1. ELECTRICAL TECHNOLOGY EET 103/4
Define and explain the meaning of current, voltage, resistance, power, conductor, and insulator

2. Resistance (R)
Opposition to current flow in a conductor – collision and friction between free electrons, ions and atoms.
The energy used to move these charges is converted into heat and raises the temperature of the conductor.
The opposition to the current flow is known as resistance (R).
The unit for resistance is ohm ()

3. Resistance (R)
For large values kiloohm (k) and megaohm (M) can be used.
1 k = 103 
1 M = 106 
microohm () and milliohm (m) are used for small values of resistance.
1 m = 10-3 
1  = 10-6 

4. Resistance (R)
Material and its unique molecular structure will react differently to pressures to establish current through its core.
Conductors – Permit generous flow of charge
Insulators – Have high resistance

5. Resistance (R)
The resistance of any material with a uniform cross-sectional area is determined by the following factors:
Material
Length
Cross-sectional Area
Temperature

6. Resistance (R)
For a constant temperature, the resistance (R) is dependent on :
Material - resistivity ()
Length (l)
Cross-sectional Area (A)

7. Resistance (R)

8. Resistance (R)
The higher the resistivity of a conductor, the higher its resistance.
The longer the length of a conductor, the higher its resistance.
The lower the cross-sectional area of a conductor, the higher its resistance.
The higher the temperature of a conductor, the higher its resistance.

9. Resistance (R) Effect of temperature
Temperature has a significant effect on the resistance of conductors, semiconductors and insulators.
For good conductors, an increase in temperature will result in an increase in the resistance level. Consequently, conductors have positive temperature coefficients.

10. Resistance (R) Effect of temperature
For semiconductor materials, an increase in temperature will result in a decrease in the resistance level. Consequently, semiconductors have negative temperature coefficients.
As with semiconductors, an increase in temperature will result in a decrease in the resistance of an insulator. The result is a negative temperature coefficient.

11. Resistor Component used in electric circuit to control the current.
General symbol.

12. Resistor types
Resistors are made in many forms but all belong in either of two groups:
Fixed resistors – are made of metal films, high-resistance wire or carbon composition
Variable resistors – have a terminal resistance that can be varied by turning a dial, knob, screw, or anything else appropriate for the application

13. Resistor types – fixed
Film resistors construction

14. Resistor types – fixed
Film resistors types

15. Resistor types – fixed
Fixed composition resistor construction

16. Resistor types – fixed
Fixed composition resistor appearance

17. Resistor types – variable
Symbol
Rheostat connection
Rheostat symbol

19. Resistor colour code
Color coding was developed to identify resistors that were too small for their resistance value to be printed on them.
Color bands are always read from the end that has the bands closest to it.

20. Resistor colour code 1st and 2nd band represent the first two digits
3rd band determines the power-of-ten multiplier (the number of zeros following the second digit)
4th band is the manufacturer’s tolerance (precision of the resistor)

21. Resistor colour code

22. Resistor colour code – example