M5
L1, L2
L3, L4
L5, L6
Questions
200
Name TWO assumptions made in projectile motion.
Only force is gravity, gravity is constant, etc. etc.
200
Around a permanent magnet, magnetic field lines travel from the ____ to ____ Pole.
BONUS: Inside an electromagnet, magnetic field lines travel from the ____ to ____ Pole.
Around magnets: North to South
BONUS:
Inside magnets: South to North (completing the loop)
200
State formula for flux.
Flux = BA
(Where B perp. A)
200
"A step down transformer has a higher number of secondary windings."
True or False?
False.
200
A DC motor has 10 turns, 10A current, 10cm sided square coil, 0.01T B field.
Find the torque it produces when the coil area is perpendicular to the B field.
0.
400
Explain how changing the width of the screwdriver handle, and length of the screwdriver shaft, affects its effectiveness.
Torque = F_perp * r
Fatter handle = larger r (force arm in plane of rotation)
Longer shaft = no effect (length not in plane of rotation)
400
Myknee Cyprus swings due NORTH-EAST on a wrecking ball, inside Earth's NORTH pointing magnetic field.
What's the direction of force on the electrons within the ball?
RHPR - reversed for electrons: vertically UP.
400
Describe Lenz's Law.
The direction of an induced emf (induces a B field that) opposes the change in magnetic flux that caused it.
400
Fix it Felix is fixing an unfelicitous DC motor which lost its split ring commutator.
What is the SRC's role in DC motors?
BONUS: How would a DC motor without an SRC behave?
SRCs reverse direction of current every half turn, flipping force on coil to maintain unidirect. torque, thus spin.
BONUS: With unidir. current and thus force, torque would flip every half turn, making rotor oscilate/turn back and forth.
400
Big yellow transformer Rumblebee has 10000 primary turns, 10000A primary current, and 1000A secondary current.
How many secondary turns does it have?
Ns/Np = Vs/Vp = Ip/Is
Ns = Np * Ip/Is
Ns = 10000 * (10000 / 1000)
Ns = 100 000
600
Compare the orbital velocities and periods of LEOs and GEOs.
v = sqrt(GM/r), i.e. v decr. as r incr.
r^3 = k * T^2, i.e. T incr. as r incr.
LEOs orbit lower than GEOs, so they have higher orbital velocity and lower orbital period.
600
Compare and contrast RHPR and RHGR.
Both describe direction not magnitude.
Palm: For forces on moving charges inside B field.
Thumb is v/I/l
Fingers is B
Palm is F
Grip: For B field induced by moving charges.
Thumb/fingers interchangable between B, I.
600
Describe the shape of the magnetic field around two parallel conductors which have currents flowing in OPPOSITE DIRECTIONS.
600
Megatorn: "PERM, by the Vp/Vs = Is/Ip equation and conservation of power, changing the primary voltage can change the secondary current!!"
Optimal Perm: "NO MEGATORN!!! The transformer windings make them mostly independent!!"
Explain who is right.
Optimal Perm is more correct.
By Vp/Vs=Np/Ns, the voltage ratio must equal turns ratio, meaning Vs will rise proportionally if Vp is increased, not Is. This already conserves power, so Is will not change.
600
DOUBLE POINTS :O
100kg copper man is trying to skate on a pair of 100A power lines. He attaches 100cm straight copper wires which run 100A DC to the bottom of both his shoes and aligns them parallel to the power lines.
How many cm of lift is copper man getting off the power lines?
F_bothpairs = F_g = mg = 980 N
F_onepair = 490 N
F/l = M/2pi * I1 I2 / r
r = l/F * M/2pi * I1 I2
r = 1/490 * 4pi *10^-7 / 2pi * 100^2
r = 2/490 * 10^-3
r = 4.08... * 10^-6 m
r = 4.1 * 10^-4 cm
800
Name ALL the other parameters you can derive from t_flight alone, assuming the ground is flat and g=9.8ms^-2.
t_maxheight
u_y
v_y
h_max
(also instantaneous v_y and s_y for any t)
800
Electrons are being shot obliquely into a magnetic field.
What happens to their path and period when the launch velocity is decreased?
mv^2 / r = qvB
r = mv / (qB)
Therefore, the loop radius and distance between loops in its helical motion both decrease.
T = 2pi*r / v
T = 2pi * m / (qB)
Therefore, the period stays the same.
800
Explain the process of what happens when a South pole is pulled away from a face-on solenoid, versus an aluminium block.
Same resisting force, but larger from block because more current mobilised.
B lines enter South pole from block/solenoid --> pulling away means LESS B OUT from block/sol. --> induces emf to induce INCREASE B OUT block/sol. --> a North face electromag. --> attraction resists pulling away.
800
Describe how the direction AND magnitude of force on each side of a DC motor's square coil varies over one rotation, if its area is initially aligned parallel to the B field. (Account for SRC)
2 Torque-exerting sides:
Magnitude is always constant.
Direction is opposite, up & down perp. to B field. SRC will flip dir. when area is perp. to B field.
2 Non-torque sides:
Magnitude varies sinusoidally. 0 when || to B field, max when perp. to B field.
Direction is either both in or both out, varied by SRC.
800
The Broker operates a wrecking ball to destroy the Betmobile, which is driving steadily around a frictionless, circular banked track. It is angled at 45deg, and spans 1960m in diameter.
What is the minimum angular velocity Broker needs to wipe out Betman?
v = sqrt(gr*tanT)
v = sqrt(9.8*980*1)
v = 98ms^-1
w = v/r
w = 98/1960
w = 1/20 = 0.05 rad/s
i.e. need more than 0.05rad/s to work.
1000
A double-sided loop de loop track accommodates 2 separate carts at once, which start at the bottom of the loop with different initial velocities (u). Both centres of mass are effectively on the track, but one cart goes over the track, while the other goes under.
What is the relationship between the top cart's u_max, and the bottom's u_min to make the loop? Explain.
Top: At peak, F_c = F_g - N
N = 0 at v_max, therefore mv^2 /r = mg
i.e. v = sqrt(gr)
Bottom: At peak, F_c = F_g + N
N = 0 at v_min, therefore same, v = sqrt(gr)
By K --> U, top v_max = bottom v_min implies top u_max = bottom u_min. (centres of mass both effectively in the track)
1000
Planet Ironball has a strong magnetic field, which is aligned parallel to the ground at the equator. A cannon there is launching a charged iron ball 45deg up from parallel to the magnetic field, over FLAT ground.
Roughly describe what happens.
It would try to perform helical motion parallel to the field, but hit the ground after half a rotation. It would travel at constant v along B field dir.
The semicircle will be slightly ovular because of gravity (it's actually an oblique helix, search 'charge in crossed/perpendicular E and B field').
1000
Brownbeard has an AC transmitting coil and a detecting coil.
Explain how he could use the principle of eddy currents to make a metal detector.
1. House detecting coil in proximity to transmitter coil, such that an AC current inducts across.
2. When transmitter coil nears conductive metal: AC current --> changing B field --> dFlux --> emf --> eddy current --> opposing B field --> reduces input to detector coil.
3. Differences/reduction to detector input = metal detected.
1000
Shooby Doo and Scaggy are hunting a poltergeist with an electric drill. But unfortunately, the poltergeist replaces the North pole in its motor with a very weak South pole.
Explain what happens to their drill.
It will spin towards faster maximum, but exerting weaker torque.
1. The drill continues spinning in same direction because the fields superimposes to mostly point in same dir.
2. Spins faster because destructive interf. of B field reduces dFlux per rotation, reducing induce back emf, allowing higher speed to be reach before back emf = supply emf
3. Less torque by lower B in T = NIABsinTheta
1000
Dr. Doofensmartz dun goofed his smarts and forgot how his mass spectrometer's electric 10N/m and 1T magnetic fields were oriented!
By firing 10kg, -10C balls into the chamber at 1m/s rightward, he observed they accelerate downward at 0.8ms^-2.
Find which directions the E and B fields are oriented.
Define up as pos., examining vert. component:
F_Net = F_E + F_B - F_g
ma = Eq + qvB - mg
m(a+g)/q = E + vB
10(-0.8 + 9.8)/(-10) = E + B
-9 = ±10 ±1
Therefore E = -10N/m, B = +1T (producing upward force on + charge)
Therefore E downward, B into page.