Grade 12 Physical Science Electromagnetism
Electromagnetism - Current and magnetic fields
Any moving charge has associated with it a magnetic field, thus any current carrying
conductor has a magnetic field around it.
The stronger the current, the stronger the magnetic field around the conductor.
Field around a straight conductor
• If we take a cross section through the conductor, the field consists of concentric
circles around the conductor in a plane at right angles to the conductor.
• The direction of the field lines depends on the direction of the current – RIGHT
HAND THUMB RULE gives us a way of finding the direction of the field lines.
Use your right hand, place your thumb in the direction of the current and wrap your fingers
around the wire. Your fingers then point in the direction of the field lines.
dot = current cross = current
out of page into page
Field around a coil
• A coil of wire carrying a current will have around it a magnetic field with the same
shape as that around a bar magnet.
• The direction of the field lines depends on the direction of the current in the coil –
RIGHT HAND RULE gives us a way of finding the direction of the field lines.
Using your right hand, grasp the coil with your fingers pointing in the direction of the current.
Your thumb will point to the end of the coil that behaves like the N-pole of a bar magnet.
http://thefutureofthings.com/upload/image/new-news/2008/december/los-alamos-magnet-lab-explores-superconductivity/magnetic-field.jpg
Electromagnets are made by winding a solenoid coil onto a bar of soft iron. Soft iron
does not retain its magnetism, so when the current is on, the magnetic effect is
enhanced by the soft iron, but once the current is switched off, the magnetic effect
disappears.
To Practice : TT 7.1 p 84 Q1+2
1
,Grade 12 Physical Science Electromagnetism
The motor effect
If two magnetic fields are superimposed, they will interfere with each other – field lines will
cancel in some areas and reinforce in other areas. This will result in a net force acting on
the components that caused the magnetic fields.
Interfering magnetic fields.
F upwards
Z Y
N S N X S
Y Z
F downwards
At Z lines cancel and at Y they reinforce, creating force F on the conducting wire and the
wire will move in the direction of that force.
So, when a conductor carries current through a magnetic field, the conductor
experiences a force.
Flemming’s LEFT HAND MOTOR rule gives us the direction of the force:
Force
B magnetic
field Place the first three fingers of your left hand at right angles
to each other. The fingers represent: Thumb = F (direction
of force), First finger = B (direction of magnetic field) and
middle finger = I (current direction)
I current
The magnitude of the force experienced by a conductor carrying a current I in a magnetic
field of strength B is given by:
F =IBℓ sinθ where ℓ is the length of conductor
θ is angle between current and magnetic field
To Practice: TT 7.1 Q 3 p 85; ebook Checkpoint 3 p 371
This effect is put to use if one of the components is able to move under the influence of the
force set up by the interfering fields. This is the principle behind the electric motor, in which,
with certain refinements, the loop can be made to rotate continuously, and so can turn some
device e.g. a fan.
B C
If the current is passed around a loop from A to B
to C to D in a magnetic field, AB will be forced
N S down into the page while CD will be pushed out of
the page and the loop will turn anti clockwise
A D
2
, Grade 12 Physical Science Electromagnetism
Direct current motor
Below is a diagram of a single loop placed in a magnetic field. This diagram represent a
simple direct current motor. Direct current (or DC) is current that moves only in one direction
due to an unchanging polarity applied across the ends of the conductor.
Images https://www.pc-control.co.uk/images/motor1.gif and http://coe.kean.edu/~afonarev/Physics/Magnetism/
The wire or carbon brushes are in fixed positions. The split ring commutator is made of half
cylinders of metal insulated from each other. The split ring commutator ensures
• that the wires don’t get tangled up.
• that the current is reversed each half turn so that
the force keeps turning the coil in the same
direction.
By increasing the magnetic force turning the coil, the power of an electric motor can be
increased. This can be done by:
• increasing the strength of the external magnetic field
• increasing the current in the coil (increasing the emf of the DC source or decreasing
the coil’s resistance e.g. using a thicker wire)
• increasing the number of turns on the coil (each coil experiences the same force
therefore increasing the total force)
• wrapping the coil around an iron core (iron is easily magnetised and will increase the
strength of the magnetic field of the coil)
3
Electromagnetism - Current and magnetic fields
Any moving charge has associated with it a magnetic field, thus any current carrying
conductor has a magnetic field around it.
The stronger the current, the stronger the magnetic field around the conductor.
Field around a straight conductor
• If we take a cross section through the conductor, the field consists of concentric
circles around the conductor in a plane at right angles to the conductor.
• The direction of the field lines depends on the direction of the current – RIGHT
HAND THUMB RULE gives us a way of finding the direction of the field lines.
Use your right hand, place your thumb in the direction of the current and wrap your fingers
around the wire. Your fingers then point in the direction of the field lines.
dot = current cross = current
out of page into page
Field around a coil
• A coil of wire carrying a current will have around it a magnetic field with the same
shape as that around a bar magnet.
• The direction of the field lines depends on the direction of the current in the coil –
RIGHT HAND RULE gives us a way of finding the direction of the field lines.
Using your right hand, grasp the coil with your fingers pointing in the direction of the current.
Your thumb will point to the end of the coil that behaves like the N-pole of a bar magnet.
http://thefutureofthings.com/upload/image/new-news/2008/december/los-alamos-magnet-lab-explores-superconductivity/magnetic-field.jpg
Electromagnets are made by winding a solenoid coil onto a bar of soft iron. Soft iron
does not retain its magnetism, so when the current is on, the magnetic effect is
enhanced by the soft iron, but once the current is switched off, the magnetic effect
disappears.
To Practice : TT 7.1 p 84 Q1+2
1
,Grade 12 Physical Science Electromagnetism
The motor effect
If two magnetic fields are superimposed, they will interfere with each other – field lines will
cancel in some areas and reinforce in other areas. This will result in a net force acting on
the components that caused the magnetic fields.
Interfering magnetic fields.
F upwards
Z Y
N S N X S
Y Z
F downwards
At Z lines cancel and at Y they reinforce, creating force F on the conducting wire and the
wire will move in the direction of that force.
So, when a conductor carries current through a magnetic field, the conductor
experiences a force.
Flemming’s LEFT HAND MOTOR rule gives us the direction of the force:
Force
B magnetic
field Place the first three fingers of your left hand at right angles
to each other. The fingers represent: Thumb = F (direction
of force), First finger = B (direction of magnetic field) and
middle finger = I (current direction)
I current
The magnitude of the force experienced by a conductor carrying a current I in a magnetic
field of strength B is given by:
F =IBℓ sinθ where ℓ is the length of conductor
θ is angle between current and magnetic field
To Practice: TT 7.1 Q 3 p 85; ebook Checkpoint 3 p 371
This effect is put to use if one of the components is able to move under the influence of the
force set up by the interfering fields. This is the principle behind the electric motor, in which,
with certain refinements, the loop can be made to rotate continuously, and so can turn some
device e.g. a fan.
B C
If the current is passed around a loop from A to B
to C to D in a magnetic field, AB will be forced
N S down into the page while CD will be pushed out of
the page and the loop will turn anti clockwise
A D
2
, Grade 12 Physical Science Electromagnetism
Direct current motor
Below is a diagram of a single loop placed in a magnetic field. This diagram represent a
simple direct current motor. Direct current (or DC) is current that moves only in one direction
due to an unchanging polarity applied across the ends of the conductor.
Images https://www.pc-control.co.uk/images/motor1.gif and http://coe.kean.edu/~afonarev/Physics/Magnetism/
The wire or carbon brushes are in fixed positions. The split ring commutator is made of half
cylinders of metal insulated from each other. The split ring commutator ensures
• that the wires don’t get tangled up.
• that the current is reversed each half turn so that
the force keeps turning the coil in the same
direction.
By increasing the magnetic force turning the coil, the power of an electric motor can be
increased. This can be done by:
• increasing the strength of the external magnetic field
• increasing the current in the coil (increasing the emf of the DC source or decreasing
the coil’s resistance e.g. using a thicker wire)
• increasing the number of turns on the coil (each coil experiences the same force
therefore increasing the total force)
• wrapping the coil around an iron core (iron is easily magnetised and will increase the
strength of the magnetic field of the coil)
3
