Download 8.3 * Resistance and Ohms Law - Mr Schmitt

8.3 – RESISTANCE AND OHMS LAW
SECTION 8.3 – RESISTANCE AND
OHM’S LAW
•
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
•
Voltage is directly proportional to current; therefore, as the voltage increases, so does the
current (I).
•
If an electric circuit has a large resistance, less current will flow if a battery with the same
voltage is connected to a lower resistance circuit.
OHM’S LAW
R = V/I
V
R = resistance (ohm - Ω)
V = voltage (volts – V)
I = current (amperes – A)
R
I
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
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 = 6 k Ω
20 k Ω  ? Ω
20 k Ω x 1000  20,000 Ω
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, 000,000
4,000,000 V  MV
4,000,000 V/1,000,000 = 4 MV
SAMPLE CALCULATIONS USING THE
OHM’S FORMULA
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 USING THE
OHM’S FORMULA
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 USING THE
OHM’S FORMULA
3. A 60 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 = 60 V/ 15 Ω
I = 4.0 A
The current through this load is 4.0 A.
PRACTICE PROBLEMS
•
Pg. 293 #1-3
CONVERTING PREFIXES
•
Prefixes are used to indicate the magnitude of a value
• Mili (m) represents one-thousandth (eg. 25mA = 25/1000 = 0.025A)
• Kilo (k) represents one thousand (eg. 5.0KΩ = 5000 Ω
• Mega (M) represents one million (example: 12MV = 12 000 000V)
•
Practice Problems Pg. 294 #1-3
•
Pg. 295 – Calculating Resistance
THE RESISTOR
•
Resistor – an electrical component that has a specific resistance.
•
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)
RESISTOR COLOUR CODE
Example: Red, blue, brown,
gold
• 260
Ω +/- 5%
Example: Green, orange, black,
silver
• 53
•
Ω +/- 10%
Circuit Diagrams
•
Pg. 296 Circuit Diagrams
RESISTORS
•
If a resistor has a fourth band, it represents the percentage of accuracy between the
indicated value and actual value
• Gold 5%
• Silver 10%
• No Color 20%