a measure of how difficult it is for electric current to travel through a material
good conductors (metals) have low resistance, while insulators (plastic, rubber) have relatively high resistance
symbol is R measured in the unit “ohms” which has
the symbol Ω
resistor an electrical device that has a specific 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 is required, a variable resistor is used
all electrical components in a circuit have electrical resistance
connecting wires and switches generally have low resistance
when charges encounter resistance the energy is usually converted to thermal energy (heat)
this can be a waste (ex: during transmission from power plants) or it can be necessary (ex: incandescent light bulb filaments)
are special materials that have no electrical resistance
current technology can only demonstrate superconductivity at extremely low temperatures
the goal is to reduce wasted thermal energy and create wires that are 100% efficient
ohmmeter a device that measures electrical resistance in a circuit
should be connected in parallel must NOT be used on a live circuit
(turn the power off first!!) symbol for an ohmmeter is: when testing a load with an ohmmeter,
the resistance 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, current and voltage is summarized in ohm’s law
“the voltage in a conductor is proportional to the current if the temperature remains constant”…so V α I
the equation can be written as:
R = V I
Calculate the resistance of a load with a voltage of 25 V and a current of 410 mA.
61 Ω