with mr mackenzie
standard grade physics is in here
Recently we looked at the way power companies calculate how much electrical energy we use in our homes. The Joule is too small a unit to measure household energy consumption, so suppliers work out our bills using a must larger unit called the kilowatt-hour (kWh).
I found this page on BBC Bitesize that has some information and a quiz to test your kWh knowledge. There is also a section on the kilowatt-hour at gcse.com.
Ria asked if a slinky really stops in mid air when it is released. Eoin helped us to find out the answer.
It’s easier to see what happens if you watch a slow motion version.
Or even slower motion.
Why does that happen?
A dog food company has launched the first TV advert aimed directly at dogs themselves. The advert’s soundtrack includes sounds with frequencies in the range 18 000 to 20 000 Hz (18-20 kHz). Dogs can hear sound with frequencies greater than 20 kHz, we call this ultrasound.
I found a copy of the advert on Youtube. The upper limit of human hearing is 20 kHz, so some young people should be able to hear the sounds for themselves even though adults can’t. Can you?
Now let’s see what happened when the BBC Newsround team filmed some dogs while the advert played in the same room.
You might have used a signal generator in class to discover your personal upper frequency limit. The video below has a soundtrack that starts at 1 Hz and sweeps up to 20 000 Hz, before sweeping back down to 1 Hz again. For how long can you hear the sounds?
We looked at generating electricity using hydroelectric and nuclear power stations today. Click on this image of a hydroelectric station to see how they work.
Nuclear fission occurs when a large nucleus splits into 2 or more smaller nuclei. In nuclear reactors, splitting of the large uranium nucleus is achieved by adding a neutron to make the large nucleus unstable.
Animation showing fission of U-235 by Stephan-Xp
The animation above shows a (blue) neutron destabilising a large (red) uranium nucleus to cause fission. When the 2 smaller nuclei (also red) are released, notice that three (blue) neutrons are produced as part of the fission process. These new neutrons can proceed to interact with other uranium nuclei and cause further fission to occur. If the reaction continues to take place, we have a chain reaction.
This youtube video uses mousetraps and table tennis balls to demonstrate a chain reaction. Notice how the reaction keeps going after the first ball is added and only stops when all the mousetraps have sprung.
Here is your homework on the resistance topic. Download the pdf with your questions by clicking on the download link below.
Please hand in your jotter no later than Tuesday 7th February.
Pupils going on the school ski trip should try hand their jotter in by the end of this week.
Here is a short test to let you find out how much you’ve learned about parallel circuits so far. Click on the circuit diagram below to open the question page. You will be asked to enter values for V1, V2, A4 & A5.
image courtesy of MATTER project
If you need some help to find all four values, click on the check answers button to view the working.
Are you ready for some more challenging questions on series and parallel circuits? Try this page.
We have been looking at electrical power this week. The man I mentioned in class today is James Watt. Here is a short biography by the BBC. I also spoke about how he calculated the power output of working horses and compared them to his steam machines. You can read more about his horsepower experiments here.
I’d say he’s a pretty famous scientist – not many people get their name on every lightbulb in the world! 😉
image courtesy of sonrisaelectrica
The section of the electromagnetic spectrum with wavelengths ranging from 10nm to 400nm is called ultraviolet radiation (uv for short). Sunlight contains uv rays and it’s those uv rays that are responsible for the suntan you get during the summer holidays. This Australian animation shows how the ultraviolet in sunlight causes our skin to tan and explains why too much uv will damage our skin. The SunSmart page has loads of information on staying safe in the sun.
The damage that uv can do to cells is put to good use in some sterilisation equipment, such as this bottle for safe drinking water and the toothbrush sanitiser shown below.
The Nobel Prize for Medicine was awarded to Niels Rydberg Finsen in 1903 for his research into the effects of ultraviolet on the bacteria that cause tuberculosis.
There is a Bank of England leaflet (pdf) with further information on the security features in our banknotes.
Remember that whenever something glows under a uv light, we’re not seeing the uv radiation itself because our eyes can’t detect ultraviolet. Instead, we see the fluoresence; visible light given out in response to the uv falling on the material.
Kuba demonstrated fluorescence in class with some uv hair gel. Here he is with some of the gel in his hair.
but we don’t see anything until we turn on the uv light.
Cool, eh?
You can even buy genetically modified tropical fish that glow under uv light.
X-rays are a form of electromagnetic radiation. They have a much higher frequency than visible light or ultraviolet. The diagram below, taken from Wikipedia, shows where x-rays fit into the electromagnetic spectrum.
image by MaterialscientistWilhelm Röntgen discovered x-rays and the image below is the first x-ray image ever taken. It shows Mrs. Röntgen’s hand and wedding ring. The x-ray source used by Röntgen was quite weak, so his wife had to hold her hand still for about 15 minutes to expose the film. Can you imagine waiting that long nowadays?
This was the first time anyone had seen inside a human body without cutting it open. Poor Mrs. Röntgen was so alarmed by the sight of the image made by her husband that she cried out “I have seen my death!” Or, since she was in Germany, it might have been “Ich habe meinen Tod gesehen!” that she actually said.
Röntgen continued to work on x-rays until he was able to produce better images. The x-ray below was taken about a year after the first x-ray and you can see the improvements in quality.
Notice that these early x-rays are the opposite of what we would expect to see today. They show dark bones on a lighter background while we are used to seeing white bones on a dark background, such as the x-ray shown below. The difference is due to the processing the film has received after being exposed to x-rays.
In hospitals, x-rays expose a film which is then developed and viewed with bright light. X-rays are able to travel through soft body tissue and the film behind receives a large exposure. The x-rays darken the film. More dense structures such as bone, metal fillings in teeth, artificial hip/knee joints, etc. block the path of x-rays and prevent them from reaching the film. Unexposed regions of the film remain light in colour.
Röntgen’s x-ray films would have involved additional processing steps. The exposed films were developed and used to create a positive. In creating a positive, light areas become dark and dark areas become light. So the light and dark areas in Röntgen’s x-rays are the opposite of what we see today. Our modern method makes it easier to detect issues in the bones as they are the lighter areas.
Röntgen was awarded the first ever Nobel Prize for Physics in 1901 for his pioneering work in this field of physics.
Medical imaging has come a long way since Röntgen’s discovery of x-rays. This promotional video from German company Siemens outlines the advances that have been made since the early 20th century.
I have attached a recording of a short BBC radio programme about the first x-ray and what people in the Victorian era thought of these new images. Click on the player at the end of this post or listen to it in iTunes.
Here is the answer to the wave equation question.
