Download magnetic field - The Physics Doctor

A –Level Physics: Magnetic
Fields
Magnetic Forces
Objectives:
FLASHBACK
FLASHBACK: Sketch a stress-strain graph for a malleable
materials and label a) the limit of proportionality b) the
elastic limit c) the ultimate tensile strength d) the breaking
point
Starter Activity
Discuss in pairs which factors/quantities affect the
force subjected upon a wire in the motor effect
Calculating the force on the wire
The strength of the force (F) on a length of wire (l) which
has a current (I) flowing through it whilst it is in a magnetic
field (B) is given by the following equation:
𝑭 = 𝑩 × 𝑰 × 𝒍 × 𝒔𝒊𝒏∅
And assuming that the angle that the current makes with
the magnetic field is 90° (perpendicular), this makes the
equation simply:
𝑭 = 𝑩𝑰𝒍
So how can you make the motor move faster (more powerful)?
You can speed up the motor by:
1. Increasing the current
2. Increasing the length of wire (number of turns)
3. Increasing the magnetic field
Calculating the force on each
individual charged particle
The force produced by the motor effect
acts on the charged particle at right
angles to its motion path AND to the field.
This makes the force centripetal and if the
particle was not constrained, it would
follow a curved path
If the particle is constrained in a wire
then this force causes the wire to
move instead!
Calculating the force on each
individual charged particle
The strength of the force on the
particle is given by a very similar
equation to that of the whole wire:
𝑭 = 𝑩𝒆𝒗
Whereby ‘e’ is the charge on the
electron/particle and ‘v’ is the
velocity of the particle
NB: remember this assumes
the angle is perpendicular
(if not it’s F=Bevsinθ)
Mass Spectrometer
Sometimes we need to identify the content of unknown
chemicals, particularly in fields such as forensic science. A mass
spectrometer can utilise the physics we have learnt so far!
1.
2.
3.
4.
A chemical is first
vaporised and then
ionised by bombarding
with electrons.
An electric field is then
used to accelerate the
particles
It’s then passed through
an electromagnet’s
magnetic field
This produces a
centripetal force on the
particle, changing its
direction
Mass Spectrometer- Analysis
Let’s first recap the equations that will be relevant:
1) INSERT THE EQUATION FOR CENTRIPETAL FORCE
2) INSERT THE EQUATION FOR FORCE ON AN INDIVIDUAL PARTICLE by
a magnetic field
As both forces are the same….
𝑚𝑣2
•
𝑟
= 𝐵𝑒𝑣
This can be rearranged into:
This is the
charge-mass
ratio and gives
us the identity
of the particle
𝑒
𝑚
=
𝑣
𝐵𝑟
The only piece of information
you need is how fast the
particle entered the
electromagnet
This is known by the calibration
of the machine! (set magnetic
flux density and radius of the
curve)
Mass Spectrometer- Analysis
As we need to know the speed they enter at, we have to look at the
acceleration by the electric field.
Energy= ½ mv2 = eV
½ mv2 = eV
This can be rearranged into:
So by altering
the accelerating
voltage (V) or
the magnet
strength (B) we
can identify all
the chemicals in
the sample!
2𝑒𝑉
𝑣=√
𝑚
So substituting into the original equation:
𝑒 2𝑉
= 22
𝑚 𝐵𝑟
More hits in one detector region=more
abundance of that particle!
Practice and I/S
Complete the exam practice questions (includes
marking and annotation)