www.bpho.org.uk Oxford 24 th June 2014 Physics Challenge AS - - PowerPoint PPT Presentation

 Physics Challenge  AS Challenge  A2 Challenge  Experimental Project  BPhO

• Round 1
• Round 2
• Training Camp
• IPhO

www.bpho.org.uk

Oxford 24th June 2014

Robin Hughes

 King’s College School Wimbledon  British Physics Olympiad www.BPhO.org.uk  Rutherford Schools Project www.Rutherford-Physics.org.uk



“Moreover a physics problem should be difficult in

• rder to entice us, yet not completely inaccessible,

lest it mock at our efforts. It should be to us a guide post on the mazy paths to hidden truths, and ultimately a reminder of our pleasure in the successful solution”. David Hilbert

rwh@kcs.org.uk

What makes a student competitive in physics and engineering?

 Problems that demand understanding?

Linguistically stylised – interpretation & recognition

Massless pulleys Infinite planes Inextensible massless string Point particles Zero friction

Etc.

Superfluous information

Occurs in the real world

 Transferable skills

Clarity of thought Perseverance The buzz of success Confidence Interest Empowering

Our people are

• ur greatest

asset

 Explanations  Computations & calculations  Estimates & Fermi problems  Technique spotting  Proofs  Bookwork  Data analysis

Recent research by SEPnet (from ASE EiS April 2011)

Employer views of the skills of physics graduates indicated that the three aspects most highly prized were those of

 mathematical competence  the ability to use equipment to produce evidence  being good at problem solving.

What was disturbing was the view that the only one that employers felt they were getting was the first.

• Requires a knowledge of physics ideas
• Requires a “feel” for some of the ideas
• Requires putting in numbers
• Requires a feel for the physics and what

seems reasonable

5012 – 4992

Is it likely that you breathe in a molecule

from Caesar's last breath? Estimate the mass of the earth's atmosphere Estimate the temperature of a newly formed star

• 0. 4

 Any good ideas?  Any numbers we know?  Is it too hard?  Is the hard way the only way?

 When a river floods, the debris that is left

behind is often seen in the form of large

• boulders. Most rivers do not flow very much

faster when the river floods as the slope of the river bed remains the same.

 What is the physics?  What are the variables?  Are they related?  What is the result?  Is this what we

• bserve?

 Mass of the boulder rolled m  Speed of the river flow v  Density of boulder (and river combined into

some density parameter) ρ

 field strength g  Derive a dimensionally homogeneous

equation for m in terms of v, ρ and g.

𝑛 = 𝑔(𝑤, ρ, g)

6 3 2 3 1 , , ] [ ] [ ] [ ] [ ] [ ] [ ] [ ] [

2 3 1

           

  

        

     

T L M T L M

• f

powers equating LT ML LT M g v M

3 6

g v k M  

Mass of rock swept down by a flooding river:

 What is the (simple) physics?  Is it a fundamental physics idea?  What are the variables?  Are they related?  What is the result?  Is this what we observe?

 An explosion produces a pressure wave and

the speed of the wave is determined by the nature of the surrounding medium and the energy of the explosion.

 Explosions producing pressure waves in the

air can be can be caused by atomic bombs, exploding petrol cans, nitroglycerine, etc.

2 5 5 2 5 1 5 1

t R E

• r

t E const R    



) , , (  t E f R 

E = 1.2 x 4.2 x 1013 J = 5 x 1013 J = 5 x 1013 / 4 x 109 T TNT = 12 kilo tonne TNT

ρair=1.2 kg m-3 1 tonne TNT = 4 x 109 J

0.006 ms 16 ms 25 ms 53 ms 62 ms 90 ms

Trinity Atomic Explosion

R5 = 4.2 x 1013 t2 (+ 6 x 109) R2 = 0.996

0.0E+00 5.0E+10 1.0E+11 1.5E+11 2.0E+11 2.5E+11 3.0E+11 3.5E+11 4.0E+11

0.002 0.004 0.006 0.008 0.01

R5 / m5

t2 / s2

Trinity Explosion

y = 0.367x + 2.7 R² = 0.997

1.8 1.9 2 2.1 2.2 2.3 2.4

• 2.4
• 2.2
• 2
• 1.8
• 1.6
• 1.4
• 1.2
• 1

Log(R/m) Log(t/s)

Trinity Explosion

 A star of uniform density is formed from a

very large cloud of gas

 The loss of gravitational potential energy

appears as thermal energy of the star

 Average stars radiate due to fusion

processes going on internally. But how does this start?

 Do the “hot” protons get close enough to

fuse, and then start the exothermic (nuclear) reaction?

GPE lost in forming a star of mass M, of radius R, and of uniform density ρ is given by

R GM 2 5 3

Mass dm falls from a great distance to radius r and forms a thin shell of thickness dr Integrate up from 0 to R to determine the total gpe lost.

 For the sun, M = 2 x 1030 kg  no. of protons, N

(1.2 x 1057 )

 Average ke of a proton (3.3 x 10-16 J ≈ 2.2

keV)

 Temp of star (1.6 x 107 k)  Closest approach of protons (3.5 x 1013 m)  Range of strong nuclear force ≈ 10-15 m  de Broglie wavelength ≈ 6 x 10-13 m

 Tea  Social event  Portfolio of questions  Pupils are the key asset  Teacher role

 Overall winner of the 1988 IPhO Competition

Conrad McDonnell (UK)

 O levels 1986  A levels 1988  Special Paper 1988  Ox Entrance Paper Nov ‘87

Overall winner of the 1988 IPhO Competition; Conrad McDonnell (UK) O levels 1986, A levels 1988, Special Paper 1988, Ox Entrance Paper Nov ‘87

O & C Special Paper 1988

Bathed in the Glow

• f Success