Lecture 26
Chapter 23
Circuits
Chapter 24
Magnetism
Quiz 4: Monday Nov. 1; Chaps. 22,23,24
26-Oct-10
Electric Circuit
• Make electric circuit by connecting voltage
source and resistive object(s) together in a loop
with metal wires that are (nearly) perfect
conductors
Resistance
+
(light bulb)
9V
Battery
Wires
(voltage source) (assumed to be perfect conductors)
• This is a closed circuit -- there is a continuous
path through conducting material from the +
terminal to the - terminal of the voltage source
Circuit
Electric Circuit
• Any closed path around which current (electrons) can flow.
• A complete circuit has a voltage source and one or more
resistances (such as light bulbs), connected by “ideal”
wires. There may be a switch to open and close the circuit.
• “Ideal” wires have no resistance.
Electric circuit.
Water circuit.
Circuit Diagram
“Ideal” wires
Could be light bulb,
heater, etc.
+ -
“Ideal” wires
Types of Electric Circuits
Circuits
• Connected in two common ways:
– series
• one single pathway for current flow from one
terminal of the battery (or generator or wall outlet),
through each resistance in turn, and back to the
other battery terminal.
– parallel
• has two or more “branches”, each of which is a
separate path for the flow of current.
Series Circuit
Resistive objects connected one after the other in
only one loop -- in series.
Same current passes through each element.
Disconnect one of the bulbs and the circuit is broken (other bulbs go out).
26-Oct-10
Parallel Circuit
There are several different paths, or “branches” that
current can follow; branches are in parallel.
Same voltage on each bulb; current from voltage
source splits, with some going through each branch.
Disconnect
26-Oct-10 one of the bulbs and the other bulbs stay light with same brightness.
Check Yourself
How do the brightnesses of the identical
light bulbs compare (which is
brightest)?
Which bulb draws the most current?
What happens if bulb A is unscrewed?
What happens if bulb C is unscrewed?
26-Oct-10
Overloading a Circuit
Electric devices in home
connect in parallel.
More appliances added to a
parallel circuit, the more
current flows.
A large current can cause
significant ohmic heating in
the wires, which is a fire
hazard.
Protect against overloading a
circuit by adding a fuse.
26-Oct-10
Fuses & Circuit Breakers
Fuse ribbon is designed to melt (due to
ohmic heating) when current is too large.
Circuit breaker does same job without
needing replacement; flip the switch to
reconnect.
20A
20A
Fuse
Circuit
Breaker
Check Yourself
If a 1200 watt hair dryer is connected to a
120 Volt line, how much current will it
draw?
How many hairdryers can you operate
before blowing a 30 amp fuse?
26-Oct-10
Check Yourself
If a 1200 watt hair dryer is connected to a
120 Volt line, how much current will it
draw?
P = IV so I = P/V = 1200W/120V = 10A
How many hairdryers can you operate
before blowing a 30 amp fuse?
Three!
26-Oct-10
Chapter 24
Magnetism
Magnetic Field & Force
• An electric charge creates an electric field around
it.
• A second charge placed in the field feels an
electric force. Coulomb’s Law describes the force.
• A moving electric charge also creates a magnetic
field around it.
• A second moving charge placed in the magnetic
field feels a magnetic force.
• Note that a set of moving charges constitutes a
current.
Magnetic Field & Force
• Electric current produces magnetic field
• Magnetic field is a disturbance in space due to a
current (current in a wire, or atomic current)
• No “magnetic charges” (magnetic “monopoles”)
have been found. There must be a current to get
a magnetic field.
• Magnetic field and magnetic force are both
vectors -- have size and direction.
Electric Current & Magnetic Field
An electric current in a wire produces a magnetic
field.
Electric
Current
Magnetic
Field Lines
26-Oct-10
Source of Magnetic Field-Current
• Magnetic field lines near current-carrying wire
and “Right Hand Rule for magnetic field lines:
• Thumb of right hand in direction of current flow;
fingers curve along magnetic field lines
Magnetic Field from Atomic Current
• All atoms have moving charges (electrons)
– Orbital motion
– Rotational motion (“spin”)
• Moving charges
– Form current loops and generate magnetic
field
– Feel magnetic force from external magnetic
field
Electron
Magnetic
Field Lines
Magnetic Fields
CHECK YOURSELF
The source of all magnetism is
A.
B.
C.
D.
E.
electron currents in atoms.
electron currents in wires
either or both A and B.
accelerating charges
none of the above.
“Magnetic” Metals
For most atoms, the magnetic fields of the different
electrons cancel out. However, Iron, Nickel, and
Cobalt atoms produce a fairly strong magnetic
field.
Spin of the
electron in
these metals
produces a
net magnetic
field
26-Oct-10
Iron, Cobalt, Nickel
Magnetic Domains
The magnetic field of an single iron atom is so
strong that interactions among adjacent atoms
cause large clusters of atoms, called magnetic
domains, to line up their magnetic fields with one
another.
A microscopic view of
magnetic domains in a crystal
of iron. Each domain consists
of billions of aligned iron
atoms. The blue arrows
pointing in different directions
tell us that these domains are
not aligned.
26-Oct-10
Magnetic domains can be aligned by external magnetic
field. The whole object becomes “magnetized.”
Permanent Magnet
(Magnetized piece of iron, nickel, or cobalt)
• Lines of magnetic field emerge from “North pole”
(N) of magnet and return into “South pole.” (S)
Magnetic Poles
• Two types of poles North (N) and
South (S).
• N pole somewhat like + electric
charge
• S pole somewhat like - electric
charge
• In all magnets—can’t have one type
pole without the other
• No single pole (“monopole”) known
to exist
Example:
– simple bar magnet—poles at
the two ends
– horseshoe magnet: bent
U shape—poles at ends
S
N
Magnetic Force between Poles
• As with electric charges,
like magnetic poles (N&N,
S&S) repel and opposites
(N&S) attract.
• As in electric polarization,
both types of poles can
attract unmagnetized
iron, steel, nickel or
cobalt objects by forming
“induced magnets”
Induced Magnets
Unmagnetized iron can be induced to align by an
external magnetic field, forming induced magnets.
S
N S
Strong
Magnet
Strong
Magnet
N
N
Strong magnetic poles attract unlike induced
magnet poles.
Permanent/Temporary Magnets
Difference between permanent magnet and
temporary magnet
• Permanent magnet
– alignment of domains remains once external magnetic field is
removed
• Temporary magnet (induced magnet)
– alignment of domains returns to random arrangement once
external magnetic field is removed
Demo: Magnetizing Iron
Magnetic domains in iron nails are
induced to align by proximity of
the strong magnet
Each nail becomes itself a magnet,
which in turn magnetizes the nail
below it, forming a chain.
When the strong magnet is
removed, most of the domains
un-align and nail lose most of
their magnetization.
26-Oct-10
Magnetic Poles
CHECK YOURSELF
A weak and strong magnet repel each other. The
greater repelling force is by the
A.
B.
C.
D.
stronger magnet.
weaker magnet.
Both the same.
None of the above.
Electric Currents and Magnetic
Fields
Wire with loop
Electromagnets
N
Electric current in a coil of wire creates a
magnetic field similar to a bar magnet.
S
Current
passing
through
loops of
coiled
wire
N
S
N
S
26-Oct-10
Electric Currents and Magnetic Fields
CHECK YOURSELF
An electromagnet can be made stronger by
A.
B.
C.
D.
increasing the number of turns of wire.
increasing the current in the coil.
Both A and B.
None of the above.
Practical Electromagnets
Electromagnet
created by
passing current
through a coil of
wire.
Electromagnet is
stronger when
an iron bar is
inserted within
the coil.
26-Oct-10
N
Iron Bar
Wire
Coil
S
Connect to
battery or
power
supply
Physics 1 (Garcia) SJSU
Check Yourself
When an object is charged with static electricity, is
that object also magnetized?
But isn’t iron is magnetized when the electrons are
aligned. If iron has electrons, then why isn’t it
charged?
26-Oct-10
Magnetic Data Storage
• “Hard” disk drive:
• Magnetic ink on checks (& paper money)
• Videotape, credit cards, ...
Key Points of Lecture 26
• Series Circuits
• Parallel Circuits
• Fuses and overload
• Magnetic Field
• Magnets: permanent, induced, electromagnets
• Magnetic force between atomic currents (poles)
• Magnetic Force on Current in Wire
• Magnetic Data Storage
z Before Friday, read Hewitt Chap. 24 (first half).
z Homework #18 due by 11:00 PM Friday Oct. 29.
z Homework #19 due by 11:00 PM on Sunday Oct. 31.