Transcript Chap 22 PPT notes

Chapter 22, 23
Electric Current and DirectCurrent Circuits
Units of Chapter 22 and 23
• Electric Current and circuit
• Resistance and Ohm’s Law
• Energy and Power in Electric Circuits
• Resistors in Series and Parallel
•Ammeters and Voltmeters
What would happen?
• More electric potential difference - more
charges move
Video - electric current animation
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Electric current
• The flow of charge particle from one
place to another.
• The electric current is defined the rate of
flow of electric charge.
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What do you need to light a bulb?
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Electric circuits - A closed path
through which charge can flow,
returning to its starting point.
• Conventional current – the flow of + charge
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Electric Current
The direction of current flow – from the positive
terminal to the negative one.
Electric Current
A battery uses chemical reactions to produce a
potential difference between its terminals. It
causes current to flow through the flashlight bulb
similar to the way the person lifting the water
causes the water to flow through the paddle
wheel.
Electric Power
Power – the rate of energy
A generator transfers kinetic energy to
electric energy each seconds.
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The electric power
• The Electric Power – the product of the
current and the potential difference.
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Example
A 120 V motor operates at 13 A. Determine
the power and the energy used in one hour
of operation.
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Resistance and Ohm’s Law
Ohm's law states that the current through a conductor between two
points is directly proportional to the potential difference across the two
points.
Resistance and Ohm’s Law
Resistance – the property determining how much
current will flow.
Solving for the resistance, we find
The units of resistance, volts per ampere,
are called ohms:
Check Your Understanding
•
Which of the following will cause the current through an electrical
circuit to decrease? Choose all that apply.
a. decrease the voltage
b. decrease the resistance
c. increase the voltage
d. increase the resistance
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Check Your Understanding
• If the resistance of a circuit were tripled, then
the current through the circuit would be ____.
a. one-third as much
b. three times as much
c. unchanged
d. ... nonsense! There would be no way to make
such a prediction.
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Resistance and Ohm’s Law
Two wires of the same length and diameter will
have different resistances if they are made of
different materials. This property of a material is
called the resistivity.
Resistance and Ohm’s Law
The difference between
insulators, semiconductors,
and conductors can be
clearly seen in their
resistivities:
Use the Ohm's law equation to determine the
missing values in the following circuits.
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Some circuit symbols used in schematic diagrams
are shown below.
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Energy and Power in Electric Circuits
In materials for which Ohm’s law holds, the power
can also be written:
This power mostly becomes heat inside the
resistive material.
Energy and Power in Electric Circuits
When the electric company sends you a bill, your
usage is quoted in kilowatt-hours (kWh). They are
charging you for energy use, and kWh are a
measure of energy.
Ammeters and Voltmeters
An ammeter is a device for measuring current,
and a voltmeter measures voltages.
The current in the circuit must flow through the
ammeter; therefore the ammeter should have as
low a resistance as possible, for the least
disturbance.
Ammeters and Voltmeters
A voltmeter measures the potential
drop between two points in a circuit.
It therefore is connected in parallel;
in order to minimize the effect on the
circuit, it should have as large a
resistance as possible.
Summary
• Electric current is the flow of electric charge.
• Unit: ampere
• 1 A = 1 C/s
• A battery uses chemical reactions to maintain a
potential difference between its terminals.
• The potential difference between battery
terminals in ideal conditions is the emf.
• Work done by battery moving charge around
circuit:
Summary
• Direction of current is the direction positive
charges would move.
• Ohm’s law:
• Relation of resistance to resistivity:
• Resistivity generally increases with
temperature.
• The resistance of a superconductor drops
suddenly to zero at the critical temperature, TC.
Summary
• Ammeter: measures current. Is connected in
series. Resistance should be as small as possible.
• Voltmeter: measures voltage. Is connected in
parallel. Resistance should be as large as
possible.