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Voltage, Current, Resistance, Ohm’s Law
10.9, 10.10
Measuring Electrical Current
Shocks from electric circuits are very serious compared to
shocks from static electricity.
Surprisingly small amounts of electric current are lethal.
An electric current is:
- the rate at which electric charges move past
a given point in a circuit.
The strength of the current is measured in:
Amperes (A)
The symbol for current in Ohm’s formula is:
Current (I) is measured with an
ammeter.
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
(like a 100W light bulb)
ELECTRIC POTENTIAL: VOLTAGE (V)
A volt is a measure of electrical pressure and is named after the
Italian scientist Alessandro Volta.
The energy of each electron is called the electric potential. Electric
potential is called voltage.
A Voltmeter is used to measure voltage.
The symbol for this unit is
V.
RESISTANCE (R)
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).
The resistance to the passage of electric current is measured with the
ohmmeter.
The unit is the ohm
Ω, named after the German scientist George Ohm.
INSULATORS have a:
CONDUCTORS have a:
high resistance.
low resistance.
Factors that affect the resistance of wires:
1. The thickness of wires
2. The length of wire
3. The material of wire
4. The temperature of the wire
OHM’S LAW
George Ohm figured out a basic law for electricity:
V=
I=
R=
Volts (electrical potential) also called “voltage drop”
Amperes (current)
Ohms (resistance) Ω
V = I x R
I = V / R
R = V / I
Increasing (↑) the voltage:
increases (↑) the current.
As resistance increases (↑):
the current decreases (↓).
Ohm’s Triangle
Practicing OHM’S LAW
1.
What is the formula for Ohm’s Law?
V = I x R
2.
If a water heater has a voltage drop of 240V and a resistance of
12.8Ω, what is the current?
I=V/R
I = 240V / 12.8Ω
I = 18.75A
The water heater has a current of 18.75 amps.
2.
If a coffee grinder has a current of 1.20A and resistance is 100Ω,
what is the voltage?
V= I x R
V= 1.20A x 100 Ω
V= 120V
The coffee grinder has a voltage of 120V.
3.
If a food dehydrator has a voltage of 320V and has a current of
18.5A, what is the resistance?
R= V/I
R= 320V / 18.5A
R= 17.29 Ω
The dehydrator has a resistance of 17.29 ohms.
4. If the current of a walkman is 4.7 A and the resistance is 26Ω, what is
the voltage?
V= I x R
V= 4.7A x 26 Ω
V= 122.2V
The voltage of the walkman is 122.2V.