Monday, August 24, 2015

Reflections on a mug of tea

A question for students (its good to have an open ended question sometimes):

A ceramic mug of hot tea sits on a wooden table. Discuss how it cools. (Yes I know the picture does not show a wooden table... :) )

Convection from the surface is important.
Conduction through the sides and then cooling by convection significant too.
Cooling by radiation happens too although the temperature difference is small.
Conduction through the base not important as the ceramic is a poor conductor and the wood is worse.

How can the cooling be enhanced?
Convection is easily speeded up by a higher flow of air (blowing, fan, breezy location etc).
Putting a spoon in the cup increases conduction loss.
Using a black mug will help a little.
Drastic methods! (if you are late for work) - pour into another vessel, add ice etc.

Tuesday, August 11, 2015

Thursday, July 30, 2015

New discovery in subatomic physics

This is a bit exotic but I have nothing else bookmarked for this month. The first pentaquark has been reported. This is a particle composed of five quarks - very strange but predicted. Normal particles such as protons are composed of three (see diagram) so it is like one and two-thirds of a proton - kind of.

Here is a good article explaining this starting from Rutherford's discovery of the compact nucleus of the atom over a hundred years ago.

Sunday, June 28, 2015

CXC Physics on-line resources update

Following changes the CXC website now offers syllabuses and past papers as downloadable ebooks through their 'store'. Past papers cost money to get but the syllabus has a cost of $0 which means it can be downloaded free but it is neccessary to create an account and go through the shopping cart thing.

Saturday, May 30, 2015

Inside a radiation detector

This picture shows the board of a gamma-ray detector known as a Geiger-Muller counter.  Gamma rays are detected by the tube, counted and the count shown on the LCD. The words at the top of the board warn of high voltage (400-500V).
The type of radiation detected depends on the tube used.
See Wikipedia for more.

Monday, March 30, 2015

Closing off modeling climate change

Have just finished the course at EdX which I mentioned in two previous posts (first, second).

As part of an assignment I looked at historical temperatures for Georgetown and Timerhi. Georgetown has one of the most complete sets of data for this part of the world. Here is a graph showing the annual average temperatures (degrees Celsius) from 1850 (Georgetown orange, Timerhi green). It is clear that there is an upward trend which has accelerated from about 1950. The trend for Georgetown is about 0.2 degrees rise per decade and the mean temperature seems to have risen by about one degree since 1950. Global temperatures show similar results.
One closing topic was short-term mitigation - what we can do now to reduce the problem. The lecturer pointed out that there are things that can be done such as reduce heat loss by homes in cold countries, change agricultural practices to trap more carbon and increase energy efficiency of transportation. Another is to change our life style and diet.

It seems to me more than ever what we need is better working together to tackle world problems and less short-term politics.