Download Section 8.3 - Resistance PP

Physics Unit
Chapter 8 – pages 268 – 303
Chapter 8
Section 8.3 Resistance and Ohm’s law
Resistance and Ohm’s Law
What is resistance?
Resistance
Is the property of any material that slows down the flow of
electrons (i.e. a load) and converts electrical energy into
other forms of energy (i.e. light or heat).
Wires that connect the battery to a load have very little
resistance; therefore, electrons can travel through the wire
with almost no loss of electrical energy.
Resistance and Current
How does resistance relate to current?
Voltage is directly proportional to current; therefore, as
the voltage increases, so does the current (I).
Now, if you use batteries with equal voltage:
 An electric circuit that has a large resistance will allow less current to
flow through.
 An electric circuit that has lower resistance will allow more current to
flow.
Ohm’s Law
R=V
I
R = resistance (ohms - Ω)
V = voltage (volts – V)
I = current (amperes – A)
V
R
I
Unit Conversions
 Divide by 1000 to convert mA (m = milli)  A
 Times by 1000 to convert A  mA
Example
5 mA  ? A
5 mA
1000
= 0.005 A
0.095 A  ? mA
0.095 A x 1000 = 95 mA
Unit Conversions
 Divide by 1000 to convert Ω  k Ω (k = kilo)
 Times by 1000 to convert k Ω  Ω
Example
6000 Ω  ? k Ω
6000 Ω
1000
=6kΩ
20 k Ω  ? Ω
20 k Ω x 1000  20,000 Ω
Unit Conversions
 Divide by 1,000,000 to convert V to MV (mega)
 Times by 1,000,000 to convert MV to V
Example
25 MV  ? V
25 MV x 1,000,000 = 25, 000,000V
4,000,000 V  MV
4,000,000 V
1,000,000
= 4 MV
Calculations Protocol
Calculations using Ohm’s Law
1.Write formula
2.Insert numbers including proper units
3.Work vertically with the = signs lined up
4.Put a box around the final answer
Sample Calculations
Question 1.
The current through a load in a circuit is 1.5 A. If the
potential difference across the load is 12 V, what is the
resistance of the load?
R=V
I
R = 12 V
1.5A
R = 8.0 Ω
The resistance of the load is 8.0 Ω.
Sample Calculations
Question 2.
The resistance of a car headlight is 15 Ω. If there
is a current of 0.80 A through the headlight, what
is the voltage across the headlight?
V = IR
V = (0.80 A)(15 Ω)
V = 12 V
The voltage across the headlight is 12 V.
Sample Calculations
Question 3.
A 0.06 V potential difference is measured across a load
that has a resistance of 15 Ω. What is the current through
this load?
I=V
R
I = .060 V
15 Ω
I = 0.004 A
The current through this load is 0.004 A.
The Resistor
What is a Resistor?
Resistor – an electrical component that has a specific
resistance.
Units – ohms (Ω)
Used to control the current or voltage in a circuit.
When charge encounters resistance, it is transformed into
other forms of energy (i.e. heat).
Resistor Colour Code
Table 8.2 Page 297
(Accuracy)
(Accuracy)
How To Read a Resistor




First coloured band – first digit (#)
Second coloured band – second digit (#)
Third coloured band – number of zeros (multiplier)
Fourth coloured band – accuracy/tolerance (+/-)
Accuracy/Tolerance:
Gold – 5%
Silver – 10%
No colour – 20%
How To Read a Resistor
Red, Black, Red, Gold
Answer: 2000 Ω +/- 5%
How To Read a Resistor
Example: Red, blue, brown, gold
 260 Ω +/- 5%
Example: Green, orange, black, silver
 53 Ω +/- 10%