ISTA – PGE & NQT Practical Sessions 1st Feb 2012 Some physics demonstrations

ISTA – PGE & NQT Practical Sessions

1st Feb 2012

Some physics demonstrations

menuRory Geoghegan. Science Education 2

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1. A block of iron in the flame

2. Steel wool in the flame

3. Metal expansion

4. Weighing air ; atmospheric pressure

5. Atmospheric pressure using syringe

6. Blowing bubbles in deep water

7. Blowing into a sealed bottle

8. Magnet on pivot acts as a compass; force on pin (3rd law)

9. Electromagnetism: magnetic effect of current; force between coil and magnet

10. Electromagnetic induction: magnet

in coil; two motors, windmill

11. Simple speaker

12. A simple spectroscope

13. Fluorescent lamps produce discontinuous spectra

14. Fraunhofer lines; star spectra

15. Colour mixing

16. Shadows

17. Measuring height of trees

18. Newton’s first law of motion

19. Another plane

20. Magnetic water

Iron in the fire


A block of iron in the flame

When a large block of iron is held in the flame for 30 seconds the most noticeable change is that it gets...

wet. The water is one of the products of

the combustion of hydrocarbons. C4H10 + 6½O2 => 4CO2 + 5H2O

(butane)


Steel wool in the flame

When fine steel wire is held in the flame it...

burns, forming iron II oxide (FeO).


Metal expansion

A thin wire in tension balances the weight of a pivoted straw.

When the wire is heated with a small flame ( e.g. a match) it gets longer and the straw tilts downward straight away.

As the wire cools again it contracts and the straw returns to its original position.

Weighing air

Air has weight (and mass); this accounts for atmospheric

pressure


Weighing air

Find the weight of three litres of air using the apparatus shown.

Calculate the mass of one litre of air (at STP ?)


Sealed syringe as ‘pressure gauge’

The 3 L bottle has a bicycle valve in the cap.

Inside there is a sealed syringe (e.g. 25 cm3)

The bottle is pumped until the air in the syringe is reduced to half its original volume.

Then the bottle contains twice as much air as it did at the start, but at twice the pressure.


Atmospheric pressure

Atmospheric pressure is due to the weight of air per unit area of the Earth’s surface.

If the atmosphere were uniformly dense it would be just 8 km deep (8000 m)

The mass of a cubic metre of air is ...1.2 kg

The weight of a cubic metre of air is ...12 N

The weight of air over each m2 of the Earth’s surface is ...12 N ×8000 1 m2


1 N/m2 = 1 pascal

Atmospheric pressure is about 100,000 N/m2

i.e. 100,000 pascal (Pa) 1 Pa = 1 N/m2

100,000 Pa might be written as 100 kPa Meteorologists prefer 1000 hPa because the

figures are about the same as they traditionally used — the millibar, now obsolete.


One ‘atmosphere’ (link)

In metric units pascal (Pa) = 101325.0270000 millibar (mb) = 1013.2502700 kilopascal (kPa) = 101.3250270 bar (b) = 1.013250270

in imperial units inch of mercury (in Hg) = 29.9213818 centimetre of mercury (cm Hg) = 76.0002548 millimetre of mercury (mm Hg) = 760.002548

in other units pound per square foot (lb/ft2) = 2116.1711775 pound per square inch (psi) = 14.6959793 atmosphere (A) = 1.0000000

http://www.bbc.co.uk/cgi-bin/weather/calculators/pressure.pl


Question

What force is required to pull the plunger of the sealed 20 cm3 syringe out to the 5 cm3 mark?

What force is required to pull the plunger out to the 10 cm3 mark?



... to the 5 cm3 mark

Approximately 32 newtons (32 N)


... to the 10 cm3 mark

Again it is approximately 32 N Why? In both cases we are pulling against the

force exerted by the atmosphere on the plunger.


Can we estimate atmospheric pressure?

On what area is the force acting? How can we find the internal area of cross-

section of the syringe? Here are two ways...

1. measure the internal diameter and calculate π r2

2. From the volume and the length calculate the area. (area = volume / length)


Finding the internal area of cross-section

The length to 20 cm3 mark is 6.3 cm so the area is 3.2 cm2. (20 / 6.3 = 3.17)

So the pressure is 32 N/3.2 cm2 = 10 N cm−2 = 100000 N m−2

Blowing bubbles

in a glass of water and

in deeper water


Blowing bubbles in a glass - easy


Blowing bubbles in deep water (1 m)

At a depth of just 1 metre it is quite difficult to blow bubbles in water.

The pressure at a depth of 10 metres in fresh water is equivalent to one atmosphere.

At 10 metres the air in your lungs would be halved in volume and would therefore provide less buoyancy.


Could you blow into a closed bottle?

Could you blow bubbles into water in a closed bottle?

Let’s see.


What happened?

It is possible to blow bubbles in the closed bottle for a while. However the extra air increases the pressure inside the bottle and it becomes increasingly difficult to add more.

When you stop blowing the excess pressure forces some of the water out of the bottle.

Is this surprising?


Extension: bottle in free fall

Blow a little air into the closed bottle, but not enough to cause the water to be forced out.

Now let the bottle fall. What happens?

Electromagnetism

The force on a coil in a magnetic field


Small coil on a card

Wind a coil of about 30 turns of fine enamelled copper wire and stick it to a piece of light card using adhesive tape.

Remove the enamel from the ends of the coil and attach an audio lead.

Connect a battery and reversing switch. Bring it near a compass or magnet on a

pivot. Switch the current on and off. Hold the card near a magnet and

switch on the current. The card is attracted to or repelled by the magnet depending on the direction of the current.


Attach an audio source

Connect the coil to an audio source.

It makes no sound...

... unless it is held near a magnet.


Parts of a speaker

Magnetic field (usually a radial field across a gap)

Coil that fits into the gap. The audio signal is fed into the coil.

A diaphragm, usually of paper or other light material. This is attached to the coil.

A simple spectroscope


The outside and the inside

A ‘transmission’ version can be made by removing the metal layer of the CD-R. (works better with some makes).

The functional item in this spectrometer is a small piece (1 cm2) cut from a CD-R.


Alternative models

A reflecting version is easier to make and is probably more effective (lower image).

ca. 45°

ca. 30°

slit

look in here

Piece of a CD, intact

Piece of a CD with metal film removed


Spectrum of small fluorescent lamp


A simple spectroscope

Atomic spectra are not continuous – unlike the continuous spectrum of incandescent solids such as the filament of a bulb.


Fraunhofer ‘lines’

By directing the spectroscope to a bright cloud or sky Fraunhofer ‘lines’ may be seen.

They are rather faint in this photograph.


Solar spectrum (sky or bright cloud)

Fraunhofer ‘lines’ can be discerned


Solar spectrum (sky or cloud)

Fraunhofer lines - somewhat clearer

Shadows

menuRory Geoghegan. Science Education 38Rory Geoghegan. Science for the Primary School: Magnetism and Electricity 38

Sunlight in wood

What do you notice?

Circles?

Why?


Shadows

Shadow of a card in different lighting conditions


Shadows

Shadow of a card in different lighting conditions


Can you predict the size of the image?


Can you predict the size of the image?



The Sun

Angular size in the sky: about 32’ = 0.53° Tan (0.53°) = 0.00925 1 ÷ 0.0095 = 108

Alternative calculation: Distance to the sun: 150 million km Diameter of the sun: 1.39 million km Ratio: 150 / 1.39 = 108

108

1




Learning points

The exercise can be used to teach, test or revise the following:

(Thinking, problem-solving) Light travels in straight lines Inverted images Pinhole camera Similar triangles Similar proportions Solving equations with fractions or proportions Tangent of an angle

Newton’s first law of motion

A ‘demonstration’


B52 over VietnamLength 48.5 mSpeed 230 m/s

(ca. 515 mph)

5 m

20 m

http://www.saceliteguard.com/images/sac_hist_019_X_x.jpg


Question

Why are the bombs directly under the plane?

The lowest one in the picture is over 20 metres below the plane and so was dropped 2 seconds before the picture was taken.


Newton’s laws of motion

In the 1st second each bomb falls 5 m. In that time the plane flies 230 m (almost five

times the length of the plane). Each bomb moves with the plane at first (because it had that speed before it was dropped) and after some seconds the effect of air resistance is noticeable.

It falls 20 m in 2 s, by which time the plane has flown almost half a kilometre.

(alt. pic.)

http://i.askask.com/2004/12/06/1965_B_52swbombsfalling6.jpg


Independence of components

The vertical and horizontal components of the motion are independent.

Neglecting some oscillation, the direction the bombs point is the direction of their velocity vectors – the sum of the horizontal and vertical velocity components.

Their path is a parabola and their general direction points along the parabola.


Relative motion

Tossing a coin looks no different in a plane or in a train than on the ground.

If a coin has a horizontal velocity when it is tossed it retains it.

To an external observer the coin would appear to move along a parabolic path. A local observer would simply see it going up and down.

Another plane

Measuring distance and speed


Dublin Bay from Seapoint


How far away is the plane? At full zoom the FZ18 camera field width is 3.4°. Tan(3.4°) = 0.06. This is the ratio of the width of the

field of view at the position of a given object and the distance to the object.

The field is 16 times the length of the plane – an Airbus 300 (length: 54 m) with Monarch Airlines livery.

So the distance is 16 × 54 ÷ 0.06 m, or about 14.4 km.

1

0.06


Google Earth

The distance can be checked using Google Earth.

The distance from Seapoint via the Ringsend chimneys to the line of the runway at Dublin Airport is 14.5 km

Shadows of the tall chimneys at Ringsend power station

Point from which the photograph was taken


The speed of the plane

The pictures are taken at three frames per second.

The plane travels about its own length (54 m) in 0.66 s

Its speed is therefore 54 m ÷ 0.66 s= 82 m/s= 294 km/h= 183 mph

Monarch Airlines, Airbus 300

http://en.wikipedia.org/wiki/Monarch_Airlines

Diamagnetism of water

Water is repelled by a magnet


Concave water surface


Water is diamagnetic

The picture shows a distorted reflection of a table lamp with a mesh in front of it.

Water in the Petri dish just covers a neodymium magnet.

The magnet repels the water; a concave depression is formed in the water surface.

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ISTA – PGE & NQT Practical Sessions 1st Feb 2012 Some physics demonstrations