Transcript 8.3 Resistance and Ohm`s Law

8.3 RESISTANCE AND OHM’S LAW
BC Science 9:
p. 290-301
Resistance
• Resistance is the property of
any material that slows
down the flow of electrons.
• Some resistant materials (ie.
loads) are able to convert
electrical energy into other
forms (eg. lamps).
Measuring Resistance
• Resistance can be
measured in circuit
using an ohmmeter.
• The unit of
measurement for
resistance is the ohm
(Ω).
Ohm’s Law
• Ohm’s law is an equation that shows how voltage,
current, and resistance are related.
• As the resistance in a circuit increases, the current
decreases (ie. slows down).
• Ohm’s Law:
– Voltage (V) = Current (I) x Resistance (R)
Ohm’s Law
• Example Problems:
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?
Ohm’s Law
• Example Problems:
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?
Ohm’s Law
• Examples Problems:
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?
Applications of Resistance
• Specific resistors are used to control the amount
of current in an electrical circuit.
– Eg. Volume controls, coils in electric stoves.
• Circuit resistors are marked with coloured bands
to indicate their resistance in ohms.
• The colours indicate different values of the
resistance.
Applications of Resistance
• The colour of the first band
indicates the first digit.
• The colour of the second band
indicates the second digit.
• The colour of the third band
indicates the number of zeros.
• The fourth band indicates the
percentage accuracy of the
resistor.
– Gold is 5%, silver is 10%, no
colour is 20%.
Applications of Resistance
• Examples:
– Violet, White, Red, Silver
• 7 900 ± 10% Ω
– Orange, Red, Brown, Gold
• 320 ± 5% Ω
– Red, Blue, Green
• 2 600 000 ± 20% Ω
Reducing Resistance
• Semiconductors have conductivity that varies
with temperature.
• Eg. Silicon
• Superconductors have zero resistance but require
very low temperatures.
• Eg. Mercury or lead alloys
• Variable resistors allow resistance to be adjusted.
• Eg. Dimmer switches and volume dials.
– Orange, Red, Brown, Gold
• 320 ± 5% Ω
– Red, Blue, Green