1. RESISTORS 1

2. There’s always a price to pay!
When electric charges move from A to B, they have to overcome an obstacle called
… RESISTANCE.
Resistance imposes impediment to the flow of charge.

3. Resistance
Think about the difference between walking through air and wading through water.

4. Resistance
Electrical resistance is the property of a material that determines how easily a charge can move through it.
Most materials have some resistance.
A good conductor such as copper has low (negligible) resistance.
A good insulator like rubber has high resistance.

5. What is a RESISTOR? Resistors are very useful in circuits. WHY?
A: They allow us to control (limit) and direct electricity through the circuit.

6. RESISTORS
What is the symbol for a resistor?

7. RESISTORS What are resistors made of?
The resistors we will use are carbon resistors.
Carbon is a conductor, but we can mix it with non-conducting material (such as epoxy resin) to manipulate the resistance of the carbon.

8. RESISTANCE IN DETAIL
The atoms of the epoxy resin interfere with the movement of electrons.
The atoms that interfere with the flow of electrons are called scattering centers.

9. RESISTANCE IN DETAIL
When electrons are scattered, the overall rate of movement is called the drift speed or drift velocity.

10. 1. RESISTANCE FACTORS
What other factors might affect resistance in a material?
Which has more resistance – a thick or thin wire?
A wire that is thin has more resistance than one that is thick.

11. 2. RESISTANCE FACTORS
Which has more resistance a long or a short wire?
A wire that is long has more resistance than a short one.
Resistance also depends on the circumference (gauge), and length of the material, as well as the type of material itself.

12. Water hoses Think of 2 hoses the same length, but one is narrower.
Which one would carry more water?
A: the wider one.
A thick copper wire allows more movement of electrons i.e. less resistance.

13. 3. RESISTANCE FACTORS
Which has more resistance a cold wire or a hot wire?
Resistance increases with higher temperatures. Why?
Atoms vibrate more and interfere with the flow of electrons.
Superconductors work at absolute zero!
What are the problems with superconductors?

14. SUPERCONDUCTORS
Scientists have known for years that some materials have almost no resistance at extremely low temperatures.
Such materials are called superconductors.

15. Superconductors What is the problem with superconductors?
A: They are very efficient – little energy is wasted when electric charge flows through them.
However …
… a large amount of energy is needed to cool them, which is expensive!

16. SUPERCONDUCTORS Superconductors have many uses:
Used in power lines to increase efficiency and conserve energy;
High-speed trains to reduce friction;
Make computers work faster;
Make better motors and generators.

17. SUMMARY Resistance depends on the following factors :
The amount of resistant material or scattering centers.
The thickness of the wire.
The length of the wire.
The temperature of the material.

18. RESISTOR CODE What is a code? Now make your own quick reference card.
Plus a mnemonic to remember the colors!

19. MNEMONIC
0 BLACK 1 BEARS 2 RIP 3 OFF 4 YOUR 5 GREENS 6 BY 7 VAULTING 8 GARDEN 9 WALLS

20. RESISTORS Resistance is measured in units called… ohms….
… in honor of George Simson Ohms (1789 – 1854), the German physicist who discovered the relationship between current, voltage, and resistance =
Ohm’s Law

21. RESISTORS

22. MULTIMETERS & RESISTANCE
There are very important rules that you MUST follow :
THERE CANNOT BE ANY ELECTRICITY FLOWING THROUGH A COMPONENT WHEN YOU ARE MEASURING ITS RESISTANCE.
RESISTANCE IS MEASURED ON COMPONENTS WHEN THEY ARE NOT IN CIRCUITS.
IF ELECTRICITY IS FLOWING, YOU WILL DAMAGE THE METERS!
TURN OFF METERS BEFORE STORING THEM AT THE END OF CLASS.

23. MULTIMETERS Rotate the dial to 2000Ω.
You should see the number 1 in the display.
The number 1 indicates that the meter is ready to measure resistance.
Now touch the two probes together and observe the display.
It should read 000. Why?

24. RESISTORS Now check the resistance of your resistors.
Compute the percentage of error for each of the four resistors. How can you do this?
For example, a 150-Ω resistor (brown/green/brown) reads 148 on the meter.
The deviation is 2 Ω.
2 ÷ 150 X 100 = 1.3%
This is well within the 5% deviation range.

25. RESISTORS
What is the relationship between the amount of resistance in the circuit and the brightness of the lamp?
Use your resistors to find out …

26. RESISTORS
There is an inverse relationship or negative correlation between the brightness and resistance in a series circuit.
In other words, the light gets dimmer as the resistance increases.
The greater the resistance in series with a lamp, the dimmer it glows.

27. Inverse Relationship – Negative Correlation