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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.
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