1. Warm up Why do electrical devices usually contain metal parts?
Why are electrical devices usually encased in plastic or glass?

2. Resistance
What is resistance?

3. What is electrical conductivity?
The ability to carry current.
When voltage is applied across a material, charge flows and you can measure current.
The amount of current varies in different materials.

4. Three Categories of Electrical Conductivity
Conductors- materials that easily carry current
Insulators - materials that mainly block current
Semiconductors – materials that are in between

5. What is resistance?
Resistance describes how easily charges flow through one object in comparison to any other object.
High resistance = difficult for current to flow
Low resistance = easy for current to flow

7. Devices that use electrical energy have resistance.
The more energy a device uses, the more resistance it has.
Resistance is measured in ohms

8. Ohm’s Law
George Ohm (1787 – 1854) experimented with circuits and discovered a mathematical relationship that is present in most circuits.
Ohm’s Law I = V/R

10. Based on Ohm’s Law What happens to current when voltage is increased?
When voltage increases current also increases
What happens when resistance increases?
current decreases

11. Temperature can effect the resistance of a material.
Resistance increases with temperature.
Why?
Atoms move around more at high temperatures and collide more often with moving charges

12. Most circuits have preset voltage sources
Current can only be varied by changing the resistance in the circuit.
This can be done by using components called resistors

13. Resistors are made of materials that keep the same resistance over a wide range of temperatures and currents