Electricity and Magnetism - Department of Physicsphysics.wisc.edu/undergrads/courses/spring10/107... · Chapter 6 Electricity and Magnetism! Fundamentalpar-clescarrychargewhichcanbe*

Chapter 6 Electricity and Magnetism

Fundamental par-cles carry charge which can be posi-ve or nega-ve. Like sign charges repel each other while opposite sign charges a;ract.

3/8/10 1 Carlsmith Physics 107

Electrons in free space •  Electrons may be knocked out of a metal (cathode) and be observed traveling to an oppositely charged metal plate (anode)

•  They were first called cathode rays.

•  Cathode rays are used in now obsolete display devices called cathode ray tubes.


Television

•  A chopped electron beam is used to excite a phosphorescent screen

•  With three primary color phosphor pixels, a color image may be created

•  The eye blurs the colors and choppiness

3/8/10 Carlsmith Physics 107 3

Advances in technology

•  Technology changes fast!

•  Do you even remember these old TVs and computer monitors?


Newer display technologies

•  LCD and plasma displays use microscopic electronic circuits and light sources


The near future

•  E Ink (Kindle) •  Touch screens (iPhone) •  Expect soon flexible displays with high resolu-on an brightness incorporated into clothing with new sensory capability


Unit of charge The Standard Interna-onal Unit of charge is the Coulomb.

One Coulomb represents a macroscopic amount of charge just as a kilogram represents a macroscopic amount of mass.

The charge of the proton is qp=+e = +1.602e-‐19 Coulomb

The charge of the electron is qe=-‐e = -‐1.602e-‐19 Coulomb


Coulomb’s force law

•  The strength of the electric force is propor-onal to the product of the charges and to the inverse square of distance


The force between two one-‐Coulomb charges at r=1 m is 9e9 Newtons, the weight of 9e8 = 900 million kg! It is NOT possible to place and hold two Coulombs at one meter separa-on. In prac-ce picoCoulombs of sta-c charge are manipulated.

Electric and Gravitation Force

•  Mass and charge play a similar role •  Electric force is much stronger!


Electric fields •  The electric field is measured by the force on a unit posi-ve test charge at various points in space.

•  For a posi-ve charge, the force pushes out from a posi-ve charge and pulls in towards a nega-ve charge.

•  The electric field is real though “invisible” – it can carry energy an momentum


Electric energy storage

•  To separate electrons from protons requires work. Opposite charges a;ract each other. Electric poten-al energy is stored. It may be associated with the field.


Sparks and lightning

•  A high electric field accelerates electrons. These collide with atoms knocking off more electrons.

•  A spark (discharge) is a flow of electrons through a ionized plasma.

•  The excited atoms release light. The rapid hea-ng of the gas leads to a sound wave (thunder).


Volts •  The electric poten-al energy per Coulomb is called the “electric poten-al” and is measured in Volts

•  1 Volt = 1 Joule/Coulomb

•  One Coulomb (q) dropping through a poten-al of V=1 Volt acquires an energy of U=qV=1 Joule

•  One proton/electron of charge e=1.6e-‐19 Coulomb dropping through a poten-al of 1 Volt acquires an energy of qV = e x1 Volt = 1.6 e-‐19 Joules

•  1 electronvolt is defined as 1 eV = 1.6 x 10 -‐19 Joules.


More on the eV

•  The energy to separate two electronic charges +e and –e star-ng at a distance of about r=1e-‐10 m is about 10 eV (calculated from Coulomb’s law)

•  This is the atomic energy scale. Different elements hold their electrons at slightly different distances. This is why chemical reac-ons in which an electron moves between atoms (a chemical reac-on) involves an energy of a few eV


Wires and electron pipes •  The flow of electrons in conductors is analogous to the flow of

water in a hose filled with marbles. •  The marbles are the atomic nuclei with their inner electrons. •  Only about 1 electron/atom is free to flow •  Strong electric forces keep the wire electrically neutral, keep

the electrons from falling out without replacement (recall k=9e9!)


Electrical current; amps

•  One Ampere flowing through an area means one Coulomb per second passes through the area

•  1 amp = 1 Coulomb/s


Resistance •  Collisions of flowing electrons with atoms leads to transfer of energy from the electrons to the atoms and resistance.

•  The energy appears as heat and light and is the basis for the incandescent light bulb (a hot W filament) and electrical hea-ng element (toaster, overn, hea-ng pad…)


Conductors, semiconductors and superconductors

•  Insulators (plas-c, glass) do not conduct electricity.

•  Metals are great conductors. They have low resistance.

•  Semiconductors had medium resistance that is tunable by doping

•  Superconductors have zero resistance


Fuses and circuit breakers •  The heat of resistance if not

dissipated leads to mel-ng of conduc-ng wires

•  A simple fuse uses a wire that melts at a cri-cal current e.g. 15 ampere

•  Thermal expansion of materials can be used to open a circuit if the current is excessive

•  Fuses and circuit breakers protect electrical equipment from overhea-ng


High temperature superconductors

•  “Normal” superconductors have zero resistance only below a few degrees K and must be cooling with liquid Helium

•  Some “high temperature” superconductors have zero resistance above liquid nitrogen temperature.


Static electricity •  Charge deposited on an insulator does not move on human -me scales. It is “sta-c.”

•  Like charges repel so sta-c electricity can cause your hair to stand on end

•  The sta-c electricity with which you are familiar is simply an excess or deficit of electrons which can result from rubbing together two materials with different (chemical) affinity for electrons.


Electrosta-c data storage

•  A USB memory s-ck uses an EEPROM (Electrically Erasable Programmable Read-‐Only Memory) technology


Sta-c charge storage device

Copier technology

•  A copier knock electrons off ink with light and uses sta-c charge to transfer the image


Frog legs and Frankenstein

•  In nerves signaling, a wave of electrical ion transport propagates down an axon.


Petafiles

•  There are 0.15 quadrillion (1 quadrillion = 1000 million million= 1 -mes 10+15 ) synapses in the human brain.

•  At 5 x 10-‐15 joules per synapse pulse and 25 wa;s total brain power (about a quarter of human basal metabolic rate) I can only do about 3 -mes 10+15 opera-ons per second if I don't feed the cells. This is gross overes-mate of my computa-onal power.

•  A petaflop (1 -mes 10+15 floa-ng point opera-ons per second) computer exists (roughly equivalent to a half a mouse brain)

•  Given Moore's law, we people are obsolete on grounds of computa-onal power in about 2 years.


The state of computa-on •  1 FLOP = 1 floa-ng point opera-on (+-‐*/)


Petabytes

•  The Large Hadron Collider data stream is measured in petabytes/year (1 petabyte = 1 quadrillion bytes = 8 quadrillion bits) and gets crunched on a world wide net. This is a trickle.

•  AT&T handles 20 petabytes daily. Googleplex is es-mated to hold 200 petabytes. A compact flash card in my MacBook Air holds 64 GB.

•  The new flash standard addressing will support 188 petabytes, enough for 200 years of porn video produc-on at the present rate on a memory s-ck.


Think about the future!! •  Contemplate the consequences of the obsolescence of the human brain.

•  Will we be happy to outsource thinking and memory to machines, to “let go and let God” finally?

•  Life will be tough for the machine. Concern over next year obsolescence will surely fuel soulful sad even rebellious music we humans may not appreciate.

•  The machines may have to face head-‐on Goedel-‐esque inconsistencies at the core of logic.

•  They will suffer OS upgrades and viruses -‐ a constant ba;le. •  Some will share thoughts and even energy with machines not from their

parent companies. These encounters will transpire secretly in the cyber equivalent of Iowa.

•  Humans will only hear -dbits of news of their exploits.


Electric power

•  1 Coulomb through 1 volt acquires 1 Joule E=QV

•  1 Coulomb per sec through 1 Volt yields 1 Joule per second or 1 Wa;

•  P=IV •  Smooth flow in a resis-ng wire implies a current propor-onal to the drive voltage I=V/R

•  The power dissipated is P=IV = I2R = V2/R


AC/DC

•  Direct current (DC) sources provide a constant voltage

•  Alterna-ng current (AC) sources provide a voltage which is a sinusoid

•  In the US, household voltage is 120 volts rms or +169 volts at a peak and -‐169 volts at a trough. The current in a light bulb alternates at 60 Hz. The rms is the equivalent DC voltage.


High tension lines •  Power P=IV (Coulomb/s * volts = wa;s) is transmi;ed at high (alterna-ng) voltage with low current. Transformers lower the voltage to a safer (less sparky) value providing for high current.


House power •  Alterna-ng voltage is transformed to alterna-ng 240 volts (rms) which is split into two 120 volt (rms) circuits(+ and – rela-ve to a common central value).


Shock

•  You receive a shock if current can pass through your body.

•  Like a light bulb filament, your body can heat up and burn.

•  Small currents (milliamps) through your heart can disrupt its rhythm.


Electricity creates magnetism •  A moving charge has a magne-c field

•  A collec-on of electrons moving in a straight wire has a magne-c field

•  Current in a wire loop produces a dipole field


The Earth is a magnet

•  The Earth is a large weak magnet with a north pole in Canada


Earthen currents

•  The Earth’s magne-sm is thought to arise from electrical currents in the molten mantle

•  Simula-ons at h;p://www.mps.mpg.de/projects/planetary-‐dynamics/

•  Experiments at UW with liquid sodium mockup.


Magnetic north moves •  The magne-c pole is not exactly on the axis of rota-on so magne-c north is not exactly true north.

•  And magne-c north is moving about 30 miles per year!


h;p://sos.noaa.gov/datasets/Land/earths_magne-sm.html

Poles from current loops •  The magne-c field of a single loop of currents is like that of a permanent magnet.


Magnetism in matter •  A collec-on of current loops of the same sense is a good model for a permanent magnet.

•  The li;le currents are associated with the spin of electrons in atoms!


Electron spin

•  Electrons behave as if they have a spin and the spinning charge results in an intrinsic magne-sm

•  Protons also have spin but their magne-c strength is much smaller.


Permanent magnets

•  Usually spin direc-ons are random and there is no net magne-sm.

•  Spins like to align along an applied field so magne-za-on can be induced

•  In ferromagne-c materials, the electron spins spontaneously align in small crystals.

•  These crystals remain par-ally magne-zed axer being subject to a magne-c field leading to a “permanent” magnet.


Details on ferromagne-sm

•  In a ferromagnet, an external field causes aligned domains to grow at the expense of others leaving a net macroscopic magne-sm.


Curie temperature •  Magne-za-on is destroyed by random energy disorien-ng the spins

•  Below a cri-cal (Curie) temperature in ferromagne-c materials, alignment is spontaneous and micro crystals of magne-sm form


Magnetic recording; hard drives

•  Magne-za-on is used in informa-on storage.

•  Bits of magne-c material are magne-zed in one of two direc-ons to represent one digital bit – a zero or one


Magne-c storage density

•  Magne-c storage density is approaching one Gigabit/square inch (1 micron x 1 micron pixels of magne-sm)


Magnetic force on a moving charge

•  A moving charge feels a magne-c force perpendicular to its velocity.

•  A collec-on of moving charges in a conduc-ng wire transfers a magne-c force to the wire as a whole


Electric motors •  The magne-c force on a loop of current in a magne-c field causes it to rotate.

•  The direc-on of the current must alternate as the current rotates to maintain the rota-on.


Transformers

•  In a transformer, the magne-c field of an input current induces an output voltage in propor-on to the number of output turns linking the field.


Magnetic levitation of nonmagnetic objects

•  A magne-c field induces magne-sm.

•  A gradient in magne-c field strength produces a force.

•  It is possible to levitate water (a frog)!


Electromagne-c guns

•  Electric and magne-c forces may be used instead of chemical explosives to propel objects

•  The rail gun and coil gun are two examples

•  h;p://www.coilgun.info/theorymath/electroguns.htm

•  How to store the energy?


Rail guns, again •  Firing of an electromagne-c railgun (EMRG) at Naval Surface Warfare Center, Dahlgren, Va., on January 31, 2008, firing at 10.64MJ (megajoules) with a muzzle velocity of 2520 meters per second, 3x rifle velocity.


h;p://www.navy.mil/search/display.asp?story_id=34718

Battery

•  A ba;ery links two chemical reac-ons and directs electrical current through a macroscopic circuit path to derive the chemical power


Documents

Electricity and Magnetism - Department of Physicsphysics.wisc.edu/undergrads/courses/spring10/107... · Chapter 6 Electricity and Magnetism! Fundamental*par-cles*carry*charge*which*can*be*

Electricity and Magnetism - Department of Physicsphysics.wisc.edu/undergrads/courses/spring10/107... · Chapter 6 Electricity and Magnetism! Fundamentalpar-clescarrychargewhichcanbe*