# GCSE Physics Unit 3

Key Points about P3, these are the important points that i always seem to forget :(

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## Moments

Moments : the TURNING effect of a force.

Can be increased by:

1. Increasing the size of the force

2. Using a spanner with a longer handle

Load - the weight of an object raised by a device used to lift the object

Effort - the force that a person must apply by a device when it is used to shift an object or shift a weight

Pivot - the point at which an object when acted on by a force that makes it turn

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## Moment Equation

The Moments Equation:

(don't have to learn it, just learn units!!!!)

Moments = force x perpendicular distance from the pivot to the line of action of the force

(Nm)    (N) (m)

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## Centre of Mass

• Centre of mass - is the point where the mass appears to be concentrated
• When a suspended object is in equalibrium, its centre of mass is directly beneath the point of suspension
• The centre of mass of a symmetrical object is along the axis of symmetry

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## Principle of Moments

The Principle of Moments states that for an object in equilbrium:

the sum of all the clockwise moments of an object = the sum of the anticlockwise moments about that point

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## Stability

1. The stability of ana object is increased by making its base as wide as possible and its centre of mass as low as possible.

2. An object will tend to topple over if the line of action and its weight is oustide its base.

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## Circular Motion

For an object moving in a circle at constant speed:

• the object's velocity is directed along the tangent to the circle
• its velocity changes direction as it moves round
• the change of velocity is towards the centre of the circle
• the object accelerates continously towards the centre of the circle

.'.  the force of the object asts towards the centre of the circle

object moving in a circle is acted on my a resultant force --> centripetal force

CF acting on vehicle moving in roundabout due to friction between tyres and road

CF acting on airplane circling due to combined effect of its weight and the lift force on it. CF is the resultant force of these two.

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## Centripetal Force

The CF needed increases as:

1. the mass or the speed increases

2. the radius of the object decreases

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## Force of Gravity

The force of gravity between two objects:

1. Is an attractive force

2. is bigger the greater the mass of each object is

3. Is smaller the greater the distance between the two objects is

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## Planetary Orbit

1. To stay in orbit at a particulat distance, a planet must move at a particular speed around the sun

2. The further a planet is from the sun, the less its speed is as it moves round the sun

3. The further a planet is from the sun, the longer it takes to make a complete orbit

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## Satellites

The period of a satellite is the time it takes to make one complete orbit

1. Geostationary Satellites:

• used for communication
• Orbit at a particular height above the equator, have a period of 24hrs
• Orit earth in the same direction as its spinn --> stay in the same place as they go round

2. Low Polar Orbits

• Used for monitering e.g. weather, military, police surveillance and environmental monitering.
• Lower orbit --> can see in more detail
• Orbit Earth once every two or three hours
• Orbit takes them over Earth's poles do they can scan the whole earth every day
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## Motor Effect

Motor Effect: when a current is passed along a wire in a magnetic field, a force may be exerted on the wire.

• Force may be exerted by:

1. increasing the current

2. Using a stronger magnet

• Force depends on the angle between the wire and the magnetic field; the force is:

1. greatest when wire is perpendicular to the magnetic field

2. zero when the wire is parallel to the magnetic field lines

• The direction of the force is reversed if the direction iof the force or the magnetic field lines is reversed.

FLEMMING'S LEFT HAND RULE

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## Electric Motor

Designed to use the motor effect.

Simple motor consists of rectangular coil of insulated wire that is forced to rotate . The coils are connected via metal or graphite 'brushes' to the battery. The brushes press into a metal 'split-ring' commutator fixed to the coil.

When a current is passed through the coil, the coil spins because:

• the force acts on each side of the coil due to the motor effect
• the force on one side is in the opposite direction to the force on the other side.

The split-ring commutator reverses the current round the coil every half turn of the coil. Because the sides swap every half turn --> the coil is pushed in the same direction every half turn. (DC current)

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## The Loudspeaker

A loudspeaker is designed to make a diaphragm attached to a coil vibrate when an alternating current passes through the coil.

• When a current passes through the coil, a force due to the motor effect makes the coil move
• Each time a current changes direction, the force reverses its direction. So the coil is repeatedly forced backwards and forwards. This motion makes the diaphragm vibrate so sound waves are created.
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## Electromagnetic Induction

A generator   - Contains coils of wire that spin in a magnetic field.

- A potential difference is created (or induced) in the wire when it cuts across the magnetic field lines.

- If the wire is part of a complete circuit, the induced p.d makes an electric current pass around the circuit.

When one end of the bar magnet is pushed into the coil, the ammeter pointer deflects this is because;

• the movement of the bar magnet causes the induced p.d in the coil
• the induced p.d causes a current because the coil is part of a complete circuit (See p.g 256 AQA)
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## Alternating Current Generator

A simple a.c generator consists of a rectangular coil which is forced to spin in a magnetic field.

The coil is connected to a centre reading meter via metal 'brushes' that press on two metal slip-rings.

When the coil turns steadily in one direction, the meter pointer deflects first one way then the opposite way and back again. This carries on as long as the coil is turning. The coil through the meter is an alternating current.

The faster the coil rotates:

• the larger the peak value of the alternating current
• the greater the frequency of the alternating current

FLEMMING'S RIGHT HAND RULE!!

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## Transformers

A transformer consists of a primary coil of wire and a secondary coil of wire wrapped around the same iron core.

When an alternating current is passes through the primary coil, an alternating p.d is induced in the secondary coil. This happens because;

• an alternating current passing through the primary coil produces an alternating magnetic field.
• the lines of the of the alternating magnetic field pass through the secondary coil and induce an alternating p.d in it.

Transformers only work with an alternating current as with a direct current there is no changing magnetic field so the secondary voltage is zero. The core of the transformer 'guides' the field lines in a loop through the coils but the field must be changing to induce a p.d in the secondary coil.

The core is layered (laminated) to cut out induced currents in the iron layers. If it wasn't laminated the efficiency of the transformer would be greatly reduced.

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## Transformers and the National Grid

The higher the grid p.d, the greater the efficiency of transferring electrical power through the grid

• p.d across the primary number of turns on the primary
• __________________     =     __________________________
• p.d across the secondary number of turns on the secondary

Step-Up transformer - number of coils on the secondary is greater

Step - Down transformer -  number of turns on the primary is greater

• primary current x primary p.d = secondary current x secondary p.d
• provided 100% efficiency
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## Galaxies

Big Bang created Universe about 13 billion years ago. Space, time and radiation were created in the Big Bang

• First,Universe was a hot, glowing ball of radiation and matter
• As it expanded, the temperature fell. A it expanded, it became transparent as radiation passed through the empty space between its atoms. Background radiation was released at this stage.
• Next few billion years, Universe was a dark, patchy and expanding cloud of hydrogen and helium which formed stars and galaxies.

Force of Gravity takes over:

• Uncharged atoms don't repel each other but can attract each other.
• During Dark Age, force of gravitational attraction was at work without opposition from repulsive forces.
• Universe became more patchy as it expanded --> denser parts attracted each other to form gigantic lumps.
• Force of Gravity turned the clumps into stars and galaxies.
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## History of a Star (1)

Low Mass Star (e.g. Sun)

• Nebula (clouds of dust and gas)
• Protostar (particles of clouds gather under own gravity. Clouds merge and merge and become more concentrated to form protostar.
• Main Stage (as protostar gets denser, it becomes hotter. If its hot enough, nuclei of hydrogen atoms and other light elements fuse together. Energy is released in process so core gets hotter and brighter and starts to shine)
• Red Giant (when it runs out of hydrogen and other light element nuclei, it cools down and turns red. Helium and other light elements fuse together to form heavier elements.)
• White Dwarf (when no more elements to fuse, fusion stops. No more radiation released and star collapses on itself. As it collapses, it heats up and turns from red to yellow to white. Becomes white dwarf, hot dense star  much smaller in diameter than it was. Stars like the sun then fade out and go cold)
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## Shining Stars

• Stars like the Sun radiate energy because of hydrogen fusion in the core. Main stage of the star, can continue for billions of years until the star runs out of hydrogen nuclei to fuse together.
• Energy is released in the core keeps the core hot so the process of fusion continues. Radiation flows out steadily from the core in all directions.
• The force of gravity that makes the star contract is balance by the outward pressure of radiation from its core. These forces stay in balance until most of the hydrogen nuclei in the core has been fused together.
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## History of a Star (2)

High Mass Star

protostar->main stage->red super giant->supernova->neutron star-> black hole (if mass is sufficient)

Bigger stars experience a supernova after the red super giant stage. It is a cataclysmic explosion that fuses elements bigger than iron and scatters them out into the universe.

The explosion compresses the core of the star into a neutron which is an extremely dense object composed only of neutrons.

If the neutron star is dense enough and massive enough, it becomes a black hole. Its gravitational field would be so strong that nothing could escape from it, not even light or any other form of electromagnetic radiation.

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## How the chemical elements are formed.

• Light elements are formed as a result of fusion in stars.

Stars like the sun fuse hydrogen nuclei into helium and similar small nuclei inc. carbon. When it becomes a red giant, it fuses helium and other small nuclei into larger nuclei.

No nuclei bigger than iron is formed because the process requires too much energy.

• Heavy elements are formed when a massive star collapses then explodes as a supernova.

The enormous force of the collapse fuses small nuclei together into nuclei larger than iron. The explosion scatters stars into space.

The debris from the supernova contains all the known elements from the lightest to the heaviest. Eventually new stars form when gravity pulls the debris together. Planets form from the debris surrounding a new star .'. such planets will be composed from all the known elements.

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Heaviest known natural element is uranium. Has a half life of 4500 million years--> presence of uranium is evidence that the solar system must have formed from the remnants of a supernova.

Search for UFO's!

• Space probes on Mars: tested the atmosphere, rocks and soil on Mars looking for microbes or any chemicals that may indicate life was once present on Mars.
• SETI - (Search for Extra-Terrestrial Intelligence) has gone for more than 40 years using radio telescopes. Signals from space would indicate the existence of living beings with technologies as advanced as ours. None have been detected...yet!
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## Sound

• Cannot travel through a vacuum
• Are longitudinal waves
• can be reflected (echoes) and refracted. Sound is refracted upwards not downwards in the daytime because the air near the ground is warmer.

Musical Sounds:

• Musical sounds are rhythmic and the sound waves change smoothly and the wave pattern repeats itself regularly.
• noise consists of sound waves that vary randomly in frequency without a pattern.

Sound:

• Increasing the loudness of a sound (amplitude)
• Increasing the frequency (pitch)
• Resonating: when sound vibrations build up in a musical instrument and cause the sound from the instrument to become much louder.
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## Ultrasound

Ultrasound are waves above 20,000Hz that the human ear cannot hear.

They are used for

1. Cleaning Devises

2. Flaw Detectors

3. Medical Scanners

Advantages of using them over X-rays:

• non-ionising, therefore harmless when used for scanning
• reflected at boundaries between different types of tissue so can be used to scan organs.
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