Section 11 - Magnetic Fields Flashcards
How is a magnetic field induced around a current carrying wire?
- The field lines are concentric circles centered on the wire
- The direction of a magnetic field around a current carrying wire can be worked out with the right-hand rule
- If you loop the wire into a coil, the field is doughnut shaped, while a coil with length (solenoid) forms a field like a bar magnet
What is magnetic flux density defined as?
The force on one metre of wire carrying a current of one amp at right angles to the magnetic field Tesla or weber per metre squared
What path do charged particles in a magnetic field take?
- Flemming’s left hand rule says that force on a moving charge in a magnetic field is always perpendicular to its direction of travek
- This is the condition for circular motion
How do you derive the radius of the circular path followed by a charged particle in a magnetic field?
- F=mv^2/r
- F=BQv
- mv^2/r = BQv
- r=mv/BQ
How are cyclotrons used in practical applications?
Produce radioactive tracers or high energy beams of radiation used in radio therapy
What are cyclotrons made up of?
Two hollow semicircular electrodes with a uniform magnetic field applied perpendicular to the plane of the electrodes, and an alternating potential difference applied between the electrodes
How do cyclotrons work?
- Charged particles are fired into one of the electrodes
- The magnetic field makes them follow a circular path and then leave the electrode
- An applied pd between the electrodes accelerates the particle across the gap until they enter the next electrode
- Because the particle’s speed is slightly higher, it will follow a circular path with a larger radius before leaving the electrode again
- The pd is reversed so the particle is accelerated again before entering the next electrode
- This repeats as the particle spirals outwards, increasing in speed, before exitting the cyclotron
What is magnetic flux density?
A measure of the strength of a magnetic field (number of field lines per unit area)
What is electromagnetic induction?
- When there is relative motion between a conducting rod and a magnetic field, the electrons in the rod will experience a force which causes them to accumulate at the end of one rod
- This induces an emf across the ends of the rod
How can you induce an emf in a flat coil or solenoid and why does that happen?
- Moving the coil towards or away from the poles of a magnet
- Moving a magnet towards or away from the coil
- Caused by the changing magnetic flux that passes through the coil
- If the coil is a part of a complete circuit, an induced current will flow through it
What does the size of the emf depend on?
- Magnetic flux passing through the coil
- Number of turns in the coil that cut the flux
What is the rate of change of flux linkage?
A change in flux linkage of one weber per second will induce an electromotive force of 1 volt in a loop of wire
What is Faraday’s law?
The induced emf is directly proportional to the rate of change of flux linkage
What is Lenz’s law?
The induced emf is always in such a direction as to oppose the change that caused it
Why is lenz’s law negative?
- Shows the direction of induced emf
- An induced emf will oppose the change that caused it
- This agrees with the principle of the conservation of energy
- The energy used to pull a conductor through a magnetic field against the resisitance caused by magnetic attraction, is what produces the induced current
How can Lenz’s law be used to find the direction of an induced emf and a current in a conductor travelling at right angles to a magnetic field
- Using flemming’s left hand rule point your thumb in the direction of the force of ressistance which is in the opposite direction to the motion of the cinductor
- Point your first finger in the direction of the field
- Second finger will give the direction of the induced emf
What is the graph of flux linkage against t?
cos graph
What is the graph for emf against t?
sin graph
How do transformers work?
- An alternating current flowing in the primary coil produces magnetic flux
- The changing magnetic field is passed through the iron core to the secondary coil, where it induces an alternating voltage of the same frequency as the input voltage
What is the difference between step-up and step-down transformers?
- Step-up increase the voltage by having more turns on the secondary coil than the primary
- Step-down reduce the voltage by having ferwer turns on the secondary coil
What are eddy currents?
- Looping currents induced by the changing magnetic flux in the core
- Create a magnetic field that acts against the field that induced them
- Reduce the field strength
- Dissipate energy by generating heat
How do you reduce eddy currents?
Laminating the core with layers of insulation
How do you minimise the resistance in the coils?
Thick copper wire is used which has a low resistance
How are transformers used in the National Grid?
- If you double the transmitted current, you quadruple the power lost
- Transformers step up the voltage for transmission through the national grid and then reduce it again for domestic use
