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

Resistance What is resistance?

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.

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

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

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

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

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

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

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

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