Introduction to Electric Circuits

Electric Circuits

• Electrical circuits are composed of electrical components that create a continuous conducting path.
• This path is connected to the terminals of a power source such as a battery.

Circuit Elements

• Voltage (V) provides the force to push charges to flow.
• Current (I) is the rate of flow of charges.
• Resistance (R) is the opposition to the flow of charge.

Charge Particles

• These particles are electrons and protons; electrons are the charges that move freely in any direction within a material.
• When electrons are controlled to move in one direction in conductors, it is called electric current or simply current.

Current

• Current is a measure of how much electric charge flows through a circuit.
• The more charge that flows, the greater the current.
• The symbol "I" is used to represent current because early scientists thought about "intensity" of electricity in a wire.
• Andre Marie Ampere, a French physicist, founded the science of electrodynamics/electromagnetism.
• The formula for current is I = Q/t, where I is current, Q is charge, and t is time.
• 1 Ampere (A) equals 1 Coulomb per second (C/s).

Electric Potential

• Electric potential is the amount of work needed to move a unit charge from a reference point to a specific point against an electric field.

Electric Potential Equation

• The equation is V = k(Q/r), where:
  • V = Elastic potential
  • Q = Point charges
  • r = Distance of separation
  • k = Coulomb constant (= 1 / (4πε₀))

Voltage

• Voltage is the electric potential difference, or pressure from an electrical circuit's power source that pushes charged electrons (current) through a conducting loop.
• Also known as electromotive force (emf).
• The unit "volt" was named after Alessandro Volta, an Italian physicist who invented the electric battery, providing the first source of continuous current.
• Formula: V = W/Q, where 1 Volt (V) = 1 Joule/Coulomb (J/C)

Resistance & Resistivity

• Electrical resistivity (ρ) is a fundamental property of a material, measuring how strongly it resists electric current.
• Resistance (R) is a measure of how difficult it is to pass current through a wire or component.

Resistance

• Resistance is a measure of the opposition to current flow in an electrical circuit
• Resistance is measured in ohms (Ω), symbolized by the Greek letter omega.
• Conductors offer very little resistance, allowing electrons to move easily.
• Insulators present high resistance, restricting the flow of electrons

Electrical Resistivity

• Electrical resistivity (ρ) is calculated by ρ = (RA) / l
• Unit of measure for resistivity is 1 Ohm-meter (Ω-m)

Factors Affecting Resistance

• Length of the Material: Resistance (R) of a wire is directly proportional to its length (L): R ∝ L
• Cross-sectional Area of the Material: Resistance (R) of a wire is inversely proportional to the cross-sectional area (A): R ∝ 1/A
• Temperature (T) of the Material: For most objects, resistance increases with an increase in temperature. For some materials like carbon, resistance decreases with an increase in temperature.
• The Kind of Material: The resistance of a material is related to it's resistivity. Resistivity ratings allow comparison of different materials' ability to conduct current.

Reading Resistor Color Codes

• Resistor color codes use colored bands to indicate resistance value, tolerance, and other parameters.
• Different colors represent different significant figures, multipliers, and tolerance levels. The chart on page 23. shows these values for each color.
• Resistors can have 3, 4, 5, or 6 bands, each providing different information.
• The "gap" between band 3 and 4 on a resistor indicates the reading direction.

Basic Circuit

• V = voltage source, is measured in volts (V)
• I = electric current, is measured in amperes (A)
• R = resistance, is measured in ohms (Ω)

Schematic Symbols

• Standard circuit symbols are used to represent components in circuit diagrams, including wires, resistors, light bulbs, batteries, and switches.

Ohm's Law

• Formula: **I = V/R (**Current equals Voltage divided by Resistance.)
• V = Voltage source with the unit: volt (V)
• I = Electric current with the unit: ampere (A)
• R = Resistance with the unit: ohm (Ω)

Ohmic and Non-Ohmic Conductors

• Ohmic Device: follows Ohm's Law for all voltages, where resistance remains constant under constant physical conditions (e.g., temperature).
• Non-Ohmic Device: does not follow Ohm's Law where resistance varies for different currents. Resistance is different for different currents passing through it. Potential difference V against current, I, has a curve, and is not a straight line. (ex. Diodes, transistors, traditional incandescent light)