MAGNETISM
INTRODUCTION
Compare and contrast electric and magnetic
interactions
How is an electric charge similar to a magnet?
How is an electric charge different from a magnet?
Magnetic Poles
- parts of the magnets where the magnetic field is strongest
- north and south poles
- magnetic monopoles do not exist
- north and south pole can never separated
- magnetic dipoles
Magnetization and Demagnetization
Magnetic domains are regions within a magnetic material where the
magnetic moments of atoms are aligned in the same direction.
Demagnetization – techniques that involve mixing up the arrangement
of molecules to cancel the polarity of the entire material.
• Hammering
• Heating
• Exposing the magnet to an alternating current.
However, molecules can be rearranged again through magnetization,
thus bringing back the material’s overall polarity.
A demagnetized magnet can be magnetized again by exposing it to
another magnet with a strong magnetic field.
OERSTED DISCOVERY
(Hans Christian Oersted)
ELECTROMAGNETISM
-moving electric charges
(current) induces a magnetic
field perpendicular to the flow of
current.
- When current is turned off , NO
MAGNETIC FIELD EXISTS, and no
force exists.
Magnetic Field Lines
- continuous loops
- outside the magnet, point out of the
north pole and then they curve toward
the south pole (N to S)
- inside the magnet, continue on
straight toward the north pole again
(S to N)
- three dimensional, curving around
the magnet in all directions around the
length of the bar
Magnetic Field
Note: 1 Tesla = 1 x 10^4 Gauss (G)
Determining the direction of B and I
Example:
1. What is the magnitude of the magnetic field 100 m away from a wire
carrying a 6.00 A current? If the current has a vector direction out of the page
(or screen), what is the direction of the magnetic field?
𝐺𝑖𝑣𝑒𝑛: 𝑟 = 100𝑚
𝐼 = 6.00 𝐴, 𝑜𝑢𝑡 𝑜𝑓 𝑡ℎ𝑒 𝑝𝑎𝑔𝑒
𝜇𝑜 = 4𝜋 𝑥 10−7 𝑇𝑚/𝐴
𝐴𝑠𝑘𝑒𝑑: 𝐵, 𝑑𝑖𝑟
𝐹𝑜𝑟𝑚𝑢𝑙𝑎: 𝐵 =
𝜇𝑜 𝐼
2𝜋𝑟
𝑆𝑜𝑙𝑢𝑡𝑖𝑜𝑛:
4𝜋 𝑥 10−7 𝑇𝑚/𝐴 6.00 𝐴
𝐵=
2𝜋 100 𝑚
𝐵 = 1.2 𝑥 10−8 𝑇
𝑐𝑜𝑢𝑛𝑡𝑒𝑟𝑐𝑙𝑜𝑐𝑘𝑤𝑖𝑠𝑒 (𝑅𝑖𝑔ℎ𝑡 𝐻𝑎𝑛𝑑 𝐺𝑟𝑖𝑝 𝑅𝑢𝑙𝑒)
Magnetic Force
The magnetic force is a consequence of the electromagnetic
force, one of the four fundamental forces of nature, and is caused
by the motion of charges.
Two objects containing charge with the same direction of motion
have a magnetic attraction force between them.
Similarly, objects with charge moving in opposite directions have a
repulsive force between them.
The magnitude of the magnetic force between them depends
on how much charge is in how much motion in each of the two
objects and how far apart they are.
Magnetic Force
q = charge (C)
v = velocity of the charge (m/s)
B = strength of magnetic field
(Tesla, T = N/A.m)
F = qvB sin ϴ
A 6.4 𝑥 10−11 𝐶 charged proton with a velocity of 9.7𝑥 104 𝑚/𝑠 is moving through a space
with a magnetic field strength of 1.2 𝑇. From the magnetic field pointing to the east, how
much force would the force feel?
𝐺𝑖𝑣𝑒𝑛: 𝑞 = 6.4 𝑥 10−11 𝐶
𝑚
𝑣 = 9.7 𝑥 104 𝑠 , 𝑠𝑜𝑢𝑡ℎ
𝐵 = 1.2 𝑇, 𝑒𝑎𝑠𝑡
𝐴𝑠𝑘𝑒𝑑: 𝐹𝑀 , 𝑑𝑖𝑟
𝐹𝑜𝑟𝑚𝑢𝑙𝑎: 𝐹𝑀 = 𝑞𝑣𝐵 sin 𝜃
𝑆𝑜𝑙𝑢𝑡𝑖𝑜𝑛:
𝐹𝑀 = 6.4 𝑥 10−11 𝐶 9.7 𝑥 104 𝑚/𝑠 1.2 𝑇 sin 90𝑜
𝐹𝑀 = 7.45 𝑥 10−6 𝑁
𝑜𝑢𝑡 𝑜𝑓 𝑡ℎ𝑒 𝑝𝑎𝑔𝑒
Ans: Upward/North
Ans: Right/East
IDENTIFY THE DIRECTION OF THE MISSING
VECTORS USING THE RHR
CHARGE
POSITIVE
FORCE
NORTH
VELOCITY
EAST
MAGNETIC FIELD
INTO THE PAGE
IDENTIFY THE DIRECTION OF THE MISSING
VECTORS USING THE RHR
CHARGE
POSITIVE
FORCE
NORTH
VELOCITY
WEST
MAGNETIC FIELD
OUT OF THE PAGE
IDENTIFY THE DIRECTION OF THE MISSING
VECTORS USING THE RHR
CHARGE
POSITIVE
FORCE
EAST
VELOCITY
SOUTH
MAGNETIC FIELD
INTO THE PAGE
Let us now take a look at particles in motion
within a magnetic field.
A magnetic field is a region in which a moving charge experiences a force.
Since electrons are moving charges, they will experience a force when they enter
a magnetic field.
If a charged particle enters a magnetic field perpendicular to the field, it will
Experience a force at right angles to its motion and it will be steered into a circular
path.
𝑚𝑣
𝑅=
𝑞𝐵
The radius of the circular path can be
calculated by dividing the product of
the mass and velocity of the particle
and the product of the absolute value
of the charge and the magnitude of
the magnetic field
The number of rotations that the particle will
make are denoted by the Cyclotron formula:
Where:
▪ 𝑓 = Frequency, Hertz (Hz)
𝑞𝐵
𝑓=
2𝜋𝑚
▪𝑞 = absolute value of the charge,
Coulombs (C)
▪ B = Magnetic field strength, Tesla (T)
▪ 𝑚 = mass, mass of the particle (kg)
▪The standard unit is Hertz (Hz) after
German physicist Heinrich Hertz.
Magnetic Flux
•total number of magnetic field
line through a given coil or area
Magnetic Flux
The unit for magnetic flux is the weber (Wb).
1 Wb = 1 T·m2
Magnetic Flux
Magnetic Flux
1. Dimension of a rectangular loop is given as 0.051m and
0.068m. B and θ are 0.02T and 47° respectively. Calculate
the magnetic flux through the given surface.
𝐺𝑖𝑣𝑒𝑛: 𝐵 = 0.02 𝑇
𝜃 = 47𝑜
𝑙 = 0.051 𝑚
𝑤 = 0.068 𝑚
𝑆𝑜𝑙𝑢𝑡𝑖𝑜𝑛:
𝐴=𝑙𝑥𝑤
= 0.051 𝑚 𝑥 0.068 𝑚
= 3.47 𝑥 10−3 𝑚2
𝐴𝑠𝑘𝑒𝑑: Φ𝐵
Φ𝐵 = 0.02 𝑇 3.47 𝑥 10−3 𝑚2 cos 47𝑜
𝐹𝑜𝑟𝑚𝑢𝑙𝑎: Φ𝐵 = 𝐵𝐴𝑐𝑜𝑠𝜃
Φ𝐵 = 4.73 𝑥 10−5 𝑊𝑏
Magnetic Force on a Current-carrying Wire
• If we put a current carrying wire in
a magnetic field, the wire as a
whole will experience a magnetic
force.
FB = IL x B ; FB = IL x B sin θ
FB = magnetic force, N
I = current, A
L = length, m
B = magnetic field, T
*direction: Palm Method or RHR 2
EXAMPLE
A current of 20 A flows East through a 50 cm long wire. A magnetic field of 4.0 T is
directed into the page. What is the magnitude and direction of the magnetic force acting
on the wire?
Given:
I = 20 A (east)
B = 4.0 T (into the page)
L = 50 cm
Conversion:
I
𝐿 = 50𝑐𝑚 ×
1𝑚
= 0.50𝑚
100𝑐𝑚
Asked: FB
Formula: FB =IL x B sin θ
Solution:
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FB
= (20 A) (0.50m) (4.0 T) sin 90o
Answer: FB = 40 N, North
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Questions?