Wednesday, November 24, 2004

Lesson 19

ELECTRICITY LESSON 19 – OHM’s Law
Lesson: 19.
Take up Hwk: Pg. 319 #1-7, collect labs pg. 75-77, take up pg. 78-79
Quiz
Measuring Electrical Energy Worksheet/Handout
More resistance questions handout
Hwk: Complete Worksheets and course pack lab pg. 80, and pg. 81.


Resistance

1. Resistance is the property of a component which restricts the flow of electric current.

2. Energy is used up as the voltage across the component drives the current through it and this energy appears as heat in the component.
3. Resistance is measured in ohms, the symbol for ohm is an omega . 1 is quite small for electronics so resistances are often given in k and M. 1 k = 1000 1 M = 1000000 .
4. Resistors used in electronics can have resistances as low as 0.1 or as high as 10 M.

Resistors connected in Series

When resistors are connected in series their combined resistance is equal to the individual resistances added together. For example if resistors R1 and R2 are connected in series their combined resistance, R, is given by:

Combined resistance in series: R = R1 + R2

This can be extended for more resistors: R = R1 + R2 + R3 + R4 + ...

Note that the combined resistance in series will always be greater than any of the individual resistances.

Resistors connected in Parallel

When resistors are connected in parallel their combined resistance is less than any of the individual resistances. There is a special equation for the combined resistance of two resistors R1 and R2:

1 = 1 + 1 + 1
__ __ __ __
Rt R1 R2 R3 ...


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