Gwrthiant

Resistance

Dydi bywyd ddim yn hawdd i electronau wrth iddynt lifo o gwmpas cylched. Er mwyn egluro’r gosodiad hwn gadewch inni ddechrau’r adran hon drwy ystyried adeiledd y dargludyddion y byddant yn teithio drwyddynt. Mae’r dargludyddion wedi’u gwneud o atomau, neu foleciwlau, sy’n agos at ei gilydd ac wedi’u trefnu mewn patrwm geometrig rheolaidd. Mae’r atomau wedi’u cloi yn eu lle o fewn y patrwm ac yn dirgrynu o gwmpas eu safle cymedrig.

Adeiledd dargludydd (electronau yw’r cylchoedd gwyrdd bach, atomau yw’r cylchoedd coch mwy).

Wrth i’r electronau ddrifftio tuag at derfynell bositif y batri, mae’n anochel y byddant yn gwrthdaro ag atomau. Mae’r atomau’n cario llawer mwy o fàs na’r electronau ac yn darparu gwrthwynebiad sylweddol i’r llif. Mae maint y gwrthwynebiad yn dibynnu ar faint a siâp y dargludydd a’r defnydd y mae wedi’i wneud ohono. Bydd dargludydd hir, main yn cynnig llawer mwy o wrthwynebiad na dargludydd byr, llydan wedi’i wneud o’r un defnydd.

Mae hyd yn oed y dargludyddion gorau’n cynnig rhywfaint o wrthwynebiad i lif electronau. Gwrthiant yw’r enw ar y gwrthwynebiad hwn.

Bydd electronau’n colli rhywfaint o’u hynni cinetig yn ystod gwrthdrawiad. Caiff yr ynni cinetig hwn ei droi’n ynni gwres, a fydd yn achosi i’r tymheredd godi o fewn y dargludydd.

Yr ohm yw uned mesur gwrthiant. Y symbol a ddefnyddir ar gyfer ohm yw’r llythyren Roegaidd omega W Cafodd yr uned gwrthiant ei henwi ar ôl y gwyddonydd o Almaenwr, Georg Ohm. Gwnaeth ei waith arloesol ef yn nechrau’r 19eg ganrif gyfraniad pwysig at ein dealltwriaeth o theori drydanol.

PRIODWEDDAU TRYDANOL DARGLUDYDDION Er mwyn gwneud cymhariaeth deg rhwng gwrthiant gwahanol fetelau rhaid inni ystyried dargludyddion o’r un maint. Mae’r tabl yn cymharu gwrthiant gwahanol ddefnyddiau ar ffurf gwifren 1 metr o hyd â thrawsfesur diamedr 1 mm.

  • Gwrthiant y Defnydd (Ω )
  • Arian 0.019
  • Copr 0.020
  • Aur 0.029
  • Alwminiwm 0.034
  • Twngsten 0.069
  • Constantan 0.62
  • Nicrom 1.27

Dylai gwifren gysylltu gynnig cyn lleied o wrthiant ag sy’n bosibl i lif electronau. Arian yw’r dargludydd gorau sydd ar gael ond mae’n llawer rhy ddrud i’w ddefnyddio fel hyn. Gwneir y rhan fwyaf o wifrau cysylltu o gopr.

Adeiladu gwrthydd:

DEFNYDDIO EFFAITH WRESOGI CERRYNT Bydd offer trydanol sydd wedi’u cynllunio i gynhyrchu effaith wresogi yn defnyddio dargludyddion â gwrthiant uchel. Defnyddir nicrom, sef aloi wedi’i wneud o nicel a chromiwm, yn yr elfen wresogi mewn offer fel tanau trydan. Mae’n rhaid gwneud ffilamentau lampau o ddefnydd sy’n gallu gwrthsefyll cael ei wresogi dro ar ôl tro i dymheredd lle mae’n troi’n wyn. Mae nicrom yn anaddas ar gyfer y defnydd hwn. Gwneir ffilament fel arfer o wifren dwngsten fain iawn.

Life is not easy for electrons as they flow around a circuit. In order to explain this statement let us start this section by considering the structure of the conductors through which they travel. The conductors are made up of atoms, or molecules, which are close together and arranged in a regular geometrical pattern. The atoms are locked in position within the pattern and vibrate about their mean position.

Structure of a conductor (electrons are shown as the small green spheres, atoms as the larger red spheres).

As the electrons drift towards the positive terminal of the battery, collisions with atoms become inevitable. The atoms carry much more mass than the electrons and provide considerable opposition to the flow. The amount of opposition depends upon the dimensions of the conductor and the material from which it is made. A long, thin conductor will offer much more opposition than a short, fat conductor made from the same material.

Even the best conductors offer some opposition to electron flow. This opposition is called resistance.

Electron lose some of their kinetic energy during a collision. This kinetic energy is converted into heat energy, resulting in a temperature increase within the conductor.

The unit of resistance is the ohm. The symbol used for an ohm is the Greek letter omega W The unit of resistance has been named after the German scientist, Georg Ohm. His pioneering work in the early 19th century made a major contribution to our understanding of electrical theory.

ELECTRICAL PROPERTIES OF CONDUCTORS In order to make a fair comparison between the resistance of different metals we must consider conductors of equal dimensions. The table compares the resistance of different materials in the form of a wire 1 metre long with a circular section of diameter 1 mm.

  • Material Resistance (Ω)
  • Silver 0.019
  • Copper 0.020
  • Gold 0.029
  • Aluminium 0.034
  • Tungsten 0.069
  • Constantan 0.62
  • Nichrome 1.27

Connecting wire should offer as little resistance as possible to electron flow. Silver is the best conductor available but is far too expensive for this application. Most connecting wires are made from copper.

Construction of a resistor:

USING THE HEATING EFFECT OF A CURRENT Electrical appliances which are designed to produce a heating effect make use of conductors with high resistance. Nichrome, an alloy made from nickel and chromium, is used in the heating element of appliances such as electric fires. Lamp filaments must be made from a metal which can withstand being repeatedly heated to a temperature at which it becomes white. Nichrome is unsuitable for this application. Filament are usually made from very thin tungsten wire.