19.2 resistance
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19.2 Resistance. pp. 700 - 707 Mr. Richter. Agenda. Review HW Chapter 19 Quiz Monday (second half) Notes: Resistance Ohm’s Law Human Resistance Resistors and Superconductors. Objectives: We Will Be Able To…. Understand the relationship between resistance, current and voltage. - PowerPoint PPT PresentationTRANSCRIPT
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19.2 Resistancepp. 700 - 707
Mr. Richter
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Agenda
Review HW Chapter 19 Quiz Monday (second half) Notes:
Resistance Ohm’s Law Human Resistance Resistors and Superconductors
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Objectives: We Will Be Able To… Understand the relationship between resistance,
current and voltage. Calculate using Ohm’s Law. Know what factors affect resistance. Define resistors and superconductors.
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Warm-Up:
If current is like water flowing in a river, what do you think resistance is?
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Resistance
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Resistance
What does resistance mean? Resistance is the measure of
how strongly an object inhibits the flow of electric current.
High resistance = low current, few electrons flow
Low resistance = high current, many electrons flow.
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Resistance
Every device that uses electrical energy adds to the total resistance to a circuit.
The more total resistance the circuit has, the less the current.
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Resistance
Remember, electron flow is bumpy and indirect.
The resistance of a material largely depends on (p. 701): length cross-sectional area type of material temperature
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Ohm’s Law
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Ohm’s Law
The resistance (R) of a material is the ratio of the applied voltage to the current that flows through.
This ratio is known as Ohm’s Law:
Resistance is measured in Ohms [Ω]
A.K.A.:
V = IR
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Ohm’s Law: Relationships
Voltage and current are directly proportional. With constant resistance
Resistance and current are inversely proportional. With constant voltage.
A.K.A.:
V = IR
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Human Resistance
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Human Resistance
Why will a 9-volt battery not shock you on your arm, but shock your tongue?
Human skin usually has a resistance of approximately 500,000 Ω.
A 9-volt battery creates a current of about 9/500,000 or ~1.8 x 10-5 A (0.000018 A)
Humans only feel current at about 0.01 A
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Human Resistance
The resistance of wet skin can decrease to as little as 1000Ω or even 100 Ω, or about 1000 times less resistance.
Water, and especially salt water, creates a better conductor. Lots of free electrons!
This creates as much as 0.09 amps of current. This is enough for human nerves to
sense. And hurt.
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Resistors and Superconductors
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Resistors
Resistors can be used to control the flow of current in a circuit.
Current can also be controlled by adjusting voltage, but voltage is usually constant (like a 120-volt outlet)
A resistor is an element that provides a specified resistance.
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Superconductors Superconductors have no
resistance below a critical temperature, usually near absolute zero.
Weird stuff: once current is established, it can continue even when the potential difference is removed.
This can create semi-permanent electromagnets, and super-efficient electric circuits.
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Wrap-Up: Did we meet our objectives?
Understand the relationship between resistance, current and voltage.
Calculate using Ohm’s Law. Know what factors affect resistance. Define resistors and superconductors.
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Homework
p707 #1-5, 7