Download 11.4 Measuring Electrical Resistance

11.4 Measuring Electrical
Resistance
(Pages 462-467)
Learning Goals
• I can solve problems involving Ohm’s Law.
• I can explain how factors such as material,
length, thickness and temperature affect
the resistance of wires.
• I can explain how superconductors and
non-ohmic conductors work.
11.4 Measuring Electrical Resistance
KEY CONCEPTS:
Ohm’s Law states that, for most conductors, the ratio of
potential difference (V) to current (I) is a constant called the
resistance (R).
A resistor is an electrical component with a specific resistance.
The unit of electrical resistance is the ohm (Ω). One ohm is
equivalent to one volt per ampere (V/A).
11.4 Measuring Electrical Resistance
(
KEY CONCEPTS:
Four factors affect the resistance of a wire: the type of material;
the length (a longer wire has greater resistance); the diameter
(a wire with a larger diameter has lower resistance); the
temperature (a hotter wire has greater resistance).
A super conductor is a material through which electric charge
can flow with no resistance.
A non-ohmic conductor does not obey Ohm’s law
Georg Ohm, a German physicist, measured the current and potential
difference in circuits that contained metal wires. Ohm found that the
ratio of potential difference to current is a constant called resistance.
VIDEO: Electrical Resistance - Georg Ohm
Ohm’s Law
• Ohm’s law: the ratio of potential difference
to current is a constant called resistance
The Units of Resistance
• Ohm (Ω): the unit for resistance,
equivalent to one volt per ampere (V/A)
Factors That Affect the Resistance of Wires
(Page 465)
Factors that affect the resistance of a wire are:
• type of material (different metals have different resistances)
• length of wire (resistance increases proportionally with length)
• diameter or gauge of wire (thicker wire has less resistance)
• temperature (resistance increases with temperature)
Higher
gauge
equals
thinner
wire,
so 12 is
thicker
than
16.
Copyright © 2010 McGraw-Hill Ryerson Ltd.
Superconductors
(Page 466)
A superconductor is a material through which electrical charges can
flow with little or no resistance. Many metals lose their electrical
resistance when they are cooled to very low temperatures.
The superconducting magnet in the Large Hadron
Collider at CERN uses a current of 21 000A but has a
potential difference of only 9 volts.
Because of its lack of resistance, a superconducting wire is very
efficient at transporting electricity. If superconductors could be made
that functioned at room temperature, enormous amounts of electrical
energy could be conserved.
Copyright © 2010 McGraw-Hill Ryerson Ltd.
Non-ohmic Conductors
(Page 466)
Conductors that do not follow Ohm’s Law are said to be non-ohmic.
Changes in current and potential difference cause changes in
resistance.
Light bulb filaments behave in a non-ohmic manner. Their resistance
increases as temperature increases, as shown in the graph below.
Copyright © 2010 McGraw-Hill Ryerson Ltd.
Sample Problem: Determining
Resistance
Sample Problem: Determining
Resistance - Solution
Sample Problem: How Many
Cells?
Sample Problem: How Many Cells? - Solution
VIDEO: Superconductors
VIDEO: Levitating Superconductor on a Möbius strip
Practice
• Page 464 # 1-6
• Page 467 # 1-8