Introduction (Electricity Class 10 Notes)

Electricity is one of the most useful forms of energy in modern life. It powers homes, schools, industries, hospitals, transportation systems, and communication devices. Almost every electrical appliance we use, such as fans, lights, televisions, refrigerators, computers, and mobile chargers, operates using electricity.

Electricity is produced due to the movement of electric charges through a conductor. In this chapter, we will learn about electric charge, electric current, electric circuits, electric potential, potential difference, Ohm’s Law, resistance, electrical energy, electric power, and the heating effect of electric current.

These notes are prepared according to the latest NCERT and CBSE syllabus with simple explanations, diagrams, formulas, solved examples, and board exam tips.

What is Electric Charge?

Electric charge is a basic property of matter that is responsible for electrical effects.

Every atom contains charged particles:

• Protons carry positive (+) charge.
• Electrons carry negative (−) charge.
• Neutrons have no charge.

Normally, an atom is electrically neutral because it has an equal number of protons and electrons.

Types of Electric Charge

There are two types of electric charges:

1. Positive Charge (+)
2. Negative Charge (−)

Rule of Charges

• Like charges repel each other.
• Unlike charges attract each other.

SI Unit of Electric Charge

The SI unit of electric charge is the Coulomb (C).

One coulomb is the amount of charge transported by a current of one ampere in one second.

Electric Current

Electric current is the rate of flow of electric charge through a conductor.

Definition:
Electric current is the amount of electric charge flowing through a conductor per unit time.

Formula of Electric Current

$$\boxed{I=\frac{Q}{t}}$$

Where:

• I = Electric Current (Ampere)
• Q = Electric Charge (Coulomb)
• t = Time (Second)

SI Unit of Electric Current

The SI unit of electric current is the Ampere (A).

Definition of One Ampere

One ampere is the current when one coulomb of charge flows through a conductor in one second.$$1A=\frac{1C}{1s}$$

Direction of Electric Current

The conventional direction of current is from the positive terminal of a cell to the negative terminal through the external circuit.

However, electrons actually move from the negative terminal to the positive terminal.

Important: In circuit diagrams and numerical problems, always use the conventional current direction.

Electric Circuit

An electric circuit is a closed conducting path through which electric current flows.

Components of an Electric Circuit

A simple electric circuit consists of:

• Electric cell or battery
• Connecting wires
• Switch (Key)
• Bulb or resistor

Closed Circuit

When the switch is ON, the circuit is complete.

Current flows.

Open Circuit

When the switch is OFF, the circuit is broken.

No current flows.

The bulb does not glow.

Electric Potential

Electric potential is the work done in bringing a unit positive charge from infinity to a point in an electric field.

It indicates the amount of electrical energy available at a point.

SI Unit of Electric Potential

The SI unit is Volt (V).

Potential Difference

Potential difference is the work done to move a unit charge from one point to another in an electric circuit.

It is the driving force that causes electric current to flow.

Formula

$$\boxed{V=\frac{W}{Q}}$$V=QW​​

Where:

• V = Potential Difference (Volt)
• W = Work Done (Joule)
• Q = Charge (Coulomb)

SI Unit

The SI unit of potential difference is the Volt (V).

Definition of One Volt

One volt is the potential difference when one joule of work is done to move one coulomb of charge.$$1V=\frac{1J}{1C}$$

Electromotive Force (EMF)

The electromotive force (EMF) of a cell is the maximum potential difference provided by the cell when no current is flowing.

It is measured in volts (V).

Measuring Electric Current

The instrument used to measure electric current is an ammeter.

Ammeter

• Symbol: Ⓐ
• Connected in series with the circuit.
• Has very low resistance.

Measuring Potential Difference

The instrument used to measure potential difference is a voltmeter.

Voltmeter

• Symbol: Ⓥ
• Connected in parallel across the component.
• Has very high resistance.

Difference Between Ammeter and Voltmeter

Difference Between Electric Current and Potential Difference

Everyday Applications

Quick Revision

✔ Electric charge is measured in Coulomb (C).

✔ Electric current is the flow of electric charge.

✔ Formula:$$I=\frac{Q}{t}$$

✔ Potential difference:$$V=\frac{W}{Q}$$

✔ Current is measured using an ammeter connected in series.

✔ Potential difference is measured using a voltmeter connected in parallel.

✔ A closed circuit allows current to flow, while an open circuit does not.

Ohm’s Law

One of the most important laws in electricity is Ohm’s Law, proposed by the German physicist Georg Simon Ohm.

Definition

Ohm’s Law states that the current flowing through a conductor is directly proportional to the potential difference across its ends, provided the temperature and other physical conditions remain constant.

$$\boxed{V \propto I}$$V∝I​

Removing the proportionality constant,$$\boxed{V = IR}$$

Where:

• V = Potential Difference (Volt)
• I = Electric Current (Ampere)
• R = Resistance (Ohm)

Formula Triangle

        V
      ─────
      I × R

To remember:

• V = I × R
• I = V/R
• R = V/I

Verification of Ohm’s Law

To verify Ohm’s Law:

1. Connect a resistor, battery, ammeter, voltmeter, and rheostat in a circuit.
2. Change the current using the rheostat.
3. Measure the corresponding potential difference.
4. Calculate the ratio $V/I$V/I.

Observation

The ratio $V/I$V/I remains constant.

Hence,$$\boxed{\frac{V}{I}=R}$$IV​=R​

This constant is called the resistance of the conductor.

V–I Graph

The graph between potential difference (V) and current (I) is a straight line passing through the origin for an ohmic conductor.

Important Points

• Straight-line graph.
• Passes through the origin.
• Slope of the graph gives the resistance.

Resistance

Resistance is the property of a conductor that opposes the flow of electric current.

Definition

Resistance is the opposition offered by a conductor to the flow of electric current.

Symbol

$$R$$

The SI unit of resistance is the Ohm (Ω).

Definition of One Ohm

A conductor has a resistance of 1 ohm if a potential difference of 1 volt produces a current of 1 ampere.$$\boxed{1\Omega=\frac{1V}{1A}}$$

Factors Affecting Resistance

The resistance of a conductor depends on four main factors.

1. Length of the Conductor (L)

Resistance is directly proportional to length.$$\boxed{R\propto L}$$R∝L​

Longer wire → Higher resistance

2. Area of Cross-section (A)

Resistance is inversely proportional to the cross-sectional area.$$\boxed{R\propto\frac{1}{A}}$$

Thicker wire → Lower resistance

3. Nature of Material

Different materials have different resistances.

For example:

• Copper → Low resistance
• Aluminium → Low resistance
• Iron → Moderate resistance
• Nichrome → High resistance

4. Temperature

For most metallic conductors:

• Increase in temperature → Increase in resistance

Formula for Resistance

Combining the first two factors,$$\boxed{R=\rho\frac{L}{A}}$$

Where:

• R = Resistance
• ρ (rho) = Resistivity
• L = Length of conductor
• A = Cross-sectional area

Resistivity

Resistivity is a property of the material of the conductor.

Definition

Resistivity is the resistance of a conductor having unit length and unit cross-sectional area.

Symbol

$$\boxed{\rho}$$ρ

SI Unit

$$\boxed{\Omega m}$$Ωm​

(Ohm metre)

Difference Between Resistance and Resistivity

Conductors and Insulators

Solved Numerical 1

Question

A current of 2 A flows through a conductor for 5 seconds.

Calculate the electric charge.

Given

• I = 2 A
• t = 5 s

Formula

$$Q=It$$

Solution

$$Q=2\times5=10C$$

Answer

Charge = 10 C

Solved Numerical 2

Question

A resistor has a resistance of 5 Ω. If the current flowing through it is 2 A, calculate the potential difference.

Formula

$$V=IR$$

Solution

$$V=2\times5$$$$V=10V$$

Answer

Potential Difference = 10 V

Solved Numerical 3

Question

A bulb operates at 12 V and draws a current of 3 A.

Find its resistance.

Formula

$$R=\frac{V}{I}$$

Solution

$$R=\frac{12}{3}=4\Omega$$

Answer

Resistance = 4 Ω

Solved Numerical 4

Question

A conductor carries 240 C of charge in 2 minutes.

Find the current.

Given

• Charge = 240 C
• Time = 2 minutes = 120 s

Formula

$$I=\frac{Q}{t}$$

Solution

$$I=\frac{240}{120}=2A$$

Answer

Current = 2 A

Formula Box

Electric Current

$$\boxed{I=\frac{Q}{t}}$$

Potential Difference

$$\boxed{V=\frac{W}{Q}}$$

Ohm’s Law

$$\boxed{V=IR}$$

Resistance

$$\boxed{R=\frac{V}{I}}$$R=IV​​

Resistivity

$$\boxed{R=\rho\frac{L}{A}}$$

Everyday Applications

Combination of Resistors

In electrical circuits, two or more resistors are often connected together. These connections are called combinations of resistors.

There are two basic types:

1. Series Combination
2. Parallel Combination

Resistors in Series

When resistors are connected one after another in a single path, they form a series combination.

Characteristics of Series Combination

• Only one path is available for current.
• The same current flows through every resistor.
• The potential difference is divided among the resistors.
• If one resistor fails, the entire circuit stops working.

Equivalent Resistance in Series

The total resistance is the sum of all individual resistances.

Formula

$$\boxed{R_s = R_1 + R_2 + R_3 + \cdots}$$Example

If:

• $R_1 = 2\,\Omega$R1​=2Ω
• $R_2 = 3\,\Omega$R2​=3Ω
• $R_3 = 5\,\Omega$R3​=5Ω

Then,$$R_s = 2 + 3 + 5 = 10\,\Omega$$

Voltage in Series

The total potential difference is:$$\boxed{V = V_1 + V_2 + V_3}$$

Current in Series

The same current flows through each resistor.$$\boxed{I = I_1 = I_2 = I_3}$$

Applications of Series Combination

• Decorative light strings (traditional)
• Electric fuse connected in series
• Torch circuits

Resistors in Parallel

When resistors are connected across the same two points, they form a parallel combination.

Characteristics of Parallel Combination

• Multiple paths are available for current.
• The potential difference across each resistor is the same.
• The current divides among the branches.
• If one branch fails, the others continue to work.

Equivalent Resistance in Parallel

$$\boxed{\frac{1}{R_p}=\frac{1}{R_1}+\frac{1}{R_2}+\frac{1}{R_3}}$$

Example

If:

• $R_1=6\,\Omega$ R1​=6Ω
• $R_2=3\,\Omega$ R2​=3Ω

Then,$$\frac{1}{R_p}=\frac{1}{6}+\frac{1}{3}$$​ $$\frac{1}{R_p}=\frac{1}{6}+\frac{2}{6}$$​ $$\frac{1}{R_p}=\frac{3}{6}=\frac{1}{2}$$ $$R_p=2\,\Omega$$

Voltage in Parallel

The potential difference remains the same across all resistors.$$\boxed{V = V_1 = V_2 = V_3}$$

Current in Parallel

The total current is the sum of branch currents.$$\boxed{I = I_1 + I_2 + I_3}$$

Difference Between Series and Parallel Combination

Why are Household Circuits Connected in Parallel?

Household electrical appliances are connected in parallel because:

• Every appliance receives the same supply voltage.
• Appliances can be switched on or off independently.
• Failure of one appliance does not affect the others.

Board Point: This is one of the most frequently asked theory questions in CBSE exams.

Heating Effect of Electric Current

When electric current passes through a conductor, electrical energy is converted into heat energy.

This phenomenon is called the heating effect of electric current.

Joule’s Law of Heating

According to Joule’s Law, the heat produced in a conductor is:

• Directly proportional to the square of the current.
• Directly proportional to the resistance.
• Directly proportional to the time.

Formula

$$\boxed{H = I^2Rt}$$​

Where:

• H = Heat produced (Joule)
• I = Current (Ampere)
• R = Resistance (Ohm)
• t = Time (Second)

Applications of Heating Effect

Electric Fuse

A fuse is a safety device that protects electrical appliances from excessive current.

It is made of a thin wire having:

• High resistance
• Low melting point

When excessive current flows:

• The fuse wire melts.
• The circuit breaks.
• Appliances are protected.

Miniature Circuit Breaker (MCB)

An MCB is an automatic electrical safety device that switches off the circuit during:

• Overloading
• Short circuit

Advantages of MCB

• Reusable
• Faster operation
• Safer than ordinary fuses
• No need to replace after tripping

Electric Power

Electric power is the rate at which electrical energy is consumed or converted.

Formula

$$\boxed{P=\frac{W}{t}}$$

Where:

• P = Power
• W = Work Done
• t = Time

Using Ohm’s Law,

Formula 1

$$\boxed{P=VI}$$

Formula 2

$$\boxed{P=I^2R}$$

Formula 3

$$\boxed{P=\frac{V^2}{R}}$$​​

SI Unit of Power

The SI unit is the Watt (W).

Kilowatt

$$1kW=1000W$$1kW=1000W

Electrical Energy

Electrical energy consumed is given by:$$\boxed{E=Pt}$$E=Pt​

Where:

• E = Electrical Energy
• P = Power
• t = Time

Commercial Unit of Electrical Energy

The commercial unit is the:$$\boxed{1\text{ kilowatt-hour (kWh)}}$$1 kilowatt-hour (kWh)​

It is commonly called one unit of electricity.

Relationship

$$\boxed{1kWh = 3.6\times10^6J}$$

Solved Numerical 1

Question

Three resistors of 2 Ω, 3 Ω, and 5 Ω are connected in series.

Find the equivalent resistance.

Solution

$$R=2+3+5=10\Omega$$

Answer: 10 Ω

Solved Numerical 2

Question

Two resistors of 4 Ω each are connected in parallel.

Find the equivalent resistance.

Solution

$$\frac{1}{R}=\frac{1}{4}+\frac{1}{4} =\frac{2}{4} =\frac{1}{2}$$R1​=41​+41​=42​=21​ $$R=2\Omega$$R=2Ω

Answer: 2 Ω

Solved Numerical 3

Question

A heater draws 5 A current through a 20 Ω resistance for 2 minutes.

Calculate the heat produced.

Given

• $I=5A$I=5A
• $R=20\Omega$R=20Ω
• $t=120s$t=120s

Formula

$$H=I^2Rt$$

Solution

$$H=5^2\times20\times120$$$$H=25\times20\times120$$$$H=60000J$$

Answer: 60,000 J

Solved Numerical 4

Question

A bulb operates at 220 V and draws 0.5 A current.

Find its power.

Formula

$$P=VI$$Solution

$$P=220\times0.5=110W$$

Answer: 110 W

Formula Box

Series Resistance

$$\boxed{R=R_1+R_2+R_3}$$

Parallel Resistance

$$\boxed{\frac{1}{R}=\frac{1}{R_1}+\frac{1}{R_2}+\frac{1}{R_3}}$$

Joule’s Law

$$\boxed{H=I^2Rt}$$

Electric Power

$$\boxed{P=VI}$$ $$\boxed{P=I^2R}$$$$\boxed{P=\frac{V^2}{R}}$$

Electrical Energy

$$\boxed{E=Pt}$$

Complete Formula Sheet

Electricity is a formula-based chapter. Learning these formulas is essential for solving CBSE board examination numericals.

Electric Current

$$\boxed{I=\frac{Q}{t}}$$

Where:

• I = Current (Ampere)
• Q = Charge (Coulomb)
• t = Time (Second)

Electric Charge

$$\boxed{Q=It}$$

Potential Difference

$$\boxed{V=\frac{W}{Q}}$$V=QW​​

Where:

• V = Potential Difference (Volt)
• W = Work Done (Joule)
• Q = Charge (Coulomb)

Ohm’s Law

$$\boxed{V=IR}$$V=IR​

Resistance

$$\boxed{R=\frac{V}{I}}$$R=IV​​

Resistivity

$$\boxed{R=\rho\frac{L}{A}}$$R=ρAL​​

Series Combination

$$\boxed{R_s=R_1+R_2+R_3}$$

Parallel Combination

$$\boxed{\frac{1}{R_p}=\frac{1}{R_1}+\frac{1}{R_2}+\frac{1}{R_3}}$$Rp​1​=R1​1​+R2​1​+R3​1​​

Electric Power

$$\boxed{P=VI}$$P=VI​ $$\boxed{P=I^2R}$$P=I2R​ $$\boxed{P=\frac{V^2}{R}}$$P=RV2​​

Electrical Energy

$$\boxed{E=Pt}$$E=Pt​

Joule’s Law of Heating

$$\boxed{H=I^2Rt}$$H=I2Rt​

Commercial Unit

$$\boxed{1kWh=3.6\times10^6J}$$1kWh=3.6×106J​

Important SI Units

Important Definitions

Electric Current

The rate of flow of electric charge through a conductor.

Potential Difference

The work done in moving a unit positive charge from one point to another.

Resistance

The opposition offered by a conductor to the flow of electric current.

Resistivity

The resistance of a conductor having unit length and unit cross-sectional area.

Electric Power

The rate at which electrical energy is consumed or converted.

Electrical Energy

The total electrical work done by an appliance over time.

Joule’s Law of Heating

The heat produced in a conductor is directly proportional to the square of the current, the resistance, and the time.

Important Differences

Current vs Potential Difference

Resistance vs Resistivity

Series vs Parallel Combination

NCERT Important Points

✔ Electric current is the rate of flow of electric charge.

✔ Conventional current flows from the positive terminal to the negative terminal.

✔ Ohm’s Law is valid only when temperature remains constant.

✔ Copper and silver are good conductors of electricity.

✔ Nichrome has high resistance and is used in heating appliances.

✔ Household appliances are connected in parallel.

✔ Fuse wire has a low melting point and protects electrical appliances.

✔ MCB automatically disconnects the circuit during overloads and short circuits.

✔ Commercial electricity consumption is measured in kilowatt-hour (kWh).

✔ One unit of electricity equals 1 kWh = 3.6 × 10⁶ J.

Assertion–Reason Questions

Question 1

Assertion (A): Ohm’s Law is applicable only when the temperature of the conductor remains constant.

Reason (R): The resistance of a conductor changes with temperature.

Answer: Both A and R are true, and R is the correct explanation of A.

Question 2

Assertion (A): Household appliances are connected in parallel.

Reason (R): Each appliance receives the same supply voltage and can operate independently.

Answer: Both A and R are true, and R is the correct explanation of A.

Question 3

Assertion (A): The heating effect of electric current is directly proportional to the square of the current.

Reason (R): This is stated by Joule’s Law of Heating.

Answer: Both A and R are true, and R is the correct explanation of A.

Case-Based Questions

Case Study 1

A student connects three resistors in series and measures the current through each resistor.

Questions

1. Is the current through all three resistors the same?
2. How is the total resistance calculated?
3. What happens if one resistor breaks?

Answers

1. Yes, the current is the same through all resistors.
2. By adding all the resistances: $R_s = R_1 + R_2 + R_3$Rs​=R1​+R2​+R3​.
3. The circuit becomes open and current stops flowing.

Case Study 2

A house has a room heater rated at 1000 W operating for 2 hours.

Questions

1. Calculate the electrical energy consumed in kWh.
2. How many units of electricity are consumed?
3. Name one safety device used in household circuits.

Solution

Given:

• Power = 1000 W = 1 kW
• Time = 2 h

Using:$$E = Pt$$E=Pt $$E = 1 \times 2 = 2 \text{ kWh}$$E=1×2=2 kWh

Answers:

1. 2 kWh
2. 2 units
3. Fuse or MCB

Solved Board Numericals

Numerical 1

Question

A bulb draws 0.5 A of current from a 220 V source. Calculate its power.

Solution

Using:$$P = VI$$P=VI $$P = 220 \times 0.5 = 110 \text{ W}$$P=220×0.5=110 W

Answer: 110 W

Numerical 2

Question

An electric iron of 1000 W is used for 30 minutes. Calculate the electrical energy consumed in kWh.

Solution

Power = 1000 W = 1 kW

Time = 30 min = 0.5 h$$E = Pt$$E=Pt $$E = 1 \times 0.5 = 0.5 \text{ kWh}$$E=1×0.5=0.5 kWh

Answer: 0.5 kWh (0.5 unit)

Numerical 3

Question

A conductor carries 480 C of charge in 4 minutes. Find the current.

Solution

Time = 4 min = 240 s

Using:$$I=\frac{Q}{t}$$I=tQ​ $$I=\frac{480}{240}=2 \text{ A}$$I=240480​=2 A

Answer: 2 A

Common Mistakes to Avoid

❌ Confusing electric current with electric charge.

❌ Using the wrong formula for power.

❌ Forgetting to convert minutes into seconds or hours where required.

❌ Mixing up series and parallel resistor formulas.

❌ Forgetting to convert 1000 W = 1 kW.

❌ Writing Joule instead of kWh for commercial electricity consumption.

Board Exam Tips

⭐ Memorize all formulas and SI units.

⭐ Practice numerical problems daily.

⭐ Learn the derivation and applications of Ohm’s Law.

⭐ Understand why household wiring uses parallel connections.

⭐ Revise Joule’s Law of Heating and electric power formulas carefully.

⭐ Draw neat and labelled circuit diagrams using standard symbols.

Last-Minute Revision Table

Chapter Summary

Electricity is the flow of electric charges through a conductor. The movement of charges produces electric current, which is measured in amperes. The potential difference between two points provides the energy required to move charges through a circuit. Ohm’s Law establishes the relationship between current, voltage, and resistance, forming the basis for solving electrical circuit problems.

The chapter also explains resistance, resistivity, and the effects of connecting resistors in series and parallel. The heating effect of electric current, described by Joule’s Law, is widely used in appliances such as electric irons, heaters, and kettles. Finally, the concepts of electric power, electrical energy, and the commercial unit of electricity (kilowatt-hour) are introduced, along with the importance of safety devices such as fuses and miniature circuit breakers (MCBs) in protecting electrical circuits.

Frequently Asked Questions (FAQs)

1. What is electric current?

Electric current is the rate of flow of electric charge through a conductor.

2. State Ohm’s Law.

Ohm’s Law states that the current through a conductor is directly proportional to the potential difference across it, provided the temperature remains constant.

3. What is the SI unit of resistance?

The SI unit of resistance is the ohm (Ω).

4. Why are household appliances connected in parallel?

They are connected in parallel so that each appliance receives the same supply voltage and can be operated independently.

5. What is the commercial unit of electrical energy?

The commercial unit of electrical energy is the kilowatt-hour (kWh), commonly called one unit.

6. What is Joule’s Law of Heating?

It states that the heat produced in a conductor is directly proportional to the square of the current, the resistance of the conductor, and the time for which the current flows.

7. Which wire is commonly used in electric heaters?

Nichrome wire is commonly used because it has high resistance and a high melting point.

8. What is the difference between a fuse and an MCB?

A fuse melts and must be replaced after operation, whereas an MCB automatically trips during overload or short circuit and can be reset without replacement.

9. What is the SI unit of electric power?

The SI unit of electric power is the watt (W).

10. Which topics are most important for the CBSE board exam?

Focus on:

• Electric current and potential difference
• Ohm’s Law
• Resistance and resistivity
• Series and parallel combinations of resistors
• Joule’s Law of Heating
• Electric power and electrical energy
• Commercial unit of electricity (kWh)
• Fuse and MCB
• Formula-based numerical problems

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