Lecture on Power Supplies and Batteries

Jul 19, 2024

Lecture on Power Supplies and Batteries

Key Concepts

  • Power Supplies: Maintain a constant potential difference (V).
  • Resistors (R): Cause current to flow and influence electric fields.
  • Electric Fields: Always run from positive to negative potential.
  • Current Flow:
    • Through power supply: Against the electric field.
    • Through resistor: In the direction of the electric field.

Mechanism of Power Supply

  • Electric field inside the supply runs opposite to current flow.
  • A mechanism (like a pump) must force current through against the electric field.

Chemical Energy in Batteries

  • VandeGraaff and Wimshurst machines: Utilize a motor or manual crank to drive current.
  • Common batteries: Use chemical energy. For example, zinc-copper plates in a solution (H2SO4).

Chemical Reaction in Batteries

  • Zinc and Copper Plates: Create potential difference (~1 volt).
  • Ions flow through a barrier from one side to another, driven by a chemical reaction overcoming the electric field.
  • SO4 Ions: Negative ions move from right to left, engaging in a chemical reaction that yields energy.
  • Copper and Zinc Ions: Precipitate and dissolve respectively on their plates, maintaining neutrality.
  • Recharging: Reverses the chemical reaction using an external power supply.

Common Battery Types

  • Car Batteries: Use lead and lead oxide with sulfuric acid.
  • Nickel-Cadmium Batteries: Rechargeable and used in consumer electronics.

Batteries in Circuits

  • Symbols: Positive and negative sides, internal and external resistance.
  • Current and Potential Difference: Governed by Ohm's Law.
  • Short-Circuiting: A dangerous condition that shows maximum possible current.

Series and Parallel Batteries

  • Series Connection: Adds potential differences (voltage) but subject to internal resistance.

Practical Demonstration

  • Creating a copper-zinc battery shows around 1 volt potential difference.
  • Connecting two such batteries in series doubles the potential difference.

Power in Resistors

  • Power (P): Energy per unit time, given by formulas: P = IV, P = I²R, and P = V²/R.
  • Heat Dissipation: Power dissipated in form of heat, higher currents lead to higher heat dissipation.

Real-World Examples

  • Light Bulbs: Incandescent light bulbs are inefficient, converting most energy to heat rather than light.
  • Electric Heaters: Generate heat without producing much light by keeping resistance large and surface area high.

Electric Company Charges

  • Energy Consumption: Measured in kilowatt-hours, which is the product of power (in kilowatts) and time (in hours).

Kirchhoff's Rules for Circuits

  • 1st Rule: Closed loop integral of electric field over distance is zero.
  • 2nd Rule: Charge conservation; current going in must equal current going out.

Solving Circuit Problems

  • Use loop currents and solve equations based on Kirchhoff's rules.
  • Calculate currents and potential differences in complex circuits.

Remarkable Battery Demonstration

  • High-Voltage Battery: Produces tens of thousands of volts using water flow and conducting cans.
  • Sparks: Observed as potential differences build up and discharge.
  • Assignment: Analyze the working of this battery.