Anyone who takes Physics AS will know that although the topics you learn about are a big contribution to the final mark of the G492 exam, they only account for 60% of all the marks. The last 40% comes from something you can prepare for. You are giving a green paper entitled ‘AS GCE PHYSICS B (ADVANCING PHYSICS G492 Understanding Processes/Experimentation and Data Handling’. Within this booklet will be three random articles of which will come up in the Section C of your exam. You can prepare for 40% of your exam this easily. For this reason, here are some revision notes for the year June/May 2012 for the three articles called 1. Quality of Measurement, 2. Measuring the Planck constant using LEDS and 3. Cavendish: Measuring the Earth’s Density. Feel free to skip to the parts most relevant to you.
If you haven’t got the article, you can find it here
1. Quality of Measurement
You can not be 100% sure what questions will come up. However, we can have a guess from the case study what type of questions will come up. From this case study, we can hint towards questions on:
- Qualities and limitations of measuring instruments (being digital and analogue ammeter).
- Resolution, sensitivity and zero error of the measuring instruments.
- Identifying the largest uncertainty.
- Ways to reduce the uncertainty.
- Zero error as a systematic error.
From the case study, we can see immediately the resolution of both ammeters. The resolution is the smallest possible value the instrument can measure (the smallest change between two values that can be measured). For the analogue ammeter, the resolution if 1 amp with the digital ammeter’s resolution at 0.01 amps. Big difference, right?
The sensitivity of the sensors can be determined through the equation change in output / change in input. E.g. if there is a 2 degrees temperature change which causes a 5V output change, 5/2 = 2.5 V/degrees. Ultimately, digital is always better.
The response time is the definition of how quickly the sensor reacts to an input.
The zero error is the definition when a measuring device does not start from exactly zero.
Digital Ammeter
- Low resolution of 0.01 amps: more accurate.
- Generally better than the analogue ammeter.
- It is easier to read from the digital ammeter. However, when recording a current, it will likely flicker a lot making it hard to determine the value. As well as this, there is a chance of the sensor having a zero error.
- We need to take into account that the sensor may have a small resistance creating a systematic error.
Analogue Ammeter
- High resolution of 1 amp.
- Hard to read because needle can be read at different heights causing the value to look different when look at different places.
- There may be a zero error.
- Hard to determine to 1d.p let alone 2.
- There may again be a resistance.
2. Measuring the Planck constant using LEDs
- ‘what the difficulties are and how the data can be processed’ – What are the difficulties with this experiment? Think about variables in a laboratory that would effect this experiment such as background light, resistance in the circuit and uncertainties.
- The biggest uncertainty is with ‘consistently and accurately judging the voltage at which the LED strikes’. This is when the observer has to make the judgement when he sees light produced by the LED. To reduce this uncertainty, the LED is shielded with a small opaque paper tube to block background light. However, there is the uncertainty of the observer their self.
- ‘Plotting an appropriate graph of the data allows a value for the Planck constant to be determined’.The gradient of this graph will be the planck constant. Therefore, we know if we have done the experiment right by comparing the gradient to the true value of the planck constant (6.6×10-^34).
To draw the graph which is strongly recommended, we need to add another column to the graph (click to zoom in):
To draw out this graph using the E = hf, we need to know two of them at least. We are trying to find out h therefore don’t need to know that. We know the frequency from the graph. However, we don’t know the Energy. To work this out, we can times the Vs values with the energy of an electron (1.6×10^-19 coulombs). This produces an eV (electron voltage) value which is energy.After completing the extra column and drawing out the graph, you should get a positive correlating line of best fit.
I’m too ashamed to show my graph because its very scruffy, but when working out the gradient, I took dy/dx which equals 2.18×10^-19 / 3.5×10^14 = 6.22×10^34. As you can see, it’s extremely close to the true value of planck’s constant.
Cavendish: Measuring the Earth’s Density
The most important aspect to the Cavendish article are the results from his experiment. There are likely to be a lot of questions on the results of his results compares to the modern accepted value of the mean density of the Earth being 5520kg/metres cubed:
- Range – The range for all 29 value is 970.
- Spread – You can work out the spread by halving the range. Therefore, it is plus or minus 485.
- Mean – From all 29 values, the mean is 5448.
- Outliers – Are there any outliers in the data? To work out if there are any outliers, you work out the values that are twice the spread from the mean.
Therefore, 2 X spread = 970. 5448 (mean) + 970 (2 times the spread) = 6418. 5448-970 = 4478.
From this, for a value to be an outlier, they need to be less than 4478 or more than 6418. There are no values that fall in this area therefore there are no outliers.
We can work out the percentage uncertainty through doing the spread / mean. This is 485/5448 = plus or minus 9%.
It is important to remember that in working out the above, I used all 29 values. However, you may be asked to do the range, spread, mean, outliers and percentage uncertainty for just the first set for the first wire or just the second wire. Therefore, just for the terms used above:
- Range – highest value – lowest value.
- Spread – range / 2.
- Mean – sum of all values / number of values there are.
- Outliers – Mean plus or minus 2 X spread. If it falls within the values, it is not an outlier.
- Uncertainty – spread / mean.
