Download Intro to Circuits

11/3/2014
Circuits
Properties of a Current
Topics:
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Current
Conservation of current
Batteries
Resistance and resistivity
Simple circuits
Sample question:
How can the measurement of an electric current passed through a
person’s body allow a determination of the percentage body fat?
Making the process continuous
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Crystal lattice model with free
electrons
To achieve a steady
flow of electric charge,
we need a device that
can maintain a steady
potential difference.
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Current and Electron
Flow
Conventional current is the
hypothetical flow of positive
charges that would
have the same effect in the
circuit as the movement of
negative charges that
actually does occur.
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Electric
current
• Free electrons
in a wire
drift in the direction of
the higher V field.
• Traditionally, the
direction of electric
current in a circuit is
defined as opposite to
the direction of the
electrons' drifting motion.
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Electric current
Batteries and emf
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What are the UNITS?
The physical size of a battery is not related to the emf
but to its storage capacity—the total charge it can
move before it must be replaced or recharged.
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The language of physics: Force and work
Conceptual Exercise 16.2
• You connect a
9.0-V battery to a
small motor.
Describe the
changes in
electric potential
in the circuit with
a graph.
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© 2014 Pearson Education, Inc.
Complete circuits
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Symbols for the elements in electric circuits
To check whether any circuit you build is
complete, trace the path of an imaginary
positive charge moving from the positive
terminal of the battery to the negative terminal.
The path must pass along conducting material
at every location.
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© 2014 Pearson Education, Inc.
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Ammeters
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An ammeter acts like a water flow meter.
If you wish to measure how much water flows
through a cross section of a pipe, the water must
pass through the flow meter.
An ammeter must be inserted into
a circuit so that the current passes
directly through it.
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Voltmeters
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To measure the voltage between two points
in a circuit, a voltmeter is connected between
the points.
A voltmeter measures
the electric potential
difference between two
points in a circuit.
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Using a voltmeter is
analogous to using a
pressure meter to
measure the water
pressure difference.
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Measuring current through and
potential difference across a resistive
wire
Resistance
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Resistance characterizes the degree to which
an object resists a current.
We can write the relationship between current
and potential difference using resistance R:
Or
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∆ =
The unit of resistance is called the ohm; 1 ohm
= 1 volt/ampere.
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Ohmic and non-ohmic circuit elements
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Ohm's law
If the resistance of a
circuit element does
not depend on the
potential difference
across it, the element
is called ohmic.
Circuit elements that
cannot be modeled
as ohmic devices are
called non-ohmic.
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© 2014 Pearson Education, Inc.
Testing Ohm's law
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Series Resistors
Testing experiment
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Parallel Resistors
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Joule's law
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Because potential difference and current are
related through Ohm's law, Joule's law can be
written in two alternate forms:
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