HSC Physics Question Thread

[winstondarmawan]

Hey!
I know this is not a physics specific question, but I just wanted to know the best possible way/structure to answer the following types of questions:
- Assess the validity of this statement.
- Assess the validity/reliability/accuracy of this experiment.

Thanks in advance!

[inu99]

The armature of a DC motor consists of 40 turns of circular coils with a diameter of 20cm. A voltage of 6.0v is supplied to the coil. If the coil has a total resistance of 0.50 ohms and the magnetic field has a strenght of 0.20T:
A) Determine the size of the torque when the coil is in the horizontal position
B) How would this torque affect the motion of the coil
c) Determine the size of the torque when the coil has moved through an angle of 30 degrees
d) Determine the size of the torque when the coil has reached th vertical position

[jakesilove]

Hey!
I know this is not a physics specific question, but I just wanted to know the best possible way/structure to answer the following types of questions:
- Assess the validity of this statement.
- Assess the validity/reliability/accuracy of this experiment.

Thanks in advance!

Hey! The best answer for ANY assess question is to weigh up to potential advantages against potential disadvantages. In this case, generally start by stating why something may be valid (ie. testing what is trying to be tested), and then go into a bit of depth as to where validity may fall down. Things to think about when it comes to validity/accuracy/reliability:

Validity
- Is the experiment testing what you are trying to test?
- Are there uncontrolled variables that may affect the data?
- Is there another explanation/factor that will affect your measurements?

Accuracy
- How good is your measuring equipment?
- How close is your values to the 'real' value?
- Are there any errors in your experiment (human error etc.)

Reliability
- If you repeated the experiment, would you get the same data?
- How many times did you repeat the experiment?
- Did you take any averages? Plot any graphs?


Hope this helped!

[jakesilove]

The armature of a DC motor consists of 40 turns of circular coils with a diameter of 20cm. A voltage of 6.0v is supplied to the coil. If the coil has a total resistance of 0.50 ohms and the magnetic field has a strenght of 0.20T:
A) Determine the size of the torque when the coil is in the horizontal position
B) How would this torque affect the motion of the coil
c) Determine the size of the torque when the coil has moved through an angle of 30 degrees
d) Determine the size of the torque when the coil has reached th vertical position

A)

The relevant formula is



We have to first current first, using




Now,



B)

The torque affects the motion of the coil by exerting a turning force. This turning force causes the coil to spin (either clockwise or anti-clockwise) around the central axis of the coil.

C)



D)

When the coil is vertical, the angle will be 90 degrees. Plugging this into the formula, the torque is zero!

[Maraos]

Need an answer quick (Physics Prac test tomorrow)

How exactly would you improve the validity of an experiment?

I understand the validity is whether or not your experimental design is adequate in solving the aim and whether or not all the controlled variables were kept and constant and the independent variable was changed.

But how could you make improvements to increase the validity of an experiment? .

Thanks!

[Maraos]

Need an answer quick (Physics Prac test tomorrow)

How exactly would you improve the validity of an experiment?

I understand the validity is whether or not your experimental design is adequate in solving the aim and whether or not all the controlled variables were kept and constant and the independent variable was changed.

But how could you make improvements to increase the validity of an experiment? .

Thanks!

After doing some of my own research and looking at past papers I think I've got it (can someone please confirm if my thinking is correct??)

Improving validity is 'making sure' that the controlled variables are 100% controlled. And removing or adding/improving unreliable equipment/methods to ensure that the experimental design is adequate in achieving the aim.

So essentially i think that it is making adjustments to your experimental design in-order to ensure that controlled variables are kept at a constant and that the experimental design is actually solving your intended aim.....

Is this right?  I dunno....

[kiwiberry]

After doing some of my own research and looking at past papers I think I've got it (can someone please confirm if my thinking is correct??)

Improving validity is 'making sure' that the controlled variables are 100% controlled. And removing or adding/improving unreliable equipment/methods to ensure that the experimental design is adequate in achieving the aim.

So essentially i think that it is making adjustments to your experimental design in-order to ensure that controlled variables are kept at a constant and that the experimental design is actually solving your intended aim.....

Is this right?  I dunno....

Hey, I think you're right! I can't think of anything other than controlling variables haha

[jamonwindeyer]

Controlling variables more completely/effectively is the best way to improve the validity of an experiment, that's what I'd jump to

[bsdfjnlkasn]

Hey there another prac related question 

This one is sort of open to anyone who has been required to do a practical task for Space. If you guys could offer me the types of experiments you did and the style of questions (will probably need to graph something) and whether there were any specific things that were integral to the experiment. I know this question is super vague but I just want to get a broader understanding of the range of experiments I could be asked to consider and what skills will help me excel in them.

I know this has probably been asked a million times but what would you recommend to be a general skill set that will cover the outcomes tested in these exams? I want to be super prepared for this exam and hopefully can get some good tips (as practice for the most part is really limited - any resources therefore would be greatly appreciated  )

Thanks again and I really hope it doesn't seem like I'm cheating. I really need to improve on my physics rank and settle in nicely before trials  so if you could help a fellow physics student out that would be lovely

[Bubbly_bluey]

hello all!
I have two questions:
1) What is a clear explanation of a back emf? My current "understanding" is that when an induced current is produced to create the spinning motion of the ammature, the law of conservation applies because it can not spin indefinitely faster. Does it act like a resistant to the normal flow of the current? I'm not what exactly it is.

2)Does Lenz's law only apply to DC and AC generators and not motors?

Thanks:)

[jamonwindeyer]

Hey there another prac related question 

This one is sort of open to anyone who has been required to do a practical task for Space. If you guys could offer me the types of experiments you did and the style of questions (will probably need to graph something) and whether there were any specific things that were integral to the experiment. I know this question is super vague but I just want to get a broader understanding of the range of experiments I could be asked to consider and what skills will help me excel in them.

I know this has probably been asked a million times but what would you recommend to be a general skill set that will cover the outcomes tested in these exams? I want to be super prepared for this exam and hopefully can get some good tips (as practice for the most part is really limited - any resources therefore would be greatly appreciated  )

Thanks again and I really hope it doesn't seem like I'm cheating. I really need to improve on my physics rank and settle in nicely before trials  so if you could help a fellow physics student out that would be lovely

Hey! It's definitely not cheating to ask for some friendly advice mate!

As far as I know, the big experiments for space will either involve a pendulum or analysis of projectiles. So be sure to revise your kinematic formulae, and the pendulum formula, \(T=\sqrt{\frac{2\pi}{L}}\) - There's a pretty good chance you'll need one of them

In terms of practice, I'd revise:
- Graphing/Lines of Best Fit
- Variables (Independent/Dependent/Controlled)
- Variability/Accuracy/Reliability
- Sources of Error

You might be able to find some practice experiment tasks here

Besides that, some general advice - Do the experiment twice if you have time (or take lots of measurements), it increases reliability. Ensure you graph your data with the independent variable on the horizontal axis and the dependent variable on the vertical axis, and use pencil (trust me, just use pencil). Human error is always a source of error and just take it slow and be careful with your setup!

[jamonwindeyer]

hello all!
I have two questions:
1) What is a clear explanation of a back emf? My current "understanding" is that when an induced current is produced to create the spinning motion of the ammature, the law of conservation applies because it can not spin indefinitely faster. Does it act like a resistant to the normal flow of the current? I'm not what exactly it is.

2)Does Lenz's law only apply to DC and AC generators and not motors?

Thanks:)

Hey!!

1. Back EMF is the name given to the induced emf in a motor that opposes the supply emf. It is formed because the coil experiences a changing magnetic flux as it spins, thus inducing an emf by Faraday's Law. It must flow to oppose the supply current that created it, to adhere to the Conservation of Energy and Lenz's Law.

^ That would be my succinct explanation. I think you are on the right track, you need to work on understanding that back emf is another, opposing voltage/current. That is, the current in your coil is given by:



I actually wrote a guide on induction last year, which contains a bit of elaboration on Lenz's Law and back emf! It might be of some help

2. Lenz's Law is a universal concept, governing the direction of all induced emf's

[kiwiberry]

Hey there another prac related question 

This one is sort of open to anyone who has been required to do a practical task for Space. If you guys could offer me the types of experiments you did and the style of questions (will probably need to graph something) and whether there were any specific things that were integral to the experiment. I know this question is super vague but I just want to get a broader understanding of the range of experiments I could be asked to consider and what skills will help me excel in them.

I know this has probably been asked a million times but what would you recommend to be a general skill set that will cover the outcomes tested in these exams? I want to be super prepared for this exam and hopefully can get some good tips (as practice for the most part is really limited - any resources therefore would be greatly appreciated  )

Thanks again and I really hope it doesn't seem like I'm cheating. I really need to improve on my physics rank and settle in nicely before trials  so if you could help a fellow physics student out that would be lovely

Just to add to Jamon's answer
I feel like the pendulum prac will most likely be tested because it's easy to set up and there's quite a lot you could ask about it in terms of calculations and stuff on validity/accuracy/reliability (but don't ignore the others!). A few things to consider with the pendulum prac off the top of my head:
- Timing 10 oscillations to increase accuracy - too many oscillations and the pendulum will be affected by air resistance, but too little swings will lead to a larger margin of error
- Only timing after the pendulum has reached a constant swing to increase accuracy as well
- Experiment repeated with different lengths of string multiple times to increase reliability

Some other random thoughts:
- There's a high chance that you'll get some sort of question which asks you to graph something, find the gradient of the graph, and then use that value to calculate something. Eg for the pendulum, you could be asked to graph L vs T² and then use this to find g using \(T=2\pi \sqrt{\frac{L}{g}}\)
- Line of best fit - only draw from the x-coordinate of your first point to the x-coordinate of your last point. It doesn't have to start and end at your first and last points - just make sure there's an even number of points on either side of it. Bring a 30cm ruler, trust me, it makes life so much easier when drawing these haha
- Calculating gradient - don't use points that you've already plotted! Pick something on your line of best fit that's in between, and show which points you pick on your graph. Omg and remember that gradients need units too!!! I've lost marks so many times for this
The teachers at my school are really picky though so you might not even have to follow all these rules

I haven't done a prac exam on Space so keep that in mind, but hope this helps a bit!

[jamonwindeyer]

OK but i cant make a connection with Lenz's labelling applied on motors. Like isn't a motor powered by the motor effect with no induced emf?

A motor is definitely 'powered' by the motor effect - It's the primary principle that allows it to work but you still get induced emf in motors, that is back emf, and what governs the direction of back emf in motors? That's Lenz's Law

[Bubbly_bluey]

A motor is definitely 'powered' by the motor effect - It's the primary principle that allows it to work but you still get induced emf in motors, that is back emf, and what governs the direction of back emf in motors? That's Lenz's Law

I'm sorry last question for today promise XD
So is the induced emf generated when the ammature cuts the magnet flux, and the system does not like that so it will create back emf to counter that?