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CONCEPTS Applied diverting it into increased kinetic energy of the positive ions and
thus increased temperature of the conductor. The energy lost to

Swinging Coils resistance is therefore reduced by lowering the current, which is
what a transformer does by increasing the voltage. Hence, electric
The interactions between moving magnets and moving
charges can be easily demonstrated with two large magnets power companies step up the voltage of generated power for eco-
and two coils of wire. nomical transmission. A step-up transformer at a power plant, for
example, might step up the voltage from 22,000 volts to 500,000 volts
1. Make a coil of wire from insulated bell wire (#18 copper for transmission across the country to a city. This step up in volt-

wire) by wrapping 50 windings around a narrow jar. Tape
the coil at several places so it does not come apart. age correspondingly reduces the current, lowering the resistance
2. Now make a second coil of wire from insulated bell losses to a more acceptable 4 or 5 percent over long distances. A
wire and tape the coil as before. step-down transformer at a substation near the city reduces the
3. Suspend both coils of wire on separate ring stands or voltage to several thousand volts for transmission around the
some other support on a tabletop. The coils should city. Additional step-down transformers reduce this voltage to
hang so they will swing with the broad circle of the 120 volts for transmission to three or four houses (Figure 6.35).
coil moving back and forth. Place a large magnet on
supports so it is near the center of each coil (Box
Figure 6.2). 6.6 CIRCUIT CONNECTIONS
4. Connect the two coils of wire.
5. Move one of the coils of wire and observe what Practically all of the electricity generated by power plants is alter-
happens to the second coil. The second coil should nating current, which is stepped up, transmitted over high lines,
move, mirroring the movements of the first coil (if it and stepped down for use in homes and industry. Electric circuits
does not move, find some stronger magnets). in automobiles, cell phones, MP3 players, and laptops, on the
6. Explain what happens in terms of magnetic fields and other hand, all have direct-current circuits. Thus, most all industry

currents at the first coil and at the second coil. and household circuits are ac circuits, and most movable or por-
table circuits are dc circuits. It works out that way because of the
present need to use transformers for transmitting large currents,
u
pp
r
Support 1
which can only be done economically with alternating currents,
and because chemical batteries are the main source of current for
dc devices.
Connecting
n
e
wires s
VOLTAGE SOURCES IN CIRCUITS
Suppport 2
C Coil of
Co o
Most standard flashlights use two dry cells, each with a potential

w wi wire 1 1
r

difference of 1.5 volts. All such dry cells are 1.5 volts, no matter
how small or large they are, from penlight batteries up to much
larger D cells. To increase the voltage above 1.5 volts, the cells
must be arranged and connected in a series circuit. A series
N N N S
connection has the negative terminal of one cell connected to
Co
Coil of
the positive terminal of another cell (see Figure 6.36A ). Th e total
wi wire 2
voltage produced this way is equal to the sum of the single cell
M Magneet 1
Ma
N S + –
Dry
cell + –
Magnet 2 + – + – + –
a
g

BOX FIGURE 6.2 Why does moving one coil result in Dry Dry Dry
cell
motion of the second coil? Dry
cell + – cell
cell
Dry
cell
Energy losses in transmission are reduced by stepping up the A B
voltage. Recall that electrical resistance results in an energy loss and
FIGURE 6.36 (A) A circuit connected with batteries in series
a corresponding absolute temperature increase in the conducting
will have the same current, and the voltages add. (B) A circuit
wire. If the current is large, there are many collisions between the connected with batteries in parallel will have the same voltage in
moving electrons and positive ions of the wire, resulting in a large the circuit of the largest battery, and each battery contributes a
energy loss. Each collision takes energy from the electric fi eld, part of the total current.
6-27 CHAPTER 6 Electricity 165