Ohm's Law

Question File Number 05



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Go here for some practice exercises Resistor Circuits |  Resistor Colour Code

Ohm's Law

Ohm's Law states that current flowing in a circuit is directly proportional to the electromotive force applied and inversely proportional to the resistance of the circuit.

As a formula it is:

I = E/R amperes

where

I = current flow, (ampere)

E = e.m.f. (volt)

R = resistance (ohm)

The symbol for volt can be either V or E. For consistency we will use E (remember Electromotive force). Resistance is abbreviated to R and ohms are written using the Greek letter Ω (Omega).

This law is fundamental to all theoretical work in radio and electronics.

The emf is expressed in a unit called the volt. A volt can be defined as the pressure required to force a current of one ampere through a resistance of one ohm.

Consider the following example: Consider the water pressure (volts) required to pass water (current) through a copper pipe of a certain small diameter (resistance).

Try to visualise water going through other pipes of varying diameters. The water pressure required will vary and the volume delivered will vary, or both.

This is Ohm's law, where E = Volts; I = current in amperes and R = resistance in ohms. By transposing we get the following:

Remember: Cover up the value you seek - and the formula to get it using the two remaining values is given

Ohmslaw


Resistors -- calculations

Many problems are straight substitution of values in one of the above three expressions. If you are given two of the values of E, I or R, find the third! Find the expression that suits your problem:

Unknown to find = (values you are given).

Remember that you must use the units: Volt, Ohm, and Ampere. If you are given millivolt, millliamp, or kohm, be careful with your arithmetic! Convert to the basic values, Volt, Amp or Ohm before you start!


Multiples and sub-multiples of electrical units

Symbol  

Unit Name  

Multiply/Divide by

Scientific Notation

T

tera

x 1 000 000 000 000

1012

G

giga

x 1 000 000 000

109

M

mega

x 1 000 000

106

k

kilo

x 1 000

103

m

milli

1 000

10-3

u

micro

1 000 000

10-6

n

nano

1 000 000 000

10-9

p

pico

1 000 000 000 000

10-12

f

femto

1 000 000 000 000 000

10-15

a

alto

1 000 000 000 000 000 000

10-18


Go here for some practice exercises Resistor Circuits |  Resistor Colour Code



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Compiled Mon Nov 29 2010 at 8:11:49pm


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