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

Chapter 4 – Ohm’s Law, Power
and Energy
Introductory Circuit Analysis
Robert L. Boylestad
4.1 - Ohm’s Law
Effect =
Cause
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 the opposition is the resistance encountered

Ohm’s Law

Simple analogy: Water in a hose
Electrons in a copper wire are as water in a hose
 Consider the pressure valve as the applied voltage and the
size of the hose as the factor that determines 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, and
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
I = E/R
Where:
I = current (amperes, A)
E = voltage (volts, V)
R = resistance (ohms, W)
4.2 - Plotting Ohm’s Law
Insert Fig 4.6
Plotting Ohm’s Law
Insert Fig 4.8
4.3 - 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

Power can be delivered or absorbed as defined
by the polarity of the voltage and the direction of
the current
power is being delivered by a dc source if the
current flows from the positive terminal
 power is being absorbed by a dc source if the
current flows into the positive terminal (a battery
being charged)

4.4 - Wattmeters
 The wattmeter is
a device used to measure the
power delivered by a source
 Power is a function of both current and voltage
levels, so four terminals must be used

2 current terminals
 (usually

larger terminals, to ensure a solid connection)
2 voltage terminals
 (sometimes
3 when there is a choice of voltage level)
4.5 - Efficiency

Efficiency (h) of a system is determined by the
following equation:
h = Po / Pi
Where:
h = 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
4.6 - 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
wattsecond, however, is too small a quantity for
most practical purposes, so the watthour (Wh) and
kilowatthour (kWh), were defined, as follows:
Energy (Wh) = power (W) X time (h)
Energy (kWh)=
 The
power (W) x time (h)
1000
Killowatthour meter is an instrument used for
measuring the energy supplied to a residential or
commercial user of electricity
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 Current 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 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 metalic water piping system of 10 ft or more, or a
buried metal plate
