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Ohms Law, Power, and Energy

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For a fixed voltage, doubling the resistance halves the current. 3. Ohm's Law ... Efficiencies vary greatly: power transformers may have efficiencies of 98 ... – PowerPoint PPT presentation

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Title: Ohms Law, Power, and Energy


1
Chapter 4
  • Ohms Law, Power, and Energy

2
Ohms Law
  • The current in a resistive circuit is directly
    proportional to its applied voltage and inversely
    proportional to its resistance.
  • I E/R
  • For a fixed resistance, doubling the voltage
    doubles the current.
  • For a fixed voltage, doubling the resistance
    halves the current.

3
Ohms Law
  • Ohms Law may also be expressed as
    E IR and R E/I
  • Express all quantities in base units of volts,
    ohms, and amps or utilize the relationship
    between prefixes.

4
R 12 x 101 120? I V/R 12V/120? 0.1A
5
Ohms Law in Graphical Form
  • The relationship between current and voltage is
    linear.

6
Open Circuits
  • Current can only exist where there is a
    conductive path.
  • When there is no conductive path we refer to this
    as an open circuit.
  • If I 0, then Ohms Law gives R E/I E/0 ?
    infinity
  • An open circuit has infinite resistance.

7
Voltage Symbols
  • For voltage sources, use uppercase E.
  • For load voltages, use uppercase V.
  • For AC voltages use lowercase e.g. v
  • Since V IR, these voltages are sometimes
    referred to as IR drops.

8
Voltage Polarities
  • The polarity of voltages across resistors is of
    extreme importance in circuit analysis.
  • Place the plus sign at the tail of the current
    arrow.

9
Current Direction
  • We normally show current out of the plus terminal
    of a source.
  • If the actual current is in the direction of its
    reference arrow, it will have a positive value.
  • If the actual current is opposite to its
    reference arrow, it will have a negative value.

10
Current Direction
  • The figures at right are two representations of
    the same current
  • Conventional current is employed (opposite
    direction to electron flow.

11
  • R 6V/25mA
  • 240?

12
Power
  • The greater the power rating of a light, the more
    light energy it can produce each second.
  • The greater the power rating of a heater, the
    more heat energy it can produce.
  • The greater the power rating of a motor, the more
    mechanical work it can do per second.
  • Power is related to energy, which is the capacity
    to do work.

13
Power
  • Power is the rate of doing work.
  • Power Work/time
  • Power is measured in watts.
  • One watt one joule per second

14
Power in Electrical Systems
  • From V W/Q and I Q/t, we get
  • P VI
  • From Ohms Law, we can also find that
  • P I2R and P V2/R
  • Power is always in watts, no matter which
    equation is used.

15
Power in Electrical Systems
  • We should be able to use any of the power
    equations to solve for V, I, or R if P is given.
  • For example

16
FIG. 4.13 Example 4.6.
Pin IV (120V)(5A) 600W
17
Power Rating of Resistors
  • Resistors must be able to safely dissipate their
    heat without damage.
  • Common power ratings of resistors are 1/8, 1/4,
    1/2, 1, or 2 watts.
  • A safety margin of two times the expected power
    is customary.
  • An overheated resistor is often the symptom of a
    problem rather than its cause.

18
Energy
  • Energy Power time
  • Units are watt-seconds, watt-hours, or more
    commonly, kilowatt-hours.
  • Energy use is measured in kilowatt-hours by the
    power company.
  • For multiple loads, the total energy is the sum
    of the energy of the individual loads.

19
Energy
  • Cost Energy cost per unit or
  • Cost Power time cost per unit
  • To find the cost of running a 2000-watt heater
    for 12 hours if electric energy costs 0.08 per
    kilowatt-hour Cost 2kW 12 hr 0.08
    Cost 1.92

20
Law of Conservation of Energy
  • Energy can neither be created nor destroyed, but
    can be converted from one form to another.
  • Examples Electric energy into heat Mechanical
    energy into electric energy
  • In energy conversions, some energy may be
    dissipated as heat, giving lower efficiency.

21
Efficiency
  • Poor efficiency in energy transfers results in
    wasted energy.
  • An inefficient piece of equipment generates more
    heat this heat must be removed.
  • Heat removal requires the use of fans and heat
    sinks.

22
Efficiency
  • Efficiency will always be less than 100.
  • Efficiencies vary greatly power transformers may
    have efficiencies of 98, while amplifiers have
    efficiencies below 50.
  • To find the total efficiency of a system
  • ?Total ?1 ?2 ?3

23
  • ?1 x ?2
  • 0.9 x 0.7
  • 0.63
  • 63

24
FIG. 4.16 Kilowatthour meters (a) analog
(b) digital.
25
FIG. 4.19 Basic components of a generating
system.
26
Fuses (a) CC-TRON (0-10 A) (b) Semitron
(0-600 A) (c) subminiature surface-mount chip
fuses.
Fuses
27
Circuit breakers.
28
FIG. 4.23 Ground fault circuit interrupter
(GFCI) 125 V ac, 60 Hz, 15 A outlet.
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