Physics For Scientists And Engineers 6E - part 221
As an example, consider a circuit in which a toaster oven, a microwave oven, and a
coffee maker are connected (corresponding to R
1
, R
2
, and R
3
in Fig. 28.30). We can
calculate the current in each appliance by using the expression ! # I "V. The toaster
oven, rated at 1 000 W, draws a current of 1 000 W/120 V # 8.33 A. The microwave
oven, rated at 1 300 W, draws 10.8 A, and the coffee maker, rated at 800 W, draws
6.67 A. If the three appliances are operated simultaneously, they draw a total current of
25.8 A. Therefore, the circuit should be wired to handle at least this much current. If
the rating of the circuit breaker protecting the circuit is too small—say, 20 A—the
breaker will be tripped when the third appliance is turned on, preventing all three
appliances from operating. To avoid this situation, the toaster oven and coffee maker
can be operated on one 20-A circuit and the microwave oven on a separate 20-A circuit.
Many heavy-duty appliances, such as electric ranges and clothes dryers, require 240 V
for their operation. The power company supplies this voltage by providing a third wire
that is 120 V below ground potential (Fig. 28.31). The potential difference between
this live wire and the other live wire (which is 120 V above ground potential) is 240 V.
An appliance that operates from a 240-V line requires half as much current compared to
operating it at 120 V; therefore, smaller wires can be used in the higher-voltage circuit
without overheating.
Electrical Safety
When the live wire of an electrical outlet is connected directly to ground, the circuit is
completed and a short-circuit condition exists. A short circuit occurs when almost zero
resistance exists between two points at different potentials; this results in a very large
current. When this happens accidentally, a properly operating circuit breaker opens
the circuit and no damage is done. However, a person in contact with ground can be
electrocuted by touching the live wire of a frayed cord or other exposed conductor. An
exceptionally effective (and dangerous!) ground contact is made when the person
either touches a water pipe (normally at ground potential) or stands on the ground
with wet feet. The latter situation represents effective ground contact because normal,
nondistilled water is a conductor due to the large number of ions associated with
impurities. This situation should be avoided at all cost.
Electric shock can result in fatal burns, or it can cause the muscles of vital organs,
such as the heart, to malfunction. The degree of damage to the body depends on the
magnitude of the current, the length of time it acts, the part of the body touched by
the live wire, and the part of the body in which the current exists. Currents of 5 mA or
less cause a sensation of shock but ordinarily do little or no damage. If the current is
larger than about 10 mA, the muscles contract and the person may be unable to
release the live wire. If a current of about 100 mA passes through the body for only a
few seconds, the result can be fatal. Such a large current paralyzes the respiratory
SECTION 28.6 • Household Wiring and Electrical Safety
881
(a)
+120 V
–120 V
(b)
Figure 28.31 (a) An outlet for connection to a 240-V supply. (b) The connections for
each of the openings in a 240-V outlet.
George Semple