Download direction of magnetic field

Electromagnetism
• Understand that an electric current creates a magnetic field around itself
• Describe the magnetic field created by a current carrying wire
• Use the Right Hand Slap Rule to predict the direction of the magnetic
force ion a current carrying wire inside a magnetic field.
• Use F = BIL and F = BILsin(θ) to calculate the size of the magnetic force on
a current carrying wire in a magnetic field.
• Use F = Bvq to calculate the size and direction of the magnetic force on a
moving charge inside a magnetic field.
• Describe the circular motion of a charged particle inside a magnetic field
• Understand electromagnetic induction in terms of the relative motion of a
wire across a magnetic field.
• Use V = BvL to calculate the voltage induced across a wire moving through
a magnetic field.
Magnetic fields
Understand that an electric current creates a magnetic field around itself
Magnetic fields are created when magnetic field
lines travel from the North pole to the South pole
of a magnet. The strength of magnetic field (B),
is measured in Tesla (T).
Magnetic field lines behave in a similar way to
electric field lines: Like poles attract and
opposite poles repel; the closer the magnetic
field lines are the stronger the magnetic field.
A uniform magnetic field can be produced
between the North and South poles of magnets.
Current Carrying wire:
Describe the magnetic field created by a current carrying wire
•
A current carrying wire creates a magnetic field. We can use the right hand
grip rule, with our thumb in the direction of the current, and our fingers in the
direction of the magnetic field, to discover the field direction.
Motor force on a current carrying wire in a
magnetic field
• When a current carrying wire is placed in a uniform magnetic field
the combination of the two magnetic fields produce a force on the
wire. This is called a motor force and the direction of the force can
be deduced by using the right hand slap rule.

+
=
Low
concentration
• The wire is pushed into the space with a
low concentration of field lines.
Right hand slap rule
Use the Right Hand Slap Rule to predict the direction of the magnetic force
ion a current carrying wire inside a magnetic field.
• Place you right thumb along the direction of the current in the wire, rotate
your hand so that your fingers point in the direction of the magnetic field
lines. The palm of the right hand gives the direction of the force acting on
the current carrying wire.
direction of
force (F)
(slap)
direction of
magnetic field (B)
direction of
current (I)
Motor force
Use F = BIL and F = BILsin(θ) to calculate the size of the magnetic force on a
current carrying wire in a magnetic field.
Force
measured in N
Magnetic field
strength
measured in T
F  BIL
Current flowing in
wire measured in A
Length of wire within
the magnetic field and
perpendicular to the
field lines.
Measured in m
Working out the direction and size of the
Motor force
• Magnetic field lines
drawn from N to S.
• Current travels from
+ve to –ve in wire.
• Using the RH slap rule
fingers are aligned
with the magnetic
field lines and the
thumb with the
direction of current.
• The palm of the hand
slaps outwards this is
the direction of the
force.
Example Exercise
S
1
B
+
I
F
What is the force
acting on a wire carrying
0.25 A that crosses a 0.20
m uniform magnetic field
with a strength of 23 mT?
F  BIL
F  0.023mT  0.25 A  0.20m
F  0.00115 N  1.2 103 N
N
Force acting on a charged Particle in a magnetic
field
Use F = Bvq to calculate the size and direction of the magnetic force on a
moving charge inside a magnetic field.
Force
measured in N
F  Bvq
Magnetic field
strength
measured in T
Velocity of the
charged particle in
the magnetic field.
Charge of particle
moving through
the magnetic field
Working out the direction and size of
the force on the charged particle
• Magnetic field lines
are going into the
page.
• The positive charge
is traveling to the
right.
• Using the RH slap
rule fingers are
aligned with the
magnetic field lines
and the thumb
with the direction
of positive charge.
• The palm of the
hand slaps upwards
this is the direction
of the force.
Example Exercise
F
B
1
v
What is the force
acting on a proton, 1.60 x
10-19 C as it moves at 8.00
kms-1 across a uniform
magnetic field of strength
27.3 μT?
F  Bvq
F  27.3 106 T  8.00 103 ms 1 1.6 1019 C
F  3.494 1020 N  3.49 1020 N
The path of a charged particle moving
through a magnetic field
Describe the circular motion of a charged particle inside a magnetic field
• As the charged particle moves
through the magnetic field a
force is produced
perpendicular to the velocity.
• This force causes the particle
to accelerate, change
direction.
• This force causes the particle
to constantly changes direction
and forces it is a circular path.
Voltage induced across a wire moving through
a magnetic field
Use V = BvL to calculate the voltage induced across a wire moving through a
magnetic field.
Voltage
measured in v
V  BvL
Magnetic field
strength
measured in T
Velocity of the
charged particle in
the magnetic field.
Length of the wire
moving through
the magnetic field
Voltage induced across a wire moving through
a magnetic field
• By aligning our thumb with the
velocity and our fingers with
magnetic field lines coming out of
the page we can work out that
positive charge is pushed towards
the right.
• As more and more positive
charge accumulates at the right
end of the wire it becomes
increasingly harder for the
positive charge to move there.
Eventually the charge slops
flowing and the right becomes
positively charged. A potential
difference, voltage is induced.
-ve
+ve
Example Exercise
1
What is the induced voltage
produced by a 12 m wire moving at 200 ms-1
through a 25 μT magnetic field?
V  BvL
V  25  106 T  200ms 1  12m
V  0.0600V  60mV
Example Exercise
1
A 20cm copper rod is
sits on conducting rails, in a 1.5
mT magnetic field. The rails are
15cm apart an connected to a
power supply supplying a current
of 1.5 A. In what direction does a
force act on the rod? What is the
magnitude of this force?
1.5 A
The current flows anti-clockwise.
The current flows down through
the rod. Using the RH slap rule a
force pushes the rod to the left.
The current only flows through
the 0.15m of the rod connected
to the rails.
F  BIL
F  1.5  103 T  1.5 A  0.15m
F  3.375  104 N  340  N
Example Exercise
2
A 30cm copper
rod is rolled on conducting
rails, through a 3.5 mT
magnetic field at 3.0ms-1.
The rails are 20cm apart. In
what direction does the
current flow? What is the
magnitude of the voltage
induced?
V
The current is pushed up
ways so it flows clockwise.
Although a voltage is induced across the whole length of the rod, only the
difference between the voltage where the rod connects the rails is measured. ∴
This is 0.20m.
V  BvL  3.5  103 T  3.0ms 1  0.20m
V  0.0021V  2.1mV
Understand electromagnetic induction in terms of the relative motion of a
wire across a magnetic field.
1.
An aeroplane with a wingspan of 26 meters flies through a 30 μT
magnetic field at 250 ms-1. What is the voltage induced across its wings?
V  BvL  30  106 T  250ms 1  26m
V  0.195V  0.2V
How will the voltage change if the plane halves its velocity?
Given that V=BvL we can see that V  v
As the voltage is directly proportional to the velocity, if the
velocity is halved the induced voltage is halved.