Download Electric Current and Resistance

Current Electricity
Chapter 22
the charges start to move
Electric Current
• Imagine a river, how do you measure
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its current? Is it the same as speed?
Electric Current = Rate at which a
charge moves through a cross section.
I = Q/t
= Coulombs/second
= Amperes
Electric Current
• When you call someone in New York,
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how do they hear you? How does the
signal get there?
How fast do electrons travel?
There would be quite a delay.
The actual speed is called Drift
Velocity and is not the same as the
speed of the signal.
Electric Current
What if you place the
Anode and Cathode from a
battery in salt water?
Which way does current flow?
Current is defined as
the flow rate of POSITIVE CHARGE
Electric Current
There are two basic types of current
•AC and DC
•Alternating and Direct Current
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Electric Current
In a wire 10,000
electrons pass by every
nanosecond. What is
the current in the wire?
Electric Resistance
•Electrical Resistance = Ratio of potential
difference to current within a material.
•Ohm’s Law
V = IR
volt = amp * ohm
•So, what does a resistor do?
It controls current by turning electrical
energy into other forms of energy.
Electric Resistance
Determine the electrical
resistance of a toaster. It
is plugged into 115 V wall
outlet and draws a current
of 8.0 A.
Electric Power
• Power = Rate of doing work = Watt (J/s)
• P = J/C * C/s
• P = Volt * Amp = V * I = Energy from the
electric company is not measured in joules
but instead is measured in kWh = kilowatt
hours
• Watt = Joules/sec so Joules = Watt sec
Electric Power
A 40 W light bulb and a 75
W light bulb operate from
120 V.
a) Which bulb has greater current?
b) Which bulb has greater resistance?
Electric Power
In one month, your family
uses 600 kWh of electricity
from the electric company.
If electricity costs
$0.20/kWh how much will
you pay?
Electric Power
If electricity costs
$0.20/kWh, how much
money will it be if you
leave a 60W light bulb
on for a whole day?