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Electric currents
Chapter 18
Electric Battery
• Made of two or more plates or rods called
electrodes.
– Electrodes are made of dissimilar metals
• Electrodes are immersed in an electrolyte
– The electrolyte will react with one metal in a way to
dissolve positive ions into the solution thus becoming
negatively charged. The other metal loses electrons to
the electrolyte becoming positively charged and a
potential difference (pressure difference) is created.
• The electrodes in an electrolyte is a cell
• Cells put together form a battery.
• battery simulation
Electrical potential and Voltage
• Electric potential (V)– the potential energy per
unit charge
– This is caused by the electrical force of repulsion.
• Voltage- a difference in electrical potential
– Considered electrical pressure
– This is measured in volts
– Flashlight batteries are 1.5V, Household outlets
are 110V or 220V
Electric current
• Current is the flow of charge through a circuit.
• I = ΔQ/Δ t
– Rate at which charge flows
• Measured in Amperes
– 1A = 1Coulomb/second
• Electron Current
– Negative electron flows from (-) to (+)
circuit simulation
• Conventional current
– Positive charge flows from (+) to (-)
Ohm’s Law/ Resistance
• For a current (flow of charge) to occur there
must be a Voltage (electric pressure)
difference. This can be produced by a battery.
• How much current there is depends on how
easily the charge can move through a circuit
(resistance).
• Current, Voltage, and resistance are all related
by Ohm’s Law
RESISTORS
• Electrical resistance is the opposition of the flow of electricity
by some object or substance.
• Resistors are used to control the flow of electricity in a circuit.
• Every conductor has some resistance depending on the
material.
• resistance simulation
• Resistance is measured in Ohms ( )
• 1  = 1V/1A
• The bands on the resistor show how much resistance the
resistor has
• Uses for resistance
– Electronic circuits (control flow of electricity)
– Toaster, heating element, variable resistors
– Lightbulbs
Ohm’s Law
• Current, Voltage, and resistance are all related
by Ohm’s Law
• V = I *R
• If you graph the relationship between voltage
and current, the slope represents the
resistance.
• Current is consistent through a circuit but
voltage changes.
Practice Ohm’s Law
• A small flashlight bulb draws 300mA from its 1.5V
battery.
– What is the resistance of the bulb?
– If the voltage dropped to 1.2V, how would the current
change.
• How many electrons would pass through the bulb in one second?
Resistivity
• How resistive a material is
– ρ = constant of proportionality/resistivity
• Measured in Ohms/meter
• Temperature dependent (α)
• ρT = ρo [1+ α(T-To)]
• Measure of the resistance in a wire.
• Depends on?
– Length, Cross-sectional area, resistivity
• R= ρ(L/A)
Resistors in series and parallel (19.1)
• When resistors are connected end to end they
are said to be in series
– Resistors can be bulbs, heating elements, …etc.
– Voltage = I Req
– For a series Circuit: Req = R1+R2+R3…
• When resistors are on their own wire they are
in parallel.
– Voltage = I Req
– For a series Circuit: 1/Req = 1/R1+1/R2+1/R3…
Practice
• Find the equivalent resistance.
R = 8 ohms
Req= 4 ohms
solution
Req=11.1 ohms
Electric Power
• Electric energy is useful because it can be
transformed into mechanical energy to do
work.
• To find the power transformed by an electrical
device: Power = energy transformed/time
QV
P
 IV
t
P  IV
P  I ( IR)  I 2R
• Measured in Watts
• Electric companies charge in kW*hrs
Practice
• Your parents yell at you for leaving the lights on in the
living room overnight. You want to find out how much
you cost them. If there were 2 75Watt bulbs running
for 7 hours and the electric company charges $0.09 per
kW*hr, how much do you owe them?