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# A2 OCR Physics Past paper question

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1. Hey guys, I'm stuck at this question in June 2014 paper. I can do the first three but don't get (d). The answer is a clockwise path but anyone can tell me why??? Thank you!!!!
2 Fig. 2.1 shows the circular path described by a helium nucleus in a region of uniform magnetic field in a vacuum. AB (b) © OCR 2014 Fig. 2.1 The direction of the magnetic field is perpendicular to the plane of the paper. The magnetic fluxdensity of the magnetic field is 0.20 mT. The radius of the circular path is 15 cm. The helium nucleushas charge + 3.2 × 10–19 C and mass 6.6 × 10–27 kg. (a) Explain why the helium nucleus
1. (i) travels in a circular path ................................ ................................ ................................ ................................ ........... ................................ ................................ ................................ ................................ ...... [1]
2. (ii) has the same kinetic energy at A and B. ................................ ................................ ................................ ................................ ................................ ................................ ................................ ................................ ................................ ................................ ................................ ................................ ............................ [1]
Calculate the magnitude of the momentum of the helium nucleus. region of uniformmagnetic field momentum = ................................ ............ kgms–1 [3]
(c) Calculate the kinetic energy of the helium nucleus. kinetic energy = ................................ ....................... J [2] (d) A uniform electric field is now also applied in the region shaded in Fig. 2.1. The direction ofthis electric field is from left to right. Describe the path now followed by the helium nucleus inthe electric and magnetic fields. ................................ ................................ ................................ ................................ ................................ ................................ ................................ ................................ ................................ ................................ ................................ ................................ ................................ ................................ ................................ ................................ ................................ ................................ ....... [2] [Total: 9]
2. in the earlier parts of the question you're just thinking about a force which acts at right angles to the particles velocity - so the resultant force is a centripetal force and the path is circular (clockwise)

in this part of the question the magnetic field is still there and is joined by an electric field which accelerates the particle from left to right... but the magnetic field is still there. They seem to be hoping to see a description that simultaneously incorporates the clockwise circular motion and the left to right acceleration

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