Year 2 Chapters 7,8,9- Electromagnetism Flashcards
When is a magnetic field induced around a conductor and what shape is it?
When current passes through a long, straight, current-carrying conductor. The field lines form concentric rings around the conductor according to the right hand thumb rule.
What is magnetic field strength/ magnetic flux density?
The force per unit current pure unit length on a current-carrying wire placed at 90 degrees to the field lines, measured in Tesla. Measure of strength of the field
What is the equation and rule for finding the force exerted on a current carrying wire in a magnetic field?
F=BIlsinϴ
Rule is Fleming’s left hand rule to find direction of the force exerted, ϴ is angle between current and magnetic field lines
How do you find the magnitude and direction of the force acting on a charged particle moving through a magnetic field?
F=BQv, use Fleming’s left hand rule with I replaced by velocity, I being the direction that the positive flow of charge is in. Velocity is perpendicular to magnetic field lines. If negatively charged, I is in opposite direction
What is always true about the force on a charged particle in a magnetic field?
Always perpendicular to the particle’s direction of motion as seen through F=BQv and Fleming’s left hand rule, resulting in circular motion of the particle as it acts as a centripetal force
What are some applications of the force acting on a charged particle in a magnetic field?
Cyclotrons and mass spectrometers
What is a cyclotron?
A particle accelerator made up of two D shaped electrodes opposite each other with a charged particle source in the centre. The electric field between the electrodes changes direction each time a particle moves from one electrode to the other, causing the particle to accelerate.
How does a cyclotron work?
Uniform magnetic field is perpendicular to the plane of electrodes and a high frequency alternating voltage is applied between electrode. Charged particle moves from centre of one dee and is deflected in a circular path inside the dee due to magnetic field (FLHR). Between the dees, it is accelerated by the E field, increasing v so the r of circular path increases
How do you show that the time of each loop in a cyclotron is independent of the radius of the loop?
Equate centripetal force and force on charged particle in magnetic field
Why is a charged particle in a cyclotron always accelerating?
In the dees, the force, F=BQv due to magnetic field is always perpendicular to direction of motion so direction of velocity is constantly changing in circular motion. Between the dees,, the particle is accelerated by the alternating electric field so magnitude of v increases, so always accelerating
How does a velocity selector in a mass spectrometer work?
After ionisation, charged particles pass between two plates with pd between them, also a uniform magnetic field perpendicular to direction of the particles motion. Force from E field = EQ, force from B field = BQv, for selected velocity EQ=BQv so v=E/B. If v too high, Fb>Fe so deflects. If v too low, Fe>Fb so deflects
What is a magnetic field?
A region around a magnet or current-carrying wire that will exert a force on any other magnet or current-carrying wire placed within it
What is magnetic flux?
A value describing magnetic field or field lines passing through an area. It is the product of magnetic flux density and the perpendicular area it passes
through.
What is magnetic flux linkage?
The magnetic flux multiplied by the number of turns, N, of the coil.
How do you find magnetic flux of a coil not perpendicular to magnetic field lines?
ϕ=BAcosθ, θ is angle between field lines and normal to plane of coil, same applies to magnetic flux linkage
What is electromagnetic induction?
When an emf is induced in a wire/ conducting rod when it is moved relative to a magnetic field
What is Faraday’s Law?
The magnitude of the induced emf is equal to the rate of change of flux linkage
What is Lenz’s Law?
The direction of an induced current is such that it opposes the change causing it
What is the x axis on an oscilloscope called and what changes when it is turned on or off?
The time base, its scale can be adjusted. When turned off, either a dot or vertical line (ac or dc) will show on the oscilloscope
What is the y axis on an oscilloscope called?
y gain, its scale shows variation of voltage
What is the peak voltage?
Distance from equilibrium to highest or lowest point
What is peak to peak voltage?
Distance from minimum to maximum point
What is the root mean square voltage?
Average of all squares of possible voltages, average value of voltage output, it has the same heating effect as dc current of the same value
What is the time period?
Distance from peak to peak
What are eddy currents?
Circular currents formed in the iron core of a transformer that generate heat and so reduce efficiency
What are some measures taken to increase efficiency in transformers?
Laminations in core, layers of insulating material to reduce size of eddy currents formed as they cannot pass through insulator
Low resistance wires in primary and secondary coils (thick wire)
Magnetically soft iron core to allow for easy magnetisation and demagnetisation
What does a step up transformer do?
Increases voltage and decreases current by having more turns on secondary coil than primary
What does a step down transformer do?
Decreases voltage and increases current by having fewer turns on secondary coil than primary
How do transformers work?
Ac in primary coil causes a changing magnetic field in the coil and core, this induces a changing magnetic field which passes through soft iron core, this links to and interacts with secondary coil as it cuts flux of magnetic field continuously due to ac, causing an alternating pd in secondary coil. If connected to a circuit, causes ac in second circuit
