1 / 21

Lecture A

Lecture A. Fundamentals and Background. Charge. “Charge” is the basic quantity in electrical circuit analysis Fundamental charge quantity is the charge of a single electron Charge will be in integer multiples of a single electron’s charge Units of charge = Coulombs (C)

feleti
Download Presentation

Lecture A

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. Lecture A Fundamentals and Background

  2. Charge • “Charge” is the basic quantity in electrical circuit analysis • Fundamental charge quantity is the charge of a single electron • Charge will be in integer multiples of a single electron’s charge • Units of charge = Coulombs (C) • One Coulomb  -6.21018 electrons

  3. Electric Fields • A charge induces an electric field (E-field) • The electric field is a vector field • Point charge E- field:

  4. Analogy: E-field vs. Gravitational field • Electric Field: • Gravitational Field:

  5. Forces on Charged Particles • A second “charge” placed in the electric field induces a force on both charges • Coulomb’s Law: • Electric field is essentially the force per unit charge placed in the field • “Like” charges repel; opposite charges attract

  6. Analogy: Mass in a Gravitational Field • Coulomb’s Law: • Newton’s Law:

  7. Demo: static electricity charge on balloon causes it to stick to wall

  8. Energy Transfer • In circuit analysis, we are primarily concerned with energy transfer • Charges move around • Moving a charge in an electric field changes the charge’s potential energy • Work to move charge from b to a:

  9. Electric Potential Difference •  Wba is the work required to move a charge from point b to point a in an electric field • Work is a form of energy  Wba is a difference in potential energy (units are Joules, J) • This difference is typically quantified as an Electric Potential Energy Difference • Electric potential difference is the electrical potential energy difference per unit charge:

  10. Voltage • Vba is generally referred to as a voltage difference; (units of Vba are volts, V) • Generally defined in terms of derivatives, for infinitesimal variations in charge and energy:

  11. Notes on Voltage • The potential energy difference is due to a physical separation (a distance) between the two points • This potential difference provides a force which can move charges from place to place. • This is sometimes called an electromotive force (emf)

  12. Charge in motion & current • Recall: • We are concerned with energy transfer  charge motion • emf (or potential energy difference, or voltage difference) can move charges • Current is the time rate of change of charge

  13. Charge Motion in Materials • Common model of materials: • Materials composed of atoms • Atoms contain protons and neutrons in a nucleus, surrounded by a “cloud” of electrons • Protons are positively charged, and are bound “tightly” in the nucleus • Electrons are negatively charged, and bound less “tightly” to the atom

  14. Charge Motion in Materials -- continued • Electrons can move from atom to atom within a material. • We can transfer charge through a material via electron motion • Current is defined as the motion of “positive” charge • Positive current is (by definition) in opposite direction to electron flow

  15. Charge motion in materials -- continued • We apply a potential difference across the material • emf causes electron motion away from negatively charged end • Current is in the direction of “positive” charge motion

  16. Current Flow in Materials • The less “tightly” bonded the electrons are to the atom, the more “easily” the material allows current to flow • The material conducts electricity more easily • The material has less resistance or higher conductivity • For example, • conductors have low resistance to current flow  low potential differences can provide high currents • insulators have high resistance to current flow  nearly no current flow, even with high potential differences

  17. Demo: touch electric fence with conductor and insulator

  18. General Passive Circuit Elements • General, two-terminal, passive circuit element • Apply a voltage difference across the terminals • This voltage difference results in current flow • Our circuit elements will be electrically neutral • Current entering the element is the same as the current leaving the element

  19. Power • Power is the rate of change of energy with time • Units of power are Watts (W)

  20. Power Generation and Dissipation • Power dissipation: • Current enters the positive voltage terminal • Examples: • Power dissipated as heat (light bulbs) • Power converted to mechanical system (electric motors, pumps) • Power generation • Current enters the negative voltage terminal • Examples: • Power generated by mechanical system (turbines, generators) • Power generated by chemical processes (batteries)

  21. Demos? • Pulling mass across surface with DC motor (point out energy added, dissipated) • Pump water through horizontal tubing (point out energy exchange)

More Related