1. A –Level Physics: Magnetic Fields Magnetic Forces

2. Objectives:

3. 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

4. Starter Activity
Discuss in pairs which factors/quantities affect the force subjected upon a wire in the motor effect
Current, Magnetic flux density, Length of wire within the field,
Angle of the wire,

5. 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)?
5.1 x N
You can speed up the motor by:
Increasing the current
Increasing the length of wire (number of turns)
Increasing the magnetic field

6. Calculating the force on each individual charged particle
5.1 x N
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!

7. 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)
5.1 x N

8. 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!
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

9. Mass Spectrometer- Analysis
Let’s first recap the equations that will be relevant:
INSERT THE EQUATION FOR CENTRIPETAL FORCE
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:
𝑒 𝑚 = 𝑣 𝐵𝑟
This is the charge-mass ratio and gives us the identity of the particle
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)

10. 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:
𝑣=√ 2𝑒𝑉 𝑚
So by altering the accelerating voltage (V) or the magnet strength (B) we can identify all the chemicals in the sample!
So substituting into the original equation:
𝑒 𝑚 = 2𝑉 𝐵2𝑟2
More hits in one detector region=more abundance of that particle!

11. Complete the exam practice questions (includes marking and annotation)
Practice and I/S
Complete the exam practice questions (includes marking and annotation)
Mass spectrometer and Flux Linkage Questions