HSC Physics Question Thread

[fun_jirachi]

Find the altitude of a satellite in orbit around Earth if its orbital speed is 5.0km/s.

Hey there

Okay but this question has me slightly confused,

"Two long, parallel, current-carrying conductors attract each other with a force of F newtons. The current in each, and the distance between them is tripled. Predict the new force between the wires"

heLP

Hey there

For some distance between them r, and for some current I₁ and I₂, the force per unit length is F. If you triple the current in one wire, the force triples, since we can see force and current are directly proportional. Similarly, if you triple the distance between them, the force is reduced by a factor of three since we can see force and current are inversely proportional. After applying all the changes specified in the question, the new force should be 3F.

[blasonduo]

Find the altitude of a satellite in orbit around Earth if its orbital speed is 5.0km/s.

Hey there

Just to add in here, if we want the orbital altitude make sure to subract Earth's radius from your answer from above

[e2503]

Hi there 

How do i make calculations of projectile motion with the consideration of air resistance/ drag forces? And would the calculations including air resistance differ immensely compared to theoretical caclucations with no air resistance?

[jamonwindeyer]

Hi there 

How do i make calculations of projectile motion with the consideration of air resistance/ drag forces? And would the calculations including air resistance differ immensely compared to theoretical caclucations with no air resistance?

Hello! So if you do want to do that (don't need to in this course!) there are a few ways you can do it, depending on how accurate you want to be with things

Air resistance is generally modelled as being proportional to the square of velocity. The constant of proportionality is the drag coefficient, and is different depending on the object, its inclination, all sorts of stuff. I've seen \(C=0.5\) used a fair bit, I think it might be the standard one for spherical objects?

So the idea is you calculate the air resistance by taking the velocity \(V\) (the vector sum of horizontal and vertical velocities), calculating the resistive acceleration as \(a_\text{res}=0.5V^2\), and then redistributing that to the two directions.



Where \(\theta\) is the velocity angle at that point in time! As you can see, it gets complicated quickly! The other thing you can do is just add a constant horizontal deceleration to the object, which effectively turns your horizontal calculations into ones with acceleration just like the vertical. And there are more - It depends on how close you want to be to reality

Edit: How much will it differ? Again, depends. Try and calculate the motion of an airplane without air resistance and things break pretty quickly! For most everyday situations with aerodynamic objects, you are fine. For example, I did an experiment with my Tutesmart class to predict the landing position of a hot wheels car launched from a ramp. The theoretically predicted position was exactly where the car landed

[e2503]

Hello! So if you do want to do that (don't need to in this course!) there are a few ways you can do it, depending on how accurate you want to be with things

Air resistance is generally modelled as being proportional to the square of velocity. The constant of proportionality is the drag coefficient, and is different depending on the object, its inclination, all sorts of stuff. I've seen \(C=0.5\) used a fair bit, I think it might be the standard one for spherical objects?

So the idea is you calculate the air resistance by taking the velocity \(V\) (the vector sum of horizontal and vertical velocities), calculating the resistive acceleration as \(a_\text{res}=0.5V^2\), and then redistributing that to the two directions.



Where \(\theta\) is the velocity angle at that point in time! As you can see, it gets complicated quickly! The other thing you can do is just add a constant horizontal deceleration to the object, which effectively turns your horizontal calculations into ones with acceleration just like the vertical. And there are more - It depends on how close you want to be to reality

Edit: How much will it differ? Again, depends. Try and calculate the motion of an airplane without air resistance and things break pretty quickly! For most everyday situations with aerodynamic objects, you are fine. For example, I did an experiment with my Tutesmart class to predict the landing position of a hot wheels car launched from a ramp. The theoretically predicted position was exactly where the car landed

Thank you so much for your help 
I will be sure to remeber your tips

[sarrahbarodawala]

Hello!
Have a question regarding magnetic fields
"What is the minimum magnitude of a magnetic field necessary to apply a force of 1 x 10^-12 to an electron moving at a speed of 500 kms^-1? What electric field is necessary to apply this magnitude force?
There are so many formulas regarding magnetic fields that I just got so confused in working it out. Please help

Thank you!
btw answers  are a) magnetic field 13T and b) electric field 6.3 x 10⁶ NC^-1

[fun_jirachi]

If there seem to be too many formulas, the quickfix is just to figure out what you have, and what you need to find. Then basically the formula with the most corresponding stuff is the one you need, otherwise you do a little substitution of one formula into another to make a derived formula you can use. In this case, it's totally unnecessary.

[david.wang28]

Hello,
I am stuck on all of the multiple choice questions in the attachment below. Can anyone please help me out? Thanks

[jmbelger]

Hey,

So we have a prac investigation assessment coming up on the connection between electricity and magnetism (electromagnetism) and we have to discuss how electricity and magnetism are linked.
I've been looking everywhere for a simple explanation without just stating that a current-carrying wire interacts with a magnetic field.

BUT all I can find are people explaining Faraday, Lenz or Maxwell's laws as well as basic principles of magnetism and electrostatics.

Is there a simple explanation for how the two are linked - using basic terminology?

Any help is greatly appreciated!
Jem 

[jamonwindeyer]

Hey,

So we have a prac investigation assessment coming up on the connection between electricity and magnetism (electromagnetism) and we have to discuss how electricity and magnetism are linked.
I've been looking everywhere for a simple explanation without just stating that a current-carrying wire interacts with a magnetic field.

BUT all I can find are people explaining Faraday, Lenz or Maxwell's laws as well as basic principles of magnetism and electrostatics.

Is there a simple explanation for how the two are linked - using basic terminology?

Any help is greatly appreciated!
Jem 

Welcome to the forums Jem!!

Electricity and magnetism are two tangible manifestations of the same electromagnetic force. Two faces, same person, sort of thing! In fact, we even have a name for the combined effects of electric/magnetic forces on an object - The Lorentz Force

If you do consider them separately, then:

- Moving electrical charges produce magnetic fields
- Changing magnetic fields produce electric fields

Those are the links between the two! to actually explain why they are the same is a fair bit above what a high school Physics student would be expected to know, so this should be plenty - The way I read that question, it wants you to explore the ways the two are connected in the physical world rather than theoretically

[jamonwindeyer]

Hello,
I am stuck on all of the multiple choice questions in the attachment below. Can anyone please help me out? Thanks

Hey David! I'll give you a quick run down, but many of these are really just about knowing the theory for this particular part of the course rather than any sort of problem solving - You might need to revisit your textbook to understand these properly!!

- Q1.1 is C, the colour of a star is related to its surface temperature. This can then be linked to the characteristic wavelength by Wien's Displacement Law.
- Q1.2 is B (as I interpret it), the dominant wavelength of a star is related to the surface temperature by Wien's Displacement Law. However, that law applies only to actual black bodies, of which stars are only an approximate. Therefore, the actual temperature is likely to be slightly different. Wien's Law is also less accurate for shorter wavelengths.
- Q1.3 is B, you just need to know this one
- Q1.4 is D, by process of elimination. Photons have different energies (clearly), the amount of energy carried by a photon is inversely proportional to wavelength, and since colour and frequency are linked, C is also untrue. If you calculate the energy per photon for blue and green light, you'll realise that you need less photons of blue light than green light to make 1 joule

[jmbelger]

Welcome to the forums Jem!!

Electricity and magnetism are two tangible manifestations of the same electromagnetic force. Two faces, same person, sort of thing! In fact, we even have a name for the combined effects of electric/magnetic forces on an object - The Lorentz Force

If you do consider them separately, then:

- Moving electrical charges produce magnetic fields
- Changing magnetic fields produce electric fields

Those are the links between the two! to actually explain why they are the same is a fair bit above what a high school Physics student would be expected to know, so this should be plenty - The way I read that question, it wants you to explore the ways the two are connected in the physical world rather than theoretically

Thank you so much! This is exactly what I needed to know - minus the convoluted textbook mumble!

The prac is meant to examine an investigation which demonstrates the relationship between the two forces so I think I'll start with considering the two forces separately then together.

Once again - thank you!
Jem 

[david.wang28]

Hey David! I'll give you a quick run down, but many of these are really just about knowing the theory for this particular part of the course rather than any sort of problem solving - You might need to revisit your textbook to understand these properly!!

- Q1.1 is C, the colour of a star is related to its surface temperature. This can then be linked to the characteristic wavelength by Wien's Displacement Law.
- Q1.2 is B (as I interpret it), the dominant wavelength of a star is related to the surface temperature by Wien's Displacement Law. However, that law applies only to actual black bodies, of which stars are only an approximate. Therefore, the actual temperature is likely to be slightly different. Wien's Law is also less accurate for shorter wavelengths.
- Q1.3 is B, you just need to know this one
- Q1.4 is D, by process of elimination. Photons have different energies (clearly), the amount of energy carried by a photon is inversely proportional to wavelength, and since colour and frequency are linked, C is also untrue. If you calculate the energy per photon for blue and green light, you'll realise that you need less photons of blue light than green light to make 1 joule

Thank you Jamon!

[louisaaa01]

Hi!

Can you please help with this question (from AC generators)?

Sketch graphs to show the relationship between:
(a) The flux through the coil and the induced
voltage.
(b) The torque acting on the coil and the induced voltage.

The answers show a sine relationship for (a), and a cosine relationship for (b) - but I can't seem to reason as to why. I thought that, for (a) - as flux increases to some maximum value, induced emf would decrease - and for (b) some sort of direct (not necessarily directly proportional) relationship would occur. Also, I'm a bit confused as to how both flux and torque can be on the x-axis (in the suggested answers) when, in a generator, they don't increase indefinitely?

Thank you in advance for your assistance!

[Coolmate]

Hi I'm in Year 11,

I am having trouble understanding the topic area of distance and displacement on horizontal planes, in particular the maths side of questions and formulas. Any guidance/ links to resources will be greatly appreciated.

Kind Regards,

Coolmate