Electricity

Electricity

Insulator won't let electricity to flow through it. Conductor allows electricity to flow through it.

Gold leaf electroscope uses: detect charge, tells approximate size, tells weather it is positive or negative.

Electric charge unit = Coulomb C. Static stays on the outside of a conductor and go's to the pointed end of it.

Coulomb's Law = Force between two charges is directly proportional to the product of the masses and inversely proportional to the square of the distance between them. Page 61 tables (inverse square law)

Permittivity: Is a measure of the effect the medium around two charges will effect the size of the force..

Electric Field: Any region where charge experiences a force other than gravity.

Potential Difference: Work done in bringing 1C of charge from one point to another ( unit = volts)

Electric current: Flow of electric charge.

Capacitance: Ratio of electric charge on a conductor to its potential. (Unit = Farad).

Parallel plate capacitor = Two parallel plates separated by an insulator called a dielectric.

Factors effecting the capacitance of a parallel plate capacitor are the area of overlap of plates, the distance between the plates and the permittivity of the diaelectric.

Three effects of electric current = heating (bulb), magnetic (compass), chemical (electrolysis of water).

Current measured using an ammeter in series with a circuit reading in amperes.

Voltage measured using a voltmeter or Galvanometer always connected in parallel to a circuit.

Current at a junction in a circuit : Current entering junction = current leaving junction.

Electromotive force (emf): This is a voltage when applied to a circuit.

Sources of Emf's: Primary cells: Is a cell that cant be recharged usually called dry cell batteries eg. zinc-carbon. A secondary cell can be recharged usually called wet cell batteries eg. Car battery, lead acid.

Resistance ®: The ratio of potential difference across a conductor to the current flowing through it. (Ohms).

Measure resistance: use an ammeter and voltmeter and V=IR, or an ohmmeter.

Resistors connected in series are added together. R = R1 + R2 + R3

Proof: V = IR (equ 1)------------------------V1 = IR1 V2 = IR2 V3 = I R3

so: V1 + V2 + V3 = I(R1 + R2 +R3) (equ 2)

Equ 1 & 2 gives: I R = I (R1 +R2 +R3) So: R = R1 +R2 +R3.

Resistors connected in parallel can be calculated using: 1/R = 1/R1 + 1/R2 +1/R3

Proof: I1 = V/R1 I2 = V/R2 I3 = V/R3

I = V/R1 + V/R2 + V/R3 I = V( 1/R1 + 1/R2 + 1/R3) From Ohm's law: I = V/R V/R = V (1/R1 +1/R2 +1/R3)

Canceling the V on both sides gives: 1/R = 1/R1 +1/R2 +1/R3

Factors effecting the resistance of a conductor include temperature, length, cross sectional area and material.

Resistance can also be measured using a Wheatstone bridge or a meter bridge (3 known and 1 unknown).

Joules Law: Rate at which heat is produced in a conductor is directly proportional to the square of the current at constant temperature.

Electricity is transported to home using high voltage because this minimizes heat loss due to high current.

Current voltage relationship for different conductors: Metallic = ohmic resistance as I increases so does V.

Filament bulb = As I increases so does V up to a point where heat causes rising resistance due to collisions so later as V increases I fails to increase any more. Semiconductor = As V increases I increases due to heat caused by current many extra conduction electrons are produced and so later as V increases I increases dramatically.

Gas = gas always has some ions being formed for conduction and so conduction starts later it stops due to all conduction ions being used up later still it conduction starts again due to potential difference in gas tube creating more conduction ions due to collisions. Vacuum = No current carriers present, but if a metal is heated in a vacuum it will undergo thermionic emission (the giving out of electrons by a hot metal) the produced conduction electrons can then be used for conduction as V increases I increases, later however as V increases I no longer increases as maximum thermionic emission is a limiting factor. Ionic solutions = Active electrodes produces ohmic resistance as V increases I increases. Inactive electrodes, a small back Emf must be overcome and once this happens as V increases I increases.

Domestic circuits: Devices with large current Eg. Oven require a Radial circuit power circuit for this device only. Sockets are usually connected using a ring circuit ie all sockets ina room connected effectively in series.

Miniature circuit breakers (MCB): Bimetallic strip and electro magnet. If the current is too high the circuit is broken.

Fuse: Melts if a current above a preset value flows through it.

Bonding and Earthing: For safety all devices connected to a live mains brown wire or devices that are simply large and metal must be connected to the earth. Thus if they become live the charge will be carried to earth.

Kilowatt hour: Amount of energy used by a 1000 watt device if it runs for an hour ( equals 1 electrical unit).

Semiconductor: Between a good conductor and an insulator. ie semiconductor diode also called a thermistor.

Intrinsic conduction: conduction ability of natural semiconductor.

Doping: Introduction of extra charge carriers into a semiconductor.

Extrinsic conduction: The improved ability of a semiconductor to conduct after doping.

N type semiconductor: Improved conductivity due to extra electrons eg: phosphorous in silicon.

P type semiconductor: Improved conductivity due to extra holes eg: Boron in silicon.

Holes: A positive area in an atom due to the absence of an electron.

PN Junction: When a p type and N type semiconductor are put together.

Depletion Layer: The moving of electron from N type into holes of p type in a PN junction.

Junction voltage: when depletion layer builds up a voltage is seen across it.

Semiconductor in a circuit:

Forward Biased: when the positive of the battery (the long end) is connected to the wide end of the diode.

Reverse Biased: when the negative end of the battery (the short end) is connected to the wide end of the diode.

When in forward biased a circuit will conduct electricity and when in reveres biased it will not.

Rectification is the conversion of alternating current AC into direct current DC for use in devices.

All electricity fomulae page 61 & 62 tables