Assignment 8 Magnetic fields and particle physics
Introduction
Aims
This assignment will test your ability to: ? answer multiple-choice questions ? describe practical situations and interpret measurements.
You do not need to have studied the physics that underlies these experiments in order to be able to complete the tasks.
Links to the assessment requirements
The assessment objectives for the A level that are relevant to this assignment are to: ? apply knowledge and understanding of scientific ideas,
processes, techniques and procedures: ? in a practical context ? when handling qualitative data ? when handling quantitative data
? analyse, interpret and evaluate scientific information, ideas and evidence, including in relation to issues, to: ? make judgements and reach conclusions ? develop and refine practical design and procedures.
How your tutor will mark your work
Your tutor will assess the following aspects of your work: ? your application of appropriate physical principles ? your use of appropriate equations ? the accuracy of your calculations ? your use of graphs and drawings where directed ? your use of appropriate units.
Are you ready to do this assignment?
Before you tackle this assignment, ensure that you have studied Section 8 of the course and Chapters 6, 7 and 8 of the textbook. In addition to the usual writing materials (or computer) you will need a sharp pencil, ruler and protractor, graph paper and a calculator.
The assignment
In calculations, use g = 9.81 m s–2 for the acceleration of free fall unless told otherwise. Data for other questions can be found in the Data booklet, which is linked to Section 6.
1 A length of wire I l is placed in a uniform magnetic field of flux density B, at an angle ?, as shown in Figure 8.1. The wire carries an electric current I. What is the force in the wire?
Figure 8.1 Illustration for Question 1
(a) BII l sin? into the plane of the page
(b) BI Il sin? out of the plane of the page
(c) BII l cos? into the plane of the page
(d) BII l cos? out of the plane of the page
(1 mark)
2 An open metal-framed window is closed. The flux linkage from the earth’s magnetic field is depicted by the graph in Figure 8.2.
Figure 8.2 Graph for Question 2
The e.m.f. in the window frame is best described as:
(a) It decreases and is zero after time t1.
(b) It is constant and is zero after time t1.
(c) It is zero until time t1, and then becomes constant and nonzero.
(d) It increases and becomes constant after time t1.
(1 mark)
3 1 GeV is equivalent to:
(a) 1.6 × 10–7 J
(b) 1.6 × 10–10 J
(c) 106 MeV
(d) 10–3 MeV
(1 mark)
4 What is the quark structure of a neutron?
(a) uds
(b) udd
(c) uss
(d) uud
(1 mark)
5 A wire of length 0.3 m, carrying a current of 2 A, is just floating in a magnetic field of flux density 5 × 10–2 T which is at right angles to the wire. Calculate the mass of the wire.
(3 marks)
6 Mains electricity in the UK is supplied at 230 Vrms. Calculate the
peak voltage in each cycle.
(2 marks)
7 In an experiment, metallic rings are placed on a pole above a coil of wire. An a.c. current is fed to the coil and the rings are observed to rise and levitate above the coil. See Figure 8.3.
Figure 8.3 Levitating rings
(a) Explain why the rings levitate, and explain why they reach a
stable height above the coil.
(6 marks)
(b) Copy Figure 8.4, which contains the shape of one wave in the coil, and sketch the waveform of the current induced in the rings on the blank axes beneath it.
(3 marks)
Figure 8.4 The waveforms in the coil and rings
8 Rutherford observed when bombarding gold foil with alpha particles that a very small number of the particles were
deflected more than 90º. Explain how this experiment supported the nuclear model of the atom.
(6 marks)
9
(a) Consider a particle of charge Q, moving perpendicular to a magnetic field of flux density B with a velocity v. Show that it will be deflected in a circular path, radius r, where: r= p BQ and p is the momentum of the charged particle.
(3 marks) (c) Electrons are accelerated through a potential difference of
2000 V. Calculate their kinetic energy in J.
(2 marks)
(d) Calculate their speed.
(2 marks)
(e) If they enter a magnetic field of flux density 1.5 mT, what will be the radius of the circle that they will move in?
(2 marks) (f) Describe how an electron beam is generated via thermionic emission.
(2 marks)
10 Using the conservation of baryon number, charge and lepton number, decide which of the following reactions is possible:
(a) p + n ? p + µ+ + µ–
(b)
(c) n ? p + e–
(d)
(12 marks)
11 A sketch of the path of a pion in a bubble chamber is shown in Figure 8.5. This shows evidence that it has taken part in a collision with another particle and that there are subsequent decay products.
Figure 8.5 Particle tracks
(a) Pions contain combinations of two quarks, one up and one down. p+ is , p– is and p0 are either . The
pion leaves a visible track in the bubble chamber; deduce which of these four pions could have been involved.
(2 marks)
(b) At point (a) the pion collided with a stationary object. What can we deduce about the charge on this object, and the charges of the products?
(4 marks)
(c) Events (b) and (c) are decays. From the visible evidence, what can we deduce about the charges on the products in each case?
(4 marks)
(d) The rest mass of p+ is listed as 140Mce2V . Calculate its rest
mass in kg. (2 marks)
Total for assignment 60 marks
Submit your assignment
When you have completed your assignment, submit it to your tutor for marking. Please use pdf format. Your tutor will send you helpful feedback and advice to help you progress through the course.
References
Figure 8.3: Wikimedia Commons/DMacks (derivative image) (2012) ‘Induction levitation’, accessed December 2016, https://commons. wikimedia.org/wiki/File:Induction_levitation_cropped.jpg