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Very quick noob Edexcel PHY5 question! watch

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    Hi guys, question 5) i) on June 07?
    I read the markscheme and it said that the hub was negative, with the rim being positive.
    Using the right hand rule, the field runs into the page. Taking the central top spoke, its motion is to the right. Hence, index finger into the page and thumb to the right = middle finger up (lol). So if the induced current is going up the central spoke, wouldn't the hub be positive and the rim negative? :confused:
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    Using Fleming's Left Hand rule:
    Looking at the top middle spoke, motion is to the right. This means that the electrons are going to the right, so the conventional current is to the left. Field is into the page. So the electrons move down the spoke towards the hub. This makes it negatively charged and the rim positively charged. Got it?
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    (Original post by discombob)
    Hi guys, question 5) i) on June 07?
    I read the markscheme and it said that the hub was negative, with the rim being positive.
    Using the right hand rule, the field runs into the page. Taking the central top spoke, its motion is to the right. Hence, index finger into the page and thumb to the right = middle finger up (lol). So if the induced current is going up the central spoke, wouldn't the hub be positive and the rim negative? :confused:
    I think that describes the flow of conventional current, so your answer should be the opposite. Im not too sure though, better wait for someone more versed in the field to reply.
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    (Original post by zedliv)
    Using Fleming's Left Hand rule:
    Looking at the top middle spoke, motion is to the right. This means that the electrons are going to the right, so the conventional current is to the left. Field is into the page. So the electrons move down the spoke towards the hub. This makes it negatively charged and the rim positively charged. Got it?
    eh? Fleming's left hand rule is when you have a field and a current, and you're determining the resultant force on the wire or whatever.
    The right hand rule is used when you have a motion in a known field, and you're determining the induced current. For this, you've gotta be using the right hand rule.
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    (Original post by bruceleej)
    I think that describes the flow of conventional current, so your answer should be the opposite. Im not too sure though, better wait for someone more versed in the field to reply.
    Frick I think that may be the answer, since it asks for the induced e.m.f not the current. Can anyone else confirm this?
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    (Original post by discombob)
    eh? Fleming's left hand rule is when you have a field and a current, and you're determining the resultant force on the wire or whatever.
    The right hand rule is used when you have a motion in a known field, and you're determining the induced current. For this, you've gotta be using the right hand rule.
    I am talking about the force on the electrons in the metal spoke. My teacher gave us this paper as a mock, then went through it with us, focussing particularly on this question. As the electrons experience a force towards the hub, this area becomes more negative, hence the induced EMF.
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    (Original post by zedliv)
    I am talking about the force on the electrons in the metal spoke. My teacher gave us this paper as a mock, then went through it with us, focussing particularly on this question. As the electrons experience a force towards the hub, this area becomes more negative, hence the induced EMF.
    Um, I'm pretty sure that you can't use the left hand rule to predict the force that electrons feel, since electron movement is the opposite of current. The left hand rule has field, force, and current all at right angles to each other. If you were to predict the force on the electron and the direction of current using the left hand rule, your thumb and middle finger should be pointing in opposite directions...which is impossible lol.

    Besides, isn't this just way easier? http://en.wikipedia.org/wiki/Fleming's_right_hand_rule
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    (Original post by zedliv)
    I am talking about the force on the electrons in the metal spoke. My teacher gave us this paper as a mock, then went through it with us, focussing particularly on this question. As the electrons experience a force towards the hub, this area becomes more negative, hence the induced EMF.
    I got this question right by fluke, cus i used flemings left hand rule, forgetting that it should be right 'cus it's a generator.
    But yea we went through this question in class and my teachers came to the same conclusion as you, and as when the electrons get to the centre they have nowhere else to go, that it charges negatively.
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    (Original post by scottnoplot)
    I got this question right by fluke, cus i used flemings left hand rule, forgetting that it should be right 'cus it's a generator.
    But yea we went through this question in class and my teachers came to the same conclusion as you, and as when the electrons get to the centre they have nowhere else to go, that it charges negatively.
    Ah okay I see what you mean. The right hand rule would predict that potential difference considering that its not a closed circuit I guess.

    BUT. In the question it says "A constant e.m.f. is induced", wouldn't that suggest a closed circuit?
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    You don't use Flemming's Left Hand Rule because it's not for induced emf. You use the Right Hand Rule. My Physics teacher said the markscheme is wrong. The hub should be positive and the rim negative.
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    i got current going upwards (at the top). just the right-hand rule. what's the problem?

    i agree with the mark scheme
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    (Original post by LethalBizzle)
    I don't understand the issue with this question. It's much easier than you are all making it out to be. Fleming's Left Hand Rule at the top: Thrust is to the right, field is into the page, so current acts downwards.

    Are you positioning your hand wrongly?
    But you can't use your left hand rule if a current is being induced?
    AFAIK, the left hand rule can only be used when you're generating a force..like a catapult field or something. The right hand rule is used for a generator, which is effectively what this is.
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    I've checked the specification and there is no mention of Fleming's Right Hand Rule there. Also, I don't believe it is in the textbooks written for our specification. Although it might be useful to use (one or my teachers likes to use it, the other insists it's not necessary) I don't think using it will be a requirement for any question on the Unit test.
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    You use flemming left hand rule with this. But cos of Lenz's law the "thrust" or "force" whatever u like to call it, OPPOSES the motion. So you point your thumb opposite to the motion, first finger is still the field, so that should be pointing down (i,e, into the page) whcih leaves your second finger pointing the dirrection of the e.m.f.
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    we were taught the right hand rule, i mean, it is GCSE stuff. and you need it here to get the right answer!
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    (Original post by nexttime)
    i got current going upwards (at the top). just the right-hand rule. what's the problem?

    i agree with the mark scheme
    Haha how on earth did you manage that? You must be good at hand gymnastics.
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    (Original post by discombob)
    But you can't use your left hand rule if a current is being induced?
    AFAIK, the left hand rule can only be used when you're generating a force..like a catapult field or something. The right hand rule is used for a generator, which is effectively what this is.
    But that works much better. I think one of the issues here is that the OP has attempted to use the left hand rule and got it wrong. I have a feeling the question is wrong, as when you use the right hand rule, it gives a different answer to the mark scheme.

    I feel sorry for the people who took this last year!
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    The markscheme is wrong. If the current goes upwards at the top the hub is positive and the rim negative.
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    you can use the right hand or left hand rule, you just gotta rember to adapt the left hand rule slightly if your gonna use that
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    (Original post by LethalBizzle)
    But that works much better. I think one of the issues here is that the OP has attempted to use the left hand rule and got it wrong. I have a feeling the question is wrong, as when you use the right hand rule, it gives a different answer to the mark scheme.

    I feel sorry for the people who took this last year!
    The problem is that I used the right hand rule and the markscheme did not concur!
 
 
 

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