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NUE 052 - Units of Measurement & Meters

The four basic units of electrical measurement are Volts, Amps, Ohms and Watts.
Volts is electrical pressure - the potential difference measured between 2 points in a circuit
Amps is electrical current - the (conventional) flow of current in a circuit from positive to negative
Ohms is electrical resistance - the opposition to current flow in a circuit
An analogue and digital multimeter
Watts is electrical power - the electrical energy in a circuit
Click here to see Ohms law using Volts, Amps, Ohms and Watts
To measure electrical units we need to use an instrument called a meter.
Meters come in all shapes and sizes, each for a different purpose. We can have special purpose meters that measure just one type of unit such as watts or an instrument called a multimeter which is capable of measuring volts, amps and ohms.
Even though the multimeter can measure three electrical units, the meter must be connected differently in the circuit.
To measure Volts - connect the meter in parallel with the circuit under measurement while connected to the supply.
To measure Amps - connect the meter in series with the circuit under measurement while connected to the supply.
To measure Ohms - connect the meter in parallel with the circuit under measurement with the supply disconnected.
Click here to learn more about meters           Click here to learn more about Cathode Ray Oscilloscopes (CRO)
 

Multiples and sub-multiples

Electrical measurements can be from thousands of amps to millionths of an amp.

Terms used for these measurements 'kilo' and 'milli' are called multiples and sub-multiples.

A few examples are listed in the following table:

Term

Symbol

Multiplier

Example

mega

M

1,000,000

107MHz = 107,000,000 hertz

kilo

k

1000

5kV = 5000 volts

milli

m

0.001

6mA = 0.006 amps

micro

u

0.000001

2uA = 0.000002 amps

Conversion Examples

1. A resistance reading is 4.7 Megohms. How many ohms does this represent?

Answer:

Mega is one million so multiply 4.7 by 1,000,000 by moving the decimal point six places to the right 4,700,000 ohms

2. There is 2.45 mA flowing in a circuit. How many amps is this?

Answer:

Milli is one thousand so divide 2.45 by 1,000 by moving the decimal point 3 places to the left 0.00245 amps

 
Scientific Notation
Scientific Notation is a way of expressing multipliers (see previous table) in mathematical form. Each multiplier is treated as a power of 10. Following are examples scientific notation using the examples from the previous table:

Term

Symbol

Multiplier

Scientific Notation

Example

mega

M

1,000,000

106

107MHz = 107,000,000 hertz

kilo

k

1000

103

5kV = 5000 volts

milli

m

0.001

10-3

6mA = 0.006 amps

micro

u

0.000001

10-6

2uA = 0.000002 amps

Conversion Examples
1. Convert 10kV to volts
Answer:

10kV

=

10 x 103 V

(k = 103)

 

=

100 x 102 V

(1 decimal place right)

 

=

1000 x 101 V

(2 decimal places right)

 

=

10000 x 100 V

(3 decimal places to right)

10kV

=

10,000 V

2. Convert 0.10mA to microamps (uA)
Answer:

0.10mA

=

0.10 x 10-3 A

(m = 10-3)

 

=

1.0 x 10-4 A

(1 decimal place right)

 

=

10 x 10-5 A

(2 decimal places right)

 

=

100 x 10-6 A

(3 decimal places to right)

0.10mA

=

100 uA

(u = 10-6)
Practice Tests Practice tests are available on the Practice Test page

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