Voltage, Current, Resistance,
Ohm’s Law
10.9, 10.10
10.9 Electric Current
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Shocks from electric circuits are very
serious in comparison to shocks from
static electricity.
Every year people are injured and
sometimes die from electrocution.
Surprisingly small amounts of electric
current are lethal.
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Electric current is a measure of the
rate at which electric charges move
past a given point in a circuit.
Unit for current is Ampere (A).
Symbol in the Ohm’s Law formula for
current is (I).
Electric Current
Human Reaction
0.001A
Perception Level
0.002A
Muscles Tingle
0.005A
Max. SAFE Current
0.016A
Muscles Contract
0.050A
Ventricular Fibrillation
(Fatal)
Heart/Breathing Stop
(severe burns)
Could suffocate 50
Adults
0.2A
0.833A
Read p 314-315
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How much current do you think someone
experiences when they are electrocuted
(death by the electric chair)?
Why is it dangerous to try to help
someone who is experiencing an electric
shock? Explain what you should do if you
wish to help that person?
Page 315 - Answer questions 1, 5
ELECTRIC POTENTIAL:
VOLTAGE (V)
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A volt is a measure of electrical
pressure and is named after the Italian
scientist Alessandro Volta.
The energy each electron has is called
the electric potential. Electric potential
is commonly referred to as voltage.
CURRENT (I)
Electric current is the quantity of
electricity going by a point in a
circuit.
 Current (I) is measured with an
ammeter. The unit used to measure
electric current is the ampere,
which is named after the French
physicist A.M Ampere.
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RESISTANCE (R) (p. 316)
The molecules of all types of
conductors impede, or resist, the
flow of electrons to some extent.
 This ability to impede the flow of
electrons in conductors is called
electrical resistance (R).
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RESISTANCE
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The resistance to the passage of
electric current is measured with the
ohmmeter. The unit is the ohm, named
after the German scientist George Ohm.
INSULATORS oppose the flow of
electric current and have a high
resistance.
CONDUCTORS have a low resistance.
Factors that affect the
resistance of wires:
1.
2.
3.
4.
The thickness of wires
The length of wire
The material wire is made of
The temperature of the wire
This might help…
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An analogy to help understand these
terms is a system of plumbing pipes. The
voltage (V) is equivalent to the water
pressure, the current (I) is equivalent to
the flow rate, and the resistance (R) is like
the pipe size.
OHM’S LAW
George Ohm figured out a basic law for
electricity
V= Volts (electrical potential)
I= Amperes (current)
R= Ohms (resistance)
V = I x R
I = V / R
R = V / I
OHM’S LAW
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Current is directly proportional to
voltage, therefore, increasing (↑) the
voltage increases (↑) the current.
The current is inversely proportional to
resistance; therefore, as resistance
increases (↑), current decreases (↓).
Question
What is the formula for Ohm’s Law?
V = I x R
If a coffee grinder has a current of 1.20A and
resistance is 100Ω, what is the voltage?
120 V
Questions
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P.319 #1,2,5
A couple of extras…
If the current of a walkman is 4.7 A and the
resistance of 26Ω, what is the voltage?
If a food dehydrator has a voltage of 320V and
has a current of 18.5A, what is the
resistance?
Answers
122.2 V
17.3Ω
Chapter 10 Review Questions
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p. 328-329
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# 2, 3, 5, 7, 8, 10, 11, 13 and 14