Transcript 900 Coulombs

Electric Current


The amount of charge that moves past a given
point in a conductor per second.
I=q
t
I = current (amperes)
q = quantity of charge (coulombs)
t = time (seconds)

Ampere is the amount of current when one
coulomb of charge moves by a point in a
conductor in one second.

Calculate the amount of current through
a toaster if it takes 900 coulombs of
charge to toast two slices of bread in 1.5
minutes.

Given
t = 1.5 minutes = 90 seconds
q = 900 coulombs
q
t
=I
900 Coulombs = 10 C/s or amps
90 seconds
Measuring devices
Electric Potential

When charges are in the presence of an
electric field they have potential energy.


The charges position in the electric field
determines how much potential energy that
charge will have.
Remember like charges repel and opposite
charges attract.

Two charges will have potential energy between
them just like objects possess potential energy
when they are raised above the earth’s surface.
Potential Difference and Voltage


Moving charges in an electric field
requires work.
Voltage is the potential difference.

Voltage = work (change in potential energy)
charge
Joules/Coulomb = Volt

One volt is the electric potential
difference between two points if it takes 1
Joule of work per coulomb to move a
positive charge from one point to another.

1 volt = 1 Joule per coulomb


A 12 volt battery means that 12 Joules of work is done
on each Coulomb of charge that flows through it.
As charges flow through conductors they
lose potential energy.


The electrical energy lost or work done by a
charge traveling through a conductor can be
calculated.
E = qV like ΔPEg = Δmgh





E = energy lost or change in energy. (Joules)
q = charge (Coulombs)
V = potential difference (voltage)
q =It substituting It for q in E =qV
E = ItV




E = energy lost (Joules)
I = current (amperes)
t =time (seconds)
V = voltage (volts)
Sources of Current

Batteries


Provide a source of electrons through a
chemical reaction.
Voltaic Cells
Two metal strips called electrodes create ions
when submerged in acid (electrolyte)
 Since they are two different metals a deficit of
electrons is created at one electrode and a surplus
of electrons is created at the other electrode.
 When these terminals are connected by a wire, the
electrons flow from one electrode to the other.

Batteries

Dry cells (Duracell, Energizer etc..)





Two electrodes are used, carbon and zinc, in a
moist paste of ammonium chloride
(electrolyte).
Easier to handle
Create 1.5 Volts
Can connect several together to increase the
voltage.
Problem with voltaic and dry cell batteries
is when the electrolyte is used up the
battery no longer functions.
Solution

Secondary or rechargeable batteries



The chemical mixture used can be reversed
when current is applied.
The chemical reactants are restored.
Nickel and cadmium are commonly used in
toys
Electromagnets

Electromagnetic generators



Moving a wire through and electric field
generates current.
Called electromagnetic induction.
Powered by water, nuclear plants, geothermal
steam or burning coal.
Other Sources of Current

Piezoelctricity

Pressing on piezoelectric material induces small charges.


Thermocoupling


A device that uses different metals attached to a loop of wire to
create electrcity when heated.
Photoelectricity


Phonograph needles connected to a piezoelectric crystal pick up
vibrations due the grooves in a record and theses vibrations are
amplified.
The photoelectric effect. Einstein used light to cause metals to
release electrons.
Solar cells

Semiconductors with a “wafer like” material is treated to form a
junction. When the junction is exposed to light a small electric
potential difference is created.



In order for current to flow through a
conductor there must be a electrical
potential difference or voltage present.
This difference in potential causes charges
to flow from a high electric potential to a
lower electric potential.
The amount of current that flows through
a conductor depends on the voltage and
the material in which the conductor is
made of.
Ohm’s Law

Relates current, voltage and resistance.
V
=IR
V= voltage (volts)
R = resistance (ohms)
I = current (amperes)

Resistance

Property of a material that inhibits the flow of
charge.
Resistance

Factors that affect the resistance of a conductor:

Length of the wire


Width of the wire



Longer wires pose more resistance than shorter ones.
Narrow wires introduce more resistance than wider wires.
Composition of the material
Temperature

Higher temperatures introduce more resistance than lower
temperatures.
Electric Power

The rate at which electrical potential is
converted to other types of energy.



Electric to light energy - light bulb
Electric to mechanical energy – motor
Power = Current x Voltage

P = IV
P = power (watts)
I = current (amperes)
V = voltage (volts)
Voltmeters must always be
connected in parallel
Ammeters must always be connect
in series