Magnetic fields Flashcards
A magnetic field is formed around a _____ carrying ____
A magnetic field is formed around a current carrying wire
How to find direction of field in current carrying wire
use right hand grip rule (thumb pointing along the direction of the current, other fingers give the direction of the field)
A current carrying wire experiences a ______ in a magnetic field
A current carrying wire experiences a force in a magnetic field
How to find direction of force, current or motion in current carrying wire
Use Fleming’s Left Hand rule (thumb force/motion, first finger field, second finger current) magnitude of force
Magnitude of force of in current carrying wire
F = BIL
A charge moving in a magnetic field experiences a ____
A charge moving in a magnetic field experiences a force
A moving charge is equivalent to
current
To find the direction of a moving charge in a magnetic field
Use Fleming’s Left Hand rule
The magnitude of the force experienced by a moving charge in a magnetic field
F = BQV
Cyclotrons use … in order to …
Cyclotrons use magnetic fields to keep charged particles in circular orbits
If a wire moves in a magnetic field then
A current is induced in a wire if it moves in a magnetic field
To find the direction of current, field, motion of wire in magnetic field
Use Fleming’s Right Hand rule (same)
Magnetic flux density =
Magnetic flux =
Magnetic flux linkage
magnetic flux density = B,
magnetic flux, phi = BA,
magnetic flux linkage = N*phi = BAN
A field is
the region around a mass, charge or magnetic in which a force can be experienced
The flux density =
the strength of the field = the force on a unit North pole or current-carrying conductor
The lines of force of a long, straight wire carrying current are
concentric circles, in the plane at right angles to the wire, with the separation of the lines 1/r. (circles around a line going up)
If you look at the end of the coil and the current is going clockwise
then you are looking at a south pole, conversely, if the current is anti-clockwise then you are looking at a north pole
Solenoid with diagram of field lines
arrows going in from south (left) to north (right) and one circle going anticlockwise at top and clockwise at bottom
Current going down at left and up and right
Strength of magnetic field in solenoid is increased by
increasing the current in the coil
increasing the number of coils in the solenoid
using a soft iron core within the solenoid
When looking at the end of the solenoid, if the current is clockwise then
you are looking at south pole
The field in a solenoid is identical to that of a bar magnet except that it continues inside the solenoid. It has the advantage over the bar magnet in that
the strength of the field can be varied, whereas a bar magnet has a fixed strength
A current carrying conductor in a magnetic field experiences a force which is
at right-angles to the field and to the wire, due to the combination of two magnetic fields
One tesla (the unit of magnetic field strength/flux density)
is the field in which a wire carrying a current of 1 amp, produces a force of 1 N for each 1 metre of the length of the wire in the field, at right angles to both the wire and the field.
A hall probe
uses the effect of the magnetic field on the electrons in a semi-conductor to give a potential difference across a slice of semi-conductor which is proportional to the field. This gives a measure of the field strength
r =
mv/BQ
What causes the path of a charged particle to curve/decrease
if B is increased or v is decreased, if particles with a larger specific charge (Q/m) are used
w = v/r =
BQ/m
Cyclotron use
The cyclotron is used in hospitals to produce high energy beams for radiation therapy
Conventional current goes in
opposite direction to electrons, same direction as positrons, +ve to -ve
Conductor, force =
BIL
Beam, for charged particles, force =
BQV
field is perpendicular to velocity
(X) means
magnetic field lines into the board
(.) means
magnetic field lines out of the board
A magnetic force only acts on particles which are ____ and ____
which are charged and moving and when the field is perpendicular to the path of the moving charge
A uniform field has
constant magnitude of force and acceleration
A magnetic force perpendicular to the trajectory of moving charge -
speed of particle is unaffected
The cyclotron frequency, f =
w/2pi = BQ/2mpi
Cyclotron cosists of
It consist fo two hollow D-shaped electrodes (dees) in a vacuum chamber. With a uniform magnetic field applied perpendicular to the plane of the dees, a high-frequency alternating voltage is applied between the dees. Each time a charged particle crosses the gap it gains kinetic energy and hence speed and momentum so the radius of the path increases
Charged particles are directed into one of the dees near the centre of the cyclotron. The charged particles are forced on a circular path by the magnetic field causing them to emerge form the dee they were directed into. As they cross into the other dee, the alternating voltage reversed so they are accelerated into the other dee where they are once again forced on a circular path by the magnetic field. On emerging from this dee, the voltage reverses again and accelerates the particles into the first dee where the process is repeated. This occurs because the time taken by a particle to move round its semi-circular path in each dee does not depend on the particle speed
A magnetic field/force line is
a line along which a free north pole would move in a field
Motor effect =
current-carrying wire placed at a non-zero angle to the lines of force of an external magnetic field experiences a force due to the field. The force is perpendicular to the wire and to the lines of force
The magnitude of the force depends on
the current
the strength of magnetic field
the length of the wire
the angle between lines of force of the field and the current direction
The force is greatest when
the wire is at right angles to the magnetic field
The force is zero when
the wire is parallel to the magnetic field
B = magnetic flux density =
= force per unit length per unit current on a current-carrying conductor at right angles to the magnetic field lines, unit is Tesla(T)=N/m/A, B and I are perpendicular
= episolon0 * I/2pi * r
Force = BQV if direction of motion of charged particle is perpendicular to the direction of the magnetic field, if not
F = BQVsin(X)
Stationary chargers in a magnetic field experience
no magnetic force
When applying Fleming’s left-hand rule to charged particles, the current direction for negative particles
is in the opposite direction to the direction of motion of the particles.
The force of the magnetic field on moving charged particles is
at right angles to the direction of motion of the particle
No work is done by the magnet on the particle as
the force always acts at right angles to the velocity of the particle
The direction of motion, kinetic energy and speed of a charged particle…
direction of motion and kinetic energy is changed by the force but not its speed
In accordance with Fleming’s left-hand rule, the magnetic force is always…. to the velocity at any point along the path meaning…
perpendicular
This means that the particle therefore moves on a circular path with the force always acting towards the centre of curvature of the circular path.
The force causes a centripetal acceleration because
its perpendicular to the velocity
The deflection of a beam of electrons in a uniform magnetic field follows a complete circle because
the magnetic field is uniform and the particle remains in the field.
Mass spectrometer
for equally charged particles travelling at the same velocity in the same magnetic file the radius of their paths are proportional to their masses, r =mv/BQ Nuclei of different isotopes of a given element could thereby be separated if they could be made to travel at the same velocity.
The mass spectrometer is used to analyse the type of atoms present in a sample. The atoms of the sample are ionised and directed in a narrow beam at the same velocity into a uniform magnetic field. Each ion is deflected in a semi-circle by the magnetic field onto a detector. The radius of curvature of the path of each ion depends on the specific charge of the ion. Each type of iron is deflected by a different amount onto the detector. The detector is linked to a computer which is programmed to show the relative abundance of each type of ion in the sample.
Electric motor
The simple electric motor consists of a coil of insulated wire which spins between the poles of a U-shaped magnet. When a direct current passes round the coil, the wires at opposite edges of the coil are acted on by forces in opposite directions; the force on each edge makes the coil spin about its axis. Current is supplied to the coil via a split-ring commutator. The direction of the current round the coil is reversed by the split-ring commutator each time the coil rotates through half a turn ensuring the current along an edge changes direction when moving poles
The trajectory of charged particles which enter at right angles to a uniform electric field is
parabolic
The trajectory of charged particles which enter at right angles to a uniform magnetic field is
circular
A square coil with its plane parallel to a uniform magnetic field induces an emf in the coil when
it is rotated about an axis through the perpendicular
A coil mounted on an axle has its plane parallel to the flux lines of a uniform magnetic field. When a current I is switched on, and before the coil is allowed to move there are no forces due to the magnetic field on
the sides parallel to this field.
Condition for F = BIL
Current is perpendicular to magnetic flux density
Charged ion is projected into a vertical magnetic field that is directed upwards, thedirection of the magnetic force that acts on the ion is
initially out of plane of diagram and subsequently it is circular in the horizontal plane
If the strength of the magnetic field is doubled then
the radius is halved
if the charge is halved then
the radius is doubled.
An electron moves due North in a horizontal plane with uniform speed. It enters a uniform magnetic field directed due South in the same plane. What happens to its motion?
It is accelerated due North
Why do ions travel in a semicircular path whilst in a magnetic field
From Fleming’s left hand rule force acts perpendicular to both magnetic field and velocity, force changes direction of velocity but not its magnitude, centripetal force remains perpendicular to velocity as direction changes
diameter is directly proportional to
mass
1/charge
If the wire is parallel to the magnetic field then
there is no force
Force =
Condition
Force = magnetic flux density x current x length = BIL when field is perpendicular to current
Two numbers for B, magnetic flux density
T = N/Am
What is magnetic flux density
The strength of the magnetic field
Couple =
BINAcosX
DC electric motor simplified
Every half turn the split ring commuter reverses current so it does not get stuck at top
Maxwell’s right hand screw rule
thumb pointing in direction of current flow (opposite to direction flow), Fingers in direction of magnetic field
Increasing strength of solenoid
Solenoid = coil of wire, increase force by adding iron core or number of coils or current to increase magnetic force strength, increase length of wire
How a bell works
switch closes causing a current to electromagnet attracting the iron causing the strike to hit thebell disconnecting the circuit causing it to stop attracting reconnecting the circuit as it rises back up starting cycle again
To determine poles of a magnetic field/direction of turn of coil/solenoid
North with arrows pointing out from tips (anticlockwise) – field going out
South with arrows pointing out from tips (clockwise) – field going in
Flux linkage (BNLV) y axis w/v x axis
y=x
induced emf y axis
w/v x axis
y=c
Use left hand rule for
Motor effect
Use right hand rule for
induction/ generators
riGht hand for Generators
