Resistance in the
Electrical System
Overview
Resistance in the Electrical
System
1. What is electrical resistance?
- opposition to the flow of electrons in
electrical circuits
- it is the most important form of
resistance in modern technology
2. How do electrons flow in a circuit?
- pushed away from the negative end and
pulled towards the positive end
Resistance in the Electrical
System
3. What causes electrons to speed up?
- the pull of the positive charge
4. What causes electrons to slow
down?
- sometimes an electron will collide with
another atom in the wire
- the electron transfers some of its
energy to the atom and slows down
Resistance in the Electrical
System
5. What is result of this collision?
- electrical resistance
6. What is the result of the atom gaining
energy from the electron?
- the wire will heat up
Resistance in the Electrical
System
7. What is Ohm’s Law?
- Electrical resistance = voltage
current
-R=V
I
- also known as V = (I)(R)
- V = voltage (volts)
- I = current (amps)
Resistance in the Electrical
System
8. What is the unit for electrical resistance?
- Ohms –
Greek letter “omega” – uppercase
9. Example – A 100-W light bulb has a current of
0.91 A when the voltage is 110 V. What is the
resistance?
R=V
I
- R = 110 / .91 = 120.9
Resistance in the Electrical
System
10. What factors effect
resistance in a wire?
- 1. Length (l) of wire
- 2. Area (A) of wire
- 3. Material (ρ) of
wire (resistivity)
Ref Table 4-2/p66
RE
l
A
Resistance in the Electrical
System
11. What is the effect of the length of
the wire?
- the longer the wire, the greater the
resistance
12. What is the effect of the area of the
wire?
- the larger the area, the lower
resistance
Resistance in the Electrical
System
13. How can you classify materials
based on their resistivities?
- all materials can fit in one of three
categories
- conductors, semiconductors, and
insulators
Resistance in the Electrical
System
14. What is a conductor?
- material with low electrical resistance
- electricity can easily flow
- these materials have several loosely
bound electrons that can travel through
the wire
- Ex. Copper (1.724), silver (1.59), gold
(2.44), solder (tin & lead)
Resistance in the Electrical
System
15. What is an insulator?
- material with very high electrical
resistance
- electrons are tightly bound to atoms,
preventing the flow of electricity
- Ex. Glass (1 x 1020), plastics, rubber,
ceramics
Resistance in the Electrical
System
16. What is a semiconductor?
- material with a mid range electrical resistance
- electrons are bound stronger than
conductors, but looser than insulators
- impurities may be added to increase or
decrease there resistance
- Ex. silicon, germanium, gallium arsenide
- used in transistors, microchips, and
integrated circuits
Resistance in the Electrical
System
17. How do you add resistors in series?
- in a series circuit, there is only one path
for current to travel
- current must travel through all the
resistors, so the resistance must be
added together
- RTOT = R1 + R2
Resistance in the Electrical
System
18. What rules must a technician follow for
series circuit?
1. Total resistance = sum of individual
resistors
2. Current through each resistor is the
same as the current through the whole
circuit
3. Sum of each of the voltages across the
resistors equal the voltage of the battery
Resistance in the Electrical
System
19. How do you add resistors in parallel?
- in a parallel circuit, current can flow
along more than one path resulting in
greater current and lower resistance
- (1/RTOT )= (1/R1) + (1/R2)
Resistance in the Electrical
System
20. What rules must a technician follow
for a parallel circuit?
1. Total resistance is less than the
smallest resistance
2. Total current is the sum of the currents
through individual resistors
3. Voltage drop is the same across each
resistor. It’s equal to the battery voltage
Resistance in the Electrical
System
21. Why is electrical resistance
important in technology?
- resistance controls the amount of
current that flows
- controlling resistance gives us the
power to control current flow
Resistance in the Electrical
System
22. What are some examples of devices
that use resistance to perform?
- light bulb – energy gained from
electrons is converted into light
- radio – volume control increases or
decreases the current
- soldering iron
- computer chips