Magnets and Magnetic Fields
Objects that align with the magnetic field of the earth.
The north pole is
magnetic south
pole
Like poles repel and
unlike poles attract
Unlike charges, poles
can not be isolate
If a magnet is cut, new poles appear 20.1
Magnets and Magnetic Fields
Magnetic field lines – look similar to electric field lines
Point in the direction of a force on a North monopole
Interaction is also similar to
electric fields
20.1
Magnets and Magnetic Fields
The earths magnetic
field –
Caused by the
iron core
Does not point to
geographic
North
Provides protection
against solar wind20.1
Electric Currents Produce Magnetic Fields
Hans Christien Oerseted – 1820 a compass needle deflects when a current is first produced in a wire
The field is in a circle around the
wire
We can determine
the direction
of a magnetic
field using the
right hand rule 20.2
Electric Currents Produce Magnetic Fields
Right hand rule #1
1. the thumb points in the
direction of the current
2. the fingers curl in the
direction of the magnetic
field
A magnetic field due to a loop
20.2
Force on an Electric Current in a Magnetic Field
Magnets exert force on a current-carrying wire (Newton’s Third Law)
The force is
1. perpendicular to the direction of current
2. perpendicular to the direction of the magnetic field
20.3
Force on an Electric Current in a Magnetic Field
Right Hand rule #2
1. fingers, direction of
magnetic field
2. thumb, direction of current
3. palm, direction of force
The force is defined as
B – magnetic field measure in tesla (T) – vector
l – length of wire in the field 20.3
sinIlBF
Force on an Electric Current in a Magnetic Field
A tesla is also called a weber per meter squared
The units in cgs is called a Gauss
20.3
2/11 mWbT
TG 4101
Force on Electric Charge Moving in a Mag Field
The equation can be determine by an analysis of the current
I is the amount of charge that passes a point in a given time
l can be defined as
Putting those in the force equation
20.4
t
qI
vtl
sinIlBF sinBvtt
qF
sinqvBF
Force on Electric Charge Moving in a Mag Field
The force is maximum when the charged particle moves perpendicular to the field
Zero when it moves parallel to the field
The angle is the angle between the velocity and the field
The right hand rule will give you the correct direction for the force on a positive charge
20.4
sinqvBF
Force on Electric Charge Moving in a Mag Field
If we establish a uniform magnetic field and the charge moves perpendicular to the field
In our example
-Field (fingers) is out of the
slide
-Velocity (thumb) is in the
direction of the blue arrow
-Force (palm) – points toward middle (assume positive charge)
The force is centripetal 20.4
Force on Electric Charge Moving in a Mag Field
A very common type of problem
The electromagnetic force is the centripetal force
Charged particles from the sun, concentrate at the poles because of this – called the Aurora Borealis
20.4
sinqvBF qvBF r
vmqvB
2
Magnetic Field Due to a Long Straight Wire
As discussed earlier, a current in a wire produces a magnetic field
This can be demonstrated by moving a compass around a current carrying wire.
20.5
Magnetic Field Due to a Long Straight Wire
The field around the wire is directly proportional to the current and inversely to the distance from the wire
0 is called the permeability of free space
20.5
r
IB
20
AmTx /104 70
Force Between Two Parallel Wires
Assuming that two wires are parallel to each other
20.6
r
IB
2
101
d
IB
2
101
sinIlBF 1222 BlIF 2210
2 2l
d
IIF
Applications
Galvanometer
1. Current flows through
the wire
2. Produces a magnetic
field
3. The magnetic field is
attracted or repelled by the permanent magnet
Found in analog voltmeters, ammeters, and ohmeters 20.10
Applications
Electric Motor
1. Current flows through
the wire
2. Produces a magnetic
field
3. The magnetic field is
attracted or repelled by the permanent magnet
4. Brushes allow the current to switch
20.10