# Physics Challenge

If you are currently finishing your 1st year of 6th form or college study, and would like to be in with a chance of winning the first prize (£100 Amazon voucher), or one of our prizes for runners-up, then have a go at the three problems below. Submit your typed or neatly-written solutions to physics@lincoln.ac.uk or by post to Physics Challenge, School of Mathematics and Physics, University of Lincoln, Lincoln, LN6 7TS.
Please include your full name, postal address and email, as well as the name and address of your school. The closing date is 5 January, 2016. See full Terms and Conditions.

Lightning striking Lincoln Cathedral (courtesy of The Lincolnshire Echo)

1. To protect tall buildings such as Lincoln Cathedral from lightning damage a ‘lightning rod’ is often placed on the highest point of the building and connected by low resistance wiring to the ground. Benjamin Franklin, who is credited with the invention in the 1760s, was actually intending to prevent lightning from striking in the first place! The idea was to allow the built-up charge in clouds to dissipate slowly rather than all at once in a bolt.

It is not well known that a Lincolnshire physicist, Edward Hussey Delaval, was on the same committee as Franklin. He suggested that a blunt tip opposed a sharp tip would actually be more receptive to lightning strikes and would offer better protection to the building if lightning were to strike. To this day it is still contended whether lightning rods work by dissipating or diverting lightning!
Why is it difficult to determine which is true? Do you agree with Delaval or Franklin; should sharp or blunt lightning rods be used?
Give your reasons, using any equations or diagrams that may help to illustrate.

2. The horse driven wheel was used in the medieval period to pull tree stumps from the ground.
Estimate how many times horsepower is increased by the use of this machine.

Scale model of a horse driven medieval stump puller (courtesy of Visual Education Project, Canada)

3. A room temperature liquid in a container, for example a plastic bottle or a metal can, can be cooled by placing the container in an ice bath. This process can be accelerated by adding salt to the ice water. Measure the rate of cooling as a function of the salt content of the ice water. Make sure your experiment is fair, plot your results and explain your conclusions.

## Notes

All solutions must be accompanied with full explanations along with any calculations you have made. You may submit partial solutions if you have managed to complete some but not all questions. You may include videos of your experiments as a part of your explanations. Submission of videos can be sent by using This is a quick and free way of sending the files to us. Please ensure that the “Friend’s email” section is filled out as physics@lincoln.ac.uk then we can download your files successfully.