Electric Current, Potential Difference, and Resistance

Page 1: Overview and Objectives of Electric Current

Learning Objectives

• Define electric current and electric current intensity.

• Explain the concepts of potential difference and electromotive force ($\text{emf}$).

• Compare connecting electric cells in series and in parallel.

• Identify different types of electrical resistors.

• Infer factors affecting conductor resistance.

• Appreciate scientific contributions in inventing and developing electrical devices.

• Design simple electric circuits using standard international symbols.

Key Concepts

• Free Electrons: Mobile electrons within conductors.

• Conductors: Materials permitting charge movement.

• Electric Current ($I$): Directed movement of charges.

• Potential Difference ($V$): Driver of charge movement.

• Electric Resistance ($R$): Opposition to current flow.

• Sources: Electric cells and batteries supplying Direct Current ($\text{DC}$) or Alternating Current ($\text{AC}$).

Inquiries & Reflection

• Why does a lamp light up as soon as the switch is pressed?

• Why do some lamps shine brighter than others?

• Can the brightness of a lamp be controlled?

Page 2: Electric Current in Metals

Conduction in Metals

• Metals such as copper and aluminum contain free electrons not bound to specific atoms.

• Open Circuit: Free electrons exhibit random motion in all directions; net current is zero.

• Closed Circuit: The energy source exerts a force, causing free electrons to drift in one direction.

• Electric Current: Defined as the directed flow of electric charges (free electrons) through a conductor.

Conditions for Electric Current

• A source of electrical energy (e.g., battery).

• A continuous, closed circuit providing a complete path from the source and back.

Page 3: Electric Current Intensity

Definition and Formulation

• Electric Current Intensity ($I$): The quantity of electric charge ($Q$) passing through a conductor cross-section per unit time ($t$).

$I = \frac{Q}{t}$

Units and Measurement

• Unit: Ampere ($\text{A}$).

$1\,\text{A} = 1\,\text{C/s}$

• Measuring Device: Ammeter ($\text{A}$), connected strictly in series within the circuit.

• Greater charge flowing per unit time corresponds to higher current intensity.

Page 4: Potential Difference and Current Direction

Concept of Potential Difference

• Electric charges flow from a point of higher electric potential to a point of lower electric potential.

• If potential difference between two points is zero, electric current ceases to flow.

Direction of Electric Flow

• Conventional Current: Flows from the positive terminal ($+$) through the external circuit to the negative terminal ($-$).

• Electron Flow: Actual physical movement of free electrons from the negative terminal ($-$) toward the positive terminal ($+$).

Page 5: Potential Difference and Electromotive Force

Potential Difference ($V$)

• Work needed to move a unit charge between two points.

$V = \frac{W}{Q}$

• Unit: Volt ($\text{V}$), where $1\,\text{V} = 1\,\text{J/C}$.

• Measurement: Voltmeter connected in parallel across the component.

Electromotive Force ($\text{emf}$)

• Total work done by the electrical source to move a unit charge of one coulomb through the entire circuit.

$\text{emf} = \frac{W_{\text{source}}}{Q}$

• Unit: Volt ($\text{V}$).

Types of Electrical Sources

• $\text{DC}$ Sources: Electric cells and batteries producing current that flows in a single direction.

• $\text{AC}$ Sources: Electric generators providing alternating current that periodically changes direction.

• Voltage serves as the driving cause that pushes charges; current is the resulting flow of charge.

Page 6: Electrical Resistance

Fundamentals of Resistance

• Resistance ($R$): The opposition encountered by electric charges while traveling through a conductor.

• Unit: Ohm ($\Omega$).

• Measurement: Ohmmeter connected directly across the resistor terminals.

Types of Resistors

• Fixed Resistors: Maintain an unvarying, constant resistance value.

• Variable Resistors (Sliding Rheostat): Allow adjustable resistance values to regulate circuit current by varying conductor length.

  • Increasing wire length increases resistance ($R$).

  • Increasing resistance causes current ($I$) to decrease.

Application

• Volume controls in audio equipment utilize variable resistors to alter circuit resistance, controlling current flow and output sound level.