11725_lecture5_2025_electricity II_(Canvas)

Lecture Overview

• Lecture conducted by J. Woolnough.

• Based on Pasquale et al. (2024), chapters 23 and 24.

• Context: Electricity II, 2025 Semester 1.

• Lecture available via Adrian Dusting at Canberra University.

Learning Objectives

• Describe the motion of electric charges in relation to the electric field.

• Explain concepts of electrical current, resistance, and power.

• Distinguish between series and parallel circuits.

• Understand and apply Ohm’s Law.

Fundamental Concepts

Electric Charges

• Like charges repel each other, while opposite charges attract.

• The electrical force increases with the magnitude of charge and decreases with distance.

• Coulomb's Law Formula:

• ( F = k \frac{Q_1 Q_2}{r^2} ) where

  • ( Q ) = charge,

  • ( r ) = distance,

  • ( k ) = constant.

Electric Potential

• Electric potential increases with distance and the amount of charge.

• Doing work (energy transfer) leads to an increase in potential.

• Voltage: Measured as potential energy per charge.

  • Formula: ( ext{Voltage} (V) = 1 ext{ J/C} )

Electric Current

• Electric current refers to the movement of charge.

• It is measured as a rate (charges per unit time).

• Coulomb: 1 Coulomb/second = 1 Ampere (1 A).

• Formula: ( I = \frac{q}{t} )

Electrical Power

• Power is the rate at which work is done and depends on work done per unit time.

• Formula: ( P = \frac{W}{t} )

  • Units: Joules/second = Watts (W).

• Other relationships include:

  • ( P = IV )

Voltage and Current

• Distinction between voltage and current:

  • High voltage, low current can be safe (e.g., Van der Graaff generator).

  • Modest voltage and current can be hazardous (e.g., household circuits).

Electric Field Strength

• Formula for Electric Field (E):

  • ( E = \frac{F}{q} )

  • Units: Newtons/Coulomb.

  • Direct relation: ( E = \frac{V}{d} ) (where (d) is distance).

  • Units: 1 N/C = 1 V/m.

Resistance and Ohm's Law

• Resistance (R) measures impediment to current flow.

  • Units: Ohms (Ω).

• Ohm’s Law states:

  • Current (I) directly proportional to electric potential (V) and inversely proportional to resistance (R).

  • Formulas:

    • ( I = \frac{V}{R} )

    • ( I \propto V )

    • ( V \propto \frac{1}{R} )

Conductivity

• Conductivity is the inverse of resistance, indicating how easily current can flow for a given voltage.

• Units of conductivity: Siemens (S).

  • 1 S = 1 Ω^{-1}.

Series and Parallel Circuits

Series Circuits

• Current remains the same throughout.

• Voltage divides across each resistor in series.

• Formula:

  • ( V_T = V_1 + V_2 + ... + V_n )

Parallel Circuits

• Current is divided among the branches.

• Voltage across each branch remains constant.

• Formulas:

  • ( I_T = I_1 + I_2 + ... + I_n )

  • Voltage: ( V_T = V_1 = V_2 )

Ohm's Law Examples

• Problems allocating current and voltage across light globes in a circuit with values provided in Ohm's law format (e.g., ( V = IR )).

Summary

• Concepts summarized include:

  • Electric potential (voltage)

  • Power

  • Current and Ohm’s Law

  • Electric field

  • Series circuit (voltage divider)

  • Parallel circuit (current divider)

• Coulomb’s Law: ( F = k \frac{Q_1 Q_2}{r^2} )

• Ohm’s Law summary: ( I = \frac{q}{t} ), ( V = IR ), ( P = \frac{W}{t} ), ( P = IV )

Course Progression

• Recap of previous lectures on forces, work/energy, fluids, and electrostatics leading up to the current lecture.

• Upcoming topics include heat, thermodynamics, waves, light, atomic physics, and nuclear physics.

Remembering Key Concepts

• “With great power comes great current squared times resistance.”

• A quote that connects to Ohm’s Law.