Download Unit 4 - Section 13 2013 Lesson 1 Current

Ordinary matter is made up of atoms that have
positively charged nuclei and negatively charged
electrons surrounding the nucleus.
There is NO significance to the circles used to
represent the proton and electron. The circles do
NOT imply size or shape.
Electrons and protons have a property called
"charge" which is the same size but opposite in
polarity (i.e., proton is + and electron is -). The proton
has 1836X the mass of the electron BUT exactly the
same size charge. The idea of charge suggests the
proton and electron will strongly attract each other.
Like charges (two protons or two electrons) strongly
repel each other. "Like charges repel and unlike
charges attract".
The rate of flow of electric charge is called ELECTRIC CURRENT.
Current is measured in amperes.
The conventional current direction is from high voltage to low
voltage. Thus electric current is from a high energy source to low
energy source.
When a conductor is attached to a battery, as shown in the diagram,
the electrons (blue) begin an orderly flow towards the battery’s
positive terminal to create an electric current. Note: In this diagram,
the current flows (…or moves…) from the positive terminal (right) to
the negative terminal (left).
OK…The flow of charge (…or charge in
motion…) is called CURRENT. In
science terminology, current is the rate
at which electric charges pass though
a conductor. NOTE: The charged
particle can be either positive or
negative.
RECALL – Current is identified as I and
the unit of measurement is amps (A).
So…what makes current (i.e., the
flowing charge)?
For a a charge to flow, it needs a push
(a force). The force is supplied by
VOLTAGE…also called the Potential
Difference.
Since charge flows from high to low, it
makes sense that the charge flows
from high potential energy difference to
a low potential energy difference.
An example…
Battery A has a potential of 12 V and
Battery B has a potential of 2 V. Thus,
there is a Potential Difference. That is,
A has higher potential energy than B.
This potential difference means there
is voltage.
In this case, the potential difference is
VA – VB
= 12 - 2
= 10
= 2V
So…we now know that if a potential
difference occurs between two regions
AND the regions are joined together, the
charge will flow.
Will charge continually flow?
If the two regions are joined, the charge
will move until…
(1) the force acting on it is reduced
to a minimum (i.e., the push cannot move
the charge) OR
(2) the voltage becomes the same
(i.e., Potential Difference is zero).
OK…
Now…What if Battery C and Battery D each
have a potential of 7 V. Is there voltage and
current?
There is no Potential Difference. Why?
The Potential Difference is:
VC – VD
=7-7
=0
Therefore, it has no voltage, AND it means
no flow of charge.
Going on, we know that…
Current (I) measures the amount of charge that passes
a given point every second. The unit for current is
ampere (A).
Let’s add more…
Charge (Q) is the unit of electric charge. It is
measured in coulombs (C).
Put together….
One amp (A) means that one coulomb (C) of charge
passes a point in the electric circuit every second.
Circuits require the correct amount of
voltage AND current to work.
To measure current, a current-measuring
device called an AMMETER must be inserted
into the circuit so that the current can flow
through it. The ammeter is measuring the
flow of electric current in a circuit.
A VOLTMETER measures the voltage
difference between two points in an electric
circuit.
So…we have a current flow and voltage.
How does our light bulb work?
We need RESISTANCE…or opposition to the
flow of electric current.
The unit of resistance is ohm and the
symbol is Ω.
Resistance varies in different materials.
Gold, silver and copper have low resistance
(i.e., current can flow easily through these
materials). Glass, plastics and wood have
very high resistance (i.e., current can not
pass through these materials easily).
Now the light bulb…
We have current (I), voltage (V), and finally,
resistance (R). R is the wire inside the light bulb.
The electric current flows through the wire in the
light bulb. The wire is a resistor…so it prevents
or limits the flow. This excites the electrons that
are the current. The product of the excitement is
heat and energy. The wire becomes hot, and
gives off heat and light energy.
Why doesn’t the wire burn? The wire is sealed in
a glass bulb. There is no oxygen in the bulb.
Without oxygen, there is no combustion.
The relationship between current, voltage
and resistance is known as Ohm's law.
The Ohm’s law suggests that there is a
proportional relationship between current,
voltage and resistance.
Confusing…let’s talk it through…
Let’s say current is constant but resistance
changes. Under Ohm’s law, voltage will
change is proportion to the change in
resistance. That is, voltage will double if
resistance doubles. If resistance is lowered
by 25%, the proportional change in voltage is
also a 25% decrease.
The scientific formula is…
V=IxR
That is Voltage = Current X
Resistance.
Well done…you have
learned the first part of
electricity.
I get it….
The flow of _______________________________ is called the
current and it is the rate at which electric charges pass
though a conductor.
The _________________________ measures the amount of
charge that passes a given point in each second.
What is the resistance of a circuit if voltage is 3 V and
current is 2 A.
____________________ ohm.
What is the current of a circuit if resistance is 3 ohm and
voltage is 15V?
____________________ A
What is the voltage of a circuit if resistance is 7 ohm and
current is 0.5 A?
____________________ V