## charged particles in magnetic fields

These Bitesize pages will help if you need to recap on the basics of magnets.  A magnetic field is produced whenever current flows through a wire.  The magnetic field is represented by a series of concentric circles around the wire, as shown below.

###### Magnetic field lines around a current carrying wire.  image: physick wiki

The direction of the arrows on these magnetic field lines is found using a left hand rule:

Point your thumb in the direction of electron flow, then wrap your fingers around the wire.  The direction in which your fingers curl is the same as the arrow direction.

A charged particle will experience a force as it moves through a magnetic field – apart from the special case where the particle enters parallel to the magnetic field lines.  There are different ways to determine the direction of the force acting on the charged particle.  The following method works for negatively charged particles.

Hold out your right hand and make a fist like Batman.…BIF!

###### Batman making a fist with his right hand. image: infinitehollywood.com

1. Straighten your thumb (B), first finger (I), and middle finger (F) so they are at 90 degrees to one another, like the x,y,z axes on a graph.
2. Align your thumb so it points in the same direction as the arrows on the magnetic field lines (B).
3. Rotate your right hand until the first finger direction matches the path of the negative particle (I).
4. Your middle finger will reveal the direction in which magnetic force (F) acts on the negative particle.

if no field lines are shown, position your thumb so it points from N to S in the magnetic field

If you’ve done everything correctly, the fingers of your right hand will look like this.

###### Right hand rule for negatively charged particles in a magnetic field.

If you are working with positive particles, there are two options:

1. use your left hand instead of the right, keeping the same BIF order for the fingers OR
2. use the right hand rule described above but reverse the direction of the force at the end.

Try both methods and decide which is best for you.
If you opt to use different hands, make sure you know which hand to use for each type of charge.

Why do we use B as the symbol for magnetic fields?  I have no idea, but there are some suggestions here.

## using linest to obtain a gradient and uncertainty

The period T of a simple pendulum can be calculated using

$T=2 \pi \displaystyle \sqrt{l \over g}$

where l is the pendulum length and g is the gravitational field strength.

Using a single value of length and period, we can determine the acceleration due to gravity.  However, it would be better experimental practise to vary the length of the pendulum and plot a graph of $T^2$ against length, determining g from the gradient of the line of best fit.

You’re going to spend the next few periods analysing your simple pendulum data.  The attached pdf will walk you through the steps.  It would be better if you used your own results but I’ve put some sample data on the first page if you’ve forgotten to bring yours.

If you are using your chromebook, there may be subtle differences from the Excel instructions I have provided.  Let me know if anything doesn’t work and I’ll try to help.

Note that if you are using your own data, there will be no random uncertainty as measurements were not repeated.

## referencing guide for AH project report

You should be thinking about getting some of your project report finished so there is less to do when the deadline approaches.  You can start writing up your underlying physics section and sort out the references you will include at the end of the report.  I’ve attached a guide on referencing in the Vancouver style.  Get back to me if you have any questions.

Thanks to Imperial College London for producing this booklet.

## using Excel’s LINEST function

The period T of a simple pendulum can be calculated using

$T=2 \pi \displaystyle \sqrt{l \over g}$

where l is the pendulum length and g is the gravitational field strength.

Using a single value of length and period, we can determine the acceleration due to gravity.  However, it would be better experimental practise to vary the length of the pendulum and plot a graph of $T^2$ against length, determining g from the gradient of the line of best fit.

We’re going to spend the next three periods analysing your simple pendulum data in the library.  The attached pdf will walk you through the steps.  It would be better if you used your own results but I’ve put some sample data on the first page if you’ve forgotten to bring yours.

## pp chain reaction in stars

Twinkle, twinkle little star,
How I wonder what you are.
Giant thermonuclear reaction;
Held by gravitational attraction.
Twinkle, twinkle little star,
You look so small ’cause you’re so far.

As you burn through constant fusion,
Your twinkle’s just an optical illusion.
That happens when your light gets near;
distorted by our atmosphere.
Twinkle, twinkle little star,
spreading light and heat so far.

As you use up fuel you’ll grow,
and give off a scarlet glow;
Maybe you’ll go supernova,
exploding elements all over.
Now I know just what you are;
and I know I’m made of stars.

Where does the Sun get its energy? A straightforward question but physicists struggled to find an answer until the 1920s, when Eddington suggested that nuclear fusion might be responsible.

A star is drawing on some vast reservoir of energy by means unknown to us. This reservoir can scarcely be other than the subatomic energy which, it is known exists abundantly in all matter; we sometimes dream that man will one day learn how to release it and use it for his service. The store is well nigh inexhaustible, if only it could be tapped. There is sufficient in the Sun to maintain its output of heat for 15 billion years. — Sir Arthur Stanley Eddington

## AH: the Hertzsprung Russell diagram

Our Sun is a typical yellow star, so its emission would be represented by the middle star in this image.

###### image courtesy of kstars, kde.org – colour is exaggerated

The colour of a star also tells us something about the expected behaviour of a star, it’s lifetime, and destiny. This is achieved by plotting stars on a Hertzsprung-Russell diagram. More about HR diagrams here.

This video clip looks at how the stars are arranged on the HR diagram.

While some HR diagrams use temperature along the x-axis, others use star classification.

## Using Excel in AH Physics

You will need the attached file when we go to the library to learn about using Excel to crunch data for your project.

## pp chain fusion in stars

Twinkle, twinkle little star,
How I wonder what you are.
Giant thermonuclear reaction;
Held by gravitational attraction.
Twinkle, twinkle little star,
You look so small ’cause you’re so far.

As you burn through constant fusion,
Your twinkle’s just an optical illusion.
That happens when your light gets near;
distorted by our atmosphere.
Twinkle, twinkle little star,
spreading light and heat so far.

As you use up fuel you’ll grow,
and give off a scarlet glow;
Maybe you’ll go supernova,
exploding elements all over.
Now I know just what you are;
and I know I’m made of stars.

Where does the Sun get its energy?  A straightforward question but physicists struggled to find an answer until the 1920s, when Eddington suggested that nuclear fusion might be responsible.

A star is drawing on some vast reservoir of energy by means unknown to us. This reservoir can scarcely be other than the subatomic energy which, it is known exists abundantly in all matter; we sometimes dream that man will one day learn how to release it and use it for his service. The store is well nigh inexhaustible, if only it could be tapped. There is sufficient in the Sun to maintain its output of heat for 15 billion years. — Sir Arthur Stanley Eddington