HSC Physics Question Thread

[bluecookie]

For the right hand grip rule, does the thumb point in the direction of conventional current flow or actual current flow?

[RuiAce]

For the right hand grip rule, does the thumb point in the direction of conventional current flow or actual current flow?

Always conventional current.

[bluecookie]

Thanks

[bluecookie]

When two wires are anti-parallel to each other, how does the left hand wire experience a force to the left?

[jakesilove]

When two wires are anti-parallel to each other, how does the left hand wire experience a force to the left?

Draw the field lines. Use crosses to signify lines into the page, and dots to signify lines out of the page. For anti-parallel wires, you'll find that you get only dots between the wires (or only crosses). As like charges repel, both wires will repel each other, resulting in the left wire experiencing a force to the left.

[Rathin]

When the coil of wire is perpendicular to the magnetic field and the current is in the same direction as the coil as its a DC circuit it will oppose the motion of torque, why is that? (split ring commutators are used to overcome this problem but I am not 100% on why this problem arises).

[Happy Physics Land]

When the coil of wire is perpendicular to the magnetic field and the current is in the same direction as the coil as its a DC circuit it will oppose the motion of torque, why is that? (split ring commutators are used to overcome this problem but I am not 100% on why this problem arises).

Hey Rathin!

Im guessing here that you are talking about how when the rotor coil changes the direction of its rotation after half a revolution (i.e. when it becomes perpendicular to the magnetic field). There's nothing that's opposing the original torque here, the only thing that has changed is the direction of the current through the rotor coil relative to the magnetic field. And according to our right hand palm rule, when the current direction reverses relative to the magnetic field, the force acting upon the coil would change as well. Be aware that the net current direction isn't changing here since a DC voltage is supplied. Current direction is only reversed RELATIVE TO THE MAGNETIC FIELD. Therefore since the direction of forces acting on the sides of the rotor coil changes, the direction of torque on the coil must change as well and thats why it starts rotating in a reverse direction.

What the commutator essentially does is it changes the direction of current flow in the rotor coil once every half a revolution to ensure that the current flow RELATIVE TO THE MAGNETIC FIELD is in a constant direction and therefore the force acting on the sides of the coil can be constant and hence torque/rotation can also be constant.

Best Regards
Happy Physics Land

[shreya_ajoshi]

Three moons around planet X have masses M,9M and 16M
If the distances of these moons form the planet's centre are R,9R and 16R respectively, calculate the ratio of their orbital speeds.


a) If the moons have the same orbital speeds, find the ratio of their orbital radii
b) If the orbital radii of the moons are the same, find the ratio of their orbital speeds

Mod edit: Merged duplicate posts. At times like this, please resort to the modify button instead of posting multiple times.

[RuiAce]

[Happy Physics Land]

Three moons around planet X have masses M,9M and 16M
If the distances of these moons form the planet's centre are R,9R and 16R respectively, calculate the ratio of their orbital speeds.


a) If the moons have the same orbital speeds, find the ratio of their orbital radii
b) If the orbital radii of the moons are the same, find the ratio of their orbital speeds

Mod edit: Merged duplicate posts. At times like this, please resort to the modify button instead of posting multiple times.

a)





Multiplying everything by 144:

[shreya_ajoshi]

Thank you!
However the answer to the first question is different to the answers the book provided.According to them, the answer is 12;4;3

[shreya_ajoshi]

Hi!
Could I please have help with this question. I have difficulty identifying t0 and tv

Star X is 8.0ly from Earth. A spaceship travels at 0.5c to reach the star.
a) Calculate how long the trip takes as measured by an observer on Earth
b) Calculate how long the trip takes as measure by the astronauts in the ship
c) Calculate the distance travelled as measure by the astronauts
d) Calculate the speed of the ship as measured by the astronauts

[Rathin]

Hi!
Could I please have help with this question. I have difficulty identifying t0 and tv

Star X is 8.0ly from Earth. A spaceship travels at 0.5c to reach the star.
a) Calculate how long the trip takes as measured by an observer on Earth
b) Calculate how long the trip takes as measure by the astronauts in the ship
c) Calculate the distance travelled as measure by the astronauts
d) Calculate the speed of the ship as measured by the astronauts

a) t=d/s
∴t=8/0.5
∴t=16 years

b) using the formula t(v)=(t(o))/sqrt((1-(v^2/c^2)))
sub in the values such that t(o)=16 and v=0.5c
∴t(v)=18.48 years

c)d=s*t
∴d=0.5c*18.48
∴d=2.77x10^9 m

d) This is a trick question as the astronauts cannot measure the speed inside their inertial reference frame unless sourced from an external reference frame.

[RuiAce]

Thank you!
However the answer to the first question is different to the answers the book provided.According to them, the answer is 12;4;3

That's correct if the masses were all M, M, M and not M, 9M, 16M. But I don't think that was what you typed at the time though...

[shreya_ajoshi]

a) t=d/s
∴t=8/0.5
∴t=16 years

b) using the formula t(v)=(t(o))/sqrt((1-(v^2/c^2)))
sub in the values such that t(o)=16 and v=0.5c
∴t(v)=18.48 years

c)d=s*t
∴d=0.5c*18.48
∴d=2.77x10^9 m

d) This is a trick question as the astronauts cannot measure the speed inside their inertial reference frame unless sourced from an external reference frame.

Thank you very much