Download Electric Current - Warren County Public Schools

Voltage, Current, Resistance
and Power
Voltage



Voltage is the energy that moves the electrons
through an electric circuit.
Voltage is measured in J/C.
In an electric circuit the voltage may be
provided by a battery, a voltage source or a
potentiometer.
Cell or Battery
Longer Line indicates
positive
Direct Current
Voltage Source
Potentiometer
Electric Current

Any flow of charge is called an electric current.
Q
I
t




Q  It
Current is measured in Amperes (Amps or A).
1 Amp = 1 C/s
In electric circuits, we assume it is positive charges flowing
through the circuit (conventional current).
In reality, it is the electrons that flow. However in gases and
liquids both positive and negative charges can flow.
Example 1: Electric Current
A steady current of 4.7 A exists in a wire for 180-s.
a) How much total charge passed by a given point in the circuit during this time?
b) How many electrons was this?
The charge on one electron is 1.60 X 10-19-C {We can ignore the sign}
Electric Circuits
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

All voltage sources have a positive “pole” and a
negative “pole”.
A battery produces current by repelling the
electrons with the negative pole and attracting
electrons with the positive pole.
Current will only be produced if there is a closed
path (circuit) for the electrons to flow from the
negative pole to the positive pole.
When there is no
But once
path is
closed
path,the
a circuit
closed
the electrons
for
the electrons
to
are
repelled
by the
follow
from the
negativepole
poleofofthe
the
negative
voltage
source
voltage
source
to and
the
attracted
positive
pole to
of the
the
positive
pole
of
voltage source, the
voltage
source.
current
will
NOT
flow!!!!
Sorry, I didn’t know.
Conventional
Current
I
+
-
Example 2: Electric Circuit
Which of the set-ups below will cause the bulb to light?
No
Now
closed
therepath
is a from
path from
the negative
the negative
pole of
pole
theofbattery,
the battery,
through
through
the bulb,
the bulb
to theand
positive
then
to
pole
theofpositive
the battery.
pole.
THE BULB LIGHTS UP!!!!!
Resistors & Resistance


A property of all conductors is their ability to hinder
the flow of electric current.
This property is called resistance, R.
V
R
I


The units for resistance are ohms, Ω.
1 Ω = 1 V/A
OR
Resistor
Ohm’s Law

Georg Simon Ohm discovered that the
resistance, R, was constant in many conductors.
V  IR



This is known as Ohm’s Law.
Materials that obey Ohm’s Law are called
ohmic.
Materials that don’t obey Ohm’s Law are called
nonohmic.
Example 3: Ohm's Law
A light bulb has 120-V across it and carries 0.012-A.
a) What is the resistance of the bulb.
b) Find the current through the bulb if the voltage across it is
now 80-V.
c) What voltage would be required to push 1.0-A through the
bulb?
Electrical Power
P  VI
Electical Power is measured in Watts
If the electrical Device is an Ohmic Resistor:
PI R
2
OR
P
V2
R
Example 4: Electrical Power
A light bulb has 110-V across it and a current of 0.5-A through it
a) What is the power of this bulb?
b) What is the resistance of this bulb?