Resistance
From ZuluNotes - Free Leaving Cert Notes
Resistance = Voltage / Current Resistance of a conductor is the ratio of the p.d across it to the current flowing through it. SI Unit: The Ohm
The Ohm: a conductor has a resistance of 1ohm if the current acroos it is 1 A when the pd across it is 1 V.
Instruments used to measure Resistance: An ohmeter; a whatstone bride; a multimeter; an ammeter and a voltmeter.
Resitors in Series: R = R1 + R2 Resitors in Parallel: 1/R = 1/R1 + 1/R2
Ohm's Law: for certain conductors, at constant temperature the current is directly proportional to the voltage. (graph of current against voltage plotted for a conductor that obeys Ohm's law is a straight line passing through the origin.)
Resistance Of A Conductor Depend On: its length, its cross-sectional area, the material from which it is made and its temperature
Thermistor: a semiconductor whose resistance decreases rapidly as the temperature increases.
Restivity: if a conductor of length l and cross-sectional area A has a resistance R, its resistivity is RA/l SI Unit: the ohm metre
Wheatstone bride: balanced when no current flows in the galvonometer. when balanced R1/R2 = R3/R4
Metre Bridge: R1/R2 = L1/L2
Advantage of Ohmeter insted of using Metre Brige: more compact, more portable and faster to use.
Disadvantage of using Ohmeter instead of Metre Bridge: less accurate and more fragile.
