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.

5CH - Square wave saturation

Went through inverting op amps with an ac input and showed via an oscilloscope that this setup can be used to produce a square wave. Also went through the solution to a past paper question on saturation.

5CH - Op amp saturation

Looked at saturation of op amps today. An op amp cannot produce an output voltage that is greater than the voltage of the supply. The output voltage will reach a maximum when the op amp saturates.

4H - Newton's Second Law

Newton's second law says that if an object has an unbalanced force applied to it then it will accelerate in the direction of that unbalanced force. The size of the acceleration depends upon the mass of the object. Objects that are bigger are harder to accelerate and more difficult to stop once they start moving. Smaller objects are easier to accelerate and easier to stop.

Remember that unbalanced forces cause accelerations and decelerations.

We worked on the second type of F = ma calculations today. These are when more than one force is given so it necessary to calculate the unbalanced force first before using the equation.

Power, Energy and Time

Each appliance has a power rating. This tells us how much energy it converts each second. We proved this relationship through experiment and worked through some examples with the E = Pt equation.

Inverting mode op amp

Talked about how to identify the inverting mode op amp today. Also proved the equation for an inverting amplifier through experiment and video contains a worked example using that equation.

Remember that the input resistor in inverting mode is connected to the negative (inverting) terminal. This resistor is R1 in the equation. Rf is the feedback resistor that 'loops' back from the output to the input. It is these resistors that determine the gain that the amplifier has i.e. how much it will increase the input voltage by.

An inverting amplifier not only changes the size of the input voltage but also changes (inverts) the sign. So a positive input will give a negative output and a negative input will give a positive output.

2007 Q26 Capacitance

The solution from today's past paper on capacitance. Some of you are still struggling with resistors in capacitive circuits. Remember that at the moment when the switch in such a circuit is closed, the capacitor is fully discharged so all the voltage of the supply is across the resistor and the current in the resistor is at a maximum. This current can also be called the charging current.

Unless the capacitor is fully charged or fully discharged then the resistor and the capacitor will each have a share of the voltage. Always read the question carefully so that you know whether the stated voltage is across the capacitor or the resistor.