introducing special relativity…

We’ve been looking at Einstein’s special theory of relativity this week.  Special relativity is tricky get get your head round.  Let’s start with a video about the speed of light.

We watched this video in class, it follows Einstein’s thought process as he worked through his special theory of relativity.

special relativity from mr mackenzie on Vimeo.
We need to consider two aspects of special relativity, time dilation and length contraction.  We’ll look at time dilation first.

time dilation

A Tale of Two Twins from Oliver Luo on Vimeo.

another take on special relativity and the twins paradox

 

…and the Glesga Physics version

 

length contraction

This video has helpful examples to explain length contraction.

Sometimes it’s easier to imagine we’re a stationary observer watching a fast moving object go whizzing past.  For other situations, it’s better to put yourself into the same frame of reference as the moving object, so that everything else appears to be moving quickly, while you sit still.  The muon example in this video shows how an alternative perspective can work to our advantage in special relativity.

Another way to think about this alternative frame of reference is that it’s hard to measure distances when you yourself are moving really quickly.  Think about it, you’d get tangled up in your measuring tape like an Andrex puppy.

Screen Shot 2016-02-09 at 23.44.47

image: trotonline.co.uk

It would be far easier to imagine you’re the one sitting still and all the objects are moving relative to your position, as if your train is stationary and it’s everything outside that’s moving.  That keeps everything nice and tidy – including your measuring tape.  Got to love Einstein’s postulates of special relativity.

Screen Shot 2016-02-09 at 23.47.59

image: mirror.co.uk

 

H tension HW answers

I’ve marked your HW jotters and will hand them back during tomorrow’s lesson.

I’ll go over the main issues in class but many of you need to review the way you attempt tension questions; use a free body diagram and only use F=ma when you know the resultant force.  These two videos should help.

HW question 4 from mr mackenzie on Vimeo.

 

Higher HW Q5 from mr mackenzie on Vimeo.

particle accelerators

An electric field can be used to accelerate charged particles.

Conservation of energy tells us that

work done by the electric field = change in the particle’s kinetic energy

The speed of the particle can be determined if its charge and the accelerating voltage (potential difference) are known.  The notes attached to the end of this post will show how to perform the calculation.

These short video clips show how to draw electric field lines for point charges and parallel plates, with example calculations for the work done by electric fields and the final speed of charged particles in electric fields.

Q1(a) Electric fields lines around point charges from mr mackenzie on Vimeo.

Q1b – Electric field between parallel plates from mr mackenzie on Vimeo.

Q2 – Work done in moving a charged particle through a potential difference from mr mackenzie on Vimeo.

Q3 – Calculating the speed of a charged particle in an electric field from mr mackenzie on Vimeo.