LOK Flashcards
Describe and explain the shape of an IV graph for a resistor.
Linear graph through origin. +ve grad
Ohmic conductor therefore constant resistance regardless of current or temperature.
Current is proportional to voltage
Resistance is constant
R is proportional to 1/grad so higher resistance creates a shallower gradient
Describe and explain the shape of an IV graph for a filament-bulb.
Gradient decreases as current increases.
Becauses resistance increases as current increases
Lattice ion vibration amplitude increaes as current increases therefore more collisions with charge carrying electrons therefore increasing resistance.
Describe and explain the shape of an IV graph for a diode.
Flat until threshold voltage is reached. at which point current increases quickly.
This is because diodes only allow one way current.
No current until threshold voltage.
When no current, its resistance is infinite
Describe and explain the shape of an IV graph for a thermistor.
Resistance decreases as current increases
Because increasing temperature increasing the charge carrier density.
So gradient increases as current increases
Relativistic motion of muons in atmosphere.
Compare the distance/velocity/time w/ a calculation
Identify that the muon travels at relativistic speeds
So time dilation occurs
Therefore increasing muon lifetime
Loopty loop mechanics question - find min height release
F = mv2 / r = mg
Don’t do it terms of energy, do it in terms of force as circular motion
Explain how transformers work. (primary has less turns than secondary)
AC in 1* coil produces a changing B field
Mag field linked to 2* coil via core
Changing B field in 2* coil induces an emf in 2* coil
More turns in 2* coil increases the emf induced (Faraday’s Law /eqn)
Explain why VT = V1 + V2
Conservation of energy / kirchoff’s law
So sum of pds in circuit must equal the emf applied
Explain why IT = I1 + I2
Conservation of charge
Sum of the current into a junction must equal to the sum of the currents out of the junction
Approach to FLHR questions
FBI
Perpendicular fields/force therefore ….
Interacting fields produce a force
Explain how a LINAC works.
Set of drift tubes in a line
Electrons accelerated by E field
Acceleration only between tubes
Adjacent tubes have opposing polarity
Alternating p.d. so tube the electron emerges from is -ve and tube infront of it is positive
Time spent in each tube must be the same (due to constant frequency of AC) so tubes get progressively longer (makes up for acceleration)
Explain how a cyclotron works.
Alternating p.d.s and E field produced
E field accelerates protons between the dees
Magnetic field perpendicular to the place of the dees
So proton path is curved by the mag field
As velocity of proton increases, radius of the path increases
So time that proton spends in a dee remains constant
Why do astronauts feel ‘weightless’ in low orbit?
Astronauts have weight in low orbit
But weight acts as centripetal force
Our experience of weight is the reaction force on the floor
So no reaction force leads them to feel ‘weightless’
B fields -> force on wire questions
Current carrying wire moves in a mag field
Therefore it experiences a force
How does current vary with time when a DC current is applied to a motor w split rings?
Current due to the switch closing/DC current being applied
So coil rotates (due to current carrying wire in mag field)
As coil rotates, cuts mag field therefore change in mag flux linkage
So emf induced due to Faraday’s law l
Opposes current that causes it due to Lenz’s law
The faster the rotation, the larger the effect of back end therefore the smaller the current
