# Multiple Choice (1 Viewer)

#### JWNH48

##### New Member
Quite sure it is constant totque since plane of coil is always parallel to the magnetic firld. It cannot be a since the torque doesnt change direction
SHoud be D, yes it should be a constant torque, but what about the emf being generated? torque=BIA the back emf will decrease I therefore decreasing torque

#### yasminee96

##### Active Member
I said torque decreases. Wouldn't back emf increase as the motor speeds up, hence decreasing current. Since t=nBIAcosø, torque would decrease?
hmm...not sure but isn't back emf only with generators <>

#### Hypem

##### Member
I rushed Q20 MC :/
I didn't even bother working out the threshold frequency and went straight to D.

If people are correct here, then I think I got 18/20

#### pls

##### New Member
hmm...not sure but isn't back emf only with generators <>
Im pretty sure that when a motor spins it becomes a 'de facto' generator, since it is effectively a conductor moving in a magnetic field and hence there will be a current created which opposes the change in flux (i.e. a current in the opposite direction of supply current) so total current decreases. Think it applies for both motors and generators (sound like im answering a fucking question in the exam, sorry) but yeah

#### tutorhunt

##### Member
torque defintely decreases, emf supplied - back emf = IR , considering R is constant, and it specifically states FROM START to RUNNING, it implies that we take into account back emf, back emf at time 0 = 0, and increases over time

#### yasminee96

##### Active Member
Im a tad confused with Q13...isnt magnetic flux density directly proportional to magnetic flux (Φ=BA)? So then shouldn't the greater magnetic flux and greater magnetic flux density be both P :/

#### AnimeX

##### Member
damnn i screwed up looking at like a 13...

#### panda15

##### Alligator Navigator
Im a tad confused with Q13...isnt magnetic flux density directly proportional to magnetic flux (Φ=BA)? So then shouldn't the greater magnetic flux and greater magnetic flux density be both P :/
Magnetic flux is how many field lines there are through a certain area, and Q has 9 field lines, whereas P only has 4, so Q has more flux, P is more flux dense.

#### superSAIyan2

##### Member
Im a tad confused with Q13...isnt magnetic flux density directly proportional to magnetic flux (Φ=BA)? So then shouldn't the greater magnetic flux and greater magnetic flux density be both P :/
P and Q are the copper rings, not the names of the magnetic fields.
theres more flux threading the plane of Q, however theres more flux per area threading P
hence D

#### Moey2200

##### New Member
20 is d, doubling the intensity only double the amount of electrons it even says it in my textbook lol intensity doesnt change the kinetic energy only frequency of the light can do that

#### blinx96

##### Member
yeh for 17 i put D lol

#### panda15

##### Alligator Navigator
20 is d, doubling the intensity only double the amount of electrons it even says it in my textbook lol intensity doesnt change the kinetic energy only frequency of the light can do that
Frequency of the light was under the threshold frequency of Y, so no photons are emitted, so it's b).

#### Menomaths

##### Exaı̸̸̸̸̸̸̸̸lted Member
20 is d, doubling the intensity only double the amount of electrons it even says it in my textbook lol intensity doesnt change the kinetic energy only frequency of the light can do that
Moey, you're forgetting about how the light's frequency is not enough to eject the electron that has a higher threshold frequency.

#### Fizzy_Cyst

##### Well-Known Member

1) b
2) d
3) a
4) a
5) d
6) c
7) a
8) d
9) b
10) c
11) b
12) a
13) d
14) b
15) a
16) d
17) d
18) c
19) c
20) b

Torque one is tricky! But, the most correct answer would have to be D as net current through the coil would be decreasing due to Back EMF

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#### hjed

##### Member
1B
2D
3B <- using right hand rule this is what I got, I remember this experiment does rotate around the magnet
4A
5D
6C
7A
8D
9B
10C
11B
12D <- both a and d are correct, but a is not completely correct as it dosen't mention balanced forces.
13D
14A -< e=V/d so as d decreases, e increases
15B -< pretty sure this is wrong, could someone explain why though?
16D
17C -< missed the back emf thing, was too busy working out that it wasn't effect by the rotation
18C
19C
20B

#### Locus

##### Member

1) b
2) d
3) a
4) a
5) d
6) c
7) a
8) d
9) b
10) c
11) b
12) a
13) d
14) b
15) a
16) d
17) d
18) c
19) c
20) b

Torque one is tricky! But, the most correct answer would have to be D as net current through the coil would be decreasing due to Back EMF
For 5, wasn't he trying to produce a spark in the receiver.
He never knew it was radio waves and wasn't it the other experiment (interference one) which showed the properties of the radio waves i.e. travels at the speed of light.

#### panda15

##### Alligator Navigator
1B
2D
3B <- using right hand rule this is what I got, I remember this experiment does rotate around the magnet
4A
5D
6C
7A
8D
9B
10C
11B
12D <- both a and d are correct, but a is not completely correct as it dosen't mention balanced forces.
13D
14A -< e=V/d so as d decreases, e increases
15B -< pretty sure this is wrong, could someone explain why though?
16D
17C -< missed the back emf thing, was too busy working out that it wasn't effect by the rotation
18C
19C
20B
At least 4 wrong.
For 15, w is the angle that the electrons make with the field. They're going across at 90 degree angles.

#### Ozzyy

##### Member
For 5, wasn't he trying to produce a spark in the receiver.
He never knew it was radio waves and wasn't it the other experiment (interference one) which showed the properties of the radio waves i.e. travels at the speed of light.
That ! That's why i was confused with this question. I was like, he had no idea what the photoelectric effect was when he observed it. It wasn't anything to do with black body radiation. So i was left debating between b and d and i was confused at d because he wasn't looking for radio waves specifically i'm pretty sure but electromagnetic radiation in general

#### hjed

##### Member
That ! That's why i was confused with this question. I was like, he had no idea what the photoelectric effect was when he observed it. It wasn't anything to do with black body radiation. So i was left debating between b and d and i was confused at d because he wasn't looking for radio waves specifically i'm pretty sure but electromagnetic radiation in general
Pretty sure it is taught as investigating radio waves, I completely missed the induction option though. In hindsight that is probably right [and with that goes my chance of getting 90RAW ]

#### tutorhunt

##### Member
14/20, wow so many silly mistakes.