Effect of Temperature on Resistance

The effect of temperature on the resistance of the conductor is an increase in the resistance value with an increase in temperature. The resistance of the conductor is proportional to the temperature. The increase in temperature of the conductor increases its resistance. The resistivity of the conductor changes with temperature and it causes an increase in resistance.

The resistance of the conductor changes with a change in its physical dimension. Conductor resistance is proportional to the length and inversely proportional to the area. Thus, the resistance of a particular value has a certain wire length and diameter. However, the conducting materials tend to increase their resistance with an increase in temperature. Contrary to this, the resistance of the insulating material decreases with an increase in temperature.

Effect of temperature on the resistance of the material

We will see the effect of the temperature on the resistance of the following types of materials.

Conductor
Insulator
Semiconductor

The effect of temperature on resistance is different for conductor, insulator, and semiconductor.

Effect of temperature on resistance of conductor

In a conductor, which already has a large number of free electrons flowing through it, the vibration of the atoms causes many collisions between the free electrons and the captive electrons. Each collision uses up some energy from the free electron and is the basic cause of resistance. The more the atoms jostle around in the material, the more collisions take place.

The reason for the change in resistivity is the temperature, and the temperature depends on the current flowing in the circuit. Basically, the flow of current is actually the movement of electrons from one atom to another under the influence of an electric field. Electrons are very small negatively charged particles that repel the negative electric charge and attract the positive electric charge. Therefore, under the application of electric potential electrons migrate from atom to atom, and finally towards the positive terminal.

Only some electrons are free to migrate, however, other electrons within each atom are held so tightly to their particular atom that even an electric field will not dislodge them. The current flowing in the material is therefore due to the movement of “free electrons” and the number of free electrons within any material compared with those tightly bound to their atoms is what governs whether a material is a good conductor (many free electrons) or a good insulator (hardly any free electrons).

The effect of heat on the atomic structure of a material is to make the atoms vibrate, and the higher the temperature the more violently the atoms vibrate. The vibration of the atom due to the collision of electrons produces heat. The collision hinders the path of electron flow and thus the resistance of the conductor increase with an increase in the temperature.

Effect of temperature on resistance of insulator

In an insulator, however, there is a slightly different situation. There are so few free electrons that hardly can cause current flow. Almost all the electrons are tightly bound to the nucleus of the atom. It is very difficult to break the bond at a normal temperature. Heating insulating material vibrates the atoms, and if heated sufficiently, the atoms vibrate violently enough to actually shake some of their captive electrons free, creating free electrons to become carriers of current.

The free electrons thus reach from valance band to conduction band and cause current to flow. Therefore at a high temperature, the resistance of an insulator falls, and in some insulating materials, the resistance falls quite dramatically.

Thus, the resistance of the insulator decreases with an increase in temperature.

Effect of temperature on resistance of semiconductor

In a semiconductor the resistance decrease with an increase in temperature. The resistance of the semiconductor material decreases non-linearly with an increase in temperature. Thus, the semiconductor devices exhibit non-linear or non-ohmic characteristics.

In a material where the resistance increases with an increase in temperature, the material is said to have a positive temperature coefficient.

When resistance decreases with an increase in temperature, the material is said to have a negative temperature coefficient.

In general, conductors have a positive temperature coefficient, whilst (at high temperatures) insulators have a negative temperature coefficient.

Different materials within either group have different temperature coefficients. Materials chosen for the construction of the resistors used in electronic circuits have a very low positive temperature coefficient. In use, resistors made from such materials will have only very slight increases in resistivity, and therefore their resistance. Using such materials for the manufacture of resistors creates components whose value changes only slightly over a given range of temperature. The resistance used as a heating element has a very low positive temperature coefficient.

The temperature coefficient of the resistance shows the change in the resistance with a change in the temperature.

We use this property of conductors for temperature measurement. The following materials have almost linear resistance and temperature graph. Therefore, the following materials are most suitable for temperature measurement.

Resistance temperature detector (RTD) -Pt-100,Cu-54
Thermistor

Example:

The resistance of a platinum resistance thermometer at C temperature is and at C it is . Its resistance at an unknown temperature is . Find the value of the unknown temperature.

Resistance of metal (or conductor) increases with increases in temperature and vice-versa.
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