Star Count Week 2011

As part of the campaign for dark skies in the UK, the CPRE are looking for people to count the number of stars they can see in the constellation of Orion.

Orion is very prominent in the winter sky and can be found most easily by looking for the three stars in Orion's 'belt'.

They want as many people as possible to look to the skies this week and count the number of stars they can see in the above constellation. More information and how to submit your total here:

http://www.cpre.org.uk/campaigns/landscape/light-pollution/start-count-week-2011

I suggest some of you go out and try this, if nothing else it'll help you find your way around the night sky!

3H - Parallel resistance calculations

Having proved the equation for parallel resistance, we looked at the three ways that we can tackle these types of calculations.

3H - Resistance in parallel

Proved the resistance in parallel equation today. A bit more complicated than the total resistance formula for a series circuit where we can just simply add the values together. The experiment showed that the total resistance in a parallel circuit actually decreases as we add more resistors so a new equation is needed. We'll do some examples with this equation next week.

5CH - 2007 Q27 Op amps

Solution to another Higher paper question with the differential op amp as a monitoring circuit. Some of you struggled with (b) (ii) - we haven't covered MOSFETs in class yet but you should still be able to figure out how changing the value of one resistor affects the potential at the points in a wheatstone bridge.

In this example, changing Rth causes an increase in the voltage across Rth. This means that resistor Z has less of a voltage across it and therefore there is less of a voltage a point Q.

5CH - Differential amplifiers as a monitoring circuit

The differential amplifier can be used to monitor changes in heat or light etc. The example we looked at was a temperature monitoring circuit. A wheatstone bridge is used to control V2-V1 and the output of the amplifier is connected to a suitable output device.



Some of you struggled a bit with this question so have a watch at the solution and make sure you understand this type of question okay. It may look complicated but if you split the circuit into the three parts as mentioned in the previous video then it is easier to understand.

5CH - Differential mode op amp

More on differential op amps today. Proved the equation through experiment last week so moved on to work through some examples. Remember that Rf/R1 must equal R3/R2 for this equation to hold.

When you are identifying a differential mode amplifier, remember that it has two input resistors. Meaning that it has two input voltages. All the amplifier does is amplify this difference in voltage (make sure you know which is V2 and which is V1 as this might not always be given).

4H - Work done

Work done is the amount of energy given to an object to make it move in a certain direction. The amount amount of work done depends on the force applied and over what distance the force is applied for. These quantities are ties together in the equation Ew = Fd.