12.3 Resistance Flashcards
what is the resistance of a component in a circuit
a measure of the difficulty of making a current pass through a component
what is resistance caused by
repeated collisions between the charge carriers in the material with eachother and the fixed positive ions in the material
what is resistance of a component defined as
the pd across the component/ the current through it
what is the unit of resistance
ohms
what is a resistor designed to do
a component designed to have a certain resistance regardless of the current passing through it
what does ohms law state
the pd across a metallic conductor is proportional to the current going through it provided that the physical conditions don’t change e.g temp
this is the same as saying that the resistance is constant
why is an ohmic conductor a resistor
because the resistance is constant and resistors are designed to have a certain resistance
resistivity =
RA/L
unit for resistivity
ohm metre
how to find area using diameter
pi d ^2 / 4
how to determine resisitivity of wire
measure diameter of wire in several different places using micrometer and fund mean value of d to find a
measure resistance at different lengths and plot graph of r against L
resistivity of wire is gradient of graph x area
what is a superconductor
a wire or device made of a material that has zero resistivity at and below a critical temperature that depends on the material
what happens when a current passes through a supercondtucer below critical temp
there is no pd because resistance is 0 and so the current has no heating effect
what happens if a superconductor is raised above its critical temp
it loses its superconductivity
what is the highest super conductor critical temp
-123 degrees
when is it referred to as a high temp super conductor
if its critical temp is above -196 degrees
what are superconductors used for
high power electromagnets that generate very strong magnetic fields in devices e.g mri scanners and particle accelerator
also used in development of new application e.g lightweight electric motors and power cables that transfer electrical energy without dissipation
what did Georg ohm discover
that the pd of a wire is proportional to the current running through it
where must the ammeter be to measure resistance
in series with the resistor so that the sane current passes through both
where must the voltmeter be plksced to measure pd of resistor
in parallel so that they have the same pd
no current should pass through volt meter or ammeter will not record exact current through the resister and therefore voltmeter should have a very high resistance
what is a variable resistor used for
to adjust current and pd as necessary to investigate the variation
change in steps and measure current and pd recorded respectively
can then be plotted on graph pd against i
