric potential difference between two points is the work done in moving a unit charge from one point to another in an electric circuit. It provides the driving force for the flow of electric current. The formula is:
[V=\frac{W}{Q}]
where V is potential difference, W is work done, and Q is charge. The SI unit of potential difference is volt (V). One volt is the potential difference when one joule of work is done in moving one coulomb of charge. Potential difference is measured using a voltmeter, which is connected in parallel across the component.
Q3. State Ohm’s Law.
Answer:
Ohm’s Law states that at constant temperature, the current flowing through a conductor is directly proportional to the potential difference across its ends.
[V \propto I]
or
[V=IR]
where V is potential difference, I is current, and R is resistance. The law was given by German physicist Georg Simon Ohm. It helps in calculating current, voltage, and resistance in electrical circuits. The graphical representation of Ohm’s Law is a straight line passing through the origin when voltage is plotted against current. This law is applicable only when physical conditions like temperature remain constant.
Q4. What is resistance? State its SI unit.
Answer:
Resistance is the property of a conductor that opposes the flow of electric current through it. It is represented by R and depends on the material, length, and thickness of the conductor. Greater resistance means less current flows through the conductor. According to Ohm’s Law:
[R=\frac{V}{I}]
The SI unit of resistance is ohm (Ω). One ohm is the resistance of a conductor when a potential difference of one volt causes a current of one ampere to flow through it. Resistance is useful in controlling current in electrical devices and circuits.
Q5. Mention the factors affecting the resistance of a conductor.
Answer:
The resistance of a conductor depends on the following factors:
- Length (L): Resistance is directly proportional to length. Longer wires have greater resistance.
- Area of Cross-Section (A): Resistance is inversely proportional to area. Thicker wires have lower resistance.
- Nature of Material: Different materials have different resistivities.
- Temperature: Resistance generally increases with temperature for metallic conductors.
Mathematically,
[R=\rho \frac{L}{A}]
where ρ is resistivity. Understanding these factors helps in designing electrical circuits and selecting suitable conductors for various applications.
Q6. What is resistivity? State its SI unit.
Answer:
Resistivity is the intrinsic property of a material that determines how strongly it opposes the flow of electric current. It is represented by the symbol ρ (rho). The resistance of a conductor depends on resistivity according to:
[R=\rho \frac{L}{A}]
Resistivity depends only on the nature of the material and temperature, not on its dimensions. Materials with low resistivity are good conductors, while those with high resistivity are insulators. The SI unit of resistivity is ohm metre (Ω m). Copper and aluminium have low resistivity and are commonly used for electrical wiring.
Q7. State two differences between resistance and resistivity.
Answer:
Resistance and resistivity are related but different quantities.
- Resistance depends on the length and area of the conductor, whereas resistivity depends only on the material.
- Resistance changes when the dimensions of the conductor change, but resistivity remains constant for a given material at a fixed temperature.
- The SI unit of resistance is ohm (Ω), while the SI unit of resistivity is ohm metre (Ω m).
Resistance tells how much a particular conductor opposes current, whereas resistivity indicates the inherent electrical property of the material itself.
Q8. What is the series combination of resistors?
Answer:
In a series combination, resistors are connected one after another so that the same current flows through each resistor. The total resistance is equal to the sum of individual resistances.
[
R_s=R_1+R_2+R_3
]
The potential difference across the combination is divided among the resistors. If one resistor breaks, the entire circuit stops functioning because the circuit becomes open. Series combinations are used where the same current is required through all components. Decorative lighting strings and some measuring instruments use series connections. The equivalent resistance of a series combination is always greater than the largest individual resistance.
Q9. What is the parallel combination of resistors?
Answer:
In a parallel combination, resistors are connected across the same two points of a circuit. The potential difference across each resistor remains the same, while current divides among different branches.
[\frac{1}{R_p}=\frac{1}{R_1}+\frac{1}{R_2}+\frac{1}{R_3}]
The equivalent resistance in parallel is less than the smallest resistor. If one branch stops working, the others continue functioning. Household electrical wiring is done in parallel because appliances receive the same voltage and can operate independently. Parallel combinations are safer and more practical for domestic use than series combinations.
Q10. Why are domestic appliances connected in parallel?
Answer:
Domestic appliances are connected in parallel for several reasons:
- Each appliance receives the same supply voltage (220–240 V).
- Appliances can be switched on or off independently.
- Failure of one appliance does not affect the operation of others.
- Each appliance draws current according to its power requirement.
If appliances were connected in series, the voltage would be divided and the malfunction of one appliance would interrupt the entire circuit. Therefore, parallel wiring ensures convenience, safety, and efficient operation of household electrical devices such as fans, lights, televisions, and refrigerators.
Q11. State Joule’s Law of Heating.
Answer:
Joule’s Law of Heating states that the heat produced in a conductor carrying electric current is directly proportional to:
- the square of the current,
- the resistance of the conductor,
- and the time for which current flows.
Mathematically,
[H=I^2Rt]
where H is heat produced, I is current, R is resistance, and t is time. This law explains why electrical appliances become hot during operation. The heating effect is useful in electric irons, heaters, kettles, geysers, and toasters. However, excessive heating can damage circuits, which is why safety devices like electric fuses are used.
Q12. What is an electric fuse? Why is it used?
Answer:
An electric fuse is a safety device used in electrical circuits to protect appliances from excessive current. It consists of a thin wire made of a material with a low melting point. When current exceeds the safe limit, the fuse wire heats up and melts due to Joule’s heating effect, thereby breaking the circuit.
This prevents overheating, fire hazards, and damage to appliances. Fuses are installed in series with the live wire. Modern electrical systems often use Miniature Circuit Breakers (MCBs), which automatically disconnect the circuit during overload conditions. Thus, a fuse ensures electrical safety in homes and industries.
Q13. Define electric power and write its SI unit.
Answer:
Electric power is the rate at which electrical energy is consumed or converted into other forms of energy. It indicates how quickly an electrical appliance performs work.
[P=\frac{W}{t}]
Using Ohm’s Law,
[P=VI]
The SI unit of electric power is watt (W). One watt is the power consumed when one ampere of current flows through a conductor with a potential difference of one volt. Larger appliances are usually rated in kilowatts (kW). Power ratings help consumers choose suitable electrical appliances and estimate electricity consumption.
Q14. Derive two expressions for electric power.
Answer:
Electric power is given by:
[P=VI]
Using Ohm’s Law,
[V=IR]
Substituting in the power equation:
[P=I(IR)]
[P=I^2R]
Again, using
[I=\frac{V}{R}]
we get:
[P=V\left(\frac{V}{R}\right)]
[P=\frac{V^2}{R}]
Thus, the three important expressions for electric power are:
[P=VI]
[P=I^2R]
[P=\frac{V^2}{R}]
These formulas are useful for solving numerical problems related to electrical appliances and circuits. (CBSE Labs)
Q15. What is electrical energy? State its commercial unit.
Answer:
Electrical energy is the total work done or energy consumed by an electrical device when current flows through it. It is calculated as:
[E=Pt]
where P is power and t is time. The SI unit of electrical energy is joule (J). However, for commercial purposes, electrical energy is measured in kilowatt-hour (kWh), commonly called one unit of electricity.
[1,kWh=3.6\times10^6,J]
Electricity bills are based on the number of kilowatt-hours consumed. Understanding electrical energy helps consumers monitor and reduce electricity usage effectively.
Q16. Differentiate between conductors and insulators.
Answer:
Conductors and insulators differ in their ability to allow electric current.
Conductors:
- Allow current to pass easily.
- Have low resistance and low resistivity.
- Examples: copper, aluminium, silver.
Insulators:
- Do not allow current to pass easily.
- Have high resistance and high resistivity.
- Examples: rubber, plastic, glass, wood.
Conductors are used in electrical wiring because they carry current efficiently. Insulators are used as protective coverings around wires to prevent electric shocks and short circuits. Both are essential for safe and efficient electrical systems.
Q17. Why is copper commonly used for electrical wiring?
Answer:
Copper is widely used for electrical wiring because it is an excellent conductor of electricity. It has very low resistivity, allowing electric current to flow with minimal energy loss. Copper is also ductile, making it easy to draw into thin wires. It possesses good mechanical strength and can withstand wear and tear for long periods.
Additionally, copper does not corrode easily and remains reliable under different environmental conditions. Its high conductivity reduces heating and energy wastage. Due to these properties, copper is preferred for household wiring, electrical cables, motors, and transformers.
Q18. What is the heating effect of electric current? Give two applications.
Answer:
When electric current flows through a conductor, part of the electrical energy is converted into heat energy. This phenomenon is called the heating effect of electric current. The amount of heat produced depends on current, resistance, and time according to Joule’s Law.
Applications:
- Electric Iron: Uses heat generated by a heating element to remove wrinkles from clothes.
- Electric Heater/Geyser: Converts electrical energy into heat for warming water or rooms.
Other applications include electric kettles, toasters, ovens, and incandescent bulbs. The heating effect is one of the most useful applications of electricity in daily life.
Q19. Why are electric bulbs filled with inert gases?
Answer:
The filament of an electric bulb is usually made of tungsten, which becomes extremely hot when current passes through it. If exposed to air, the filament would oxidize and burn quickly. Therefore, bulbs are filled with inert gases such as argon or nitrogen.
These gases do not react chemically with the hot filament and reduce the rate of tungsten evaporation. As a result, the filament lasts longer and the bulb works efficiently for an extended period. The use of inert gases increases the lifespan and performance of incandescent bulbs.
Q20. Explain the difference between kilowatt and kilowatt-hour.
Answer:
A kilowatt (kW) is a unit of power, while a kilowatt-hour (kWh) is a unit of energy.
- Kilowatt (kW): Measures the rate at which electrical energy is used.
[1,kW=1000,W] - Kilowatt-hour (kWh): Measures the total electrical energy consumed.
[1,kWh=3.6\times10^6,J]
For example, a 1000 W heater running for one hour consumes one kilowatt-hour of energy. Electricity bills are calculated in kWh, commonly known as units of electricity. Thus, power and energy are related but represent different physical quantities.