Transcript electric current

Chapter 34
Electric
Current
34.1 Flow of Charge
• Potential difference – difference
in potential (voltage) across the
ends of a conductor
• Conductor – a substance
through which electricity can
flow
Flow of Charge
• Water analogy: Water flows from high
pressure (higher position) to low pressure
(lower position) until water pressure
equalizes
• To keep water flowing, keep pumping water
back to high pressure region (higher level)
34.2 Electric Current
• Electric current – the flow
of electric charge
–Measured in AMPERES
–= 1 coulomb per second
Electric Current
• We use the symbol I for
current.
•I ~ V
• Potential Difference can
cause flow of charge
34.3 Voltage Sources
• Voltage source – an “electrical
pump” that can provide a
sustained potential difference
• Can be dry cells, wet cells or
generators
34.3 Voltage Sources
• Electromotive force – (EMF)
the potential difference
(voltage) that provides the
“electrical pressure” to move
electrons between the
terminals in a circuit
Voltage Sources
• Voltage is impressed across a
circuit, but charge flows through
the circuit
• Water Analogy (again):
Water flows from high pressure to low
pressure, but only water flows, not the
pressure.
34.4 Electrical Resistance
• Electrical resistance (R) – the
resistance of a material to the
flow of an electric current
through it
–Measured in OHMS
• Named after Georg Simon Ohm
Electrical Resistance
• Depends on:
• conductivity of the wire material
• thickness of the wire
• length of the wire
• I ~ 1/R
34.5 Ohm’s Law
voltage
• Current =
resistance
volt
Units: ampere = ohm
34.5 Ohm’s Law
• Another way to say this law:
•V = voltage
•I = current
•R = resistance
V=IR
34.6 Ohm’s Law and Electric
Shock
What causes electric shock
in the human body?
Current flowing through the
body
0.07 A can disrupt heart
rhythm and kill
34.6 Ohm’s Law and Electric
Shock
What effects current?
voltage and resistance
The voltage of the current’s
source and the resistance of
your skin
34.6 Ohm’s Law and Electric
Shock
The resistance of your skin can
range from 100 ohms to about
500,000 ohms
100 ohms if you’re soaked in salt
water
500,000 ohms if your skin is very
dry
34.6 Ohm’s Law and Electric
Shock
Why is it dangerous to work
with electrical equipment
near water?
34.6 Ohm’s Law and Electric
Shock
In order for a shock to
occur, there must be a
difference in electrical
potential between one body
part and another.
34.6 Ohm’s Law and Electric
Shock
This is why a bird can stand
on a live electrical wire and
not get electrocuted.
(Notice both feet are touching
the wire at ALL times!!)
34.7 Direct Current and
Alternating Current
• Direct current – (DC) a
flow of charge (electric
current) that is always
in one direction
–EX: battery-operated
34.7 Direct Current and
Alternating Current
• Alternating current (AC) –
electric current that rapidly
reverses direction
–EX: electricity in our homes
• Rates:
• 60 hertz (in North America)
• 50 hertz (most everywhere else)
vs.
DC
current
current
DC
AC
AC
time
time
34.8 Converting AC to DC
• Diode – a tiny electronic
device that acts as a oneway valve to allow
electron flow only in one
direction
34.9 Electron Speeds in Circuits
• Electric Fields propagate through a
conductor at near the speed of light.
• However, electrons are another matter:
in direct current they drift through the
circuit very slowly (3 hours for a meter of
wire)
in alternating current there is no net
movement at all – back and forth only
34.10 Source of Electrons
• Electric power sources provide ENERGY,
not electrons!
• Energy is carried by the electric field,
causing the electrons within an appliance to
vibrate back and forth (AC) or drift in one
direction (DC).
• Your don’t get electric shock from electrons
entering your body, but from the electric
field making your own electrons move.
34.11 Electric Power
• Electric power =
current · voltage
ampere · volt =
C/s · J/C =
J/s =
watt
Chapter 34 Vocabulary
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•
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Alternating current
Ampere
Diode
Direct current
Electrical
resistance
• Electric current
• Electric power
• Electromotive
forces
• Ohm
• Ohm’s law
• Potential
difference
• Voltage source