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

Datalogger diagram

Learn this diagram. In this case it has a thermistor connected but you might be asked for a different piece of equipment.

Diode-resistor combinations in paralle

This type of circuit could cause problems. Here's how you do it:
Resistor R1 gets the full 12 Volts. It's a parallel circuit. We can calculate the current in Ammeter 2 like this:
I=V/R = 12V/6 Ohms = 2 Amps
Now being in parallel, the diode and R2 combination also get to share 12V.
The key to this is to realise that the diode takes up 0.6 Volts just to get started, which leaves 11.4 Volts for R2. After that, we can just ignore the diode.

So ammeter 3 has this reading: I = V/R = 11.4V/4 Ohms = 2.85 Amps.

Add up the two ammeter readings to get the reading for ammeter 1: 2 + 2.85 = 4.85 Amps

Hard homework questions

Page 6: Q4b You need to find the charge Q by using Q=It. Then use V=W/Q.

Page 7 Q10. This is the circuit described.
But it then says that the voltmeter has a resistance, so we could re-draw the circuit like this:

All you have to do now is find the share of the voltage taken by the parallel combination. You start by working out the combined resistance of the two parallel resistors and go from there.

Units for resistance and conductance.

Resistance is calculated as R=V/I.

You know that the units are OHMS.

However, because R=V/I, we could say that resistance is measured in Volts per Amp (VA-1)

Conductance is calculated as G=I/V.

The units are Siemens.

However, because G=I/V, we could say that conductance is measured in Amps per Volt (AV-1)

If you bear this in mind, you'll be able to do Q1 on page 5. (but note that they use milliAmps per Volt mAV-1)

IV and VI graphs

This is the normal way that we do graphs with voltage on the x-axis and current in the y-axis. Wire A lets a lot of current through with small voltages. We say that it conducts well - it has a big CONDUCTANCE. (or a small resistance). Hence GRADIENT MEANS CONDUCTANCE. This is the old fashioned way of presenting the same information. Notice that the axes are the other way round. Notice that for B, even a large voltage produces only a small current. It has a high resistance. So now GRADIENT MEANS RESISTANCE.

This is our filament bulb graph. Notice that as the voltage increases, the gradient gets less. In other words, it doesn't conduct as well when it gets hot, because resistance increases.

Now look at the filament bulb when the axes get changed around. On this graph, gradient means resistance, so it needs to get steeper to show higher resistance.

YOU NEED TO CAREFULLY LOOK AT THE AXES ON ANY GRAPH AND THINK ABOUT IT. DON'T ASSUME!

Presentations for Mr Skidmore

These are the groups:

Group1: Michaela, Tom W, Ashley, Greg doing HADRONS

Group 2: John, Andrew, Martin W, Tom M doing LEPTONS

Group 3: Naomi, Hannah, Martin F, Oliver doing QUARKS

Your sheets tell you what he wants in the presentations.

I'm saying:

• Powerpoints with white background
• Font size 40 or 44 is always good.
• Do NOT copy and paste things you don't understand. I can always tell!
• Put some whiteboard questions at the end to check that the rest of the class were awake during your presentation.

I'm saying you MUST include a handout!

I've put some worksheets that I've used with classes before on the school network:

Shared area; Read only; Year 12; Science; Physics; Particle physics.

They might help or they might not...

I will ask you in class next week about your progress and make sure that everyone in the group has a job. You can ask me about stuff you don't understand.

I will make you put your powerpoint onto my area of the network on Weds 8 October so that I can check work has been done.

Annihilation and pair production

Everyone has seen this equation. It's not strictly on the syllabus this year: ie you don't need to do calculations with it. However, it explains a lot. It tells us that pure energy (E) can be changed into particles of mass m. Or particles of mass m can be destroyed and turned into energy E.
This is annihilation. A positron and an electron are destroyed and two gamma ray photons are made.

• Positive and negative because charge has to add up to zero. The energy that is made has no charge.
• Equal and opposite motion so that momentum before is zero.
• Two gamma ray photons so that momentum is zero after the annihilation.
  

In reverse, two gamma ray photons can come together to make a positron and an electron. This is pair production.

Cladding

The question on the January exam about curved bit of glass with 3 rays going through (P,Q,R) raised some interesting ideas about cladding. Cladding is a layer of transparent material wrapped around the outside of the central fibre (the core). The purpose of the cladding is to stop the central glass core getting scratched. Scratched cores leak light more easily because it changes the angle of the outside wall of the glass and thus changes the angle of incidence.

Cladding makes it more difficult for the light to stay in the glass:

We will be using this equation. But note that at the critical angle, the second angle is zero.When there is no cladding, there is air around with refractive index of n = 1. So With the cladding, we get this equation:

Simple calculation will show that the critical angle becomes bigger. There are now fewer angles at which the light can hit the glass for TIR, so it is much easier for light to escape into the cladding.

Finally, be careful how you define critical angle.

• The angle of incidence for which the angle of refraction is 90 degrees.
• The minimum angle of incidence for which you get total internal reflection.
  
  
  

Making IV characteristics

In this circuit, a varaible resistor is being used to change the voltage and current through a bulb. This is not a good circuit to use for an IV characteristic. Here's why:

• Suppose the bulb has a resistance of 20 Ohms and we use a 6 Volt battery.
• The variable resistor can go from zero up to a maximum of 20 Ohms.
• When the variable resistor has zero Ohms, then the bulb will take all of the energy and the voltmeter reading will be at a maximum of 6 Volts.
• When the variable resistor has 20 Ohms, then the bulb and the resistor will have the same resistance and they will have equal shares of the energy. Voltmeter will read 3 Volts.
• Hence by using a variable resistor like this, there is a limited range of voltages you can use. In this case it is between 3V and 6V. You can't get down to 0 V.
• I suppose that if you use a variable resistor with a maximum resistance massively bigger than the bulb, you'd get a better range.

It is much better to use this set up, called a potentiometer. All 3 connections on the variable resistor (rheostat) are used and you can have all the voltages from 0V up to 6V.
This is the set up we used in class. (see IV characteristics booklet). We used a version of a potentiometer when we had the wire along the meter stick in the observation room so that every 10 cm represented 1V.

The plum pudding model

Before Geiger and Marsden did the important gold leaf experiment for Ernest Rutherford, the best idea about the inside of an atom was called the "Plum Pudding" model.

• They had just discovered the electron so they knew that there had to be negative particles inside the atom.
• If there were negatives, there had to be positive as well.
• They imagined the positive charge as being thinly smeared all over the atom. There were no definite positive particles in this model.
• The positive is supposed to be like the dough in a Christmas Pudding, and the electrons are like the raisins.

Any alpha particles fired at it should go straight through because

• alpha particles are much bigger and heavier than electrons so they would knock them out of the way.
• the positive charge is so thinly spread that there is no chance of repelling the doubly positive alpha particle.

Then the gold leaf experiment was done and Rutherford invented the Solar System model with the positive nucleus and orbitting electrons. The Plum Pudding model was consigned to the dustbin of history.