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)  pow er(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
Typical wattage ratings of some common
household items
Insert Table 4.1
4.7 - Circuit Breakers, GFCIs, and
Fuses
 Power coming into any facility or item must be limited to
ensure that the current through the lines or electrical
equipment is not above the rated value.
 Fuses or circuit breakers are installed where the power
enters the installation.
 Fuses have an internal metallic conductor which begins to melt if
the current exceeds the fuse rated value on the case.
 In recent years fuses have been replaced with circuit breakers.
Circuit breakers have an electromagnet that, when the current
exceeds the rated value, has sufficient strength to draw the
connecting metallic link out of the circuit and open the path.
Circuit Breakers, GFCIs, and
Fuses
 National
Electrical Code requires that outlets in the
bathroom and other sensitive areas be of the Ground
Fault Circuit Interrupt (GFCI) variety.
 GFCIs are designed to trip more quickly than the
standard circuit breaker.
GFCI senses differences in input and output currents to
the outlet, and trips if they are not the same.
4.8 - Applications
 Microwave ovens
 Most microwaves are rated at 500 W to 1200 W at
a frequency of 2.45 GHz.
 Heating occurs because the water molecules in the
food vibrate at such a high frequency that the friction
with neighboring molecules causes the heating effect.
 Most microwaves are between 50% and 60%
efficient.
Applications
 Household wiring
 Most older homes, without electric heating, have a
100 A service.
 Power is broken down into different circuits utilizing
15 A, 20 A, 30 A and 40 A protective breakers.
 Maximum load on each breaker should not
exceed 80% of its rating (12 A of a 15 A circuit
breaker).
Applications
 The correct gauge of wire must be used with
the right circuit breaker – #14 wire up to a 15 A
breaker, #12 wire up to 20 A, #10 wire up to 30 A.
 Grounding is a very important part of safety.
 The National Electric Code requires that the neutral
wire of a system be grounded to an earth-driven rod,
a metallic water piping system of 10 ft or more, or a
buried metal plate.