PRO (PSA4) Revision Notes

These are like the ten commandments of PRO. There’s more than ten but anyway… learn these things and you will be able to work out the rest in your head when you do past papers. The equations you need to know off by heart are the first bullet points

What’s electrostatic force? F = kQ₁Q₂/r² and k = 1/4πε₀

What’s centripetal force? F = mv²/r

Derive expression for a. F = ma → ma = mv²/r → divide both sides by m → a = v²/r

What’s angular velocity? It’s how quick something rotates, angle/time, dθ/dt, in rad s^-1

How big are atoms? Atoms have a diameter of about 1 nm or 10^-9 m

What’s the electric field from a point charge? E = kQ/r²

Electric fields in particle accelerators. Why? To make particles go faster

Magnetic fields in particle accelerators. Why? To make particles turn corners

What are mesons? Quark-antiquark pair (always the same type of quark)

What are baryons? Quark threesomes

How did we predict the top quark existed? Symmetry in the Standard Model

Can you do log-log graphs? Try a question on one from PRO, or look at www.ben.mills.btinternet.co.uk/physics/A2.html.

Essentially, make a table of y, x, log(y) and log(x), then plot a graph of log(y) vs. log(x). The gradient is the power in the power law and the y-intercept is the coefficient.

For extra help read the maths reference at the back of the A2 Salters Horners textbook

What’s the de Broglie wavelength? λ = h/p

What’s the kinetic energy of a non-relativistic particle? E_k = p²/2m

What’s its momentum? Rearrange the previous equation → √(2mE_k) = p

Do you know the principle of conservation of momentum? p_before = p_after

What’s always conserved in particle collisions? Charge, energy and momentum

Why are high energies needed to break particles into their constituents and see fine structure? Smaller gaps need smaller wavelength waves to diffract through them, and smaller wavelength waves are higher energy. Since c = fλ and E = hf, E = hc / λ → smaller wavelength (λ) means you’re dividing hc by less, making E more.

Small particles like quarks are held together by the strong force, which is zillions of times stronger than the electromagnetic force which holds bricks together. Consequently you have to hit small particles with a lot of force to break them, and that means high energies.

When particles go nearly the speed of light (like say 0.1c), what happens? All the equations we’re used to like KE = ½mv² don’t work anymore.

Electronvolts! What’s that in real money? 1e = 1.6 x 10^-19 C → 1eV = 1.6 x 10^-19 J

Radians! What’s that in real money? 5 rad = 5 x (2π/360)° and 90° = 90 x (360/2π) rad

How much energy is mass worth? E = mc² → one unit of mass is worth c² units of energy

How did Rutherford discover the nucleus?

• It was thought the atom was a large cloud of positive charge with electrons dispersed in lumps throughout the atom (the Plum Pudding model of the atom)

• Geiger and Marsden fired alpha-particles at gold foil, expecting small-angle scattering (like alphas going up or down a bit as they came out the other side)

• Rutherford suggested they look for alpha particles deviated at large angles (bouncing back towards the side of the gold foil they came from

• Almost all the alphas went straight through (not what they expected) and a tiny proportion were deflected backwards

• Rutherford realised that only an atom with all its positive charge located in a tiny nucleus (centre) could explain the results of the experiment

The new nuclear model of the atom