Physics 10
Lecture 23A
"Love and electricity are one in the same, my dear.... if
you do not feel the jolt in your soul every time a kiss is
shared, a whisper is spoken, a touch is felt, then you're
not really in love at all...."
--C. J. Franks
Current
Up until now, we have dealt with electrostatics
(i.e. charges that do not move).
We now define electric current, I, to be the
rate at which electric charge passes through a
surface or volume.
The SI unit for current is the Ampere.
Current
What causes electric current to flow?
An electric potential difference, usually
created by some sort of battery. The battery
uses chemical energy to create ΔV.
Current is defined as positive charges moving
in a certain direction.
I
+
+
+
If negative charges are actually moving, then
current is defined as moving opposite to the
motion of the negative charges.
I
–
–
–
Current
What physically flows through wires: positive
charges or negative charges?
It is the tiny electrons that move in the wire.
They move more easily than the massive
protons.
But due to historical reasons, we use positive
current when performing calculations.
How fast do electrons move in a wire under
current flow?
Not fast at all, electrons move about 0.5mm/s
in a conducting wire.
Current
Current
If electrons move so slowly, then why do lights
turn on exactly when I hit the switch?
The electron at the switch isn’t the one that
will give energy to the lights.
When you turn on a light switch you are
establishing an electric field that moves all the
free electrons in the wire circuit.
You need a closed loop with the wire. An open
loop means no electric field and no current
flow. This is known as an open circuit.
Current
There are two types of current flow.
Direct Current (DC) is current that flows in one
direction.
Alternating Current (AC) moves back and forth.
The sources of DC must be
constantly providing energy or
voltage (such as a battery).
The sources of AC must be
switching energy to push one
way, then another.
Generators and alternators
provide AC.
Current
When a battery is placed in a closed loop an
electric field is established inside the
conducting wire.
This electric field starts on the positive terminal
and ends on the negative terminal.
Positive charges will move
with the electric field
toward the negative
terminal.
When the circuit is open,
the positive charges don’t
see a negative terminal and
thus, no current.
Batteries
The cause of current flow is an electric potential
difference, ΔV.
Batteries supply energy to keep “pumping”
charge carriers.
We term this “pumping” as emf, ɛ. (It stands for
electromotive force, problem not a force).
Most materials do not like charges to flow
through them. They are said to resist current
as it tries to flow through them.
We define this property as electrical resistance.
Resistors
Resistance comes from friction (electrons
rubbing on electrons).
You can increase the resistance of a resistor by
either increasing its length (more friction) or
decreasing its area (less pathways).
A thin wire has a greater resistance than a thick
wire and a long wire has a greater resistance
than a short wire.
When current flows through a
resistor, it can heat up quickly.
Some resistors heat up so much
that they glow, this is how the
light bulb was created.
Ohm’s Law
No matter how good your conductor is, you are
always going to have a minute slowing of
current due to the conductor.
Ohm’s Law quantifies the ability of a given
material to resist the flow of charge for a
given electric potential difference.
where R is called the resistance and is
measured in Ω (Ohm’s).
[Ω] = [Volt]/[Ampere]
Ohm’s Law
Ohm’s Law is not really a law in the technical
sense. It merely defines the resistance for
certain materials.
Certain materials obey Ohm’s Law, these are
called ohmic materials.
There is a linear
relationship between
current and electric
potential.
Certain materials do not
obey Ohm’s Law, these are
called nonohmic materials.
Ohm’s Law
A good conductor will have a low resistance, this
means that current will easily flow through it.
A good insulator will have a high resistance, this
means that current will have a very hard time
flowing through it.
With Ohm’s Law, as you increase electric potential
(ΔV), then current will increase linearly.
The brightness of a light bulb will be represented
by the electric power of the light bulb:
Power = current × voltage
Household Circuits
In your house, you have parallel wiring
between your appliances.
If you had wiring in series, if one light bulb
went out the whole house would stop
consuming power (not a complete circuit).
Each appliance is independent of the others.
The power company supplies 120V of electric
potential.
Also, if current were to get too high (~100A),
the wires would physically melt.
Household Circuits
To avoid this, fuses are available on every circuit.
Fuses disconnect circuits (make them open
circuits) when current gets too high (Fuses
typically blow at about 10A).
Caution: Never become part of a household
circuit. Death can occur at about 1A.
Especially be careful of cuts on the skin around
electricity.
Dry skin has a resistance of about 1MΩ, but just
under your skin you have a resistance of about
10Ω.
Circuits
Light bulbs are resistors that you can
measure the power dissipated by them with
their brightness.
The brighter the bulb, the more power
dissipation of the light bulb.
I have two light bulbs, one 100W and one
60W. If I hook them up separately to the a
given battery which one will be brighter?
Correct, the 100W bulb was brighter!
What if I hook them up in series to the same
battery, which one will be brighter then?
Clicker Question 23A-1
I have two light bulbs, one 100W and one
60W. If I hook them up in series to the
same battery, which one will be brighter?
A) The 100W bulb.
B) The 60W bulb.
C) They will have the same brightness.
D) It doesn’t matter what power rating they have it
will always be the one that is closest to the positive
terminal of the battery.
Circuits
The 60W?????????????????
Realize that the power rating on light bulbs
only represent the power dissipated when they
are plugged into the wall.
A 100W bulb will not always put out 100W of
power dissipation.
You have to know what the potential difference
source is and what other elements are in the
circuit.
When examining a circuit you need to know all
of the elements in the circuit (think globally).
For Next Time (FNT)
Read Chapter 24.
Start the homework for Chapter
23.