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
R = resistance (ohm - Ω)
V = voltage (volts – V)
I = current (amperes – A)
V
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
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%
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