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Ohm’s Law and Power Notes

•  An electric circuit is a complete path through which electric current travels.

•  A good example of a circuit is the one found in an electric toaster.

•  Current only flows when there is a complete and unbroken path, or a closed circuit.

•  Flipping a switch to the “off” position creates an open circuit by making a break in the wire.

Voltage and Current •  Voltage is the energy which causes electrical

energy to flow. Current flows from higher voltage to lower voltage.

•  Current is what actually flows (the electrons) and does work (lights a light bulb, turns a blade in a blender, heats a coil).

•  You cannot have current without voltage but you can have voltage without current!

Resistance •  Resistance

controls how much current flows for a given voltage.

•  When resistance is low, current flows easily.

8.2 Resistance •  Any object that uses

electricity has resistance! (for example: light bulbs)

•  Electrical resistance is

measured in ohms.

This light bulb has a resistance of 1 ohm.

• Light bulb: 240 ohms.

• Dry skin has higher resistance, wet skin has lower resistance.

• Conductors have lower resistances, insulators have higher resistances.

Resistance of Common Objects

What 5 changes could you make to a closed circuit consisting of a battery, a light bulb, and a switch that would increase the amount of current?

1. You could increase the number of batteries, which would increase total voltage

2. You could replace the light bulb with one that had a lower resistance

3-5. You could use thicker wires, shorter wires, wires made from a material with higher conductivity.

•  Voltage and current are directly proportional

•  Doubling the voltage doubles the current.

•  Resistance and current are inversely proportional.

•  Doubling the resistance cuts the current in half.

•  If you know two of the three quantities, you can use Ohm’s law to find the third.

Ohm’s law

Electrical Power •  Electrical power measures how much

energy (current and voltage) is used in electrical devices. Measured in watts.