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Electric Charges and
Current
Spring 2008
CHARGE!!!!
• Protons (+) will repel other protons
• Electrons (-) will repel other electrons
• Protons and Electrons are attracted to
each other.
• SO…charges that are the same
REPEL…charges that are opposite
ATTRACT!
• One BIG difference though…electric
charges can exist on their own…
Electric Fields…
• An electric charge exerts a force through
the electric field that surrounds the charge.
• The electric field extends outward from
every charged particle.
• When a charged particle is placed in the
electric field – it is either pushed or
pulled…depends on the charge.
More on Electric Fields…
• Electric fields are strongest at the source
of the charge.
• The field decreases as you move away
from the charge.
• When you add electrical charges, the
electric field is increased.
STATIC…OUCH!
• In most objects – the charge is canceled out –
depending on the atomic structure of the
object…this is why you are not shocked by
EVERYTHING!
• The build up of charges is called static electricity.
• Static electricity behaves differently than electric
current.
• In an electric current – charges move
CONTINUOUSLY…in static electricity the
charges build up , but they do not flow.
Transferring Charges…
• 3 ways charges can build up: friction, conduction,
and induction.
• Friction – rubbing an object to build up static
(balloon on hair)
• Conduction – direct contact
• Induction – caused by an electric field of another
object.
• Charges are not created or destroyed – they are
only transferred…LAW OF CONSERVATION OF
CHARGE…
STATIC CLING…
• Static electricity is responsible for static
cling. The clothing in a dryer rub against
each other and create the electricity.
• Fabric softener creates a barrier around
the clothing preventing static electricity.
• Static electricity is also responsible for
“cling wrap” and what draws the toner into
a piece of paper in a photocopier.
Static Discharge…
• When a negatively charged object and a
positively charged object are brought
together, electrons move until both objects
have the same charge.
• The loss of static electricity is called static
discharge.
• This is what you feel when someone
“shocks” you.
Factors of Static Discharge…
• Humidity can prevent static charges from
building up.
• Lightning is a dramatic example of static
discharge.
• During a storm particles move to areas of
negative to positive – the resulting factor is
a bolt of lightning.
• Most lightning charges are caused by
induction.
Detecting Charges…
• Electroscopes can detect electric charge
since they are invisible.
• The electroscope reacts only if a charge is
present.
Electrical Potential…
• Electrons in a circuit have potential energy.
• The potential energy per unit of electric
charge is called electrical potential.
VOLTAGE!!!
• Electrical charges, just like energy, move from
areas of high concentration to areas of low
concentration.
• The difference in electrical potential between 2
places is called potential difference.
• This is what provides the force to push the
charge through the circuit.
• We measure potential difference in volts, so
potential difference is commonly called
VOLTAGE!
More on VOLTAGE!
• Voltage causes current to flow through an
electric current.
• An electric current requires a device to maintain
a potential difference or voltage.
• A voltage source creates a potential difference in
an electric circuit – batteries and generators are
examples of voltage sources.
• A voltage source has 2 terminals (positive and
negative).
A Little More…
• Some voltage sources are stronger than
others…
• An increase in voltage causes a greater
flow of electric current.
Resistance…
• Current depends on more than just
voltage…it depends on resistance…
• Electrical resistance is the opposition to
the flow of charge.
• The greater the resistance, the less current
there is for a given voltage.
• Resistance depends on the thickness and
length of the wire…long wires have more
resistance…short wires less…
More on Resistance…
• Thin wires have more resistance than thick
wires…
• Resistance is also dependent upon the
type of conductor used.
• Temperature is another factor of
resistance…electrical resistance
decreases as temperature decreases – the
opposite is true as well…
Path of Least Resistance…
• When given the option – a current will flow
through the path with the least amount of
resistance.
Ohm’s LAW…
• Due to extensive research by Georg Ohm
in the 1820’s – the law governing
resistance bears his name…Ohm’s Law…
• The unit of measure for resistance is called
the ohm and it looks like this…Ω.
• In order to measure the resistance of a
wire – you need to measure 2 points of
voltage…
Ohm…continued…
• Voltage is measured with a voltmeter –
measured in amps.
• Current is measured in amps with a device
called a ammeter.
• These 2 devices are commonly combined
into a “multimeter” that can determine both.
One More Time…
• Ohm found that resistance does not solely
depend on voltage.
• Conductors will obey Ohm’s Law.
• Ohm’s Law states that the resistance is equal to
the voltage divided by the current or Ohms =
volts / amps.
• It is commonly written: R = V / I…where R =
Resistance; V = voltage; I = current.
• The formula can also be rewritten to show:
I = V / R or V = IR…if any 2 values are known,
you can solve for the third…
Key Notes about Ohm’s Law…
• If voltage is doubled – then current is
doubled.
• So, the greater the voltage – the greater
the current.
• If resistance is doubled, then the current
will be cut in half.
• So, for greater resistance, the current is
less.
Light Bulbs…
• Some resistors do not obey Ohm’s Law…
• For example, the resistance of a light bulb
increases when the bulb is turned on and
the filament heats up.
• A filament has the lowest resistance before
it heats up, so a cold filament conducts the
most current.
• This is why a bulb typically burns out the
instant you switch it on!
Series Circuits…
• If all the parts of an electric circuit are
connected one after the other, the circuit is
a series circuit.
• In a series circuit, there is only one path for
the current to take.
• For example, a switch and the device it
controls are connected in series with each
other.
Series Circuits…again…
• So what happens???
• If one light in a series circuit goes
out…they all go out!
• Also, in a series circuit, as more lights are
added the lights become dimmer. The
resistance continues to increase.
Parallel Circuits…
• In a parallel circuit, the different parts of the
circuit are on separate branches.
• In a parallel circuit, there are several paths
for circuits to take.
• If a light burns out – the rest will still stay
lit. The current still moves!
• As more paths are added, the electric
current more paths to follow, so resistance
decreases.
Hey, who turned out the…
• The circuits in your home are parallel
circuits.
• Electricity is fed into your house by heavy
wires – called lines.
• These lines have VERY LOW resistance.
They branch out to the wall sockets, etc.
• The voltage in your home is 120 volts.
• All switches are connected to a circuit
breaker box in your home.
Hey…don’t touch…
• Downed wires – chances are they are still
“live”.
• If you touch a “live” wire – you become part
of the circuit and we cannot withstand that
many volts at one time.
• A short circuit is a connection that allows
current to take an unintentional path.
No touchy!
• The soles of your shoes will provide some
protection between you and the Earth.
• The current would not be enough to kill
you, but could definitely do harm…
• Grounding wires protect people from being
shocked as well – the charge goes directly
into the Earth.
More No Touchy!
• Third prongs are actually grounding
prongs.
• Lightning rods are metal rods mounted on t
he roof of a building in order to protect it.
The rod is connected to a grounding wire.
When lightning strikes the rod, charges
flow through the rod, into the wire, and
then down to the Earth.
Keep it Safe!
• If you are caught out in an electrical storm…
• Do not stand under a big tree – it acts light a
lightning rod WITHOUT a grounding wire!
• Do not stand around holding a metal handled
umbrella – again it acts light a lightning rod and
YOU are the grounding wire!
• Stay LOW and DRY!
Fuses and Circuit Breakers…
• In order to prevent circuits from overheating, devices
called fuses and circuit breakers are added to
circuits.
• A fuse is a device that contains a thin strip of metal
that will melt if too much current flows through it.
When the fuse melts or “blows” the circuit stops the
current.
• The overload can be reset using the circuit breaker.
• Circuit Breaker is a safety device that uses
electromagnets to shut off the circuits when the
current gets too high. You can reset the circuit
breaker by flipping a switch.
Electric Shocks…
• The severity if an electric shock depends
on the current.
• A current of .01 amp is almost
undetectable, but .1amp and .2 amp can
be deadly!
• These types of current can cause an
irregular heart beat or even stop your
heart!
Electric Shock – take 2!
• The human body does have a certain
degree of resistance from electric shock.
• One factor is body tissue – living cells have
a low resistance to current cause by the
fluid in the cells that act like a conductor.
• Another factor would be if your skin is wet
or dry. You are much more likely to suffer a
shock if your skin is wet!
THE END!!