Electromotive Force (EMF) and Ohm’s Law - Notes

1. Electric Current and Potential Difference:

• Electric Current: Flow of electric charges (movement of electrons)
  • Conventional Current: Positive charges moving from higher to lower potential
  • Electron Flow (Actual Current): Negative charges moving from lower to higher potential
• Potential Difference: Electrical energy difference between two points
  • Measured in volts (V)
  • Potential difference across a component = Work done in moving a unit positive charge from one point to another
• Relationship: I = V/R
  • I = Current (in Amperes, A)
  • V = Potential difference (in Volts, V)
  • R = Resistance (in Ohms, Ω) Reference: NCERT Class 12, Chapter 1: Electric Charges and Fields

2. Resistance:

• Opposition to the flow of electric current
  • Measured in Ohms (Ω)
• Factors affecting resistance:
  • Length: Resistance directly proportional to length
  • Area of cross-section: Resistance inversely proportional to the cross-sectional area
  • Temperature: Resistance of most metals increases with temperature (PTC materials are exceptions)
  • Material: Resistance depends on the material’s resistivity
• Types of resistors:
  • Fixed Resistors: Constant resistance
  • Variable Resistors: Variable resistance (e.g., potentiometers, rheostats) Reference: NCERT Class 12, Chapter 13: Current Electricity

3. Electromotive Force (EMF):

• Energy supplied by a source to maintain a potential difference between its terminals
• Measured in Volts (V)
• Sources of EMF:
  • Voltaric Cells (Batteries): Chemical energy to electrical energy
  • Thermocouples: Heat energy to electrical energy
  • Generators: Mechanical energy to electrical energy
• Internal Resistance: Resistance within the source of EMF that opposes the flow of current
• Terminal Potential Difference: Potential difference across the terminals of a source Reference: NCERT Class 12, Chapter 13: Current Electricity

4. Kirchhoff’s Laws:

a) Kirchhoff’s Current Law (KCL):

• Sum of currents entering a junction equals the sum of currents leaving the same junction
• Conservation of charge b) Kirchhoff’s Voltage Law (KVL):
• Algebraic sum of potential differences around a closed loop is equal to zero
• Applicable to any closed loop or mesh in a circuit Reference: NCERT Class 12, Chapter 13: Current Electricity

5. Power:

• Rate of energy transfer or consumption in an electrical circuit
• Measured in Watts (W)
• Power (P) = Current (I) x Potential difference (V)
• Power Dissipation: Conversion of electrical energy into other forms (e.g., heat, light) Reference: NCERT Class 12, Chapter 13: Current Electricity

The notes above provide a detailed summary of the important subtopics related to Electromotive force and Ohm’s law for the JEE exam. Remember, it’s essential to refer to the prescribed syllabus, additional study materials, and practice questions to gain a thorough understanding of the concepts.