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Chapter 27
Magnetism
Exam 3 Study Guide is posted online
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26-5 Circuits Containing Resistor and
Capacitor (RC Circuits)
Example 26-12: Discharging RC circuit.
In the RC circuit shown, the battery has fully charged the capacitor, so
Q0 = C E. Then at t = 0 the switch is thrown from position a to b. The
battery emf is 20.0 V, and the capacitance C = 1.02 μF. The current I
is observed to decrease to 0.50 of its initial value in 40 μs. (a) What is
the value of Q, the charge on the capacitor, at t = 0? (b) What is the
value of R at 40 μs? (c) What is Q at t = 60 μs?
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26-5 Circuits Containing Resistor
and Capacitor (RC Circuits)
Example 26-14: Resistor in a
turn signal.
Estimate the order of magnitude
of the resistor in a turn-signal
circuit.
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26-7 Ammeters and
Voltmeters
An ammeter measures current; a voltmeter measures
voltage. Both are based on galvanometers, unless they
are digital.
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The current in a circuit passes through the
ammeter; the ammeter should have low
resistance so as not to affect the current.
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26-7 Ammeters and Voltmeters
A voltmeter should not affect the voltage
across the circuit element it is measuring;
therefore its resistance should be very large.
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26-7 Ammeters and
Voltmeters
An ohmmeter measures
resistance; it requires a
battery to provide a current.
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26-7 Ammeters and
Voltmeters
Summary: How to connect Meters?
An ammeter must be in series with
the current it is to measure;
A voltmeter must be in parallel
with the voltage it is to measure.
27-1 Magnets and Magnetic
Fields
Magnets have two
ends – poles – called
north and south.
Like poles repel;
unlike poles attract.
https://www.youtube.com/watch?v=Mp0Bu75MSj8
27-1 Magnets and Magnetic Fields
However, if you cut a magnet in half, you don’t get a
north pole and a south pole – you get two smaller
magnets.
27-1 Magnets and Magnetic
Fields
Magnetic fields can be visualized using magnetic
field lines, which are always closed loops.
Magnetic Fields
similarities with Electric Fields
Electric
•
•
•
•
Positive and Negative Charges
Like Charges repel
Opposite Charges attract
Field lines move from + to -
Magnetic
•
•
•
•
North and South poles
Like poles repel
Opposite poles attract
Field lines outside the
material move from N to S
Magnetic Fields
similarities with Electric Fields
Electric Field
•tangent to the field
lines
•the strongest where the
field lines are the closest
Magnetic Field
•tangent to the field
lines
•the strongest where the
field lines are the closest
27-1 Magnets and Magnetic Fields
The Earth’s magnetic field
is similar to that of a bar
magnet.
Note that the Earth’s
“North Pole” is really a
south magnetic pole, as
the north ends of magnets
are attracted to it.
Canadian Artic
27-1 Magnets and Magnetic Fields
A uniform magnetic field is constant in
magnitude and direction.
The field between
these two wide poles
is nearly uniform.
27-2 Electric Currents Produce
Magnetic Fields
https://www.youtube.com/watch?v=tKxFLH2Nhe4
27-2 Electric Currents Produce
Magnetic Fields
Experiment shows that an electric current
produces a magnetic field. The direction of the
field is given by a right-hand rule.
27-2 Electric Currents Produce
Magnetic Fields
Here we see the field
due to a current
loop; the direction is
again given by a
right-hand rule.
27-2 Electric Currents Produce
Magnetic Fields
https://www.youtube.com/watch?v=F1PWnu01IQg
B
A magnet exerts a
force on a currentcarrying wire. The
direction of the force
is given by a right-hand
rule.
27-3 Force on an Electric Current in a
B
Magnetic Field; Definition of B
The force on the wire depends on the
current, the length of the wire, the
magnetic field, and its orientation:
In vector notation:
27-3 Force on an Electric Current in a
Magnetic Field; Definition of B B
Unit of B: the tesla, T:
1 T = 1 N/A·m.
Another unit sometimes used: the gauss (G):
1 G = 10-4 T.
Directions of the Magnetic Field:
27-3 Force on an Electric Current in
a Magnetic Field; Definition of B B
Example 27-1: Magnetic Force on a
current-carrying wire.
A wire carrying a 30-A current has a
length l =12 cm between the pole faces of
a magnet at an angle θ = 60, as shown.
The magnetic field is approximately uniform
at 0.90 T. We ignore the field beyond the
pole pieces. What is the magnitude and
direction of the force on the wire?
Problem 8
8.(II) A long wire stretches along the x axis
and carries a 3.0-A current to the right (+x).
The wire is in a uniform magnetic field
B  0.20 ˆi  0.36 ˆj  0.25 kˆ T.
Determine the components of the force on the
wire per cm of length.


27-3 Force on an Electric Current in
a Magnetic Field; Definition of B B
Example 27-2: Measuring a magnetic field.
A rectangular loop of wire hangs vertically as shown. A
B is directed horizontally, perpendicular
magnetic field B
to the wire, and points out of the page at all points.
The magnetic field is very nearly uniform along the
horizontal portion of wire ab (length l = 10.0 cm) which
is near the center of the gap of a large magnet
producing the field. The top portion of the wire loop is
free of the field. The loop hangs from a balance which
measures a downward magnetic force (in addition to the
gravitational force) of F = 3.48 x 10-2 N when the wire
carries a current I = 0.245 A. What is the magnitude
of the magnetic field B?