1. Properties of Resistors
Current of Electricity (Part 2)

2. Topics Ohm’s Law Ohmic Conductor Filament Lamp Semiconductor Diode
Resistivity

3. Previously from Part 1…
Definition of Resistance: ratio of potential difference to the Current
R = V/I
Note that this is the definition of resistance. This is always true, no matter what the values of V and I are.
We will now study something which is potentially confusing if you are not strong with this concept

4. Ohm’s Law
Ohm’s Law states that the current passing through a metallic conductor is directly proportional to the potential difference across it, provided that physical conditions (such as temperature) remains constant
Important note: Ohm’s Law only holds true if it is a metallic conductor AND temperature is constant

5. Ohmic Conductor
Current
P.D.

6. Filament Lamp
Current
P.D.

7. Filament Lamp
As current increases, the filament generates more heat, increasing the temperature
(Note: Ohm’s Law no longer applies when temperature increases)
When temperature increases, the resistance increases

8. Semiconductor Diode
A semi-conductor is a non-metal, and it does not obey Ohm’s law
Some semiconductor diodes emit light, and they are called LEDs (Light Emitting Diodes)
LEDs are prevalent in technology, recently televisions also use LED technology
An ideal diode has zero resistance in one direction, but infinite resistance in the opposite direction

9. Ideal Diode
Current
P.D.

10. Real Life Diode
Current
P.D.

11. Semiconductor Diode Diodes act as one-way only gates
They are extremely useful in electronics
We will come across diodes again when we study AC Electricity

12. Resistivity
This part of the lesson only concerns solid metal conductors
Two principles:
1) The resistance of a conductor is directly proportional to its length
2) The resistance of a conductor is inversely proportional to its cross-sectional area

13. Length
Resistance R
Length L
Resistance 2R
Length 2L

14. Cross-Sectional Area Cross-Sectional Area A Resistance R