CBSE Class 10 Science Notes: Electricity

Definitions:

• Electric Current (I): The rate of flow of electric charge (usually electrons) through a conductor. Measured in Amperes (A).
• Potential Difference (Voltage, V): The work done in moving a unit positive charge from one point to another in an electric field. Measured in Volts (V).

Core Principles:

Electric current flows only when there is a potential difference (voltage) and a closed circuit.

Formulaes:

• $I = \frac{Q}{t}$ (where I is current, Q is charge in Coulombs, and t is time in seconds)

Units:

• Current (I): Ampere (A)
• Potential Difference (V): Volt (V)
• Charge (Q): Coulomb (C)

Statement: At a constant temperature, the current (I) flowing through a conductor is directly proportional to the potential difference (V) across its ends.

Formula:

• $V = IR$ (where V is voltage, I is current, and R is resistance)

Examples:

If a 12V battery is connected to a resistor with a resistance of 4 ohms, the current flowing through it will be 3A ($I = \frac{V}{R} = \frac{12V}{4\Omega} = 3A$).

Definitions:

• Resistance (R): The opposition to the flow of electric current. Measured in Ohms ($\Omega$).
• Resistivity ($\rho$): A measure of the resistance of a specific material for a given length and cross-sectional area. Measured in Ohm-meters ($\Omega \cdot m$).

What Affects Resistance?

• Length (L): Resistance is directly proportional to the length of the conductor. Longer wire = higher resistance.
• Area (A): Resistance is inversely proportional to the cross-sectional area of the conductor. Thicker wire = lower resistance.
• Material: Different materials have different resistivities. Copper has low resistivity (good conductor), while nichrome has high resistivity (used in heating elements).
• Temperature: For most conductors, resistance increases with increasing temperature.

Formulaes:

• $R = \rho \frac{L}{A}$ (where R is resistance, $\rho$ is resistivity, L is length, and A is area)

Practical Implications:

Understanding resistance and resistivity is crucial for designing electrical circuits, choosing appropriate wires, and understanding how heating elements work.

Core Principles:

• Series Combination: Resistors are connected end-to-end. The same current flows through each resistor. The total resistance is the sum of individual resistances.
• Parallel Combination: Resistors are connected across each other. The voltage across each resistor is the same. The reciprocal of the total resistance is the sum of the reciprocals of individual resistances.

Formulaes:

• Series: $R_{total} = R_1 + R_2 + R_3 + …$
• Parallel: $\frac{1}{R_{total}} = \frac{1}{R_1} + \frac{1}{R_2} + \frac{1}{R_3} + …$

How Current and Voltage Divide:

• Series: Current is the same in all resistors. Voltage divides proportionally to the resistance of each resistor.
• Parallel: Voltage is the same across all resistors. Current divides proportionally to the inverse of the resistance of each resistor.

Everyday Examples:

• Series: Old Christmas tree lights (if one bulb fails, the whole string goes out).
• Parallel: Household circuits (each appliance can operate independently; a blown bulb doesn’t affect others).

Core Principles:

When an electric current flows through a conductor, some of the electrical energy is converted into heat. This is due to the collisions of electrons with the atoms of the conductor.

Formulaes:

• $H = I^2Rt$ (where H is heat energy, I is current, R is resistance, and t is time)
• $H = VIt$ (where V is voltage, I is current, and t is time)
• $H = \frac{V^2}{R}t$

Common Applications:

• Electric Heater: Uses a high-resistance heating element (e.g., nichrome wire) to generate heat.
• Fuse: A safety device that melts and breaks the circuit if the current exceeds a safe limit, preventing damage to appliances.
• Electric Iron: Uses a heating element to generate heat for ironing clothes.

Definitions:

• Electric Power (P): The rate at which electrical energy is consumed or dissipated in an electrical circuit. Measured in Watts (W).

Relation between P, V, I, and R:

• $P = VI$ (Power = Voltage x Current)
• $P = I^2R$ (Power = Current squared x Resistance)
• $P = \frac{V^2}{R}$ (Power = Voltage squared / Resistance)

Calculating Energy Consumption:

• Energy (E) = Power (P) x Time (t) (in kilowatt-hours, kWh if P is in kW and t is in hours)

Simple Problems:

A 100W light bulb is used for 5 hours. Energy consumed = 100W * 5 hours = 500 Wh = 0.5 kWh.

Definitions:

• Alternating Current (AC): An electric current that periodically reverses direction.
• Frequency: The number of complete cycles of AC per second. Measured in Hertz (Hz).
• Typical Mains Frequency: 50 Hz or 60 Hz (depending on the country). This means the current changes direction 50 or 60 times per second.

Core Principles:

AC can be easily stepped up or stepped down using transformers. This is not possible with DC.

Step-up/Step-down:

• Step-up Transformers: Increase voltage and decrease current, reducing energy loss during transmission over long distances.
• Step-down Transformers: Decrease voltage and increase current for safe use in homes and appliances.

Simple Reasons:

AC’s ability to be easily transformed makes it the most efficient way to transmit electricity over long distances. DC transmission suffers significant energy losses due to voltage drop over distance.

Components:

• Fuse: A safety device that protects the circuit from overcurrents. It melts and breaks the circuit if the current exceeds a certain limit.
• Earthing: Connecting the metal casing of an appliance to the earth to provide a low-resistance path for fault currents, protecting users from electric shock.
• Live Wire (L): Carries the current from the power source.
• Neutral Wire (N): Completes the circuit, carrying the current back to the power source.
• Earth Wire (E): A safety wire connected to the metal body of appliances.

Safe Practices:

• Always use properly insulated wires and appliances.
• Never touch electrical appliances with wet hands.
• Ensure all appliances are properly earthed.
• Do not overload circuits (plugging too many appliances into one socket).
• Replace fuses with the correct rating.
• Get electrical work done by qualified electricians.