NCERT notes for Class 10 Physics Chapter 12, Electricity, are crucial for building a strong foundation for future concepts. This chapter forms the basis for many topics you'll encounter later, and a solid understanding of it is essential, as many exam questions are likely to be based on this material.
Chapter 12 of the NCERT Class 10 Physics textbook focuses on electricity. It covers key topics such as electric current, circuits, power, resistance in series and parallel, and Ohm's Law. These notes provide a comprehensive summary of the chapter, highlighting essential concepts, formulas, and topics that are crucial for exam success.
What is Electricity?
Electricity is a form of energy caused by the movement of electrons or charged particles through a conductor, used for powering devices, transmitting information, and generating electrical power.
- Electricity is a crucial energy source in modern society, powering a wide variety of sectors, including business, transportation, and residential homes. In our everyday lives, electricity powers essential systems such as lighting, fans, and heating.
- It's also a key component in transportation, with electric trains relying on it for operation. Beyond this, electricity is indispensable in industrial processes and other forms of transportation, making it a vital energy source across multiple domains.
- Electricity is generated by the movement of tiny particles called electrons. Electrons can carry either a negative or a positive charge. Like charges repel each other; so negative charges repel negative charges, and positive charges repel positive charges. Conversely, opposite charges attract, meaning negative charges are drawn to positive charges. This movement of electrons creates the flow of electricity.
- Electricity can be generated when two different materials are rubbed together. For example, when a silk cloth is rubbed against a glass rod, the silk cloth becomes positively charged, while the glass rod takes on a negative charge.
- Electricity serves a wide range of purposes, from powering equipment, appliances, and lighting to enabling information transmission and communication.
Electric Charge
Electric charge is a fundamental property of matter that causes it to experience a force when placed in an electric and magnetic field. It is a property of subatomic particles, such as electrons and protons, and can be positive or negative.
- The coulomb (C) is the unit of electric charge. Protons, which carry a positive charge, and electrons, which have a negative charge, are fundamental particles found in all matter.
- One coulomb is the amount of electric charge that pushes or pulls with a force of 9 × 109 newtons when another equal charge is placed 1 meter away from it.
- Through contact, friction, or induction, we can transfer charge between objects without direct contact. When two objects with different charges are brought together, charge flows from the object with more charge to the one with less, until both objects reach an equal charge. This phenomenon is known as static electricity.
- For example, if you rub a balloon against your hair, the balloon becomes negatively charged, while your hair becomes positively charged. The opposite charges attract, causing the balloon to stick to your hair.
Note:
- An electron carries a negative charge of 1.6 × 10⁻¹⁹ C, while a proton has an equal but positive charge of 1.6 × 10⁻¹⁹ C.
- The total charge of 6.25 × 10¹⁸ electrons is equivalent to one coulomb (C), which is the SI unit of electric charge.
Learn more about, Electric Charge
Example: Calculate the number of electrons constituting one coulomb of charge.
Solution:
We know that the charge of an electron is 1.6 × 10-19 coulomb (or 1.6 × 10-19 C).
Now, If charge is 1.6 × 10-19 C, No. of electrons = 1
If charge is 1 C, then No. of electrons = 1 / (1.6 × 10-19 C) = 6.25 × 1018 electron
Electric Current
An electric current is the movement of charged particles, like electrons or ions, through a conductor or across a space.
- Amperes (A) are the units used to measure electric current. One ampere is defined as the flow of one coulomb of charge per second.
- For example, 1 coulomb of charge will flow through a wire if a current of 1 A runs through it for 1 second.
- Many items in our homes and workplaces, including lights, appliances, and computers, are powered by electric current.
- The magnitude I of the electric current flowing through a conductor if a charge of Q coulombs flows through it in time t seconds is given by:
Current, I= Q/t
- The symbol "A" stands for ampere, which is the SI unit of electric current.
- Electric current is considered to be 1 ampere when 1 coulomb of charge passes through a conductor's cross-section in 1 second.
\bold{\text{1 Ampere}=\frac{\text{1 Coulomb}}{\text{1 Second}}}
Ammeters are devices used to measure electric current. To measure the current, the ammeter is connected in series with the circuit.
Learn more about, Electric Current
Example: An electric bulb draws a current of 0.25 A for 20 minutes. Calculate the amount of electric charge that flows through the circuit.
Solution:
Current, I = 0.25 A
Charge, Q = ? (To be calculated)
And Time, t = 20 minutes
⇒ t = 20 × 60 seconds
⇒ t = 1200 s
Thus, I = Q /t
⇒ Q = It
⇒ Q = 0.25(1200)
⇒ Q = 300 C
Thus, the amount of electric charge that flows through the circuit is 300 coulombs.
Electric Circuit
An electric circuit is a closed loop that creates an uninterrupted path for electric current to flow, enabling electricity to travel from a power source to a device or location where it does work.
- A battery can cause electrons to move, but they need a pathway to travel. An electrical circuit provides this path, allowing electrons to flow from the battery to a component, like a light bulb.
- A circuit typically consists of three main components: a battery, a conductor, and a device. The battery provides the energy needed to move the electrons, while the conductor serves as the pathway for electron flow. A device, such as a lightbulb, uses the energy from the moving electrons to perform a specific function.
- When all the components of a circuit are connected, electrons flow from the battery to the device and then return to the battery. This is how a circuit operates.
- Batteries have positive and negative terminals. A copper wire connects the positive terminal to the positive end of a light bulb, while the opposite end of the light bulb is connected to the negative terminal.
- A switch is placed between the two terminals. When the switch is closed, a complete circuit is formed, allowing current to flow. This current lights up the light bulb.
- When the switch is flipped off, the circuit is broken, meaning the path for the current is no longer complete. Without a complete circuit, the current cannot flow, and the light bulb will stop glowing.
Read more about, Electric Circuit
Electric Circuit Symbols
Electrical circuits are depicted using circuit diagrams. These diagrams use standardized electrical symbols to represent the connections and relationships between different components in the circuit.
Electric Circuit Symbols
Electric Potential
Electric potential is the amount of electric potential energy per unit charge at a specific point in an electric field. It represents the work needed to move a positive test charge from a reference point (usually infinity) to that point, without accelerating the charge.
- When a positive charge is placed near a negative charge, it experiences an attractive force pulling it toward the negative charge. To separate the positive charge from the negative charge, work must be done, as the force between them resists their separation.
- The electric potential at a specific place indicates how much effort is needed to transport a positive charge from infinity to that position. The greater your potential, the more work you must put in.
- The unit of measurement for electrical potential is the volt. One volt (V) is defined as the amount of work required to move one coulomb of charge over a distance of one meter.
Potential Difference
Potential difference, or voltage, is the difference in electric potential between two points within an electric field. It indicates the amount of work required to move a unit of electric charge from one point to another.
The potential difference V between the two points is given by the formula if W joules of work must be done to transport Q coulombs of charge from one place to the other.
\bold{V=\frac{W}{Q}}
Where,
- W is the work done, and
- Q is the quantity of charge moved.
- The volt, represented by the symbol V, is the unit of potential difference in the SI system. Sometimes, potential difference is also referred to by the abbreviation p.d.
- A device known as a voltmeter is used to measure the potential difference.
A voltmeter connected in parallel with conductor AB to measure the potential difference across its ends.Example: How much work is done in moving a charge of 2 coulombs from a point at 118 volts to a point at 128 volts?
Solution:
Q = 2C (coulombs)
Thus, V = 128 - 118
⇒ V = 10 volts
and V = W / Q
⇒ 10 = W / 2
⇒ W = 20 joules
Thus, 20 joules of work is done in moving a charge of 2 coulombs from a point at 118 volts to a point at 128 volts.
Read more about, Different between EMF and Potential Difference
Ohm’s Law
Ohm's Law describes how voltage, current, and resistance are related in an electrical circuit. It states that the current (I) flowing through a conductor is directly proportional to the voltage (V) applied across it, as long as the resistance (R) stays constant.
- Ohm's law establishes a connection between current and potential differences.
- At constant temperature, the current flowing through a conductor is precisely proportional to the potential difference across its ends, according to Ohm's law.
- If I denotes the current flowing through a conductor and V denotes the potential difference (or voltage) at its ends, then Ohm's law states:
I ∝ V (At constant temp.)
⇒ V ∝ I
⇒ V = R × I
Where R is the conductor's "resistance" constant. The value of this constant is determined by the conductor's type, length, cross-sectional area, and temperature. The equation above can also be written as:
\bold{\frac{V}{I}=R}
Where,
- V is the Potential difference,
- I is the Current, and
- R is the Resistance (which is a constant).
Example: Potential difference between two points of a wire carrying a 2 ampere current is 0.1 volt. Calculate the resistance between these points.
Solution:
According to the OHM Law, V = R × I
⇒ 0.1 = R × 2
⇒ R = 0.1 / 2
⇒ R = 0.05 ohm
Graph Between V and I
- If a straight line is drawn between the potential difference readings (V) and the corresponding current values (I), the resulting graph will be a straight line that passes through the origin.
- A straight-line graph can only be produced when the two quantities are directly proportional to each other. Since the "current versus potential difference" graph forms a straight line, we can conclude that current is directly proportional to the potential difference.
- Graph OA clearly demonstrates that as the potential difference (V) increases, the current (I) also increases. However, the ratio of V/I remains constant. This constant is referred to as the resistance of the conductor.
Resistance of a Conductor
Resistance is a property of a conductor that opposes the flow of current through it. The resistance of a conductor is equal to the potential difference between its ends divided by the current flowing through it.
\bold{R=\frac{V}{I}}
Where,
- V is the Potential difference,
- I is the Current, and
- R is the Resistance.
The resistance of a conductor is influenced by its length, thickness, material, and temperature. The SI unit of resistance is the ohm, represented by the symbol omega (Ω).
Factor Affecting Resistance
There are various factors that affect the resistance of the conductor,
- Length of the Conductor
- Area of Cross-Section of the Conductor
- Nature of Material of the Conductor
- Temperature
Let's discuss these effects in detail as follows:
Effect of Length of the Conductor
A conductor's resistance (R) is proportional to its length (l).
R ∝ l
Effect of Area of Cross-Section of the Conductor
A conductor's resistance (R) is inversely proportional to its area of cross-section (A).
R ∝ 1/A
Effect of the Nature of Material of the Conductor
The resistance of a wire depends on the material it is made from. Some materials, like copper, allow electricity to flow easily, while others, like nichrome, resist the flow. If two wires of equal length and thickness are compared; one made of copper and the other made of nichrome; the nichrome wire will have approximately 60 times greater resistance than the copper wire.
Effect of Temperature
- When pure metals are heated, their resistance increases, and when they are cooled, their resistance decreases. This happens because heating causes the metal's atoms to move faster, making it harder for electrons to pass through the metal.
- The temperature generally has little effect on the resistance of alloys such as manganin, constantan, and nichrome. This is because the atoms in these alloys have difficulty moving, even when heated, which helps maintain a stable resistance.
- Alloys with a low temperature coefficient of resistance are often used in applications that require stable resistance, such as in electrical meters and precision instruments. This ensures that the resistance remains consistent despite temperature changes.
Learn more about, Factors Affecting Resistance
Resistivity
As we discussed the effects of various parameters on resistance, let's derive a formula using all those parameters.
As we know, the resistance (R) of a conductor is proportional to its length (l).
R ∝ l … (1)
and resistance (R) of a conductor is inversely proportional to its area of cross-section (A).
R ∝ 1 / A … (2)
By combining the relations (1) and (2), we get:
R ∝ l / A
⇒ R = (ρI) / A
⇒ ρ = (RA) / I
Here, ρ (rho) represents a constant known as the resistivity of the material of the conductor. Resistivity, also called specific resistance, is a property that characterizes how strongly a material opposes the flow of electric current.
Example: A copper wire of length 2 m and area of cross-section 1.7 × 10-6 m2 has a resistance of 2 × 10-2 ohms. Calculate the resistivity of copper
Solution:
Resistance, R = 2 × 10-2 ohm
Area of cross-section, A = 1.7 × 10-6 m2
Length, l = 2 m
⇒ ρ = (2 × 10-2 × 1.7 × 10-6) / 2
⇒ ρ = 1.7 × 10-8 ohm-meter
Combination of Resistors
The combination of resistors occurs in two ways :
- Resistors in Series
- Resistors in Parallel
Let's discuss these in detail ,
Resistors in Series
If we want to raise overall resistance, we connect the individual resistances in series
Any number of resistances connected in series have a combined resistance that is equal to the sum of the individual resistances. As an illustration, the total resistance R is calculated as follows if several resistances R1, R2, R3,... etc. are connected in series:
R = R1 + R2 + R3 +...…
Example: A resistance of 6 ohms is connected in series with another resistance of 4 ohms. A potential difference of 20 volts is applied across the combination. Calculate the current through the circuit.
Solution:
Total resistance,
R = R1 + R2
R = 6 + 4
R = 10 ohms
Now, Total resistance, R = 10 ohms
Potential difference, V = 20 volts
and, Current in the circuit, I = ? (To be calculated)
So, applying Ohm’s law to the whole circuit, we get :
V / I = R
20 / I = 10
I = 2 amperes
Thus, the current flowing through the circuit is 2 amperes.
Resistors in Parallel
if we want to decrease overall resistance, we connect the individual resistances in parallel.
The sum of the reciprocals of all the individual resistances is equal to the reciprocal of the combined resistance of a number of resistances connected in parallel.
For example, the formula yields the total resistance R if a series of resistances, R1, R2, R3, etc., are connected in parallel.
R = 1/R1 + 1/R2 + 1/R3 + …
Example: Calculate the equivalent resistance when two resistances of 3 ohms and 6 ohms are connected in parallel.
Solution:
1/ R = 1 / R1 + 1 / R2
1 / R = 1 / 3 + 1 / 6
1 / R = (2+1) / 6
1 / R = 3 / 6
R = 2 ohm
Heating Effect of Electric Current
The heating effect of an electric current is the process in which electrical energy is transformed into heat energy as an electric current passes through a conductor.
- When an electric current passes through a wire, it heats up. The amount of heat generated depends on the wire's resistance and the current flowing through it. For the same amount of current, a high-resistance wire, like a nichrome wire, will heat up more than a low-resistance wire.
- The increased resistance in a high-resistance wire, such as nichrome, causes more heat to be generated as it resists the flow of current. The heating effect of electric current occurs when electrical energy is converted into heat energy.
- Just as some of the energy required to push a heavy object is converted into heat due to friction, similarly, some electrical energy is converted into heat by resistance when an electric current flows through a wire.
- Both friction and resistance work to oppose movement. While friction hinders physical objects from moving past each other, resistance restricts the flow of electrical current.
- The amount of heat generated by friction or resistance depends on the force applied and the distance over which it acts.
- Although resistance and friction are both unwanted forces, they can be useful. For instance, friction allows us to walk and drive cars, while resistance is essential for the operation of electrical equipment.
The work done by a current I when it flows through a resistance R for time t. Now, when an electric charge Q moves against a potential difference V, the amount of work done is given by
W = Q × V
According to the OHM Law, V = R × I
and Current , I = \frac{Q}{t}
⇒ Q = I × t
Thus, W = I × t × R × I
⇒ W = I2Rt
We can substitute "Heat produced" for "Work done" in the equation above if we assume that all electrical work completed or electrical energy used is transformed into heat energy.
W = I2Rt
Example: A potential difference of 250 volts is applied across a resistance of 500 ohms in an electric iron. Calculate the heat energy produced in joules in 10 seconds.
Solution:
Here, Potential difference, V = 250 volts
Current, I = ? (To be calculated)
t = 10 seconds
Resistance, R = 500 ohms
⇒ 250 / I = 500
⇒ I = 1 / 2
⇒ I = 0.5 ampere
The heat energy in joules can be calculated by using the formula : H = I2 × R × t
⇒ H = (0.5)2 × 500 × 10
⇒ H = 1250 joules
Electric Power
Electric power is the rate at which electrical energy is transferred or converted in a circuit. It essentially measures how quickly energy is being consumed or provided within the circuit.
- Electric current is the flow of electrons through a conductor. This movement of electrons requires energy, which can be used to perform tasks like heating a stove or lighting a bulb.
- Power measures how quickly tasks are completed. Electric power refers to the rate at which electrical energy is used or depleted in a circuit.
- The watt is the unit of electrical power. One watt is equal to one joule of work done per second.
- The watt is the measure of electrical power. One joule of work is equal to one watt of power every second.
For example, a 100-watt light bulb consumes 100 joules of energy every second.
P = W / t
The work done W by current I when it flows for time t under a potential difference V is given by :
W = V × I × t
Thus, P = V × I × t / t
And I = Q / t
⇒ Q = I × t
Thus, P = VQ / t
⇒ P = V × I
⇒ P = V2 / R
Example: What will be the current drawn by an electric bulb of 40 W when it is connected to a source of 220 V?
Solution:
P = V × I
Here, Power, P = 40 watts
Voltage, V = 220 volts
And, Current, I = ? (To be calculated)
Now, putting these values in the above formula, we get :
⇒ 40 = 220 × I
⇒ I = 40 / 220
⇒ I = 0.18 ampere
Learn more about, Electric Energy and Power
Applications of the Heating Effect of Current
- Electric heating appliances rely on the heating effect of electric current. Examples of such devices include electric irons, kettles, toasters, ovens, room heaters, and water heaters.
- Electric lightbulbs work by heating the tungsten metal filament inside. Due to tungsten's very high melting point, it can withstand the heat generated by the current. As the filament heats up, it reaches a temperature where it glows and emits light.
- Electric fuses are safety devices designed to protect residential wiring and electrical appliances from damage caused by high currents. A fuse contains a thin wire that melts when a certain current flows through it. This melting breaks the circuit, preventing damage to the wiring or appliance.
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