Hey there! Sign in to join this conversationNew here? Join for free
x Turn on thread page Beta
    • Thread Starter
    Offline

    15
    ReputationRep:
    (Original post by spread_logic_not_hate)
    SUVAT eqautions apply only to bodies moving with constant accelleration, i.e. accelleration that does not change in time.

    eg if a car accellerates from 1ms-1 to 10ms-1 with accelleration a = 2ms-2, then SUVAT is ok.

    If the car accellerates from 1ms-1 to 10ms-1 with accelleration a = (1 + t)ms-2, where t is time, then SUVAT is not ok as the accelleration is no longer constant in time.

    Also SUVAT is only valid for 1d motion, as all of the quantities are scalars. It may be possible to use it to describe 2 or 3d motion, by assigning each direction its own set of SUVAT equations, but im not sure on this and anyway a vector based approach would be far easier there.
    Thanks for your reply mate do you mind explaining what you meant by "canceling effect" you mentioned earlier?
    Offline

    8
    ReputationRep:
    Thanks for your reply mate do you mind explaining what you meant by "canceling effect" you mentioned earlier?
    Sure - imagine a field as a series of arrows arranged in space, where the direction of the arrow tells you which way the field is aligned and the length of the arrow tells you the size (or magnitude) of the field at a given point.

    Now imagine getting two of these fields and superimposing them. At each point, the resultant field (i.e. that actually felt by a physical object) is going to be the vector sum of these arrows; that is, you add them taking into account their size and direction.

    A partial cancellation refers to the case where the arrows point in opposite directions - when they are added together, the result will be smaller than that if the larger of the fields was to be considered seperately.

    As always a diagram is worth a thousand words so...

    \rightarrow + \rightarrow =  \Longrightarrow Fields reinforce

    \Longrightarrow + \leftarrow =  \rightarrow Fields partially cancel

    \rightarrow + \leftarrow =  0 Fields fully cancel

    Hope that clears it up a bit!
    Offline

    10
    (Original post by spread_logic_not_hate)
    Also SUVAT is only valid for 1d motion, as all of the quantities are scalars. It may be possible to use it to describe 2 or 3d motion, by assigning each direction its own set of SUVAT equations, but im not sure on this and anyway a vector based approach would be far easier there.
    Yes it works fine for 2d and 3d motion problems. You just resolve into x and y (and z) components and apply SUVAT independently to those.

    I would disagree that "all the quantities are scalar". In 1 dimension, the directional element is within the sign. A velocity can be + or -. This indicates its direction in one dimensional space.
    Offline

    8
    ReputationRep:
    Yes it works fine for 2d and 3d motion problems. You just resolve into x and y (and z) components and apply SUVAT independently to those.
    Thanks for clarifying, I figured it would...

    I would disagree that "all the quantities are scalar". In 1 dimension, the directional element is within the sign. A velocity can be + or -. This indicates its direction in one dimensional space.
    This is true, although I think that the SUVAT equation  v^2 = u^2 + 2as is definitly a scalar equation as u = -2 gives the same v as u = 2 for fixed a and s, hence the directionality info about the starting velocity is lost. For this to work in the system you suggest requires the equation to be written as

     v \bullet v = u \bullet u  + 2as

    where the angle between + and - is understood to be 180 degrees.
    • Thread Starter
    Offline

    15
    ReputationRep:
    Okay let me see if i got this right now. This is on the difference for motor effect and generator effect.

    The motor effect is when basically current is induced due to the continuously changing magnetic flux and of course it will induce a current if the flux linking the conductor is connected to a complete circuit. This uses the Fleming's Left hand rule. Alternating voltage is used.

    The generator effect. This uses the right hand rule(though i am not required to know this at all) and this is what i don't get what happens here. So someone please explain?
    • Thread Starter
    Offline

    15
    ReputationRep:
    anyone?
    Offline

    8
    ReputationRep:
    The motor effect is when basically current is induced due to the continuously changing magnetic flux and of course it will induce a current if the flux linking the conductor is connected to a complete circuit. This uses the Fleming's Left hand rule. Alternating voltage is used.
    Think this is some combination of the two you describe here...

    Motor effect: A loop of wire carrying an alternating current (AC) is placed in a magnetic field. The static field causes a force to be felt by the wire loop (as described by the Biot Savart law) and causes the loop of wire to spin, where the direction of motion is found by the left hand rule. The frequency of current is chosen such that as the loop gets to the point at which the force would reverse direction, the current reverses direction meaning that force continues to push the wire in the same direction.

    Generator effect: A length of wire is placed in a time varying magnetic field; this field induces (or generates) an electric current in the wire by Faraday's law of induction.
    Offline

    10
    (Original post by spread_logic_not_hate)


    This is true, although I think that the SUVAT equation  v^2 = u^2 + 2as is definitly a scalar equation as u = -2 gives the same v as u = 2 for fixed a and s, hence the directionality info about the starting velocity is lost. For this to work in the system you suggest requires the equation to be written as

     v \bullet v = u \bullet u  + 2as

    where the angle between + and - is understood to be 180 degrees.
    Of course. You get a scalar if you do the dot product of vectors.
    • Thread Starter
    Offline

    15
    ReputationRep:
    (Original post by spread_logic_not_hate)
    Think this is some combination of the two you describe here...

    Motor effect: A loop of wire carrying an alternating current (AC) is placed in a magnetic field. The static field causes a force to be felt by the wire loop (as described by the Biot Savart law) and causes the loop of wire to spin, where the direction of motion is found by the left hand rule. The frequency of current is chosen such that as the loop gets to the point at which the force would reverse direction, the current reverses direction meaning that force continues to push the wire in the same direction.

    Generator effect: A length of wire is placed in a time varying magnetic field; this field induces (or generates) an electric current in the wire by Faraday's law of induction.
    AHh yess, Faraday's law of induction for generator effect and a wire carrying an electric current into a magnetic field is magnetised causing the coil to rotate. Thanks mate!!!!
    • Thread Starter
    Offline

    15
    ReputationRep:
    I have a few questions from a past paper i need answering, i will ask them in a minute let me find those questions.
    • Thread Starter
    Offline

    15
    ReputationRep:
    "A rectangular piece of plastic film, width 2.0cm and thickness 1.0mm, was subjected to an increasing tensile force. Polarised light was directed at it and the transmitted light was viewed though an analyser. Each successive time the film appeared dark the value of force was noted.

    order of film appearing dark: 0 1 2 3
    Phase difference alpha/rad : 0 2pi 4pi 6pi
    Force/N: 0 19 40 66

    sigma = k*alpha/2pi

    Use this formula to that the average value of K is about 10^6N m^-2"

    This is i just switched around to get K which i think is just F/A and this gives me a value of 9.5*10^5

    Then it asks "Calculate the percentage uncertainty in this value of K" I got no idea tbh, the answer they give is the following

    +/- 1*10^5 or any attempt to find the % difference

    +/- 10%"

    Someone please explain. Thanks in advance.
    • Thread Starter
    Offline

    15
    ReputationRep:
    anyone?
    • Thread Starter
    Offline

    15
    ReputationRep:
    no-one?
    • Thread Starter
    Offline

    15
    ReputationRep:
    anyone??????
    • Thread Starter
    Offline

    15
    ReputationRep:
    someone must be able to help me on this!!!!!
    Offline

    8
    ReputationRep:
    Use this formula to that the average value of K is about 10^6N m^-2"
    It has something to do with this being an average value, but im not exactly sure what in this case... does your post contain all the info from the question? e.g. are there any mentions of precision of the measurements of the force/area?
    Offline

    10
    As you have 4 different sets of values to use for the calculation, the maximum and minimum values obtained come from F=66N and F=19N (40N gives a value equal to the average they quote).
    The percentage error comes from the difference between the highest and lowest values obtained with the data. (At least, that's what it seems they are looking for here.)
    A better way of doing this would have been a graph using the 4 points and the best value of the slope used to find k. A best and worst slope would give the uncertainty.)
    • Thread Starter
    Offline

    15
    ReputationRep:
    (Original post by spread_logic_not_hate)
    It has something to do with this being an average value, but im not exactly sure what in this case... does your post contain all the info from the question? e.g. are there any mentions of precision of the measurements of the force/area?
    Nope that's all the info i am given mate..... so i should be using stress = F/A but as an average instead to find the value for k???
    • Thread Starter
    Offline

    15
    ReputationRep:
    (Original post by Stonebridge)
    As you have 4 different sets of values to use for the calculation, the maximum and minimum values obtained come from F=66N and F=19N (40N gives a value equal to the average they quote).
    The percentage error comes from the difference between the highest and lowest values obtained with the data. (At least, that's what it seems they are looking for here.)
    A better way of doing this would have been a graph using the 4 points and the best value of the slope used to find k. A best and worst slope would give the uncertainty.)
    it's only 2 marks for the percentage error, i doubt we have to use a graph but i wanna focus more on using the highest and lowest value.... so all i do is (66-19)/the value of k????..... is my value for K correct?
    Offline

    10
    If you find k using 66N, and then find k using 19N, you get two slightly different answers. The difference between them is the uncertainty in the answer.
    How did you find the average when you did this initially?
 
 
 
Turn on thread page Beta
Updated: February 2, 2010
Poll
Do you agree with the proposed ban on plastic straws and cotton buds?

The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE

Write a reply...
Reply
Hide
Reputation gems: You get these gems as you gain rep from other members for making good contributions and giving helpful advice.