Download Current, Voltage and Resistance

Current, Voltage and
Resistance
Electricity
Current Electricity
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What do turning on a light, turning on a
radio, and turning on your television have in
common?
In each case, you are completing an electric
circuit. In the last section, we studied static
(stationary) charges.
we are going to learn about charges that are
on the move and the paths they follow!
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Alessandro Volta’s work led to the
invention of the electric battery (voltaic
cell), which produced the first steady
flow of charged particles.
Conversion of Energy
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Generator – coverts KE to electrical
energy
Battery – converts chemical energy to
electrical energy
Photovoltaic cell – converts solar
energy to electrical energy
Electric Circuit
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a closed loop of a device
(battery, generator,
photovoltaic cell), that will
increase the PE of electric
charges, connected to a
device that reduces the PE
of the charges (radio, lamp,
hair dryer) while converting
electrical energy into a form
of "useful" energy (sound,
light, heat).
In order to keep charges
flowing, a potential
difference must be
maintained.
Electric Current
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The rate of flow of electric charge is
called electric current.
Electric current in a wire can be defined
as the net amount of charge that
passes through it per unit time at any
point.
Symbol “I” unit “amperes”,“amps ,“A”
Calculating Electric Current
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I = ΔQ
Δt
I - electric current ( C/s or A)
Q – amount if charge passing through
conductor (C )
t – time interval (s)
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Conversion tips
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1 A = 1 C/s
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Often measured in mA and μA
Sample Problem
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A steady current of 3.5 A flowed in a
wire for 2.0 minutes. How much charge
passed through the circuit?
Give it a go…
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A battery was charged using a current of
5.7 A. How long did it take to charge the
battery if 1.2 x 105 C of charge passed
through it?
( answer =5.8 h)
What is the current in amperes if 2000 Na+
ions were to flow across a cell membrane in
9.8 µs? The charge on a sodium ion is the
same as on an electron, but positive.
(answer = 3.3 x 10-11 A)
Resistance and Ohm’s Law
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Players so far: current (I) and potential
difference (V)
Georg Ohm experimentally determined
that I was proportional to V. (I  V)
For example:
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If you connect a wire to a 9 V battery, the current
will be three times what it would be if the wire
were connected to a 3 V battery.
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The amount of current that flows in a circuit
is also dependent on the resistance offered
by the circuit.
hallway example
Current (I) is inversely proportional to
resistance (R) I  1/R
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as resistance increases, current decreases
Ohm’s Law
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I = V/R
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Usually written and V = IR
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I = current (A)
V = potential difference (V)
R = resistance (Ω) (Greek letter omagha)
Resistors
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Connecting wires generally have very low
resistance compared to the coils or wire
filaments in some electrical devices like
heaters and light bulbs.
Resistors are devices designed to have a
specific resistance and are often used in
electronic devices to control the amount of
current that flows..
Rresistor
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The first two colors represent
the first two digits in the value
of resistance,
the third represents the power
of ten that it must be multiplied
by
the fourth is the manufactured
tolerance.
Red is 2,
green is 5,
orange is 103
silver indicates a tolerance of
10%.
This resistor, then, has a
resistance of 25 x 103 , or 2.5 k
, give or take 10%.
Sample Problems
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What is the resistance of a toaster if 110 V
produces a current of 3.1 A?
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A 4.5 V battery is connected to a bulb whose
resistance is 2.5 Ω. What is the current?
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A hair dryer draws 11.0 A when plugged into
a 120 V line. How much charge passes
through it in 10 min?
Resistance in a Metal Wire
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resistance of a metal
wire is directly
proportional to its
length, and inversely
proportional to its crosssectional area, A:
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R – resistance (Ω)
ρ - resistivity(Ω m)
L – length (m)
A – cross sectional area
(m2)
How to Find Cross-Sectional
Area of a Wire
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The cross-sectional area of a circular
wire can be calculated using
where d is the diameter of the wire or
where r is the radius of the
wire.
Sample Problem
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What is the resistance of a 3.5 m length
of aluminum wire 1.5mm in diameter?
Give these a go…..
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What is the length of a copper wire that has a
cross-sectional area of 3.4 x 10-6 m2 and a
resistance of 7.1 x 10-2 Ω ?
(answer – 14 m)
What is the radius of a 1.00 m length of
tungsten wire whose resistance is 0.25 Ω ?
(answer – 2.7 x 10-4 m)