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.

Rutherford scattering

In this experiment, a very narrow beam of alpha particles was fired at a very thin piece of gold foil.

• The thick sheets of lead are to stop the beam of alpha spreading out like the beam on a car headlight. They keep the beam narrow. The arrangement is called a collimator.
• The detector is moved around the outside of vacuum chamber and readings taken for different angles.
• The beam has to be narrow because this gives a very definite spot on the foil from which to measure the angle, as shown on the diagram below: The moment of genius was when it was decided to move the detector round onto the same side as the alpha source. No one in their right mind would expect to find alpha particles on this side, but that is excatly what they did discover. It's called backscattering. We can tell that the nucleus is positive because the positive alpha particles are repelled.
• We can tell that the nucleus is heavy by thinking of what happens in snooker when the cue ball hits a coloured ball. The cue ball stops but the coloured ball moves on. Momentum is transferred here because both balls have the same mass. However, the alpha particle bounces back and the nucleus is unmoved. This means that the nucleus is much heavier.
  
  

What happens when a photon carries too little energy?

They seem to be asking a lot of questions about what happens when a photon arrives carrying too little energy for an electron to reach the first level.

The correct answer is that NOTHING HAPPENS.

The electron does not jump up half way and then fall back. It just sits there as if nothing has happened. It does not absorb the photon at all.

Work function

Remember that we model an electron as being at the bottom of the well. In the model, energy is needed to work against gravitational attraction in the same that energy is needed to work against eectrostatic attraction between the positive nucleus and the negative electron.

The work function is the minimum energy required to escape from the atom.

It is possible to give more energy. That just means that the escaping electrons have extra kinetic energy.

That extra kinetic energy is measured in a circuit like this:

Notice the weird extra battery positioned at the top of the circuit. It is the wrong way round. Any electrons that are realeased by the light are pulled back across the gap, stopping the flow so the ammeter reads zero.

The more kinetic energy given to the electons the bigger the backwards voltage V needed to stop the current. This is called the stopping potential.

This is the graph that is produced by this equipment. Change the frequency of the light and measure the stopping potential in eV. Turn that into Joules to give you the kinetic energy. The threshold frequency is obvious.

How to describe the composition of baryons

Baryons are made of 3 quarks. If the question asks for the general composition, it doesn't want specific details of up, down or strange. So you need to write "3 quarks (qqq)", where q stands for any quark.

Clearly, this is how you would have to write a baryon anti-particle: