Please can someone help me clarify a few things (1 Viewer)

[izzyheslop]

Hi! I have been doing a few practice papers for physics and have a bunch of questions I am confused about! Does anyone know:

1. What is the best definition for an uncontrolled/controlled chain reaction. I would have said the number of fissions subsequently initiated in comparison to the number of original fissions but have also see it defined in terms of rate.
2. Why can't a particle in free fall move straight past Earth? What if gravity was pulling in two opposing directions and these cancelled out?
3. How does precision work for experiments and stuff I can find a standard answer literally nowhere
4. In back emf graphs why is back emf proportional to motor speed if emf changes periodically as the rotor rotates

If you could explain it would be greatly appreciated!

 

[notme123]

Hi! I have been doing a few practice papers for physics and have a bunch of questions I am confused about! Does anyone know:

1. What is the best definition for an uncontrolled/controlled chain reaction. I would have said the number of fissions subsequently initiated in comparison to the number of original fissions but have also see it defined in terms of rate.
2. Why can't a particle in free fall move straight past Earth? What if gravity was pulling in two opposing directions and these cancelled out?
3. How does precision work for experiments and stuff I can find a standard answer literally nowhere
4. In back emf graphs why is back emf proportional to motor speed if emf changes periodically as the rotor rotates

If you could explain it would be greatly appreciated!

1. controlled is when 1 nuclear fission reaction creates thermal neutrons that trigger another nuclear fission reaction. This is ensured by the control rods which absorb the energy of 1 neutron to make it slow enough to trigger another reaction. uncontrolled is when 1 nuclear fission reaction causes more than 1 nuclear fission reaction. another way to look at this is the critical coefficient, or the multiplicative effect of 1 reaction on the next wave of reactions. controlled, K=1. Uncontrolled, K>1
2. im not sure what you mean here but since gravitational force is inversely square proportional, the heaviest and closest body will have a dominant acceleration on a particle in free fall. if the particle was at a point were net forces equal 0, yes the gravity forces would cancel out. e.g. there is a point where if a particle was directly between the earth and moon, the opposite and equal forces would cancel out but since the moon is lighter, it exterts a weaker force, so the particle needs to be closer to the moon to balance this out.
3. precision is the spread of scores, like the standard deviation if you do maths, you get for an experiment. higher precision means the spread is smaller and indicates your experiment is potentially valid and has a lack of random error. this is different from accuracy, where accuracy measures how close your experimental results are to the actual or calculated results. so you can be precise but inaccurate at the same time.
4.this is actually a good q. the reason has to do with the change in mag flux, not the amount of mag flux present at one moment. the faster a motor spins, the greater its angular velocity, so the cross-sectional area changes faster. the amount of mag feild lines going through the area changes more often as motor speed increases, so the change in flux increases, so back emf increases proportionally. back emf does not change periodically, i think youre thinking of ac generators.

 

[wizzkids]

Controlled vs. Uncontrolled.
This is usually defined by the neutron generation rate. It would be wrong to define it in terms of the number of fissions because each fission releases 2.5 (on average) neutrons, and you have to absorb 1.5 neutrons. For controlled chain-reaction, the neutron generation rate must be exactly 1. This level of stability is actually incredibly difficult to achieve with just one source of prompt neutrons, a bit like trying to balance on the head of a pin. In practice, a power reactor is run at sub-critical, i.e. a prompt neutron generation rate less than 1, and the balance of the neutrons to reach 1.0 is made up of delayed neutrons that come from radioactive daughter products (this goes way beyond the syllabus). So a uranium power reactor is actually run at a sub-critical neutron generation rate by the use of control rods, and then the delayed neutrons top up the neutron flux to get to exactly 1.0 generation rate. Thank goodness for delayed neutrons, because without them, controlling the power output of a uranium-fuelled power reactor would be a nightmare.
Equal and opposite gravitational attraction.
Yes, this is certainly possible. Try to imagine modelling a gravitational field as being a bit like a topographic map of hills and valleys. Hills represent regions of high gravitational potential energy (GPE), and valleys represent areas of low GPE. The angle of the slope represents the strength of the local gravitational field. A free-falling object will roll down into a valley, or spiral into a depression. Any contour line across a topographic map that has the same elevation represents an equipotential path in a gravitational field, i.e. following that path does not require any change of GPE. Now imagine in our landscape of hills and valleys, there are two straight valleys, and they must be separated by a straight ridge. If you can stick to the ridge, you are traversing two gravitational valleys on either side. However, this is intrinsically unstable because you could fall to the left or the right at any moment, and you would fall into the strong gravitational attraction of one of the valleys. (If you are motivated, go look up Lagrange Points)
What do we mean by precision?
A working definition of precision is, how many significant figures can you quote in the result? The more significant figures, the more precise is the result. For example, a moving-coil voltmeter has a precision of 1% of the full scale reading, so 2 significant figures. A digital voltmeter with four digits has a precision of +/- 1 last digit, so that is 4 significant figures. However, you can have a very precise result, but it can also be inaccurate. Clearly, what we want is both a precise result AND an accurate result.
Back EMF graphs?
Yes, those graphs don't make sense, unless you realise they are averaging the amplitude of the back-EMF around one rotation of the armature. The average amplitude of the back-EMF is proportional to motor speed. If you hooked up a high speed oscilloscope to the motor, you would be able to see the fluctuations in the back-EMF around one rotation, and you would see it has a synchronous character.

 

[idkkdi]

Hi! I have been doing a few practice papers for physics and have a bunch of questions I am confused about! Does anyone know:

1. What is the best definition for an uncontrolled/controlled chain reaction. I would have said the number of fissions subsequently initiated in comparison to the number of original fissions but have also see it defined in terms of rate.
2. Why can't a particle in free fall move straight past Earth? What if gravity was pulling in two opposing directions and these cancelled out?
3. How does precision work for experiments and stuff I can find a standard answer literally nowhere
4. In back emf graphs why is back emf proportional to motor speed if emf changes periodically as the rotor rotates

If you could explain it would be greatly appreciated!

4. i think we assume a curved magnet.