2.  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

3.  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

4.  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)

5.  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

6.  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

7.  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

8.  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

9.  Calculate the resistance of a load with a voltage of 25 V and a current of 410 mA.  61 Ω