29/09/2020 Part 1A - Electromagnetics Cambridge University Engineering Department 1 Michaelmas Term 2020 Prof T. D. Wilkinson (tdw13@cam.ac.uk) Handout I - The physical principles of electronics (PPE) (you own the gaps, you fill 'em ... ) Electrostatics - charges can't move 2 YOU OKAY? ZZZAAPP YEAH, FINE. I THINK THERE'S SOMETHING WRONG WITH YOUR COMPUTER? IMPOSSIBLE! THIS IS A MAC! cobow.com http://bradcolbow.com/archive/ 1
29/09/2020 Electrostatics = charges fixed Charges and Coulomb's law In electromagnetics one of the most important concepts is that of charge and charge movement (current). Charge is a difficult concept to visualize but one of the simplest ways of understanding it is as an imbalance in the electronic structure within a material. Charge unit - electron (e) Charges are similar to Mass 3 In the case of insulators (such as dielectrics) charge motion is restricted within the material, however there may still be an imbalance of electrons throughout the molecules of the material leading to a net overall charge. Asymmetry = polarisation In conductors charges are free to move throughout the material leading to currents. We always assume that all the charges have finished moving 4 2
29/09/2020 the nucleus http://atomic.lindahall.org/what-is-an-atom.html orbits electrons Nicotine http://jagdeep-chemistry.blogspot.co.uk/2012/07/chemical-bond-and-moleculer-structure.html 5 The interaction of charges generates forces and force fields. Such as E and D Charges of the same sign - Repel - - Charges of the opposite sign - Attract - + The forces between charges and fields often mean that they are able to move. In this section we are dealing with electrostatics so it is assumed that the charges are stationary. (or trapped) Later they move = current 6 3