6 Magnetic fields Flashcards
magnetic field
a field surrounding a permanent magnet or a current-carrying conductor in which magnetic objects experience a force
how can you detect the presence of a MF?
with a small plotting compass
the needle will deflect in the presence of a MF
magnetic field lines
map magnetic field patterns around magnets and current-carrying conductors
- the arrow is the dir in which a free north pole would move- points N to S
- equally spaced and parallel mfl represent a uniform field
- MF is stronger when the mfl are closer.
- like poles repel and unlike poled attract
electromagnetism
when a wire carries a current, a MF is created around the wire
field is created by elecs moving within the wire
any charged particle that moves creates a magnetic field in the space around it
for the mf of a bar magnet, the mf is created by the elecs whizzing around the iron nuclei
current-carrying conductors
current-carrying wire- mfl are concentric circles centered on the wire and perpendicular to it
the dir of the mf can be determined using the right hand grip rule
right hand grip rule
thumb- dir of conventional current
fingers curl- dir of field
magnetic field patterns of single coil and a solenoid
both the coil and solenoid produce N and S poles at their opposite faces
magnetic field pattern outside solenoid is similar to that for a bar magnet, and at the centre of the core of the solenoid it is uniform
Fleming’s left hand rule
when a current-carrying conductor is placed in an external mf, the two fields interact
the dir of the force experienced by the conductor placed perpendicular to the external mf can be determined using Fleming’s LHR
-first finger gives dir of external mf
-second finger gives the dir of the conventional current
-thumb gives the dir of motion (force) of the wire
magnetic flux density
the strength of the field
Tesla T
1T= 1Nm-1A-1
mfd is 1T when a wire carrying a current of 1A placed perpendicular to the mf experiences a force of 1N per metre of its length
force on a current-carrying conductor equation
F=BILsinθ
B is mfd. L is length of wire. θ is angle between mf and current direction
when the wire is perpendicular to the mf θ=90 degrees and sinθ=1
therefore F=BIL
B=F/IL
determining magnetic flux density in the lab
two magnets placed on top-pan balance
the mf between them is almost uniform
a stiff copper wire is held perpendicular to the mf between the two poles
the length L of the wire in the mf is measured with a ruler
using crocodile clips, a section of the wire is connected in series with an ammeter and a variable power supply
the balance is zeroed when there is no current in the wire
with a current I the wire experiences a vertical upwards force
according to newtons third law of motion, the magnets experience an equal downward force, which can be calculated from the change in the mass reading
the mfd B between the magnets can then be determined from the equation B=F/IL
an electron deflection tube
shows that a charged particle moving in a magnetic field experiences a force
the force on a beam of elecs can be predicted using flemings LHR
the beam of elecs is moving from left to right into a region of uniform magnetic field
as the elecs enter the field they experience a downwards force
the elecs change direction but the force F on each elec always remains perpendicular to its velocity
the speed of the elecs remains unchanged because the force has no component in the direction of motion
once out of the field the elecs keep moving in a straight line
a current carrying wire in a uniform mf experiences a force as each elec moving within the wire experiences a tiny force
find the force acting on a charged particle of charge Q moving at a speed v at right angles to a uniform mf of flux density B
F=BQv
F=Bev
circular orbits of charged particles
charged particle of mass m and charge Q moving at right angles to a uniform mf of flux density B
the particle will describe a circular path because the force acting on it is always perpendicular to its velocity
BQv=mv2/r
velocity selector
a device that uses both electric and magnetic fields to select charged particles of specific velocity
consists of two parallel horizontal plates conncted to a power supply
they produce a uniform electric field of field strength E between the plates
a uniform mf of flux density B is also applied perpendicular to the ef
the charged particles travelling at diff speeds to be sorted enter through a narrow slit
the ef and mf deflect them in opposite directions- only for particles with a specific speed v will these deflections cancel so that they travel in a straight line and emerge from the second narrow slit Z
for an undeflected particle:
electric force=magnetic force
EQ=BQv
v=E/B