Electric Power Systems: A Conceptual Introduction - Comprehensive Notes

Electric Power Systems

Key quantities for understanding electricity: charge, voltage, current, resistance, and electric/magnetic fields.
Charge is a basic dimension of physical measurement along with mass, distance, time, and temperature.
Electrical phenomena are often perceived as mysterious due to their abstract nature.
Direct experience with electricity includes electric shocks and static cling.

There are two types of charge: positive and negative.
Like charges repel, opposite charges attract.
Charges can be transferred through friction.
Atoms consist of a nucleus (protons & neutrons) surrounded by electrons.
- Protons: positive charge; Electrons: negative charge; Neutrons: no charge
Electrical attraction balances electrons' tendency to escape, which arises from kinetic energy and mutual repulsion.
Materials are electrically neutral when they have an equal number of protons and electrons.
Ions are atoms or groups of atoms with a net charge due to an imbalance of protons and electrons.
The practical unit of charge is the coulomb (C).
- 1 \text{ C} = 6.25 \times 10^{18} \text{ protons}
- Charge of one proton: 1.6 \times 10^{-19} \text{ C}
- Charge of one electron: -1.6 \times 10^{-19} \text{ C}
Charge is denoted by the symbol Q or q.

Like charges repel, therefore charge has a natural tendency to spread out.
Accumulation of local charge creates "tension".
Electrical potential energy is analogous to mechanical potential energy.
Electric potential (voltage) is potential energy per charge.
Voltage can be positive or negative, implying repulsion or attraction of positive charge, respectively.
A reference location is needed for voltage, typically zero potential or ground.
Voltage units are volts (V), equivalent to joules per coulomb.
- 1 \text{ V} = 1 \text{ J/C}
Voltage is denoted by E, e, V, or v.

Ground is an electrically neutral place with zero voltage.
It has the ability to absorb/disperse charge.
The literal ground outdoors serves as a vast reservoir of charge.
Circuit "ground" is created by a pathway for charge into the earth, often via metal water pipes in homes.

In conductors, some electrons are free to travel, allowing the material to conduct electricity.
Metals are important conductors because of their microscopic structure.
Water with dissolved ions is also a conductor.
Pure distilled water does not conduct electricity.
Air can become temporarily conductive through ionization, forming a plasma.
Superconductivity occurs in some materials at very low temperatures, allowing electrons to travel with no loss of energy.

Electric current is the flow of charge through a material.
Current is measured in amperes (A), where 1 \text{ A} = 1 \text{ C/s}.
Negative charge flowing in one direction is equivalent to positive charge flowing in the opposite direction.
Conventional current flow is labeled as positive from positive to negative potential.
The propagation speed of current is very fast, near the speed of light.
A circuit treated as sufficiently small so that the speed of current is not an issue is called a lumped circuit.

Ohm’s law states the linear relationship between voltage and current: V = IR
- V is the voltage, I is the current, and R is the resistance.

Resistance (R) is the proportionality constant in Ohm's law, measured in ohms (Ω).
Resistance depends on material composition and shape.
For a wire, resistance increases with length (l) and decreases with cross-sectional area (A).
Intrinsic material property is resistivity (ρ).
- R = ρ \frac{l}{A}
Units of resistance: ohms (Ω).
Units of resistivity: ohm-meters (Ω-m).

Conductance (G) is the inverse of resistance: G = \frac{1}{R}.
Conductivity (σ) is the inverse of resistivity: σ = \frac{1}{ρ}.
Units of conductance are mhos (℧) or siemens (S), where 1 \text{ mho} = \frac{1}{Ω}.
- G = σ \frac{A}{l}

Insulating materials have high resistance and low conductance, also known as dielectric materials.
Insulator resistance is ideally infinite, blocking current flow.
Insulators have a specific voltage regime, beyond which they may break down and become conductive.

Electric circuits provide a sustained current flow by recycling charge and using an electromotive force (emf).
A battery connected to a light bulb is an example of a simple circuit.

Voltage drop is the voltage difference between two points in a circuit.
It's proportional to the current flowing through a component multiplied by its resistance: V = IR.
High demand causes greater voltage drop and possible "brownouts".

Occurs when a current flows through the body, which requires a voltage drop across it.
Severity depends on voltage, current, and individual’s resistance (e.g., skin moisture).

Electric current flowing through resistance creates heat.
Heat corresponds to work done by charge carriers in traveling to lower potential.
This heat generation is either intended (heating appliances) or unintended (power lines).

Heat (power) dissipated in a resistor:
- P = IV (Power = Current × Voltage)
- P = I^2R (Power = Current^2 × Resistance)
Units of power: watts (W), equivalent to joules per second.

Resistive losses cause increasingly high voltage levels in power transmission lines.
Higher voltage reduces current, thereby reducing I2R losses.

Fields explain how objects exert forces on each other at a distance.
They map hypothetical effects across space, describing properties of space itself.
Gravitational field is an example.

Electric field maps the electric force experienced by a charge at any location.
It represents the potential gradient.
Described graphically with field lines - direction of arrows indicates direction that a "test charge" would be pushed or pulled.
Strength of the force - proximity of field lines, closer together implies a strong force.

Changing magnetic fields exert an electromotive force (emf ) on charges causing current to flow