Magnetic Fields (Electromagnetism) Flashcards
What is the use of Fleming’s Right Hand rule?
Making a thumbs up shows the orientation of magnetic field lines created by a current through a wire (through the curl of your fingers).
Define Magnetic Flux Density.
The Magnetic Flux Density of a magnetic field is 1 Tesla if a 1 metre length of wire place perpendicular to the field lines with a current of 1 Amp running through it experiences 1 Newton of force.
What equation shows the relationship of the force on a wire, the current length of the wire, the magnetic flux density and the orientation of the wire?
F = BILsinθ
How are magnetic fields different to gravitational and electric fields?
The force exerted by the magnetic field is perpendicular to the field lines, whereas electric and gravitational fields exert forces parallel to their field lines. Also, particles must not be stationary for magnetic fields to exert a force, whereas electric and gravitational fields act on stationary particles.
What equation describes the force exerted by a magnetic field on a charge particle (assuming particle is moving perpendicular to field lines)?
F = Bqv
What is a cyclotron and what is a modern day application of it?
A cyclotron is a particle accelerator and is still used in radiotherapy, despite having been overtaken in terms of particle acceleration.
What do the fingers used for Fleming’s Left Hand rule denote respectively?
Thu’M’b = Motion (Force);
‘F’irst Finger = Field;
Se’C’ond Finger = Convential Current.
Describe how a cyclotron works.
A cyclotron consists of two semi-circular boxes called dees which are separated by a small gap and which have a magnetic field flowing into them perpendicular to their plane. They are contained within an evacuated chamber and the magnetic field is uniform. Charged particles (protons) are produced by a device in the gap between dees and move into the first dee (D1) where they move in a semicircular path due to the magnetic field. A high frequency alternating voltage in the form of a square wave is applied between the dees creating an electric field in the gap between dees but not within the dees themselves. Initially D1 is negative with respect to D2 so the protons are attracted towards D1. However, the alternating voltage alternates just as the charged particles leave the dees so that they are always attracted and accelerated towards the next dee, meaning the pick up speed and the radius of the semicircular path becomes bigger each time. As they reach their desired radius the charged particles are deflected by a charged metal plate towards the target area.
What equation describes the radius of charged particles moving through a magnetic field?
r = mv/Bq
What is a mass spectrometer and why is it so useful?
A mass spectrometer is a machine made to measure the masses of individual atoms and is so sensitive that it can distinguish between different isotopes of the same element.
Describe and explain how a mass spectrometer works.
A mass spectrometer is made up of three main parts: the ion accelerator, velocity selector and ion separator. In the ion accelerator, the atoms are vaporised to become ions and accelerated by an electric field. The ions then pass into a chamber consisting of crossed electric and magnetic fields, this is the velocity selector. The chamber only allows ions of a particular velocity to pass through the chamber into the ion separator. After leaving the velocity selector the ions are collimated into a narrow beam which passes through a strong perpendicular magnetic field in the ion separator. They are then deflected into semicircular paths where the radius is dependent on mass, velocity, Magnetic Flux Density and and charge on the particle which means the radius depends only on the ratio of mass over charge of the ions (as velocity and Magnetic Flux density are constant).
What is the unit of magnetic flux?
The Weber (Wb).
What is magnetic flux?
Magnetic Flux (Φ) is the number of magnetic field lines in a given area.
What is Lenz’s Law?
When an e.m.f. is generated by a change in magnetic flux in accordance with Faraday’s Law, the polarity of the induced e.m.f. is such that it produces a current to generate a magnetic force that opposes the change which produced the e.m.f.
How is an emf induced across a conductor moving through a magnetic field?
In the example of a length of wire, each individual electron experiences a force to one end of the wire and so electrons begin to accumulate at that end. After a while the accumulation of electrons provide a strong enough repulsive force on all other electrons that the other electrons are held in equilibrium and so the emf becomes constant. The wire is now analogous to parallel plates.
