Rate in the
Electrical Systems
Overview
Rate in the Electrical System

What is the prime mover in the
electric system?
•


voltage difference
What is electric rate?
•
amount of charge (q) that flows through
a conductor in a unit of time (t)
What term in given to electric rate?
•
current
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Rate in the Electrical System



What symbol is used to represent electric
current?
•I
What unit is used to measure current?
• amperes (amps)
Current = the rate of movement of
charges, or
• I = charge moved/elapsed time = q/t
• Amperes = coulombs/second
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Rate in the Electrical System

Example – A DC motor operates at a
rate of 10 amperes for 10 seconds.
• How much charge (q - coulombs) moves
•
through the motor?
Solve
Amps = coulombs/time or I = q/t
Substitute values, then:
10 amps = q/10 sec
Thus
q = 100 coulombs
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Rate in the Electrical System

There are 2 types of electric
current – AC & DC. What is the
difference?
DC current flows only one way – from
the negative electrode to the positive
AC current flows both ways alternately,
with the electrons constantly change
direction
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Rate in the Electrical System

Why is AC current used in homes and
businesses?
• it is much more economical to transfer AC
current over long distances than DC current
• DC power suffers from substantial voltage
decay over long distances
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Rates in Electrical Systems

Both voltage
and current are
important
measurements
in an electric
circuit.
• How do you
measure voltage
in a circuit?
use a voltmeter that is
connected across a load
component in the circuit (VA)
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Rates in Electrical Systems

How do you
measure current
(amps) in an
electrical circuit?
use an ammeter that is
connected in series (in-line)
with the circuit (CT)
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Rate in the Electrical System

Measuring devices
• VOM (Volt-ohm-milliampere meter)
• a handheld device that measures
both current, resistance & voltage
• Multimeter
• Voltmeter
• Ohmmeter
• (Milli)ammeter
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Rate in the Electrical System


•
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•
Frequency and period
are 2 fundamental
parameters of AC
current
What is frequency ?
period
measure of how often
the current alternates
(cycles/sec)
What unit is used to
measure frequency?
hertz (Hz) – both SI
and English
frequency
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Rate in the Electrical System

How is frequency calculated?
•
f = n / t, where
f = frequency
n = number of cycles
t = time
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Rate in the Electrical System

Example – A generator produces
an electrical signal that has 240
cycles in 4 seconds.
• What is the frequency of the signal?
•f=n/t
• f = 240 cycles / 4 seconds
• f = 60 Hz
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Rate in the Electrical System

The period is the time it takes for one
cycle to occur (seconds/cycle) – how
long for a pattern to repeat.
• How are frequency and period related?
•f=1/T
• T = period (usually in seconds)
•T=1/f
• Their relationship is reciprocal!
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Rate in the Electrical System

Example – The frequency of an
electrical wave is 60 Hz.
• What is the period of the wave for one cycle?
•T=1/f
• T = 1 / 60 Hz
• T = 0.0167 sec
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Rate in Electrical Systems

Refer
• Student exercises p69
• Math skills laboratory p71
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