Electron capture

This process happens for nuclei which have a high ratio of protons compared to neutrons. (They call it a superabundance of protons).

• An electron from the inner electron level is absorbed by the nucleus and interacts with a proton.
• The proton becomes a neutron by changing a u quark to a d quark, and a neutrino is released.
• This must be a WEAK interaction because there is a change of quark flavour.
• It is also called K-capture because the electron that is captured is said to have been in the K orbital.

Link to AQA website for markschemes

Here's the link to the AQA website: http://www.aqa.org.uk/qualifications/a-level/science/physics-a/physics-a-key-materials

Choose the "Past papers and mark schemes".  Then there's a drop down box to choose the year.  We are studying PHYA2 at the moment.  The resits are PHYA1.

Revision notes link

Barnard Castle School's website has some excellent Physics revision notes for our syllabus: http://extranet.barneyschool.org.uk/myphysicsweb/page2aaaaaaaaa.html Scroll down the page to find them!

rate of dissipation of energy

Dissipation means "given off" - usually meaning wasted.

A resistor gets hot and dissipates heat. This is waste: it is unintentional.

"Rate of energy" means Joules per second. On other words, it is POWER in WATTS.

So this question merely wants you to calculate the power. Why didn't they just say so??

Bouncing ball

This graph shows the motion of a ball held in a hand and then dropped so that it falls to the floor and repeatedly bounces. At A, it is in the hand, so it has zero velocity. As it falls, its velocity increases. At B, it hits the floor. It changes direction, but is still moving, so it has a velocity but it becomes negative. As it rises back up, it slows down due to gravity. At the top, it has velocity zero (C) before it falls again, with an increasing velocity to hit the ground at D etc. The gradient of a velocity-time graph gives you the acceleration, so here the gradient = 9.81 m/s/s/

This next graph is for a rocket. As it burns fuel, the thrust upwards is bigger than the weight downwards, so it accelerates. At C it runs out of fuel, so the weight starts to make it decelerate. It goes slower and slower until it stops at E, ready to fall back. E is the highest point that it reaches.

Multiple slit interference

This time we can't easily work out the fringe directions by marking out the points where the semi-circular ripples cross over. I've drawn ripples from four slits on my diagram but I could have added more. If I had done, the ripples from the top slit and the bottom slit would not even have overlapped. So what you have to do here is figure out directions in which there is constructive interference. We do this by drawing lines across the ripples in the way that Huygen's invented. This creates a wavefront where there will be constructive interference (you are joining up peaks and peaks!). To get the direction, draw a ray at 90 degrees to the wavefront. Notice that to reach the wavefront, the red wave has gone one wave further than the blue wave etc. Between the waves from one slit and the next on this wavefront, there is a path difference of one whole wavelength.

The next diagram shows the same situation with the ripples removed. Since the rays travel (almost) parallel, the marked section shows the path difference. If the path difference is an integer multiple of the wavelength, you will get constructive interference in that direction.

Finally we extract the bottom triangle, marked X on the previous diagram and label it up as shown to get the diffraction grating equation.

Diffraction with wider slits

These pictures show what happens if we repeat diffraction with wider slits. Notice that the waves no longer bend round as far. This means that the area of overlap in which interference can occur is reduced. This means fewer fringes are seen. (In the second picture, I've dotted the fringes that are lost). But notice that the fringes that remain still have the same spacing.

Slit separation and fringe width

You can use the Double Slit equation to prove that a smaller slit width results in a bigger fringe separation but these diagrams are a more "physics" way of doing it. Pick one wave a count the number of other waves that it crosses. The further apart the slits, the more waves it crosses so there are more places for constructive interference and thus more fringes in the same space. The closer the slits, the fewer times a wave overlaps another wave, so the directions of constructive interference are more spread out.

The photoelectric effect

Look at the curcuit diagram. There is a big gap in the circuit so no current should flow. There are two metal plates on the sides of the gap. One is made of gauze so we can shine light through it to hit the other plate.

A weird thing happens:

1. If we start with a low frequency of light we get no current, but if we increase the frequency we suddenly get to a point where there is a current. Light releases electrons to cross the gap.

2. If we keep increasing the frequency above the threshold frequency but keep the brightness the same, we get exactly the same ammeter reading but the electrons can be measured to have more kinetic energy.

3. If I pick a fixed frequency below the threshold and keep making it brighter, there will still be no current. Yes, there will be more energy and the bright light might even feel hot, but there will still be no electrons.

4. If I pick a fixed frequency above the threshold and keep making it brighter, the current will go up and up. However, the measured kinetic energy will stay the same.

Points 3 and 4 are evidence for the particle nature of light - that it is made of lumps of energy called photons.

Photons are like getting money from a cash machine. I can have as much as I like but it will be made up of £20 notes. So I can't get a £100 note, but I can get five £20s. I can't get a £280 note but I can get fourteen £20s.

Brighter light is like this. Below the threshold I get more lumps of insufficient energy. An electron can only collide with one photon at a time so it can never get enough energy.

Above the threshold, there will be more lumps of energy available so more electrons will be able to absorb the necessary energy at once so more electrons will escape and the current goes up.

Past papers

This is a new course. There are no past papers because your exam will be the first ever.

However, you can use the questions from the old syllabus. They will probably be the same people setting and marking the new syllabus. You have already done most of these questions as homeworks and tests.

Use this link. You need the electricity parts of Module 3. The mark schemes are also available.

http://www.aqa.org.uk/qual/gceasa/phya_assess.php