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CONCEPTS Applied 6.3 MAGNETISM

The ability of a certain naturally occurring rock to attract iron

Shocking Costs

has been known since at least 600 b.c. The early Greeks called
You can predict the cost of running an electric appliance this rock Magnesian stone, since it came from the northern
with just a few calculations. For example, suppose you Greek county of Magnesia. Knowledge about the iron-attracting
want to know the cost of using a 1,300 watt hair dryer for properties of the Magnesian stone grew slowly. About a.d. 100,
20 minutes if the utility charges 10 cents per kilowatt- the Chinese learned to magnetize a piece of iron with a Magne-
hour. The equation would look like this:
sian stone, and sometime before a.d. 1000, they learned to use
(1,300 W)(0.33 h)($0.10/kWh)
___ the magnetized iron or stone as a direction fi nder (compass).
cost = W _
1,000 Today, the rock that attracts iron is known to be the black iron

kW oxide mineral named magnetite.
Find answers to one or more of the following questions Magnetite is a natural magnet that strongly attracts iron and
about the cost of running an electric appliance: steel but also attracts cobalt and nickel. Such substances that are
• What is your monthly electrical cost for watching attracted to magnets are said to have ferromagnetic properties,
television? or simply magnetic properties. Iron, cobalt, and nickel are con-
• What is the cost of drying your hair with a blow dryer? sidered to have magnetic properties, and most other common
• How much would you save by hanging out your materials are considered not to have magnetic properties. Most
clothes to dry rather than using an electric dryer? of these nonmagnetic materials, however, are slightly attracted
• Compare the cost of using the following appliances: or slightly repelled by a strong magnet. In addition, certain rare
coffeemaker, toaster, can opener, vegetable steamer, earth elements, as well as certain metal oxides, exhibit strong
microwave oven, and blender. magnetic properties.
• How much does the electricity cost per month for the
use of your desk lamp?
• Of all the electrical devices in a typical household, MAGNETIC POLES
which three have the greatest monthly electrical cost?
Every magnet has two magnetic poles, or ends, about which the
force of attraction seems to be concentrated. Iron filings or other

small pieces of iron are attracted to the poles of a magnet, for
EXAMPLE 6.7 example, revealing their location (Figure 6.18). A magnet sus-
pended by a string will turn, aligning itself in a north-south
What is the cost of operating a 100 W lightbulb for 1.00 h if the utility direction. The north-seeking pole is called the north pole of the

rate is $0.10 per kWh?
magnet. The south-seeking pole is likewise named the south pole

of the magnet. All magnets have both a north pole and a south
SOLUTION
pole, and neither pole can exist by itself. You cannot separate a

The power rating is given as 100 W, so the volt and amp units are not north pole from a south pole. If a magnet is broken into pieces, each
needed. Th erefore,
new piece will have its own north and south poles (Figure 6.19).
IV = P = 100 W You are probably familiar with the fact that two magnets
exert forces on each other. For example, if you move the north
t = 1.00 h
pole of one magnet near the north pole of a second magnet,
rate = $0.10/kWh
each will experience a repelling force. A repelling force also
cost = ?
occurs if two south poles are moved close together. But if the
(watts)(time)(rate)
__

cost = _ north pole of one magnet is brought near the south pole of a
watts

1,000 second magnet, an attractive force occurs. The rule is that like

kilowatt
magnetic poles repel and unlike magnetic poles attract.
___ A similar rule of like charges repel and unlike charges attract
(100 W)(1.00 h)($0.10/kWh)
= _
1,000 W was used for electrostatic charges, so you might wonder if there

kW is some similarity between charges and poles. The answer is that

(100)(1 .00)(0.10) W _ _ _ _ they are not related. A magnet has no effect on a charged glass
__
h
$
kW

= × × ×

1,000 1 1 kWh W rod, and the charged glass rod has no effect on either pole of a
= $0.01 magnet.
The cost of operating a 100 W lightbulb at a rate of 10¢/kWh is 1¢/h.

MAGNETIC FIELDS
EXAMPLE 6.8 A magnet moved into the space near a second magnet expe-
riences a magnetic force as it enters the magnetic fi eld of the
An electric fan draws 0.5 A in a 120 V circuit. What is the cost of oper-

ating the fan if the rate is $0.10/kWh? (Answer: $0.006, which is 0.6 of second magnet. A magnetic field can be represented by mag-

a cent per hour) netic fi eld lines. By convention, magnetic field lines are drawn to
indicate how the north pole of a tiny imaginary magnet would
154 CHAPTER 6 Electricity 6-16