Electric Current
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Electric Current is the flow of charges from a high electric potential energy state to a lower electric potential
energy.
This can be compared to the flow of water (WATER CURRENT) from a high
gravitational PE to a lower gravitational PE
With water this reduced potential energy can be converted to
 Kinetic energy, the water simply flows faster
and / or

Some form of work
o various mills that are water powered
o hydroelectric power from dams that store the PE
o Erosion (movement) of soil long river banks
o ECT.
Electric Current differs from water current in one very fundamental way. It has two directions for losing
potential energy depending on the charge that is flowing
 Positive charge
 Negative charge
The current flow is measured across the change in electric potential (Δ voltage or ΔV).
Recall that the traditional change in electric potential is measured from high potential to low potential by the flow
of positive (+) charges.
High
Potential
Low
Potential
+
+
-
As a positive charge naturally flows towards the negative, from high potential to low potential, left to
right in the above diagram, its change in PE is measured as
ΔPE = q(ΔV) = q (Vf-Vo)
ΔPE = (+q)(-V) = - ΔPE a reduction in PE
As a negative charge naturally flows towards the positive, from low potential to high potential, right
to left in the above diagram, its change in PE is measured as
ΔPE = q(ΔV) = q (Vf-Vo)
ΔPE = (-q)(+V) = - ΔPE a reduction in PE
In both of theses situations the lost PE is converted into
 Kinetic energy, the charge simply flows faster
and / or

Some form of work: Heat, turn electric motor, light bulb, ect.
With electric charges the negative charge (electrons) flows with much greater ease than the positive charges
(protons). The flowing negative charges are the valance electrons of the conductor. They are easily released
from one conductor atom (which is often a metal) to second conductor atom which would be found in an area
of the electric field that has a HIGHER electric potential (V).
Think about this.
Write a brief sentence stating why an electron naturally flows towards a the higher potential side of
an electric field.
Problems
1. What is the change in electric PE of a -8.00 x 10-16 C as it moves across a change in potential of
-36.0V? (+2.88 x 10-14J) Explain why the answer is positive
2. What is the change in kinetic energy for the charge listed in problem #1? (-2.88 x 10-14J)
3. A charge consisting of 3.00 x 105 electrons moves from an electric potential of 24.0 V to 54.0V.
What is the change in electric PE for this charge? (1.44 x 10-12J)
Classification of electric current
There are two types of charge flow in electric current:
 conventional current: the flow of protons through an electric field. They flow from
the electric field’s high potential to its low potential.
 electron current: (often called the electron flow) the flow of electrons through an
electric field. They flow from the electric field’s low potential to its high potential.
o Explain why the electrons flow in this manner.
Electric current is also classified by direction of charge flow.
There are two types of electric current flow:


direct current (dc). In this type of flow charges flow in one direction only
through a conductor. http://www.school-for-champions.com/science/dc.htm
alternating current (ac). In this type of current charges alternate the direction in
which they flow through a conductor.
http://www.school-for-champions.com/science/ac.htm
measurement of current flow rate (I)
measure as flow of charges per time unit
𝑞
 I=
𝑡

SI measurement is
I=
𝐶
I=
1𝐶
1 ampere = 1A
𝑠
𝑠
 1 ampere is the flow of one C of charge past a point in 1 second
Named for Andre Ampere
 1A; 1 Amp, 1mA; milliamps,
1μA; microamp,
1nA; nanoamp
Electric Current Velocity
Charges move through a conductor. (often a metal)
Charges move to reduce their PE. Therefore a negative charge (free electron) will move towards the
positive end of an electric field. (It moves to the higher potential side of the electric field)
The electric field forms between a positive plate and a negative plate or oppositely charged point
charges
If there is no potential difference between two points in a conductor there is no electric field and the
charges (free electrons) do not flow.
The free electrons have millions of collisions with the metal atoms within their conductor. This cuse
the electron movement to be completely random
Without the potential difference the free electrons move randomly. The same number of electrons
passes through a point in the conductor in both directions. There is NO net current.
With the establishment of a electric potential difference (ΔV) the electrons begin to flow through the
conductor. There movement has a net flow in the direction of the positive end of the electric field.
neg
pos
electric field
The velocity of the current is dependent on the potential difference (ΔV). The greater the ΔV the
greater the velocity for a given conductor in a given environment.
The symbol ΔV will be replaced by the symbol V for the rest of the electric lessons
The flow of free electrons is NOT directly from one end of the conductor to the other end of the
conductor. The free electrons still have millions of collisions with the atoms within the conductor
When the light switch is turned on it could take hours for the electrons at the swatch to reach the
light. The light comes on immediately. Why?
The free electrons with in the wire (conductor) all start to move (DRIFT) the same time. The cause of
their movement is the electric field which moves through the conductor at near light speed with the
closing of the light switch.
POWER SUPPLY BATTERY
general types of power supplies today
A) battery
B) electric generator
a. coal or oil fired
b. wind
c. hydroelectric
increasingly common power supplies
A) solar
Battery: converts chemical energy into electric energy
Consists of:
 two dissimilar metal electrodes that are immersed in a
 anode is positive electrode
 cathode is the negative electrode
 electrolyte solution that can conduct an electric current
 the electrodes are separated by a membrane that is impermeable to the ions that form
in the electrolyte
each electrode ends with an exposed terminal that can be connected to wire that carries electrons
through a circuit from the cathode to the anode. The wire passes through some device that extracts
energy fro the electrons as the move back towards the anode(positive terminal)
when the battery is NOT connected to a circuit the electric potential between the terminals is called
the electromotive force (emf). It is represented by the symbol (ℇ) This is measured in volts.
When the battery IS connected to a circuit the electric potential between the terminals is called the
terminal voltage. The terminal voltage (V) is the most important. It is less than the emf due to the
internal resistance of the battery as the electric current flows through it.
V = ℇ - Ir
I = electric current moving through the battery and r = internal resistance of the
battery.
The terminal voltage is some number of volts that matches the battery’s stated voltage as it leaves the
battery. The voltage MUST BE ZERO when the current returns to the battery. This will be discussed
further in the next chapter.
Terminal voltage (V)
Electromotive force (emf)(ℰ)
Multiple batteries can be connected together. They can be placed in series or in parallel
When the batteries placed in series the total voltage
is equal to the algebraic sum of all of the batteries
The current must flow through each of the batteries
to complete the circuit
Note that the +
connect to the the next battery
When using Ohm’s Law (V = IR) the voltage is the sum of
all the batteries
When the batteries of the same voltage are placed in
parallel the voltage through the circuit is the same as
any one battery.
Each of the batteries supplies only a fraction of the current
going through the circuit
Symbols for Circuits basic symbols
Note that the +
connects to the +
of the next battery
&
the – connects to
the – of the next
battery