# Physics Unit 3

Magnetism and stars, Forces

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## Gravity

Gravity is the centripetal force that keeps planets in orbit

• ·         The larger the masses, the greater the force of attraction.
• - An orbit is possible when there is a balance between the forward motion and the gravitational force.
• - The orbits are elliptical (elongated circles)
• - To counteract the stronger gravity, planets near the sun move faster, covering their orbit quicker.
• -Artificial satellites were set up by humans for 4 main purposes:
• ·         Monitoring of the earth – weather and climate
• ·         Communications – phone and tv
• ·         Space research – hubble
• ·         Spying on baddies.
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## Satellites

There are 2 main types of orbits useful for satellites:

• Geostationary satellites are used for communication
• ·         They are in high orbits over the equator and take exactly 24 hours to complete.
• ·         This means they stay above the same point on the earth’s surface.
•
• Low polar orbit satellites are for weather and spying
• ·         The satellite sweeps over both poles whilst the earth rotates beneath it.
• ·         Each orbit only takes a few hours.
• ·         This means the whole surface of the earth can be monitored each day.
• ·         These are good for weather.
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## Circular motion/centripetal force

• - Circular motion means the velocity is always changing.
• - Velocity is the speed and direction of an object.
• - If it is travelling in a circle, it is constantly changing direction and so it is constantly accelerating.
• - This means there must be a force acting on it, this is the centripetal force. It acts inwards towards the centre of the circle.
• - The force providing the centripetal force could be gravity, tension or friction.
• Centripetal force depends on mass, speed and radius.
• -          Centripetal force depends on mass, speed and radius.
•  -          The faster an object is moving, the bigger the C.F

-

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## Moment and centre of mass

-          A moment is the turning effect of a force

• -          Moment = Force x perpendicular distance (m) between line of action and pivot
• -          A larger force would mean a larger moment
• -          To get the maximum moment you need to push at right angles to the spanner (eg).
• -          Pushing at any other angle would make it smaller because the perpendicular distance would be smaller.
• -          The centre of mass is directly below the point of suspension
• -          The centre of mass is where the whole of the mass is concentrated.
• -          A freely suspended object will swing until its centre of mass is directly below the point of suspension.
• -          To find the centre of mass you can use the plumline experiment.
• -          For simple shapes, you can tell by where the lines of symmetry cross
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## Images

-          A real image is where all the light from an object comes together to form an image on a ‘screen’.

-          A virtual image is when the rays are diverging, so the light from the object appears to be coming from a completely different place.

-          When you look in the mirror, you see a virtual image because the object appears to be behind the mirror.

-          You can get a virtual image when looking at an object through a magnifying lens; the image looks bigger and further away than the object actually is.

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## Lenses

Light is refracted when it enters and exits glass prisms

·         In a glass rectangular block, the ray bends towards the normal as it enters the medium, and further away as it exits the medium.

·         Different wavelengths of light refract by different amounts so white light disperses into different colours as it passes through a triangular prism.

·         2 main types of lens converging and diverging.

·         Converging is convex – It bulges outwards and makes rays join together.

·         Diverging is concave – it caves inwards causing light to diverge.

·         Each lens has a focal point behind and in front of the lens.

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## Refraction rules

Three rules for refraction in a converging lens:

1.      An incident ray parallel to the axis refracts through the lens and passes through the focal point on the other side.

2.      An incident ray passing through the focal point refracts through the lens and travels parallel to the axis.

3.      An incident ray passing through the centre of the lens carries on in the same direction.

Three rules for refraction in a diverging lens:

1.      An incident ray parallel to the axis refracts through the lens and travels in line with the focal point.

2.      An incident ray passing towards the focal point refracts through the lens and travels parallel to the axis.

3.      An incident ray passing through the centre of the lens carries on in the same direction.

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## Magnification and Cameras

Uses – Magnification and Cameras       -          Magnifying glasses use convex lenses,  You can’t project a virtual image onto a screen, The work by creating a magnified virtual image

Magnification formula

Magnification =  Image height / Object height

Taking a photo forms an image on the film

- The light from the object travels to the camera and is refracted by the lens and forms an image on the screen.

- The image on the screen is a real image because the rays actually meet there.

- The image is smaller than the object because the object is a lot further away than the focal length of the lens.

- The image is also inverted (upside down)

- This is the same as in our eyes, a real, inverted image is formed on the retina but the brain is smart enough to turn it around.

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## Sound waves

- The waves of sound are caused by vibrating objects, they are known as longitudinal waves.

- Because they are caused by vibrating particles, the denser the medium, the faster the sound travels through it.

- Sounds generally travels faster through solids than liquids, and liquids than gases.

- Sound waves can reflect and refract

- They can be reflected by hard, flat surfaces, things like carpets or curtains will absorb sound as opposed to reflect it. - Sound waves refract as they enter a different media.

Humans hear sound between 20 – 20000 Hz

- Sound does not travel in vaccums.

- Loudness increases with amplitude and pitch increases with frequency.

- Frequency is the number of complete vibrations each second.

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## Ultrasound

- It is a sound with higher frequency than we can hear (20,000 hz+)

- You can use CRO traces to compare amplitudes and frequencies, they look the same as normal sound but each square represents a much shorter time (e.g. 0.000001s)

- As the waves pass from one medium to another, they get partially reflected and partially transmitted.

- The time it takes for reflections to reach a detector can be used to measure how far away the boundary is.

You can also use oscilloscope traces to find boundaries

-You can work out how long it takes to travel between two pulses.

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## Uses for ultrasound

Ultrasound vibrations are used in industrial cleaning

- It can be used to clean delicate mechanisms without them needing dismantling

- The high frequency vibrations of ultrasound make the dirt on the equipment vibrate and be ‘shook off’

It is also used in industrial quality control

-It can be used to detect the quality of internal structure based on the reflections and echoes.

It is also used for pre natal scanning of a foetus

-The virtual image is produced according to the way in which the waves are reflected back.

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## Magnetic fields

A magnetic field is a region where magnetic materials and also wires carrying currents experience a force acting on them.

-          If there is a wire carrying a current, there will be a magnetic field of concentric circles around it.

-          Right hand rule – thumb in direction of current, the way the fingers go round is the direction the magnetic field goes.

-          The magnetic field inside a solenoid (coil of wire) is strong and uniform.

-          Outside the solenoid, the magnetic field is the same as around a bar magnet.

-          The ends of a solenoid act as a north and south pole.

-          You can tell which end is north or south (form an N or S depending on the direction of the current).

-          Iron, steel and Nickel are all magnetic.

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## The motor effect, the simple electric motor

The motor effect

-          A current in a magnetic field experiences a force

-          Fleming’s left hand rule tells you which way the force acts.

The simple electric motor

-          Factors which speed it up – More current, more turns on the coil, stronger magnetic field, a soft iron core in the coil.

-          The direction of a motor can be reversed either by swapping the polarity of the DC supply or swapping the magnetic poles over.

-          With an electric motor, if you link a coil to an axle, the axle will spin round. This is the basis for how all electric motors work.

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## Electromagnetic induction

-          Electromagnetic induction: The creation of a voltage in a wire which is experiencing a change in magnetic field.

-          Moving a magnet in a coil of wire induces a voltage.

-          If you move the magnet in the opposite direction, the current will be reversed.

-          If you keep moving the magnet then this will create an AC current.

-          You can create the same effect by turning a magnet inside a coil of wire.

-          Four factors affect the size of the induced coil – Strength of the magnet, area of the coil, number of turns on the coil, speed of the movement.

-          The faster you move/turn the magnet, the higher the frequency/pitch of the wave.

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## Generators and Transformers

Generators

-          Just turn the coil and there’s a current! They produce AC voltage

-          Dynamos involve turning a magnet instead of a coil, they are a different kind of generator.

-           It still causes the field through the coil to swap every half turn so the output is just the same as a generator.

Transformers

-          Transformers change the AC voltage only

-          Step-up transformers – More turns on the secondary coil than the primary coil.

-          Step-down transformers – Less turns on the secondary coil than the primary coil.

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## Transformers

-          They work by electromagnetic induction

1.       The primary coil produces a magnetic field which stays within the iron core.This means nearly all of it passes through the secondary coil.

2.       Because there is AC in the primary coil, the field in the iron core is constantly changing direction.

3.       The rapidly changing field is felt by the secondary coil.

4.       The secondary coil then has the same voltage as the primary coil.

5.       The relative number of turns on the 2 coils determines whether it is a step up or steo-down transformer.

6.       If you supplied DC current to a transformer, you would get nothing out of the secondary coils as it stays contained in the iron core.

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## Transformers

The iron core carries magnetic field not current  and no electricity flows around the iron core.

Primary voltage/secondary voltage = No. of turns on primary/no. of turns on secondary

This formula can be used either way up, as long as the primary’s/secondary’s are on the same side.

-          An issue with high current is a lot of energy loss due to resistance of cables.

-          The formula to work this out is P = I^2 R

-          The cheapest thing to do is boost the voltage up (400,000v) and keep the current very low.

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## Stars and the solar system

Stars and solar systems form from clouds of gas and dust

-          These spiral together due to gravitational attraction

-          Gravity compresses the matter so much that an intense heat builds up and sets off nuclear fusion reactions.

-          The star then begins emitting light and other radiation.

-          At the same time that a star is forming, lumps are orbiting round, when all of these fuse together they can form a planet.

-          The universe is pretty damn huge!

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## Early universe

-          The early universe only contained hydrogen.

-          There was only hydrogen, then the atoms formed together to make stars.

-  These helium nuclei then fuse to form carbon, and then carbon forms with helium to make oxygen and neon. This happens in all red giant stars.

-  Eventually, the helium runs out and the oxygen, neon and carbon form to make silicon. In massive stars, this carries on until iron is formed.

-  At the end of their lives, the massive stars explode (supernova) and release all of the elements into the universe which then form to make pretty much every element in the universe.

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## Life cycle of stars

1.       Stars form from clouds of dust and gas

2.       Force of gravity makes gas clouds come spiralling in and as they do, gravitational energy is converted into heat energy and the temperature rises.

3.       When the temperature is high enough and nuclear fusion forms helium nuclei, extreme amounts of light and heat are let out.

4.       It goes through a long period of being stable – the heat emitted out acts against the gravity pulling in. In this stable period, it is called a main sequence star.

5.       Eventually, the hydrogen runs out and the star then swells into a red giant, it is red because the surface cools.

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## C F continued

The heavier the object is, the larger the C.F

• - The smaller circle you want the object to make, the larger C.F needed
• - If the anticlockwise moments are equal to the clockwise moments, the object won’t turn.
• - If the total anticlockwise moments do not equal the total clockwise moments then there will be a resultant force.
• - Low and wide objects are most stable.
• - The object will not tip over until its centre of mass is beyond the edge of the base.
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## Images continued

- Reflection of light is what allows us to see objects.

- When it reflects off an uneven surface, you get diffuse reflection. When it reflects off an even surface, you get clear reflection.

- Law of reflection : Angle of incidence = Angle of reflection

- Refraction of light is when the waves change direction as they enter a different medium.

- This is caused by a change in speed of the waves.

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## Cycle of stars continued

6. A small star like the sun will then begin to cool and contract into a white dwarf and then when the light fades completely it is a black dwarf.

7. Big stars however begin to glow brightly again as they undergo more fusion and contractions. Eventually they’ll explode in a supernova.

8. The exploding supernova throws out layers of dust and gas leaving a very dense core called a neutron star. Or if the star is big enough, a black hole.

9. The dust and gas thrown off will form into second generation stars like the sun.

10. The matter from which neutron stars, white dwarfs and black dwarfs are made is millions of times denser than any matter on earth because the gravity is so strong, it even crushes the atoms.

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