Physics of Technology PHYS 1800

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Physics of TechnologyPHYS 1800

• Lecture 33
• Electromagnetism

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PHYSICS OF TECHNOLOGY Spring 2009 Assignment
Sheet
Homework Handout
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Physics of TechnologyPHYS 1800

• Lecture 33
• Electromagnetism

Magnetism and Currents
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Magnetic Effects of Electric Currents

• Oersted discovered that a compass needle was
  deflected by a current-carrying wire.
• With the wire oriented along a north-south line,
  the compass needle deflects away from this line
  when there is current flowing in the wire.

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Magnetic Effects of Electric Currents-Right Hand
Rule

• The magnetic field produced by the current is
  perpendicular to the direction of the current.
• The magnetic field lines produced by a straight,
  current-carrying wire form circles centered on
  the wire.
• The right-hand rule gives the direction of the
  field lines with the thumb in the direction of
  the current, the fingers curl in the direction of
  the field lines produced by that current.
• The effect gets weaker as
• the compass is moved
• away from the wire.

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Magnetic Effects of Electric Currents-Right Hand
Rule

• Two parallel current-carrying wires exert an
  attractive force on each other when the two
  currents are in the same direction.
• The force is proportional to the two currents (I1
  and I2) and inversely proportional to the
  distance r between the two wires
• One ampere (A) is the amount of current
• flowing in each of two parallel wires
• separated by a distance of 1 meter that
• produces a force per unit length on each
• wire of 2 x 10-7 N/m.

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Two long parallel wires carry currents of 5 A and
10 A in opposite directions as shown. That is
the magnitude of the force per unit length
exerted by one wire on the other?
Magnetic Effects of Electric Currents-Right Hand
Rule

1. 2.0 x 10-6 N/m
2. 5.0 x 10-6 N/m
3. 2.0 x 10-4 N/m
4. 50 N/m
5. 1000 N/m

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Two long parallel wires carry currents of 5 A and
10 A in opposite directions as shown. What are
the directions of the forces on each wire?
Magnetic Effects of Electric Currents-Right Hand
Rule

1. The wires exert an attractive force on each
   other.
2. The wires exert a force repelling each other.
3. Each wire exerts a force on the other in the
   direction of the other wires current (the red
   arrows shown).
4. Each wire exerts a force on the other in the
   direction opposite to the other ones current.
5. The wires exert no force on each other.

The wires repel each other.
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Two long parallel wires carry currents of 5 A and
10 A in opposite directions as shown. What is
the total force exerted on a 30-cm length of the
10-A wire?
Magnetic Effects of Electric Currents-Right Hand
Rule

1. 2.0 x 10-6 N
2. 3.0 x 10-6 N
3. 2.0 x 10-5 N
4. 6.0 x 10-5 N
5. 2.0 x 10-4 N

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Physics of TechnologyPHYS 1800

• Lecture 33
• Electromagnetism

Magnetic Forces
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Magnetic Forces

• Magnetic forces are exerted by magnets on other
  magnets, by magnets on current-carrying wires,
  and by current-carrying wires on each other.
• The force exerted by one wire on the other is
  attractive
• when the currents are flowing in the same
  direction and
• repulsive when the currents are flowing in
  opposite
• directions.
• The magnetic force exerted on a moving charge of
  an electric current is perpendicular to both the
  velocity of the charges and to the magnetic
  field.
• This force is
• proportional to the
• quantity of the charge
• and the velocity of the
• moving charge and to
• the strength of the
• magnetic field

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Two long parallel wires carry currents of 5 A and
10 A in opposite directions as shown. What is
the strength of the magnetic field produced by
the 5-A wire at the position of the 10-A wire?
Magnetic Forces

1. 2.4 x 10-6 T
2. 2.0 x 10-5 T
3. 1.2 x 10-5 T
4. 1.2 x 10-4 T
5. 2.4 x 10-4 T

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Magnetic Forces

• For this relationship to be valid, the velocity
  must be perpendicular to the field.
• This actually defines the magnetic field as the
  force per unit charge and unit of velocity
• units 1 tesla (T) 1 N/A?m

• If the index finger of the right hand points in
  the direction of the velocity of the charge, and
  the middle finger in the direction of the
  magnetic field, then the thumb indicates the
  direction of the magnetic force acting on a
  positive charge.

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Magnetic Forces

• The force on a moving positively charged particle
  is perpendicular to the particles motion and to
  the magnetic field, just as the force on a
  current is perpendicular to the current and to
  the field.
• The force on a negative charge is in the opposite
  direction of the force on a positive charge q ?
  -q.
• Because the force is perpendicular to the
  velocity of the particle, the force does no work
  on the particle.
• It cannot increase the particles kinetic energy
  it only serves to change the direction of the
  particles motion.
• It provides a centripetal acceleration.
• If the charge is moving perpendicular to a
  uniform magnetic field, the particle will follow
  a circular path.

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Two long parallel wires carry currents of 5 A and
10 A in opposite directions as shown. What is
the direction of the magnetic field produced by
the 5-A wire at the position of the 10-A wire?
Magnetic Forces

1. Perpendicular to the plane of the page and into
   the page
2. Perpendicular to the plane of the page and out of
   the page
3. Upward
4. Downward
5. Inward toward the other wire
6. Outward away from the other wire

Perpendicular to plane of page and into page
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A straight wire with a length of 15 cm carries a
current of 4 A. The wire is oriented
perpendicularly to a magnetic field of 0.5 T.
What is the size of the magnetic force exerted on
the wire?
Magnetic Forces

1. 0.3 N
2. 0.48 N
3. 0.6 N
4. 1.0 N
5. 2.0 N

The direction of this force will be perpendicular
to both the current in the wire and to the
magnetic field, as described by the right-hand
rule.
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Physics of TechnologyPHYS 1800

• Lecture 33
• Electromagnetism

Current Loops
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Magnetic Effects of Current Loops

• When a current-carrying wire is bent into a
  circular loop, the magnetic fields produced by
  different segments of the wire add to produce a
  strong field near the center of the loop.

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Magnetic Effects of Current Loops

• The magnetic field produced by a current loop is
  identical to one produced by a short bar magnet
  (a magnetic dipole).
• In fact, in an external magnetic field, a current
  loop will experience a torque just as a bar
  magnet would.

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Magnetic Effects of Current Loops

• Consider a rectangular loop
• Each segment of the rectangular loop is a
  straight wire.
• The force on each segment is given by FIlB.
• Using the right-hand rule, you can verify that
  the loop will tend to rotate in the direction
  indicated.
• The forces on the two ends of
• the loop produce no torque
• about center of the loop,
• because their lines of action
• pass through the center of the
• loop.
• The forces on the other two
• sides combine to produce a
• torque that tends to line up the
• plane of the loop perpendicular
• to the magnetic field.

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A current-carrying rectangular loop of wire is
placed in an external magnetic field as shown.
In what direction will this loop tend to rotate
as a result of the magnetic torque exerted on it?

1. Clockwise
2. Counterclockwise

The loop will rotate counterclockwise. The
forces on the long arms are outward and because
they do not share a common line of action, impart
a counterclockwise torque on the loop.
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• Since the magnetic forces on the loop segments
  are proportional to the electric current flowing
  around the loop, the magnitude of the torque is
  also proportional to the current.

• Thus, the torque on a current-carrying coil can
  be used for measuring electric current.
• An electric meter consists of a coil of wire, a
  permanent magnet, and a restoring spring to
  return the needle to zero when there is no
  current flowing through the coil.

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• This torque is also the basis of operation for
  electric motors.
• The current must reverse directions every half
  turn to keep the coil turning.
• This can be achieved by using alternating
  current, or by using a reversing direction of dc
  current with a split ring commutator.

• One design for a simple dc motor consists of a
  wire-wound rotor mounted on an axle between the
  pole faces of a permanent magnet.
• The split ring causes the current to reverse
  directions every half turn, thus keeping the coil
  turning the same direction.

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• The magnetic field produced by a coil of wire
  will be stronger than one produced by a single
  loop carrying the same current.
• The magnetic field produced by each loop all add
  together.
• The resulting field
• strength is proportional
• to the number of turns
• N that are wound on
• the coil.
• The torque on the coil,
• when placed in an
• external magnetic field,
• is also proportional to
• both the current and
• the number of turns in
• the coil.

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Can we utilize the similarities between a
current-carrying coil of wire and a magnet?

• By winding a coil around a steel needle or nail,
  the magnetic field produced is enhanced.
• The nail then behaves like a magnet that is
  stronger than most natural magnets.
• This is an electromagnet.

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Physics of TechnologyPHYS 1800

• Lecture 33
• Electromagnetism

Faradays Law
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Faradays Law Electromagnetic Induction

• We have seen that an electric current produces a
  magnetic field.
• Can magnetic fields produce electric currents?
• Faraday tried, at first unsuccessfully, to detect
  a current in a coil as a result of a current in a
  nearby coil.
• The primary coil was connected to a battery to
  produce a current.
• The secondary coil was connected to a
  galvanometer, a device to detect magnitude and
  direction of current.

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Physics of Technology

• Next Lab/Demo Electric Circuits
• Magnetism
• Thursday 130-245
• ESLC 46
• Ch 13 and 14
• Next Class Friday 1030-1120
• BUS 318 room
• Read Ch 14