Thermistors

 Introduction :

Thermistor is a word formed by combining thermal with a resistor. Thermistors such as RTDs are temperature-sensitive resistors. Thermistors are non-linear devices. Their resistance will decrease with increase in temperature but at a much faster rate than RTDs. The resistance can change by more than 1000 times. As a result, thermistors can sense minute change in temperature which are undetected by RTDs and thermocouples.

Metals used in thermistors :

Thermistors are small and inexpensive devices which are most commonly made of metal oxides such as those of chromium, nickel, manganese and cobalt. The metal are oxidized through a chemical reaction, ground to a fine powder, than compressed and subjected to high heat. Theses oxides are semiconductor.

Types of thermistors :

1)Beads type

2)metallized surface-contact type

1)Beads type :

Bead types have platinum wires sintered into a ceramic body(bead)

2)metallized surface-contact type :

In contrast to bead types, lead are not sintered directly into the ceramic. Instead , the sintered ceramic is coated with a metallic contact. Either the chip manufacturer or user attaches leads to this contact. One advantage of chip thermistors over bead types is that chip are easily trimmed by cutting or grinding. Thus, they are easy to match and therefore, they are interchangeable. While matched bead thermistors are available, they more than interchangeable chip.

Application :

1)It is used in the measurement of thermal conductivity.

2)It is used in the measurement of composition

3)It is used in the measurement of flow and pressure of liquids

4) It is used in time delay circuits

D-Alembert's Principle

 D-Alembert's Principle :

Consider arigid body acted upon by a system of forces. The system may be reduced to a single  resultant force (P) acting on the body whose magnitude is given by the product of the mass of body (m) and the linear acceleration (a) of the centre of mass of the body.

Newton's Laws of Motion

Newton's Laws of Motion 

Following are the Newton's three laws of motion:

1) Newton's first law of motion. It states that everybody continues in the state of rest or of uniform motion, in a sraight line, wnless it is acted upon by some external force.

2) Newton's second law of motion. It states that the rate of change of momentum is directlyr popotional to the inmpressed force, and takes place in the same direction, in which the force acts

3)Newton s third law of motion. It states that to every action, there is always an equal and opposite resction

Laws of Friction

 Laws of Static Friction :

Following are the laws of static friction:

       1)The force of friction always acts in a direction, opposite to that in which the body tends to move

      2)The magnitude of force of friction is exactly equal to the force, which tends the body to move

      3)The magnitude of the limiting friction bears a constant ratio to the normal reaction between the two surfaces.

     4)The force of friction is independent of the area of contact between the two surfaces.

     5)The force of friction depends upon the roughness of the surfaces.

Laws of Dynamic or Kinetic Friction :

Following are the laws of dynamic or kinetic friction:

    1) The force of friction always acts in a direction, opposite to that in which the body tends to move.

    2) The magnitude of the kinetic friction bears a constant ratio to the normal reaction betweenh the two surfaces.

    3) For moderate speeds, the force of friction remains constant. But it decreases slightly with the increase of speed.

Friction

 Friction :

A force acting in the opposite direction to the motion of the body is called force offriction simply friction. It is of the following two types:

 1. Static friction; and

 2. Dynamic friction.

Static friction :

The friction, experienced by a body, when at rest, is known as static friction

Dynamic friction :

The friction experienced by a body, when in motion, is called dynamic friction. It is also call kinetic friction. It is of the following two types:

(a) Sliding friction; and

(b) Rolling friction.

Sliding friction :

The friction, experienced by a body, when it slides over another body, is known as sliding friction

Rolling friction : 

The friction experienced by a body, when balls or rollers are interposed a surfaces  known as rolling friction.