Transcript Electric Current

ELECTRIC
CURRENT,
VOLTAGE, AND
RESISTANCE
ELECTRICAL PRESSURE
• It is a flow of charged particles.
• Charges flow from high voltage to low voltage
• Voltage = electrical pressure that pushes
charge
• Voltage difference (V) is the push that causes
charges to move
• Voltage is measured in volts
• Flow of current stops when the differences are
equal.
• How could you keep flow moving?
FLOW OF CHARGE
• Charge will flow when there is a potential difference
between the ends of a conductor
VOLTAGE
• Provides the “electric
pressure” to move the
e- between terminals
in a circuit
• If there is a voltage
difference ACROSS a
wire, then charge will
flow THROUGH it.
• If voltage difference
increases, current
increases
BATTERIES
• Dry Cell:
• Produces voltage
difference between
positive and
negative terminals
• The voltage
difference between
the terminals
causes current to
flow though a
closed circuit
• Wet Cell:
• Contains two
connected plates
consisting of
metallic
compounds in a
conducting solution
RESISTANCE:
• The tendency for a material to oppose the flow of
electrons.
• This happens because electrical energy is changed
into thermal and light energy.
• Causes electric current to lose energy as it moves
through the filament
• Resistance is measured in ohms (Ω)
• Electrical conductors have a lower resistance than
insulators.
RESISTANCE (CONT.)
• The size, temperature, and length of the wire affects
resistance
• Electrons move more efficiently through thick wires than thin
wires (the thicker the wire, the less resistance).
• The longer the wire, the greater the resistance because
more collisions occur.
• The resistance increases as temperature increases.
ELECTRIC RESISTANCE
• Resistance decreases the amount of e- that can
flow through a conductor
• Increase resistance = decrease current
• Factors affecting resistance:
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Conductivity of material
Thickness of wire
Length of wire
temperature
ELECTRIC CURRENT:
• The rate of flow of electric charge
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Symbolized by: I (capital i)
Measured in coulombs per second
1 coulomb per second is an ampere (A)
1 C/s = 1 A
• We can measure it with a device called an
ammeter.
CLOSED CIRCUITS
• A circuit is closed, conducting path
• Electric Current is the flow of charges through a wire
or any conductor
• Current (I) is measured in Amperes (A)
• Electrons are the charges that move because they
are not locked in the nucleus but move around the
atom
OHM’S LAW:
Current = Voltage Difference
Resistance
I (A) = V(V)
R(Ω)
OHM’S LAW AND ELECTRIC SHOCK
• Touching surfaces of differing potential you
become a pathway for current
• Electric shock can overheat (cook) tissues or disrupt
nerve functions = disrupt breathing…remove with
non-conducting material and apply artificial
respiration
FUSES:
• When a fuse is blown, a small piece of metal melts if
the current becomes too high.
• This causes a break in the circuit, thus stopping the
flow of current through the overloaded circuit.
• Circuit breakers have a piece of metal that bends
instead of melting and can be reset by changing it
back to the on position.
ELECTRICAL POWER
• The rate at which electrical energy is converted to
another form of energy
• Power = current x voltage
• Watts = Amps x volts
CALCULATING ENERGY:
• Depends on two things:
• Power required
• How long they piece of equipment is used
• Energy = power x time
• E (kWh) = P (kW) x t(h)
• Unit is kilowatt-hr (kWh)
ELECTRIC CIRCUITS:
• Series:
• The current has only one
loop to flow through.
• Flashlights and some
holiday lights
• Amount of current is the
same everywhere.
• When any part of a series
circuit is disconnected,
no current flows through
the circuit (open circuit).
• Parallel:
• Contain two or more
branches for current to
move through.
• The current splits up to
flow through the different
branches.
• Voltage difference is the
same in each branch.
• More current flows
through the branches
with lower resistance
CIRCUIT DIAGRAMS (SCHEMATICS)
BATTERY/
SWITCH
RESISTOR
VOLTAGE
SOURCE
PROBLEMS
• A 110 volt wall outlet supplies power to a
strobe light with a resistance of 2200 ohms.
How much current is flowing through the
strobe light?
• 0.05 A
• A CD player with a resistance of 40 ohms
has a current of 0.1 amps flowing through it.
Sketch the circuit diagram and calculate
how many volts supply the CD player.
• 4.0 V
• A 120-volt power source supplies a lamp
with a resistance of 192 ohms. What is the
current flow of the circuit?
• 0.625 A
PROBLEMS:
• What is the resistance of the circuit conductors
when the conductor voltage drop is 3 volts and the
current flowing through the conductors is 100
amperes?
• 0.03 Ω
PROBLEMS:
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In a series circuit with two 3-ohm light bulbs
powered by a 6 V battery:
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What is the resistance in the circuit?
What is the current in the circuit?
What is the voltage drop across each resistor?
Which bulb is brightest and why?
What would happen to the following if I added
a 6-ohm light bulb after the second 3-ohm
bulb?
The resistance
The current in the circuit
The voltage drop across each resistor
Which bulb is now brightest?
PROBLEMS:
• Consider a circuit with a 2-ohm, 4-ohm and
8-ohm resistor in parallel to each other,
being supplied by a 24V battery:
• What is the voltage drop across each resistor?
• What is the current in each branch?
• 2-ohm
• 4-ohm
• 8-ohm
• Find the current in the circuit
• What is the equivalent resistance of the circuit?
I
V
R
P  IV
P  I 2R
E  I 2 Rt