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Electromagnetism 2
SPH4U – Grade 12 Physics
Unit 1
Tribal Challenge

a)
Determine the direction of the electric current in
(a) and the force on the wire in (b)
b)
Tribal Challenge

a)
N
Determine the direction of the electric current in
(a) and the force on the wire in (b)
b)
Review – Mini Quiz from last day




Moving electric charges will create a magnetic
field (T / F)
Moving magnetic fields will create an electric
current (T / F)
The magnetic field around a solenoid resembles
a bar magnet (T / F)
A current carrying wire in a magnetic field will
experience a force if you place it in an external
magnetic field. (T / F)
Review – Mini Quiz from last day




Moving electric charges will create a magnetic
field (T / F)
Moving magnetic fields will create an electric
current (T / F)
The magnetic field around a solenoid resembles
a bar magnet (T / F)
A current carrying wire in a magnetic field will
experience a force if you place it in an external
magnetic field. (T / F)
Calculating Magnetic Force

From last day, we learned that a current carrying
wire would have it’s own magnetic field, and
that it would experience a force in the presence
of an external magnetic field.

Today we will discuss how to calculate that
force.
Measuring Magnetic Field
Strength

Measuring Magnetic Field
Strength

Some common magnetic fields:

common refrigerator magnet: 0.001 Tesla
 magnetic field near the Earth’s surface:
5 x 10-5 Tesla
The force on a moving charge


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As we discussed last day, a current carrying wire
will experience a force in the presence of a
magnetic field.
This is because there are moving electrons
that make up the current that cause another
magnetic field to be created.
We said that this is because electricity and
magnetism are both fundamentally caused by
electric charges.
The force on a moving charge

In truth, magnetic forces act on all types of
moving charges: electrons, protons, and ions.

When one of these moves in the presence of a
magnetic field, there will be a magnetic force
exerted on it, as long as the charge is in
motion.
The force on a moving charge

The force on a moving charge

The force on a moving charge

Right hand rule:
If you point your right thumb in
the direction of the velocity of
the charge and your straight
fingers in the direction of the
magnetic field then your palm
will point in the direction of the
resulting magnetic force. This
gives the direction when the
charge is positive. If the
charge is negative, the particle
will go in the opposite
direction.
The force on a moving charge

The magnetic force will always
be directed at a 90º angle to
the velocity of the particle and
the magnetic field. That is
independent of θ, the angle
between the magnetic field and
the velocity of the particle. The
angle θ only helps determine
the magnitude of the force.
Example 1

An electron moving at a velocity of 6.7x106 m/s
[E] enters a magnetic field with a magnitude of
2.3T directed at an angle of 47º to the direction
of motion and upward in the vertical plane. Find
the magnitude and direction of the magnetic
force on the electron.
Example 1

An electron moving at a velocity of 6.7x106 m/s [E] enters a magnetic field with a
magnitude of 2.3T directed at an angle of 47º to the direction of motion and upward
in the vertical plane. Find the magnitude and direction of the magnetic force on the
electron.
Magnetic force on Current Carrying
Conductor

Since a current carrying wire is a collection of
moving charges, some of the same principals we
have just been discussing for a single charge
apply to the situation where we have a current
running through a wire.
Magnetic force on Current Carrying
Conductor

Magnetic force on Current Carrying
Conductor

Magnetic force on Current Carrying
Conductor

Much like with charges, this formula only tells us the
magnitude of the force. Also, the angle θ only
determines the magnitude of the force, not the direction.

The direction of the force is determined by the right hand
rule which we discussed last day:
If the fingers of your open right hand point in the
direction of the external magnetic field, and your thumb
points in the direction of the conventional current, then
your palm faces in the direction of the force on the
conductor.
Magnetic force on Current
Carrying Conductor

Some implications:
A
magnetic field does not exert a force on a
current moving parallel to the direction of the
magnetic field. (This would make sinθ = 0 in
our formula, cancelling the force out).
 The magnetic force is greatest when the
current moves perpendicular to the magnetic
field. (sinθ = 1).
Example 2

Earth’s magnetic field exerts a force of 1.4x10-5
N on a 0.045 m segment of wire in a truck motor.
The motor wire is positioned at an 18° angle to
the Earth’s magnetic field, which has a
magnitude of 5.3x10-5 T at the truck’s location.
Calculate the current in the wire.
Example 2

Earth’s magnetic field exerts a force of 1.4x10-5 N on a 0.045 m segment of
wire in a truck motor. The motor wire is positioned at an 18° angle to the
Earth’s magnetic field, which has a magnitude of 5.3x10-5 T at the truck’s
location. Calculate the current in the wire.
Video

Another recommended video by Derek
Owens:
 “Magnetic
Force on Moving Charges” http://www.youtube.com/watch?v=XWkhUwX
4D5s
 “The Motor Effect” http://www.youtube.com/watch?v=3BYXuYyb
aNg
Cool Stuff

Video of ferromagnetic liquid (ferro fluid)
 http://www.youtube.com/watch?v=PvtUt02zV
As
Homework
Read Sections 8.2, 8.3 (again)
 Make additional notes to supplement the
lesson notes.
 Complete the following questions:

 Pg.
391 # 3, 4, 5, 7
 Pg. 396 # 1, 2, 4