Current Electricity 2025 (1)
Page 1: Introduction to Current Electricity
Topics Covered:
Current Electricity
Potential Difference
Resistance
Page 2: Electric Current
Charge Flow:
Current flows when there is a potential difference (voltage).
No potential difference means no current flow.
Current-Carrying Wire:
A current-carrying wire has no net charge.
Potential difference causes a flow of charge.
Page 3: Voltage Sources
Sustained Current:
Requires a suitable "electric pump" for sustained potential difference.
Voltage Source:
Provides potential difference.
Examples: Dry cells, wet cells, solar cells, generators.
Not Suitable Sources:
A metal sphere charged positively and negatively develops voltage but isn’t practical.
Comparison:
e.m.f. (electromotive force) vs. P.d. (potential difference).
Page 4: Effect of Current on the Body
Current in Amperes vs. Effects:
0.001 A: Can be felt
0.005 A: Painful
0.010 A: Causes spasms
0.015 A: Loss of muscle control
0.070 A: Can disrupt heart function if through the heart.
Bird Example:
A bird can stand harmlessly on one wire; reaching another provides a dangerous current.
Page 5: Ohm's Law Symbols
Basic Definitions:
Ohm: unit of resistance (R)
Volt: unit of potential difference (V)
Ampere: unit of current (I)
Page 6: Drift Velocity
Example with Marbles:
Marble in a groove analogy shows drift velocity.
Key Point:
Drift speed is slow (about 10^-4 m/s) while the information travels at the speed of light.
Page 7: Drift Velocity and Charge Carriers
Electric Field:
Electrons move opposite to the electric field direction.
Movement Pattern:
Electrons do not move in a straight line due to collisions.
Equations:
Drift Velocity relates to current with I = nAvq where v is drift velocity.
Page 8: Derivation of Current Equation
**Parameters:
Length (l), Cross-sectional area (A), Current (I), Charge (e), Drift velocity (v).
Volume Analysis:
Volume of section = Number of free electrons = Total charge = Time taken.
Current Relation:
Current (I) is defined as Q/t.
Page 9: Calculating Drift Velocity
Example Calculation:
Drift Velocity for copper wire with A = 0.8 x 10^-6 m² and 5.0 A current.
Free Electrons: 10²⁹ per m³; charge of electron = 1.60 x 10^-19 C.
Comment on result: Very low, takes 40 minutes to travel 1 m.
Electric Field Immediate Effect:
Instant establishment of the electric field across the conductor.
Page 10: Drift Velocity in Different Wires
Drift Velocity Comparison:
Higher in thin wires than in thick wires under the same current.
Lower number density in semiconductors leads to higher drift velocity compared to conductors.
Page 11: EMF and Potential Difference
Definitions:
EMF: Measure of energy converted to electrical energy per charge.
P.d: Energy converted to heat, light, etc., for a charge through an appliance.
Page 12: Differences between EMF and P.D.
Comparative Points:
EMF is the maximum potential difference; P.d is the difference under load.
EMF independent of circuit resistance; P.d proportional.
Uses of terms: 'emf' applies to the source while P.d applies across points in a circuit.
Page 13: Potential Difference Illustration
Page 14: Circuit Diagram Example
Elements: E, R, r, I, V drop around a circuit.
Page 15: E.M.F and P.D Relation
E = PD + Lost Volts:
Use in circuit analysis for various components.
Page 16: Internal Resistance Analysis
Resume of Circuit Variables:
E = EMF source, V = P.D., description of circuit elements with values.
Page 17: Internal Resistance Concept
Relationships between V and E, illustrating the gradient = r (internal resistance).
Page 18: Resistance and Temperature Effect
Impact of Temperature on Resistance in Metals:
Resistance increases with temperature due to more collisions among free electrons and metal atoms.
Definitions:
Resistance depends on length, area, and material properties.
Page 19: Resistivity Equations
Key Terms:
R = resistivity (ρ), L = length, A = cross-sectional area.
Page 20: Definition of Electrical Power
Electrical Power:
Defined as the rate of doing work.
Page 21: Semiconductors Characteristics
Charge Carrier Density in Semiconductors:
Much lower than good conductors.
High resistivity and negative temperature coefficients where resistance decreases with increasing temperature over certain ranges.
Page 22: Change in Resistivity with Temperature
Resistivity Values at 20°C:
Material
Resistivity
Change
Silver
1.6 X 10^-8
Increases
Copper
1.7 X 10^-8
Increases
Lead
2.1 X 10^-7
Increases
Graphite
8.0 X 10^-6
Decreases
Germanium
5.0
Decreases
Silicon
2.5 X 10^3
Decreases
Glass
10^12
Decreases
Page 23: Ohm’s Law Principles
Resistance Dependence:
Length, area, material, temperature.
Experiments with Resistance:
Resistance often constant over a range of applied P.d.
Page 24: Resistance Behavior in Conductors
Metallic Conductors:
Resistance (V/I ratio) remains constant at constant temperature.
Resistance increases in filament lamps as V increases due to temperature changes.
Diodes and Thermistors:
Diodes provide low resistance in forward bias; thermistors’ resistance decreases with temperature increases.
Page 25: Thermistors Overview
Types of Thermistors:
NTC (Negative Temperature Coefficient): resistance decreases as temperature increases.
PTC (Positive Temperature Coefficient): resistance increases as temperature increases.
Page 26: Body’s Resistance
Resistance Characteristics:
Typical resistance is about 500,000 Ω when skin is dry.
Resistance decreases significantly with wet skin or saltwater exposure (can drop to as low as 100 Ω).
Page 27: Dangerous Resistances
Risks of Low Resistance:
Low bodily resistance under high potential differences can be hazardous.
Ion conductivity in perspiration increases risk of electric shock during stress tests.
Page 28: Example Problem Solving with Currents
Calculate:
a) I in the battery
b) Resistance Rz
c) EMF E
Results: I = 2.4 A; Rz = 0.67 Ω; E = 8.4 V.
Page 29: Kirchhoff's Laws
First Law:
Sum of currents entering a junction = sum of currents leaving.
Second Law:
Total E.m.f. in a closed loop = sum of potential drops.
Page 30: Examples of Kirchhoff's Laws
Page 31: More Examples of Kirchhoff's Laws
Page 32: Further Circuit Analysis Examples
Page 33: Resistor Current Flow Calculation
Analyze current through resistors and solve.
Page 34: Voltage Determination within Circuit
Convert supplied voltages into equations and solve for unknowns.
Page 35: Reading Ammeter Values
Provide current readings of ammeters in a given circuit context.
Page 36: Reading Voltmeter Values
Analyze voltmeter measurements within a circuit containing uniform resistances.
Page 37: Lamp Circuit Analysis
Conduct calculations based on the behavior and specifications of lamps under voltage.
Page 38: Circuit Analysis for Currents
Solve for currents, potential differences, and terminal pandas based on provided circuit information.
Page 39: Resistors: Series and Parallel
Requires derivation of expressions for resistors in both configurations.
Utilize diagrams and Kirchoff’s laws for clarity.
Page 40: Series Resistor Voltage Drop
Discuss p.d. across multiple resistors in series arrangement discussing ratios.
Page 41: Voltage Drops Ratio Illustration
Calculate potential differences across specific points in a series circuit based on measured voltage levels.
Page 42: Potentiometer Analysis
Application of potentiometer principles to find unknown e.m.f. based on balance point measurements.
Page 43: Wheatstone Bridge
Explore null deflection principle in resistance measurement through Wheatstone bridge.
Page 44: Wheatstone Bridge Components
Understand balancing method for measuring unknown resistance using a Wheatstone bridge approach.
Page 45: Wheatstone Bridge Equation
Writing the correct equation for a balanced Wheatstone bridge.
Page 46: Unknown Resistor Calculation in Bridge
Example problem calculating unknown resistance in a balanced Wheatstone bridge setup.