Transcript Chapter 4 – Ohm`s Law, Power and Energy

Chapter 4 – Ohm’s Law, Power
and Energy
Introductory Circuit Analysis
Robert L. Boylestad
4.2 - Ohm’s Law
Cause
Effect 
Opposition
 Every
conversion of energy from one form to
another can be related to this equation.
 In electric circuits the effect we are trying to
establish is the flow of charge, or current. The
potential difference, or voltage between two
points is the cause (“pressure”), and resistance
is the opposition encountered.
Ohm’s Law
 Simple analogy: Water in a hose
Electrons in a copper wire are analogous to water in a
hose.
 Consider the pressure valve as the applied voltage and
the size of the hose as the source of resistance.

The absence of pressure in the hose, or voltage across the wire
will result in a system without motion or reaction.
 A small diameter hose will limit the rate at which water will flow,
just as a small diameter copper wire limits the flow of electrons.

Ohm’s Law
 Developed in 1827 by Georg Simon Ohm
 For a fixed resistance, the greater the voltage (or
pressure) across a resistor, the more the current.
The more the resistance for the same voltage, the
less the current.
 Current is proportional to the applied voltage and
inversely proportional to the resistance.
Ohm’s Law
E
I
R
Where:
I = current (amperes, A)
E = voltage (volts, V)
R = resistance (ohms, )
4.3 - Plotting Ohm’s Law
Plotting Ohm’s Law
Insert Fig 4.8
4.4 - Power
 Power is an indication of how much work (the
conversion of energy from one form to another)
can be done in a specific amount of time; that is,
a rate of doing work.
Power
W
P
t
1 Watt (W)  1 joule / second
 Power can be delivered or absorbed as defined by the
polarity of the voltage and the direction of the current.
4.5 - Energy
 Energy (W) lost or gained by any system is
determined by:
W = Pt
 Since power is measured in watts (or joules per
second) and time in seconds, the unit of energy is
the wattsecond (Ws) or joule (J)
Energy
 The
watt-second is too small a quantity for most
practical purposes, so the watt-hour (Wh) and kilowatthour (kWh) are defined as follows:
Energy (Wh)  power (W)  time (h)
power (W)  time (h)
Energy (kWh) 
1000
 The
killowatt-hour meter is an instrument used for
measuring the energy supplied to a residential or
commercial user of electricity.
4.6 - Efficiency
 Efficiency () of a system is determined by the
following equation:
 = Po / Pi
Where:
 = efficiency (decimal number)
Po = power output
Pi = power input
Efficiency
The
basic components of a generating (voltage) system
are depicted below, each component has an associated
efficiency, resulting in a loss of power through each
stage.
Insert Fig 4.19