R Ch 34 Electric Current pg 1
Text Qs pg 545
RQ 1, 3-19,21 ,22
R 34.1 Flow of Charge pg 2
• Charges flow in a conductor when there is a
difference in voltage (another name for
voltage is potential). The flow will continue
until both locations have the same voltage
R 34.2 Electric Current pg 3
• Electric current is the flow of electric
charge.
• Solids only electrons flow
• Liquids electrons & positive ions flow
• Gasses electrons & positive ions flow
• The overall or net charge of a conductor is
zero, as a new charge enters another charge
leaves
R 34.3 Voltage Sources pg 4
• Cells are what we call batteries. Cells
convert chemical energy to electrical energy
• Dry cell = flashlight battery
• Wet cell = car battery
• Two cells hooked together = battery
• Generators like the alternator in your car &
power plant convert mechanical energy to
electrical energy
R 34.3 Voltage Sources pg 5
• The electrons have potential energy leaving
the negative terminal heading to the positive
terminal.
• The electrons feel pressure to move. This
pressure is called the voltage or EMF
(electromotive force)
R 34.3 Voltage Sources pg 6
• At home the power company provides a 120
volts of potential difference or pressure
between the two holes in the socket.
• This means there is 120 Joules of energy
given to each Coulomb of charge (6 e18
electrons)
R 34.3 Voltage Sources pg 7
R 34.4 Electrical Resistance pg 8
• Electrical Resistance is a measure of how difficult
it is for the electrons to flow through a wire &
depends on;
•
•
•
•
1) material type, different metal has diff. resistanc
2) wire thickness, thicker wire = less resistance
3) length of wire, longer wire = more resistance
4) temperature, higher temp = more resistance
R 34.5 Ohm’s Law pg 9
• Ohm’s law is the V = I x R
R 34.5 Ohm’s Law & Electric
Shock pg 10
• Skin, shoes & clothes all provide a good
deal of resistance to current electricity. Dry
skin has about 500,000 ohms of resistance.
• If current travel through your body two
things can happen;
• 1) overheating (cooking) tissue
• 2) disrupt nerve function, stopping the heart
& or breathing
R 34.6 Ohm’s Law & Electric
Shock pg 11
• To help someone who is being electrocuted;
• 1) think
• 2) use an insulator (probably wood or plastic) to break
contact between the body and the wire
• Birds contact wires all the time but do not get
shock because;
• 1) their scaly feet are good insulators
• 2) both feet are on the same wire, there is no
difference in voltage so no current flows from one
foot to the other
R 34.7 Direct Current &
Alternating Current pg 12
• Direct Current or DC current the charges
only flow in one direction.
• Alternating Current or AC the charges
vibrate back & forth and never move out of
the wire
R 34.7 Direct Current &
Alternating Current pg 13
• In North America the electron vibrates back
and forth 60 times each second (due a change
in voltage polarity) with a 120 volts of EPE
• Europe uses a 220 volt system because it is
more efficient.
R 34.7 Direct Current &
Alternating Current pg 14
• The 60 vibrations a second is called 60
cycle or 60 Hertz electricity.
• It means that for 1 / 120th of a second the
wire is positive and for the next 1 / 120th of
a second the wire is negative.
R 34.7 Direct Current &
Alternating Current pg 15
• At your house there is a circuit breaker box
call the electric service drop.
• A 120 volt negative wire and a 120 volt
positive wire enter your house from the
neighborhood electric line.
• Normally these are kept separate but they
can be combined to make 240 volts for the
air conditioning etc.
R 34.8 Converting AC to DC
pg 16
• To convert the AC current that comes out of
the wall socket to DC needed to run
household devices requires three things;
• 1) a transformer, to lower or raise the
voltage
• 2) a diode a one-way valve to eliminate the
vibration back and forth
• 3) a capacitor to fill in the missing electrons
R 34.8 Converting AC to DC
pg 17
R 34.9 The Speed of Electrons in
a Circuit pg 18
• DC current electrons moves about 0.01 cm/sec
because the electrons keep bouncing off the
positive nucleus which are in the way.
• AC current electrons just vibrate back and forth
and don’t go anywhere
• An electric field is what really moves through the
wire at close to the speed of light
R 34.10 The Source of electrons
in a Circuit pg 19
• So the truth is electrons do not come out of
a wire into a light bulb. An electric field
comes out of the wire and pushes and or
pulls on the electrons in light bulb.
• The electric field caused the electrons to
move and do work. Just like a magnet
would push or pull a metal ball bearing.
R 34.10 The Source of electrons
in a Circuit pg 20
• When you get an electric “shock” what you
feel is the electrons in your body being
pushed & pulled 60 times each second with
120 Joules of EPE.
R 34.11 The Source of electrons
in a Circuit pg 21
• Electric power is the rate at which electric
energy is converted to heat, light or
mechanical energy.
• The electric company sells you Kilowatthours of energy, (not Joules of energy)
• So 10 100 watt light bulbs burning for one
hour is 1 kw-hr of electric energy.