a measure of how difficult it is for electriccurrent to travel through a material
good conductors (metals) have low resistance,while insulators (plastic, rubber) have relativelyhigh resistance
symbol is R
measured in the unit “ohms” which has thesymbol Ω
resistor an electrical device that has aspecific resistance value
circuit symbol:
depending on the desired effect in the circuit,different level resistors can be used
if a smooth steady change in current isrequired, a variable resistor is used
all electrical components in a circuit haveelectrical resistance
connecting wires and switches generally havelow resistance
when charges encounter resistance the energyis usually converted to thermal energy (heat)
this can be a waste (ex: during transmissionfrom power plants) or it can be necessary (ex:incandescent light bulb filaments)
are special materials that have no electricalresistance
current technology can only demonstratesuperconductivity at extremely lowtemperatures
the goal is to reduce wasted thermal energyand create wires that are 100% efficient
ohmmeter a device that measures electricalresistance in a circuit
should be connected in parallel
must NOT be used on a live circuit (turn thepower off first!!)
symbol for an ohmmeter is:
when testing a load with an ohmmeter, theresistance should read low
there are 4 main factors that affect resistance:
1.type of material
conductor vs insulator
2.length of wire
longer wire provides more resistance
3.diameter
increased thickness will decrease resistance
4.temperature
higher temperatures actually increase resistance
the relationship between resistance, currentand voltage is summarized in ohm’s law
“the voltage in a conductor is proportional tothe current if the temperature remainsconstant”…so V α I
the equation can be written as:
R = V
I
Calculate the resistance of a load with avoltage of 25 V and a current of 410 mA.
61 Ω