12: Magnetic Fields Flashcards
Which direction to field lines go on magnets?
North to south
What is a transformer?
Devices that make use of electromagnetic induction to change the size of the voltage for an alternating current
What is meant by a magnetically soft material
A material whose magnetisation disappears after the current is removed
how do transformers work
- an alternating current flowing in the primary coil causes the core to magnetise, demagnetise and remagnetise continuously in opposite directions
- this produces a rapidly changing magnetic flux across the core
- the rapidly changing magnetic flux in the iron core passes through the secondary coil, where it induces an alternating voltage of the same frequency
Why is magnetically soft material needed for the core of a transformer?
due to the rapidly changing magnetic flux across the core
- could be iron or a special alloy
How to work out the voltage across primary coil
Vp = Np x ΔΦ/Δt
How to work out the voltage across the secondary coil
Vs = Ns x ΔΦ/Δt
What is the equation for an ideal transformer
Ns/Np = Vs/Vp
What do step-up transformers do
Increase the voltage by having more turns on the secondary coil than the primary
What do step-down transformers do
Reduce the voltage by having fewer turns on the secondary coil than the primary
What are eddy currents
- looping current induced by the changing magnetic flux in the core
- they create a magnetic field that acts against the field that induced them, reducing the field strength
- they also dissipate energy by generating heat
How can you reduce the effects of eddy currents?
laminating the core
- this involves having layers of the core separated out by thin layers of insulator, so a current can’t flow
other ways transformers can be inefficient (apart from eddy currents) and how to fix it
- Heat is generated by resistance in the coils
- this can be reduced by using wires with low resistance (thick copper wires)
- energy is needed to magnetise and demagnetise the core, and this energy is wasted as it heats the core
- this can be reduced by using a magnetically soft core
how do you work out the efficiency of a transformer
Is x Vs/ Ip x Vp
What is an alternating current
A current that changes direction with time
how can you measure alternating current using an oscilloscope
- the trace is made by an electron beam moving across a screen
- the vertical height of the trace shows the input voltage at that point
- you can control the volts per division of the grid using the Y-gain control dial
What is the pattern produced by an alternating current source
A regularly repeating sinusoidal waveform
What is the pattern produced by a direct current source
The source is always at the same voltage, so you get a horizontal line
What happens if you turn off the time base of an oscilloscope
- ac displayed as a vertical line
- dc displayed as a dot
What are the three pieces of information you can get from an ac oscilloscope trace
- time period T
- peak voltage V0
- peak-to-peak voltage
what is the purpose of a root mean square voltage
creates an average ac voltage so it can be compared with dc
How do you work out the Vrms of a sine wave
Vrms = V0/sqrt(2)
where V0 is the peak voltage
How do you work out the Irms of a sine wave
Irms = I0/sqrt(2)
where I0 is the peak current
How do you work out the average power of an ac supply
average power = Irms x Vrms
What is the Vrms of UK mains voltage
230
What is Faraday’s law
Induced emf is directly proportional to the rate of change of flux linkage
What does the gradient of a graph of flux linkage against time represent
magnitude of emf
What does the area under a graph of magnitude of emf against time represent
flux linkage change
What is Lenz’s law
the induced emf is always in such a direction as to oppose the change that caused it
How can you alter the shape of the graph of induced emf
- Increasing the speed of rotation will increase the frequency and increase the maximum emf
- increasing the magnetic flux density B will increase the maximum emf, but will have no effect on frequency
How do you induce emf in a rotating coil
- when a coil rotates uniformly in a magnetic field, the coil cuts the flux and an alternating emf is induced
- the amount of flux cut by the coil is given by NΦ = BANcosθ
- as the coil rotates, θ changes so the flux linkage varies sinusoidally between +BAN and -BAN
How can you investigate flux linkage
- with a search coil
- the stretched metal spring acts as a solenoid when connected to an alternating power supply
- the alternating supply means the magnitude field field of the solenoid is constantly changing, so the flux is changing therefore an emf is induced
- search coil is connected to an oscilloscope to record the induced emf
- position the search coil so that it is about halfway along the solenoid and within the inside of the solenoid
- ## rotate the search coil so its angle to the solenoid and magnetic flux lines changes by 10 degrees and the record the induced emf
what is magnetic flux
- the magnetic flux density multiplied by the area
- the number of magnetic field lines passing through an area
What is the equation for magnetic flux
Φ = BA
What is electromagnetic induction
- when there is a relative motion between a conducting rod and a magnetic field, electrons in the rod experiences a force which causes them to accumulate at one end of the rod
- this induces an electromotive force across the ends of the rod
what is flux linkage
the magnetic flux in a coil multiplied by the number of turns on the coil
what is the equation for flux linkage
NΦ = BAN (cosθ) if not normal
what is the equation for a force acting on a single particle moving through a magnetic field
F = BQv
what is the equation for the radius of the path followed by charged particles in a magnetic field
r = mv/BQ
what does the radius equation tell you
- the radius of curvature increases if the mass or velocity of the particle increase
- the radius of curvature if the strength of the magnetic field or the charge on the particle increase
the frequency of rotation of a charge particle in a magnetic field…
is independent of its velocity
what are some uses of cyclotrons
- producing radioactive tracers
- high-energy beams of radiation for use in radiotherpay
what is a cyclotron made up of
- two hollow semicircular electrodes with uniform magnetic field applied perpendicular to the plane of electrodes, and an alternating potential difference applied between the electrodes
how does a cyclotron work
- charged particles are produced and fired into one of the electrodes, where the magnetic field makes them follow a (semi)circular path and then leave the electrode
- an applied potential difference between the electrodes then accelerates the particle across the gap until they enter the next electrode
- as the particle’s speed is slightly higher, it will follow a circular path with a larger radius before leaving the electrode again
- at this point the direction of the potential difference is reversed so the particle is accelerated again before entering the next electrode
- this process repeats as the particle spirals outwards, increasing in speed, before eventually exiting the cyclotron
how can you investigate the force on a current-carrying wire
- use a top pan balance
- the dc current should be connected to a variable resistor so that you can alter the current
- use the variable resistor to change the current and record the new mass
- repeat until you have tested a large range of currents
- convert mass into force by xg
- do a graph of F against I
- gradient = Bl
the closer together magnetic field lines are…
the stronger the field is
what does your thumb represent in right hand rule
current
what do other fingers represent in right hand rule
magnetic field
what does your first finger represent in the left hand rule
magnetic field
what does your second finger represent in the left hand rule
current
what does your thumb represent in the left hand rule
motion/force
what is magnetic flux density
the force on one metre of wire carrying a current of one amp at right angles to the magnetic field
how do you work out the force of a current-carrying wire perpendicular to a magnetic field
F = BIl
