Download Electric Current and Circuits

Electric Current and Circuits
Even more of the world explained
What is current?
• Current is the flow of electric charge
• Remember, in a circuit (or most anywhere
else), it is the electrons that move.
• So for an electric circuit, it is the flow of
electrons that is the current.
• Current flows because there is a
VOLTAGE
• You can think of voltage as the pressure in
the circuit that makes the electrons move
A picture is worth a thousand
words
• K, pull out your books and open to page
532 (figure 34.1).
• In the picture on the left, water would
simply flow to the right side of the
container to equal itself out
• But if there is a pump that takes water
from the right and puts it on the left, you
can maintain a height difference between
the sides.
Current flow
• Current flows
because the
outermost electrons
of the atoms in a
conductor are “free”
• This means they can
wander away from
their atoms easily
• They move around
the other atoms in the
conductor as they
generally flow in the
same overall direction
http://www.powerworks.com.au/images/atomicflow.gif
Check for understanding
• Once again with the volunteers
Voltage Sources
• Current will not flow if there is no voltage source
• Voltage provides the “pressure” for current to
flow in a circuit
• So if you just have a plain loop of wire, even
though there are electrons in it, they will not flow
as current
• But if you put a battery into the circuit, current
will flow
• A battery is what acts as the “pump” that we saw
in figure 34.1
What creates voltages?
• Batteries create voltages by using chemicals
• You create a voltage by shuffling your feet
across the carpet and building up extra electrons
on your body
• Clouds create voltage with respect to the ground
by having much more negative charge at the
bottom of the cloud than the ground has.
• Voltages all come from a PHSYICAL
SEPARATION OF CHARGES
Man, the Internet is great…
• One instructive example of the nature of voltage is the
fact that a bird can sit on a high-voltage wire without
harm, since both of its feet are at the same voltage. You
can also see that the bird is not "grounded" -- you will not
be shocked by touching a high voltage if there is no path
for the current to reach the Earth or a different voltage
point. Typically if you touch a 120 volt circuit with one
hand, you can escape serious shock if you have
insulating shoes which prevent a low-resistance path to
ground. This fact has led to the common "hand-in-thepocket" practice for engineers and electrical workers. If
you keep one hand in your pocket when touching a
circuit which might provide a shock, you are less likely to
have the kind of path to ground which will result in a
serious shock.
http://hyperphysics.phy-astr.gsu.edu/hbase/electric/shock.html#c1
Check for Understanding
• Once again with the volunteers
Electrical resistance
• Resistance is what it
sounds like: it resists
the flow of electricity
• Electricity flows more
easily through some
materials than others
• Also, the skinnier the
wire, the greater the
resistance.
• Specially designed
resistors (right) are
used in electric
circuits
http://blog.makezine.com/img413_204-1.jpg
Ohm’s Law
• Ohm’s law is a relationship among
voltage, current and resistance:
• V = IR
• V = voltage
• I = current (don’t ask my why it isn’t “c”,
because I don’t know)
• R = resistance
Exploring Ohm’s Law
• If a 12 V battery is connected to a 24 Ohm
resistor, what is the current flowing in the
circuit?
• If Voltage is held constant and you lower
the resistance, what happens to the
current?
• If Voltage is held constant and you raise
the resistance, what happens to the
current?
Series versus Parallel
• There are two basic types of circuits:
series and parallel
• In a series circuit, there are no branches.
All the current has to go through all the
elements
• In a parallel circuit, there are branch points
and the current splits.
Examples of Each
Series
Parallel
Series and Parallel
• In a series circuit, the CURRENT stays the
same the whole time and the VOLTAGE
changes after each resistor
• In a parallel circuit, the VOLTAGE stays
the same in each branch and the current
splits up
Check for Understanding
• Once again with the volunteers
Your body’s resistance: Don’t write
this all down
•
There are a lot of factors involved and not every person has the same electrical
resistance. For instance, men tend to have lower resistance than women. Just like
for the resistors used in electronics, the resistance of a person’s arm depends on the
arm’s length and diameter. Resistance goes up with length and down with
diameter. Since men tend to have thicker arms and legs (more muscle), they usually
have lower resistance. (An implication of this is that the lethal current for men is
higher than that for women.) A rough value for the internal resistance of the human
body is 300-1,000 Ohms. Naturally, the resistance also depends on the path that
electricity takes through the body - if the electricity goes in the left hand and out the
right foot, then the resistance will be much higher than if it goes in and out of adjacent
fingers.
Within the body, the tissues with the greatest resistance are bone and fat - nerves
and muscle have the least resistance. That said, the majority of the body’s
resistance is in the skin - the dead, dry cells of the epidermis (the skin’s outer layer)
are very poor conductors. Depending on the person, the resistance of dry skin is
usually between 1,000-100,000 Ohms. The skin’s resistance is much lower if it is wet
or burnt/blistered. This means that when a person is electrocuted in real life, the
body’s resistance drops as the skin is burned. To determine a person’s total
resistance, just add together the resistance of each part of the body - remember that
the electricity must pass through the skin twice (on the way in and on the way out), so
the total resistance is:
Rtotal = Rskin(in) + Rinternal + Rskin(out)
http://van.physics.illinois.edu/qa/listing.php?id=6793
Now let’s lighten the mood some
more
Current
Effect
1 mA
Tingling
5 mA
Max
500 V
harmless
Can’t let go 1000V
10-20 mA
If body
If body
100K Ohms 1000 Ohms
100 V
1V
5V
10V
100-300 mA Heart stops 10,000V
100V
6 amps
6,000V
defibrillator
600kV
http://hyperphysics.phy-astr.gsu.edu/hbase/electric/shock.html#c3
Direct Current vs. Alternating
Current
• Direct current is the
same value all the
time and only goes in
one direction around
the circuit
• Alternating current
changes value all the
time and reverses
direction at 60
times/sec
http://www.physics4kids.com/files/art/elec_current3_240x180.gif
A note on Ground
• “Ground” has a special meaning for electricity.
• Ground always has zero voltage. So if
something has a high voltage and is then
connected to ground, current will flow to ground
• There is a ‘ground’ in a circuit and also the literal
ground outside. Both are the same for this
explanation of voltage.
• If you provide a path between a high voltage
source and the physical ground, the current will
pass through your body to get to the ground.
Wrapping it all up: Water analogy
• Let’s say you have water flowing in a
series of pipes.
• The amount of water flowing past a certain
point per second is the CURRENT
• The water pressure in the pipes (provided
by a water pump) is like the VOLTAGE
• The difficulty water has going through a
pipe (clogged, skinny pipe, whatever) is
the RESISTANCE