← Back 🏠 Home

Science: Class 10 — Chapter 11: Electricity

11.1 Electric Current and Circuit

11.1.1 Current, Charge, and Units
  • Electric Current: The rate of flow of electric charges. It is expressed by the amount of charge flowing through a particular area in unit time: I = Q/t.
  • SI Unit of Charge: The coulomb (C), which is equivalent to the charge contained in nearly 6 × 1018 electrons.
  • Electron Charge: An electron possesses a negative charge of 1.6 × 10-19 C.
  • SI Unit of Current: The ampere (A), defined as the flow of one coulomb of charge per second (1 A = 1 C/s).
  • Direction of Current: Conventionally taken as opposite to the direction of the flow of electrons.

11.1.2 Electric Circuits and Instruments
  • Electric Circuit: A continuous and closed path of an electric current.
  • Switch: A device that makes or breaks the conducting link between the cell and the load.
  • Ammeter: An instrument used to measure electric current in a circuit. It is always connected in series.
11.2 Electric Potential and Potential Difference

11.2.1 Potential Difference and Voltmeter
  • Potential Difference (V): The work done to move a unit charge from one point to another in an electric circuit: V = W/Q.
  • SI Unit: The volt (V), named after Alessandro Volta. One volt is defined when 1 joule of work is done to move a charge of 1 coulomb (1 V = 1 J/C).
  • Voltmeter: An instrument used to measure potential difference. It is always connected in parallel across the points of interest.
11.3 Circuit Diagram

11.3.1 Common Electrical Symbols
  • Electric Cell & Battery: Represented by long (positive) and short/thick (negative) parallel lines.
  • Plug Key / Switch: Represented as open —( )— or closed —(•)—.
  • Resistor: Represented by a zigzag line.
  • Rheostat / Variable Resistance: Represented by a resistor symbol with an arrow through or across it.
11.4 Ohm's Law

11.4.1 Relationship and Resistance
  • Ohm's Law: The potential difference $V$ across the ends of a metallic wire is directly proportional to the current $I$ flowing through it, provided its temperature remains constant: V = IR.
  • Resistance (R): The property of a conductor to resist the flow of charges through it. Its SI unit is the ohm (Ω): R = V/I.
  • Variable Resistance: A component used to regulate current without changing the voltage source.
11.5 Factors on which Resistance Depends

11.5.1 Relationships and Resistivity
  • Length: Resistance is directly proportional to the length of the conductor (R ∝ l).
  • Area of Cross-section: Resistance is inversely proportional to the area of cross-section (R ∝ 1/A).
  • Resistivity (ρ): A constant of proportionality representing the characteristic material property. Combined resistance formula: R = ρ(l/A).
  • SI Unit of Resistivity: The ohm-meter (Ω m). Metals and alloys have very low resistivity, whereas insulators have extremely high resistivity.
11.6 Resistance of a System of Resistors

11.6.1 Resistors in Series
  • Equivalent Resistance: The sum of individual resistances: R_s = R_1 + R_2 + R_3.
  • Current: The current remains constant and is the same through every part of the circuit.
  • Voltage: Total potential difference is divided: V = V_1 + V_2 + V_3.

11.6.2 Resistors in Parallel
  • Equivalent Resistance: Calculated by the sum of reciprocals: 1/R_p = 1/R_1 + 1/R_2 + 1/R_3.
  • Current: Total current splits across parallel branches: I = I_1 + I_2 + I_3.
  • Voltage: The potential difference remains the same across all parallel branches.
11.7 Heating Effect of Electric Current

11.7.1 Joule's Law of Heating
  • Heating Effect: When current flows through a purely resistive circuit, the source energy is continuously dissipated entirely as heat.
  • Joule's Law: The heat produced in a resistor is directly proportional to the square of the current, the resistance, and the time of current flow: H = I2Rt.

11.7.2 Practical Applications
  • Heating Devices: Electric laundry irons, toasters, ovens, and kettles use high-resistivity alloys that do not oxidise easily at high temperatures.
  • Electric Bulb: Uses tungsten (melting point 3380°C) as a filament to radiate heat and light, typically filled with inactive nitrogen/argon gas.
  • Fuse: A safety device placed in series to protect circuits by melting and breaking the circuit when current exceeds limit.
11.8 Electric Power

11.8.1 Power and Commercial Units
  • Electric Power (P): The rate at which electrical energy is consumed in a circuit: P = VI = I2R = V2/R.
  • SI Unit: The watt (W), where 1 W = 1 V × 1 A.
  • Commercial Unit of Energy: The kilowatt hour (kW h), commonly known as a "unit". 1 kW h = 3.6 × 106 J.