Transcript Use the equations for electric power to

Chapter 18
Electric Currents
Objectives
After studying the material of this chapter, the student should be able to:
Use the equations for electric power to determine the power and energy
dissipated in a resistor and calculate the cost of this energy to the
consumer.
Remember:
P= W / t
(Power=Work/time)
• Remember: W= VQ and I = Q/t
So: P= I V
18.5 Electric Power
Power, as in kinematics, is the energy
transformed by a device per unit time:
(18-5)
Hewitt – Electric Power
Electric Power, P= I V
Known as Joule’s Law
P: is the power consumed by a resistor, R.
Unit: Joule/s= Watt
Derive the following using P=IV
and Ohm’s Law:
P=V2/R
P=I2R
18.5 Electric Power
The unit of power is the watt, W.
For ohmic devices, we can make the
substitutions:
(18-6a)
(18-6b)
Problem 1
• The current through a car motor is
150A. The battery used is 12V. How
much energy is supplied by the
battery in 5s?
Problem 1
• The current through a car motor is
150A. The battery used is 12V. How
much energy is supplied by the
battery in 5s?
Answer: 9,000J
kWh
kiloWatt hour
What does the kWh measure,
a) Energy or b) Power ?
18.5 Electric Power
What you pay for on your electric bill is not
power, but energy – the power consumption
multiplied by the time.
We have been measuring energy in joules, but
the electric company measures it in kilowatthours, kWh.
Problem 2
A small desktop radio has a resistance
of 8,000 Ω. The voltage is 120V.
a)How much current does it draw?
b)How much power does it use?
c)How much does it cost to run the radio
for 12 hours, if 1kWh costs $0.15?
Problem 2
A small desktop radio has a resistance
of 8,000 Ω. The voltage is 120V.
a)How much current does it draw?
b)How much power does it use?
c)How much does it cost to run the radio
for 12 hours, if 1kWh costs $0.15?
Answers:
a)0.015A, b)1.8W, c) $0.00324
18.6 Power in Household Circuits
The wires used in homes to carry electricity
have very low resistance. However, if the current
is high enough, the power will increase and the
wires can become hot enough to start a fire.
To avoid this, we use fuses or circuit breakers,
which disconnect when the current goes above
a predetermined value.
Household Circuits
 Household
circuits are:
parallel circuits
 Typically designed to carry 15 Amperes of
current - maximum.

Overloading Circuits
 Since
adding resistance to a parallel
circuit causes current to increase,
adding appliances to a household circuit
causes current in the house wiring to
increase.
 This means wires must dissipate more
heat energy (P = I2R).
Short Circuits
A
short circuit is a very low resistance
path in a circuit.
 A short circuit will also cause large
amounts of current to flow in a circuit
and overload the circuit.
Fuses & Circuit Breakers
 Fuses
and circuit breakers act as
automatic switches in a circuit to
prevent dangerous currents from
flowing.
18.6 Power in Household Circuits
Fuses are one-use items – if they blow, the
fuse is destroyed and must be replaced.
When the current
exceeds a certain
value, the metallic
ribbon melts and
the circuit opens.
then the fuse must
be replaced.
18.6 Power in Household Circuits
Circuit breakers, which are now much more
common in homes than they once were, are
switches that will open if the current is too
high; they can then be reset.
18-6 Power in Household
Circuits
Example 18-11: Will
a fuse blow?
Determine the total
current drawn by all
the devices in the
circuit shown.
Example 18-11: Will a fuse blow?
Determine the total current drawn by all the
devices in the circuit shown.
20-A fuse or 30-A fuse
If the circuit is designed for a 20-A fuse, what will
happen?
Hopefully the fuse will blow. What if it does not?
It can overload the wires, which will get hot and can
start a fire.
What do you do to prevent this from happening?
Move one of the devices to another circuit.
If the circuit is designed for a 30-A fuse, what will happen?
If it has a heavier wire and a 30-A fuse, the fuse should not blow.
If it does, a short could be the problem, which could be in cord.
18-6 Power in Household
Circuits
Conceptual Example E: A dangerous extension
cord.
Your 1800-W portable electric heater is too
far from your desk to warm your feet. Its
cord is too short, so you plug it into an
extension cord rated at 11 A. Why is this
dangerous?
18-6 Power in Household
Circuits
Conceptual Example E: A dangerous extension
cord.
Your 1800-W portable electric heater is too
far from your desk to warm your feet. Its
cord is too short, so you plug it into an
extension cord rated at 11 A. Why is this
dangerous?
Solution: An 1800-W heater operating at 120 V
draws 15 A of current, which exceeds the rating
of the extension cord. This creates a risk of
overheating and fire.
Homework
Chapter 18 Problems: 26, 28, 29, 31, 33
Inside a Flashlight (optional)
Closure
Kahoot 18-5 and 18-6