Understanding Voltage and Current: Atoms, Electrons, and Batteries, Slides of Electrical Circuit Analysis

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Chapter 2 – Voltage and Current

2.2 – Atoms and Their Structure

• Nucleus
  • Protons
  • Electrons
  • Neutrons

2.3 - Voltage

• The flow of charge is established by an external

“pressure” derived from the energy that a mass

has by virtue of its position: Potential energy

• Energy: the capacity to do work
  • If a mass (m) is raised to some height (h) above a reference plane, it has a measure of potential energy expressed in joules (J) that is determined by
  • W (potential energy) = mgh where g is the gravitational acceleration (9.8 m/s^2 )

Voltage

• A potential difference of 1 volt (V) exists

between two points if 1 joule (J) of energy is

exchanged in moving 1 coulomb (C) of charge

between the two points

• The unit of measurement volt was chosen to

honor Alessandro Volta

Voltage

• Notations for sources of voltage and loss of

potential

• E - Voltage sources (volts)
• V - Voltage drops (volts)
• Potential – The voltage at a point with respect

to another point in the electrical system.

Typically the reference point is the ground,

which is at zero potential.

Voltage

• Potential difference: The algebraic difference in potential (or voltage) between two points of a network.
• Voltage: When isolated, like potential, the voltage at a point with respect to some reference such as ground.
• Voltage difference: The algebraic difference in voltage (or potential) between two points of a system. A voltage drop or rise is as the terminology would suggest.
• Electromotive force (emf): The force that establishes the flow of charge (or current) in a system due to the application of a difference in potential.

2.4 - Current

• The free electron is the charge carrier in a copper

wire or any other solid conductor of electricity

• With no external forces applied, the net flow of

charge in a conductor in any one direction is zero

• Basic electric circuit

Current

• Safety considerations
  • Even small levels of current through the human body can cause serious, dangerous side effects
  • Any current over 10 mA is considered dangerous
  • currents of 50 mA can cause severe shock
  • currents over 100 mA can be fatal
  • Treat electricity with respect – not fear

Voltage Sources

• dc Voltage sources
  • Batteries (chemical action)
  • Generators (electromechanical)
  • Power supplies (rectification)

Voltage Sources

• Batteries: combination of two or more similar

cells

• A cell being a fundamental source of electrical energy developed through the conversion of chemical or solar energy
• All cells are divided into Primary and Secondary types - Primary type is not rechargeable - Secondary is rechargeable; the cell can be reversed to restore its capacity - Two most common rechargeable batteries are the lead-acid unitDocsity.com

Voltage Sources

• Alkaline primary cells
  • Powered zinc anode (+)
  • Potassium (alkali metal) electrolyte
  • Manganese dioxide, carbon cathode (–)

Voltage Sources

• Lead-acid secondary cell
  • Sulfuric acid is the electrolyte
  • The electrodes are spongy lead (Pb) and lead peroxide (Pb0 2 )

Voltage Sources

• Solar cell
  • A fixed illumination of the solar cell will provide a fairly steady dc voltage for driving loads from watches to automobiles
  • Conversion efficiencies are currently between 10% and 14%

Voltage Sources

• Ampere-hour rating
  • Batteries have a capacity rating in ampere-hours
  • A battery with an ampere-hour rating of 100 will theoretically provide a steady current of 1A for 100 h, 2A for 50 h or 10A for 10 h
  • Factors affecting the rating: rate of discharge and temperature - The capacity of a dc battery decreases with an increase in the current demand - The capacity of a dc battery decreases at relatively (compared to room temperature) low and high temperatures