Current and magnetic field into & out of page notation
Right hand grip rule
How a solenoid electromagnet works
Motor effect/ Flemings left hand rule
Magnetic fields around 2 adjacent bar magnets with same and opposite poles
Motor effect formula
How motor works
Split ring commutator role in motor
Motion of electron in mag field going into page
frequency of rotation of a charged particle in a magnetic field going into page
Magnetic flux
Flux linkage
Faradays law
Straight wire (length l) moving through magnetic field (b) at velocity v - faraday
Rotating coil of wire - faraday
lenz law: Straight wire (length l) moving through magnetic field (b) at velocity v
Magnet moved towards a solenoid - lens law
when the north pole of a magnet is moved towards the end of a solenoid there is a change is flux linkage in the solenoid (flux linkage increases as the magnet is moved closer) This change in flux linkage induces an emf across the solenoid. If the solenoid is part of a complete circuit, a current flows through the solenoid in a direction that causes the end of the solenoid nearest the north pole of the permanent to also become a north pole. The two poles repel, slowing down the relative motion between the magnet and the solenoid and hence reducing the change in flux linkage - i.e. opposing the change in flux caused by the relative motion.
(If a south pole were produced at the end of the solenoid nearest the magnet the two would attract each other, the relative speed of approach would increase, the rate of change of flux linkage would increase, the induced emf would increase, the current in the solenoid would increase, and we would be in an impossible positive feedback loop where energy was being created out of thin air!)
Magnet dropped down a copper pipe - lens law
as magnet fallsthere is a rate of change of fluxz linkage within the copper tube. This induces an EMF & hence current in the tube. This produceds a secondary magnetic field which opposes the magnetic field of the falling magnet causing a repulsive force making it move down more slowly
transformer derivation
ways to reduce Inefficiencies in a transformer
- The windings, are made of extremely low-resistance coppers to minimise energy loss due to heat generated in the coils.
- Eddy currents is another cause of energy loss in a transfer. They resist the field that formed them due to Lenz’s law, reducing the flux density of the field and generating heat, resulting in energy loss. using a laminated iron core, which consists of layers of iron sandwiched between layers of an insulator can reduce inefficiencies, because eddy currents cannot flow through the insulator, their amplitude is reduced
- Also in order to magnetise and demagnetise the core, energy is required, and this energy can be wasted as it heats up the core. A magnetically soft material that magnetises and demagnetizes quickly should be used to minimize this effect.
Eddy currents - what are they
Cyclotron
A cyclotron consists of two hollow semi-circular electrodes (sometimes known as “dees” due to their D-shape) with an uniform magnetic field applied perpendicularly to the electrodes’ plane and an alternating potential difference applied between them.
Charged particlescome from the source into one of the electrodes, where they follow an oval/circular path and eventually exit the electrode due to the magnetic field. An applied p.d. between the electrodes accelerate the particles across the gap until they enter the next electrode.
The direction of the p.d. will have been reversed when the particles enter the next electrode (i.e. the charge of the electrodes will switch so that the negative electrode will now be the positive electrode or vice versa, attracting the particle), and the particle will accelerate again before entering the next electrode. The process repeats as the particles spiral outwards, increasing in speed, before eventually exiting the cyclotron.
Because the particle’s speed increases slightly each time it crosses the electrodes, it will follow a larger-radius circular path before exiting the electrode.
We need an alternating p.d. because if the p.d. wasnโt alternating the particle would slow down after leaving the second electrode. As a result, every half turn, we’ll have to reverse the polarity of the two electrodes.
The particles will always spend the same amount of time in each electrode or cross the gap at the same time since the frequency of circular motion is independent of radius. The alternating p.d. will have a constant frequency if ๐ต,๐, and ๐ are constant.
velocity selector
Mass-Spectrometer
