Electricity is one of the most important chapters of Class 10 Science. It explains how electric charges flow through a circuit, what causes current to flow, how resistance controls current, how resistors are connected, and how electrical energy is converted into heat and other forms of energy.
These notes cover the important concepts of the NCERT chapter in a clear, exam-oriented manner.
Importance of Electricity
Electricity is one of the most important topics in Class 10 Science because it explains how electric current flows and how electrical devices work. This chapter introduces important concepts such as current, potential difference, resistance, Ohm’s Law, electric power and electrical energy. A strong understanding of these concepts helps students solve board-exam questions, numerical problems and application-based questions with confidence.
Before You Study Electricity: Basic Concepts
Before studying Electricity, it is useful to understand a few basic ideas about matter, atoms, electric charges, ions and conductors.
Electric Charge
Matter is made up of atoms containing protons, neutrons and electrons. Protons carry positive charge, electrons carry negative charge, while neutrons have no charge.
An object becomes charged when it gains or loses electrons. The movement of electrons is especially important in understanding electric current.
Ions
An ion is an atom or group of atoms having a net electric charge because it has gained or lost electrons.
- Loss of electrons → Positive ion (cation)
- Gain of electrons → Negative ion (anion)
Ions are particularly important when studying electric conduction in liquids and electrolytes.
Conductors
Materials through which electric charge can move relatively easily are called conductors.
Examples: copper, aluminium, iron and graphite.
Metals conduct electricity mainly because they contain mobile or relatively free electrons that can move through the material when a potential difference is applied.
Non-Conductors or Insulators
Materials that strongly resist the movement of electric charge are called insulators or non-conductors.
Examples: rubber, glass, plastic and dry wood.
In these materials, electrons are tightly bound to their atoms, so there are very few freely moving charge carriers available to produce an electric current.
Mass and Charge of Electron, Proton and Neutron
| Particle | Symbol | Mass (kg) | Charge (C) | Nature of Charge |
|---|---|---|---|---|
| Electron | e⁻ | 9.11 × 10⁻³¹ kg | −1.6 × 10⁻¹⁹ C | Negative |
| Proton | p⁺ | 1.67 × 10⁻²⁷ kg | +1.6 × 10⁻¹⁹ C | Positive |
| Neutron | n⁰ | 1.675 × 10⁻²⁷ kg | 0 C | No charge |
Key Point:
The magnitude of the charge on an electron and a proton is the same, 1.6 × 10⁻¹⁹ C, but their charges are opposite in nature. A neutron has no electric charge.
Why This Matters in Electricity
The basic idea is simple:
Electric charge + movable charge carriers + a potential difference → electric current
Understanding these basics makes it easier to understand electric current, resistance, Ohm’s Law, conductors, insulators and electric circuits in the Electricity chapter.
1. Electric Current and Electric Circuit
When electric charge flows through a conductor, we say that an electric current is flowing through the conductor.
A simple electric circuit generally consists of:
- A cell or battery
- Connecting wires
- A switch or plug key
- An electrical component such as a bulb or resistor
A closed and continuous conducting path through which electric current flows is called an electric circuit.
When the switch is open, the circuit is broken and current does not flow. When the switch is closed, the conducting path is complete and current can flow.
Electric Current
Electric current is defined as the rate of flow of electric charge through a conductor.
If a charge Q flows through a conductor in time t, then:
I = Q/t
Where:
- I = electric current
- Q = electric charge
- t = time
The SI unit of electric current is ampere (A).
One ampere is the current produced when one coulomb of charge flows through a conductor in one second.
1 A = 1 C/s
Smaller currents are expressed in:
- 1 mA = 10⁻³ A
- 1 μA = 10⁻⁶ A
Direction of Electric Current
In metallic conductors, electrons are the moving charges. Electrons carry negative charge.
The actual movement of electrons is from the negative terminal towards the positive terminal of the source through the external circuit.
However, by convention, the direction of electric current is taken as opposite to the direction of electron flow, that is, from the positive terminal towards the negative terminal.
Ammeter
An ammeter is an instrument used to measure electric current.
It is always connected in series with the component through which the current is to be measured.
2. Electric Potential and Potential Difference
A question naturally arises: What makes electric charges flow through a conductor?
A useful analogy is the flow of water. Water flows from a region of higher pressure towards a region of lower pressure.
Similarly, electric charges flow through a conductor when there is a difference in electric potential between two points.
This difference is called potential difference.
A cell or battery produces potential difference between its terminals through chemical reactions. When the circuit is connected, this potential difference drives the movement of charges.
Potential Difference
The potential difference between two points is defined as the work done in moving a unit charge from one point to another.
V = W/Q
Where:
- V = potential difference
- W = work done
- Q = charge
The SI unit of potential difference is volt (V).
1 V = 1 J/C
Thus, a potential difference of 1 volt means that 1 joule of work is required to move 1 coulomb of charge between two points.
Voltmeter
A voltmeter is used to measure potential difference between two points.
It is connected in parallel across the component whose potential difference is being measured.
Important Difference
Ammeter → measures current → connected in series
Voltmeter → measures potential difference → connected in parallel
3. Circuit Diagram and Symbols
Instead of drawing actual electrical components, we use standard symbols to represent them in circuit diagrams.
Common components include:
- Electric cell
- Battery
- Switch or plug key
- Resistor
- Variable resistor or rheostat
- Ammeter
- Voltmeter
- Connecting wires
- Electric bulb
Students should learn the standard circuit symbols because circuit diagrams are important for board examinations.
4. Ohm’s Law
The relationship between potential difference and current through a conductor was studied by the German physicist Georg Simon Ohm.
According to Ohm’s Law:
At constant temperature, the potential difference across the ends of a conductor is directly proportional to the current flowing through it.
Therefore,
V ∝ I
or
V/I = constant
The constant is called resistance (R).
Hence,
V = IR
This is the mathematical form of Ohm’s Law.
Resistance
Resistance is the property of a conductor that opposes the flow of electric charge.
From Ohm’s Law:
R = V/I
The SI unit of resistance is ohm (Ω).
A conductor has a resistance of 1 Ω when a potential difference of 1 V produces a current of 1 A through it.
1 Ω = 1 V/A
Effect of Resistance on Current
From:
I = V/R
For a fixed potential difference:
- Greater resistance → smaller current
- Smaller resistance → greater current
For example, if resistance is doubled while voltage remains constant, current becomes half.
5. Resistance of a Conductor
Different conductors offer different amounts of opposition to electric current.
The resistance of a uniform conductor depends mainly on:
- Length of the conductor
- Area of cross-section
- Nature of the material
- Temperature
For a uniform conductor:
R ∝ l
and
R ∝ 1/A
Therefore,
R = ρl/A
Where:
- R = resistance
- ρ = resistivity of the material
- l = length of conductor
- A = area of cross-section
Effect of Length
Resistance is directly proportional to length.
R ∝ l
Therefore:
- Length doubled → resistance doubled
- Length tripled → resistance tripled
- Length halved → resistance halved
A longer wire provides greater opposition to the movement of charges.
Effect of Area of Cross-section
Resistance is inversely proportional to the area of cross-section.
R ∝ 1/A
Therefore:
- Larger area → lower resistance
- Smaller area → higher resistance
A thick wire generally has lower resistance than a thin wire of the same material and length.
6. Resistivity
The constant ρ in the relation
R = ρl/A
is called electrical resistivity.
Resistivity is a characteristic property of the material of a conductor.
Its SI unit is:
ohm metre (Ω m)
Unlike resistance, resistivity does not depend on the dimensions of the conductor. For a given material at a given temperature, its resistivity has a definite value.
Low and High Resistivity
Materials having low resistivity generally conduct electricity easily.
Metals such as copper and aluminium have low resistivity and are therefore good conductors.
Materials such as glass and rubber have very high resistivity and act as insulators.
NCERT notes that resistivity also varies with temperature.
Resistance vs Resistivity
| Resistance | Resistivity |
|---|---|
| Depends on length and area | Characteristic property of material |
| Depends on dimensions of conductor | Does not depend on dimensions |
| SI unit: Ω | SI unit: Ω m |
| Represented by R | Represented by ρ |
7. Why Are Alloys Used in Heating Devices?
Alloys generally have higher resistivity than their constituent metals and do not oxidise readily at high temperatures.
Therefore, alloys are commonly used in heating devices such as:
- Electric irons
- Toasters
- Electric heaters
- Heating coils
Nichrome is a commonly used alloy for heating elements.
8. Why Is Tungsten Used in Electric Bulb Filaments?
The filament of an electric bulb must become extremely hot to emit light.
It therefore requires a material that:
- Can withstand very high temperature
- Has a high melting point
- Can be made into a thin wire
Tungsten is used for bulb filaments because of its very high melting point.
NCERT gives the melting point of tungsten as about 3380°C.
9. Combination of Resistors
In electrical circuits, resistors can be connected in different ways.
The two important combinations are:
- Series combination
- Parallel combination
10. Resistors in Series
When resistors are connected end-to-end so that the same current passes through each resistor, they are said to be connected in series.
For three resistors:
R₁, R₂ and R₃
the equivalent resistance is:
Rₛ = R₁ + R₂ + R₃
Characteristics of Series Combination
In a series circuit:
- The same current flows through every resistor.
- The total potential difference is divided among the resistors.
- The total potential difference is:
V = V₁ + V₂ + V₃
- Equivalent resistance is greater than any individual resistance.
Important Point
If one component in a series circuit fails or the circuit breaks, the entire circuit is interrupted.
This is one reason why series connection is not suitable for domestic electrical appliances.
11. Resistors in Parallel
When resistors are connected between the same two points of a circuit, they are connected in parallel.
For three resistors:
R₁, R₂ and R₃
the equivalent resistance is given by:
1/Rₚ = 1/R₁ + 1/R₂ + 1/R₃
Characteristics of Parallel Combination
In a parallel circuit:
- The potential difference across each resistor is the same.
- The total current is divided among different branches.
- Total current is:
I = I₁ + I₂ + I₃
- The equivalent resistance is less than the resistance of the smallest individual resistor.
12. Series vs Parallel Combination
| Series | Parallel |
|---|---|
| Same current flows through all components | Current divides among branches |
| Potential difference is divided | Same potential difference across each branch |
| Equivalent resistance increases | Equivalent resistance decreases |
| Failure of one component can break the entire circuit | Other branches can continue working |
| Not suitable for domestic circuits | Used in domestic electrical circuits |
13. Why Are Household Appliances Connected in Parallel?
Electrical appliances in homes are connected in parallel because:
- Each appliance receives the required potential difference.
- Appliances can be operated independently.
- Switching off one appliance does not normally stop the others.
- Different appliances can draw different currents according to their resistance and power requirements.
This makes parallel connection more suitable for domestic electrical circuits.
14. Heating Effect of Electric Current
When electric current passes through a resistor, electrical energy is converted into heat energy.
This phenomenon is called the heating effect of electric current.
For a purely resistive circuit, the electrical energy supplied by the source is dissipated as heat in the resistance.
If:
- Current = I
- Resistance = R
- Time = t
then the heat produced is:
H = I²Rt
This is known as Joule’s law of heating.
Joule’s Law of Heating
The heat produced in a resistor is:
- Directly proportional to I², for fixed R and t.
- Directly proportional to R, for fixed I and t.
- Directly proportional to t, for fixed I and R.
Therefore:
H ∝ I²
H ∝ R
H ∝ t
Other Forms of Heat Formula
Since:
V = IR
we can write:
H = VIt
and also:
H = V²t/R
These forms are useful for solving numerical problems.
15. Applications of Heating Effect
The heating effect of electric current is useful in many electrical appliances.
Examples include:
- Electric iron
- Electric heater
- Electric toaster
- Electric oven
- Electric kettle
- Heating coils
The heating effect is also used in electric bulb filaments and safety devices such as fuses.
16. Electric Fuse
A fuse is a safety device used to protect an electrical circuit from excessive current.
A fuse is connected in series with the circuit.
It contains a wire made of a suitable metal or alloy. When current becomes higher than the specified safe value, the fuse wire gets heated and melts.
This breaks the circuit and stops the flow of excessive current.
Working of a Fuse
Excess current → Heating of fuse wire → Fuse melts → Circuit breaks → Appliances protected
The working of a fuse is based on the heating effect of electric current.
17. Electric Power
Electric power is the rate at which electrical energy is consumed or converted into other forms of energy.
The formula for electric power is:
P = VI
Using Ohm’s Law:
P = I²R
and
P = V²/R
Thus, the three important power formulas are:
P = VI
P = I²R
P = V²/R
The SI unit of electric power is watt (W).
1 W = 1 V × 1 A
A larger unit is:
1 kilowatt (kW) = 1000 W
18. Electrical Energy
Electrical energy consumed by an appliance is given by:
E = Pt
Since:
P = VI
we can also write:
E = VIt
Electrical energy may be expressed in joules when SI units are used.
For commercial purposes, electrical energy is measured in kilowatt-hour (kWh).
Commercial Unit of Electrical Energy
The commercial unit of electrical energy is:
1 kWh
It is commonly called one unit of electricity.
Since:
1 kW = 1000 W
and
1 hour = 3600 s
therefore:
1 kWh = 1000 × 3600 J
1 kWh = 3.6 × 10⁶ J
19. Electrical Energy and Electricity Bill
Electrical energy consumed depends on:
- Power of the appliance
- Time for which it operates
For example, if an appliance has a power of 1000 W = 1 kW and operates for 2 hours:
Energy = Power × Time
= 1 kW × 2 h
= 2 kWh
Therefore, the appliance consumes 2 units of electrical energy.
The cost of electricity can be calculated as:
Cost = Energy consumed in kWh × Rate per kWh
20. Important Relationships for Numericals
Students should be comfortable changing one formula into another.
From Ohm’s Law
V = IR
Therefore:
I = V/R
and
R = V/I
From Electric Power
P = VI
Therefore:
I = P/V
and
V = P/I
Using Ohm’s Law:
P = I²R
and
P = V²/R
From Electrical Energy
E = Pt
Also:
E = VIt
From Heating Effect
H = I²Rt
Also:
H = VIt
and
H = V²t/R
21. Important Graph: V–I Relationship
For a conductor obeying Ohm’s Law, potential difference V is directly proportional to current I, provided temperature remains constant.
Therefore, a graph of V against I is a straight line passing through the origin.
The constant ratio:
V/I = R
represents the resistance of the conductor.
Board Exam Point
If the V–I graph is a straight line through the origin, the conductor follows Ohm’s Law under the given conditions.
22. Rheostat
A rheostat is a variable resistance device used to control current in an electric circuit without changing the voltage source.
By changing the resistance of the circuit, the current can be increased or decreased.
It is therefore useful when a controlled variation of current is required.
23. Important NCERT-Based Conceptual Points
Why does a thick wire have lower resistance?
A thick wire has a larger cross-sectional area. Since:
R = ρl/A
a larger area results in lower resistance.
Why does a long wire have greater resistance?
Resistance is directly proportional to length:
R ∝ l
Therefore, increasing the length increases resistance.
Why is copper used for electrical transmission?
Copper has low resistivity and therefore offers low resistance to the flow of current. It is consequently a good conductor.
Why are heating elements made from alloys?
Heating elements require materials that have suitable resistance and can withstand high temperatures. Alloys generally have higher resistivity and do not oxidise readily at high temperatures.
Why is the series arrangement unsuitable for domestic circuits?
A series circuit carries the same current through all components, while domestic appliances generally require different currents. Also, failure of one component can interrupt the entire circuit.
24. Common Board Exam Mistakes
Students should avoid these common mistakes:
- Writing current and charge as the same quantity.
- Connecting an ammeter in parallel.
- Connecting a voltmeter in series.
- Forgetting to convert minutes into seconds.
- Forgetting to convert watts into kilowatts when calculating electricity consumption in kWh.
- Using the series formula for a parallel circuit.
- Forgetting the square in I²R.
- Writing kWh as a unit of power instead of electrical energy.
- Omitting units from numerical answers.
- Using R = ρA/l instead of the correct formula R = ρl/A.
25. Quick Revision Table
| Quantity | Formula | SI Unit |
|---|---|---|
| Electric Current | I = Q/t | Ampere (A) |
| Potential Difference | V = W/Q | Volt (V) |
| Resistance | R = V/I | Ohm (Ω) |
| Resistance of Wire | R = ρl/A | Ohm (Ω) |
| Series Resistance | R = R₁ + R₂ + R₃ | Ohm (Ω) |
| Parallel Resistance | 1/R = 1/R₁ + 1/R₂ + 1/R₃ | Ohm (Ω) |
| Electric Power | P = VI | Watt (W) |
| Electric Power | P = I²R | Watt (W) |
| Electric Power | P = V²/R | Watt (W) |
| Electrical Energy | E = Pt | Joule (J) |
| Electrical Energy | E = VIt | Joule (J) |
| Heat Produced | H = I²Rt | Joule (J) |
| Commercial Energy | 1 kWh = 3.6 × 10⁶ J | kWh |
26. Key Points to Remember
- Electric current is the rate of flow of charge.
- Conventional current flows from positive to negative terminal through the external circuit.
- An ammeter is connected in series.
- A voltmeter is connected in parallel.
- Potential difference provides the driving force for charge flow.
- Ohm’s Law is V = IR, when temperature remains constant.
- Resistance depends on length, area of cross-section and material.
- Resistivity is a characteristic property of a material.
- In series, current remains the same.
- In parallel, potential difference remains the same across each branch.
- Domestic circuits use parallel connections.
- Heating effect is described by H = I²Rt.
- Electric power is P = VI.
- Commercial electrical energy is measured in kWh.
- 1 kWh = 3.6 × 10⁶ J.
- A fuse protects circuits against excessive current.
These concepts form the core of the Class 10 Electricity chapter and provide the foundation for solving the chapter’s numerical, conceptual, diagram-based and application-based questions. The current NCERT chapter itself includes worked examples and exercises covering resistance, resistor combinations, power, heating effect and electrical energy.