Page 293 - Schaum's Outline of Theory and Problems of Applied Physics

278 ELECTRIC CURRENT [CHAP. 24

The unit of electric current is the ampere (A), where
coulomb
1 ampere = 1
second

OHM’S LAW
For a current to exist in a conductor, there must be a potential difference between its ends, just as a difference in
height between source and outlet is necessary for a river current to exist. In the case of a metallic conductor, the
current is proportional to the applied potential difference: Doubling V causes I to double, tripling V causes I to
triple, and so forth. This relationship is known as Ohm’s law and is expressed in the form
V
I =
R
potential difference
Electric current =
resistance
The quantity R is a constant for a given conductor and is called its resistance. The unit of resistance is the ohm
( ), where
volt
1 ohm = 1
ampere
The greater the resistance of a conductor, the less the current that flows in it when a certain potential difference
is applied.
Ohm’s law is not a physical principle but is an experimental relationship that most metals obey over a wide
range of values of V and I.

SOLVED PROBLEM 24.1
Since electric current is a flow of charge, why are two wires rather than a single one used to carry current?
If a single wire were used, charge of one sign or the other (depending on the situation) would be permanently
transferred from the source of current to the appliance at the far end of the wire. In a short time so much charge
would have been transferred that the source would be unable to shift further charge against the repulsive force of
the charge piled up at the appliance. Thus a single wire cannot carry a current continuously. The use of two wires,
however, enables charge to be circulated from source to appliance and back, so that a continuous one-way flow of
energy can take place.

SOLVED PROBLEM 24.2
Which solids are good electric conductors and which are good insulators? How well do these substances
conduct heat?

All metals are good electric conductors. All nonmetallic solids are good insulators, for instance, glass, wood,
plastics, rubber. In general, solids that are good conductors of electricity are also good conductors of heat, and solids
that are good electric insulators are poor conductors of heat. Metals are good conductors of heat and electricity because
both are transferred through a metal by the freely moving electrons that are a characteristic feature of its structure.

SOLVED PROBLEM 24.3

A wire carries a current of 1 A. How many electrons pass any point in the wire each second?
The electron charge is of magnitude e = 1.6×10 −19 C, and so a current of 1 A = 1 C/s corresponds to a flow of
1 C/s
18
= 6.3 × 10 electrons/s
1.6 × 10 −19 C/electron