One of the truly remarkable features of superconductors is that once a current is
set up in them, it persists without any applied potential difference (because R " 0). Steady
currents have been observed to persist in superconducting loops for several years with
no apparent decay!
An important and useful application of superconductivity is in the development of
superconducting magnets, in which the magnitudes of the magnetic field are about
ten times greater than those produced by the best normal electromagnets. Such
superconducting magnets are being considered as a means of storing energy.
Superconducting magnets are currently used in medical magnetic resonance imaging
(MRI) units, which produce high-quality images of internal organs without the need
for excessive exposure of patients to x-rays or other harmful radiation.
For further information on superconductivity, see Section 43.8.
27.6 Electrical Power
If a battery is used to establish an electric current in a conductor, there is a continuous
transformation of chemical energy in the battery to kinetic energy of the electrons to
internal energy in the conductor, resulting in an increase in the temperature of the
conductor.
In typical electric circuits, energy is transferred from a source such as a battery, to
some device, such as a lightbulb or a radio receiver. Let us determine an expression
that will allow us to calculate the rate of this energy transfer. First, consider the simple
circuit in Figure 27.13, where we imagine energy is being delivered to a resistor.
(Resistors are designated by the circuit symbol
.) Because the connecting
wires also have resistance, some energy is delivered to the wires and some energy to the
resistor. Unless noted otherwise, we shall assume that the resistance of the wires is so
small compared to the resistance of the circuit element that we ignore the energy
delivered to the wires.
Imagine following a positive quantity of charge Q that is moving clockwise around
the circuit in Figure 27.13 from point a through the battery and resistor back to point a.
We identify the entire circuit as our system. As the charge moves from a to b through
the battery, the electric potential energy of the system increases by an amount Q !V while
the chemical potential energy in the battery decreases by the same amount. (Recall from
Eq. 25.9 that !U " q !V.) However, as the charge moves from c to d through the
resistor, the system loses this electric potential energy during collisions of electrons with
atoms in the resistor. In this process, the energy is transformed to internal energy
corresponding to increased vibrational motion of the atoms in the resistor. Because we
have neglected the resistance of the interconnecting wires, no energy transformation
occurs for paths bc and da. When the charge returns to point a, the net result is that
some of the chemical energy in the battery has been delivered to the resistor and
resides in the resistor as internal energy associated with molecular vibration.
The resistor is normally in contact with air, so its increased temperature will result
in a transfer of energy by heat into the air. In addition, the resistor emits thermal
SECTION 27.6 • Electrical Power
845
A small permanent magnet
levitated above a disk of the
superconductor YBa
2
Cu
3
O
7
, which
is at 77 K.
Courtesy of IBM Research Laboratory
At the Active Figures link
at http://www.pse6.com, you
can adjust the battery voltage
and the resistance to see the
resulting current in the circuit
and power delivered to the
resistor.
Active Figure 27.13 A circuit consisting of a resistor of
resistance R and a battery having a potential difference !V across
its terminals. Positive charge flows in the clockwise direction.
b
a
c
d
R
I
∆V
+
–
▲
PITFALL PREVENTION
27.6 Misconceptions
About Current
There are several common
misconceptions associated with
current in a circuit like that in
Figure 27.13. One is that current
comes out of one terminal of the
battery and is then “used up” as it
passes through the resistor,
leaving current in only one part
of the circuit. The truth is that
the current is the same everywhere
in the circuit. A related miscon-
ception has the current coming
out of the resistor being smaller
than that going in, because some
of the current is “used up.”
Another
misconception
has
current coming out of both
terminals of the battery, in
opposite directions, and then
“clashing” in the resistor, deliver-
ing the energy in this manner.
This is not the case—the charges
flow in the same rotational sense
at all points in the circuit.