Download - ELECTROMAGNETIS M LECTURE#11 Instructor: Muhammad Mateen Yaqoob.

ELECTROMAGNETISMLECTURE#11

Instructor:Muhammad Mateen Yaqoob

Resistors

Most electric circuits use circuit elements called resistors to control the current level in the various parts of the circuit.

Two common types of resistors are the composition resistor, which contains carbon, and the wire-wound resistor, which consists of a coil of wire.

Ohmic materials and devices have a linear current–potential difference relationship over a broad range of applied potential differences.

In fact, most modern electronic devices, such as transistors, have nonlinear current–potential difference relationships; their proper operation depends on the particular way in which they violate Ohm’s law.

Mateen Yaqoob Department of Computer Science

Resistor Color Code


Resistor 4-band color code



Example

What is the resistance and tolerance of each of the four-band resistors?

5.1 k W ± 5%

820 k W ± 5%

47 W ± 10%

1.0 W ± 5%

Tolerance= 0.255KΩ

4.845------------5.355

Resistance and Temperature

Over a limited temperature range, the resistivity of a conductor varies approximately linearly with temperature according to the expression


Resistance and Temperature


Superconductors

There is a class of metals and compounds whose resistance decreases to zero when they are below a certain temperature Tc , known as critical temperature. These materials are known as superconductors.

The resistance–temperature graph for a superconductor follows that of a normal metal at temperatures above Tc. When temperature is at or below Tc, resistivity drops suddenly to zero.


Superconductors

This phenomenon was discovered in 1911 by the Dutch physicist Heike Kamerlingh-Onnes (1853–1926) as he worked with mercury, which is a superconductor below 4.2 K.

Two kinds of superconductors are recognized. The more recently identified ones are essentially ceramics with high critical temperatures, whereas superconducting materials such as those observed by Kamerlingh-Onnes are metals. If a room-temperature superconductor is ever identified, its impact on technology could be tremendous.

The value of Tc is sensitive to chemical composition, pressure, and molecular structure. It is interesting to note that copper, silver, and gold, which are excellent conductors, do not exhibit superconductivity.


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.

Let us determine an expression that will allow us to calculate the rate of this energy transfer. First, consider the simple circuit in Figure, where we imagine energy is being delivered to a resistor.


Electrical Power

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 circuit element that we ignore energy delivered to the wires.

Let us consider now the rate at which the system loses electric potential energy as the charge Q passes through the resistor:


Electrical Power
