DC Fundamentals ELTR-114-Electrical Circuits

1. DC Fundamentals
IRFAN SULTAN

2. Specific Resistance / Resistivity
Specific Resistance is the inherent property of a material. It is defined as “the resistance offered
per unit length and unit cross sectional area of that material when a known quantity of
voltage is applied at its end”
Mathematically specific resistance is given by,
𝜌 = (𝑅𝐴)/𝐿
◦ Where,
◦ p is Specific resistance/ resistivity
◦ R is Resistance
◦ A is cross sectional area of the material
◦ L is length of the material
Specific resistance of a conductor is the resistance of a conductor of unit length and unit area of
cross section.

3. Example
If a 1 m × 1 m × 1 m solid cube of
material has sheet contacts on two
opposite faces, and the resistance
between these contacts is 1 Ω, then the
resistivity of the material is 1 Ω⋅m.

4. Electrical conductivity /specific
conductance
Electrical conductivity or specific conductance is the reciprocal of electrical resistivity, and
measures a material's ability to conduct an electric current. It is commonly represented by the
Greek letter σ (sigma), but κ (kappa) (especially in electrical engineering) or γ (gamma) are also
occasionally used. Its SI unit is siemens per metre (S/m).
It is also defined as “The Conductance of a unit volume of a conductor”.

5. Resistor and its types
Fixed Resistor
◦ Carbon Composition Resistor
◦ Thick and Thin Film Resistor
◦ Metal Film Resistor
◦ Wire Wound Resistor
Variable Resistor
◦ Potentiometer
◦ Rheostat

6. Carbon Composition Resistor

8. Wire Wound Resistor

10. Potentiometer
A potentiometer is a manually adjustable variable
resistor with 3 terminals. Two terminals are connected
to both ends of a resistive element, and the third
terminal connects to a sliding contact, called a wiper,
moving over the resistive element. The position of the
wiper determines the output voltage of the
potentiometer. The potentiometer essentially
functions as a variable voltage divider.

11. Potentiometer

13. Rheostat
A rheostat is a variable resistor which is used to
control the current flowing in a circuit.