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# AQA GCSE Physics P3

What is a 'moment,' and what is it measured in? The turning effect of a force, in Newton metres.
How can the moment be increased? By increasing the force, or increasing the distance to the pivot.
What is the centre of mass? The place on an object where its mass can be thought to be concentrated.
Where will the centre of mass be on a freely suspended object? Directly below the point of suspension.
Where is the centre of mass on a symmetrical object? Along its axis of symmetry - if it has more than one, then it is where they meet.
What does it mean if an object is in equilibrium? The sum of all of the anticlockwise moments is equal to the sum of all of the clockwise moments.
What acts through the centre of mass? THe line of action of the weight of an object.
What happens if the line of action of the weight of an object is outside the object? It will tend to topple over.
How can the stability of an object be increased? By making its basee wider and its centre of mass lower.
What is the continuous change in velocity, and therefore acceleration, of an object moving in a circle called? Centripetal acceleration
What is the resultant force acting towards the centre of the circle that causes acceleration called? Centripetal force
What happens if the centripetal force stops acting? The object moves at a tangent to the circle.
How can the centripetal force by increased? By increasing the mass of the body or the speed of the body, or decreasing the radius of the circle.
What happens to the gravitational force between two bodies when the distance between them increases? It will decrease.
What is weight? The force of gravity acting from the Earth on an object.
What shape are planetary orbits? They are slightly squashed circles, or ellipses.
What must happen to a planet's speed depending on its distance from the Sun to stay in orbit? The further away from the sun, the less speed needed, the closer to the sun, the more speed needed.
What happens to the length of a planet's orbit as it gets closer to the sun? It will decrease.
Name 5 uses of artificial satellites. Monitoring climate and the weather, spying, space research, navigation and communication.
What type of orbit is a communication satellite placed in, and whjat does this mean? A geostationary orbit - They stay at the same place above the earth's surface, as they have an orbit above the equator that takes 24 hours to complete.
What are communication satellites used for, and how high above the Earth are they? Sending telephone and TV signals, and at about 36000 km above the Earth.
What type of orbit does a monitoring satellite have, and what does this mean? Low polar orbit - an orbit that passes over both poles while the Earth rotates underneath.
How long do these orbits last? Two or three hours - the height of the orbit is much lower than a geostationary orbit.
What is the incident ray, and the reflected ray? The incident ray is the one travelling towards the mirror, and the reflected ray is the one coming away.
What is the normal? The line at right angles (perpendicular) to the mirror at the point where the incident ray hits the mirror
What are the angles of incidence and reflection? The angle of incidence is the angle between the incident ray and the normal, and gthe angle of reflection is the angle between the reflected ray and the normal. They are equal.
What is the name for a reflection that occurs when light reflects from an uneven surface at lots of different angles? A diffuse reflection
What is the name of the reflection that occurs when light reflects from an even surface, such as a mirror? A regular reflection
What are four properties of an image in a plane mirror? Upright, the same size as the object, the same distance behind the mirror as the object in front, virtual
What is the difference between a real and virtual image? A real image can be formed on a screen because the rays of light that produce the image actually pass through it, while a virtual image cannot be formed on a screen because the rays of light only appear to pass through it.
What is the focal length of a mirror? The distance from the mirror to the principle focus.
What type of image is produced when an object is placed beyond the principal focus of a mirror? Real, inverted
What type of image is produced when an object is placed between the principal focus and the mirror? Virtual, upright - behind the mirror and larger than the object.
What type of image does a convex mirror produce? Virtual, upright - smaller than the object and behind the mirror.
What are convex mirrors used for, and why? Rear view mirrors in cars, they produce a wider field of view than a plane mirror.
Why does refraction happen? Waves cahnge speed when they cross a boundary - the wavelength also changes, but not the frequency.
What does the change in speed of the waves cause? A change in direction
What happens when light enters a more dense substance? It slows down and the ray bends towards the normal
What happens when light enters a less dense substance? It speeds up and the ray bends away from the normal.
What is it called when a ray of white light is shone onto a triangular glass prism and causes a spectrum? dispersion
Which lights are refracted the least and the most? Violet and Red
What happens to parallel rays of light that pass through a convex (converging) lens? They converge to a point, called the principal focus
What type of image is formed when the ovject is further away from the convex lens than the principal focus? A real, inverted image
What tyope of image is formed if the object is closer to the lens than the principal focus? A virtual, upright image behind the object
What happens to parallel rays of light that pass through a concave (diverging) lens? They are refracted so that they diverge away from the principal focus
What type of image is always produced by a diverging lens? Virtual
What is the line on ray diagrams through the centre of the lens and at right angles to it called? The principal axis
What are the three construction rays needed on a ray diagram? A ray parallel to the principal axis, refracted through the principal focus, a ray through the centre of the lens travels straight on, a ray through the principal focus, refracted parallel to the principal axis.
What sort of lens is used on a camera? A converging lens, forms a real image of an object on a film or array of pixels.
What sort of lens is used on a magnifying glass? Converging lens, to form a virtual, magnified image of an object, on the same side of the lens as the object.
What is sound caused by, and how does it travel? Mechanical vibrations in a substance, as a wave.
What can sound not travel through, and how is this tested? A vacuum, listening to a ringing bell in a bell jar
Where do sound waves travel fastest and slowest? Solids and gases
What are the range of frequencies able to be heard by humans? 20-20,000 Hz
What sort of waves are sound waves? What sort of waves are light waves? What is the difference? Sound waves are longitudinal, and the direction of the vibrations is the same as the direction in which the wave travels. Light waves are transverse, and vibrate at right angles to the direction that they travel in.
Which things reflect (echo) and absorb sounds? Hard, flat surfaces reflect sound, while things like carpets, curtains and furniture absorb sound.
Where does the refraction of sound waves take place? At the boundaries between layers of air at different temperatures.
What does increasing the amplitude of a sound do? The wave carries more energy, and makes the sound louder.
What happens if you increase the frequency of a sound wave? The pitch increases.
What sort of waveform does a tuning fork or generator produce? A pure waveform
What sort of waveform do musical instruments produce? One that is a mixture of different frequencies - instruments have different sounds because they produce different waveforms.
What is a sound wave above 20,000Hz called? Ultrasound
How do ultrasound waves produce images? When a wave meets a boundary between two different materials it is reflected and travels back through the material to a detector. The time it takes to reach the detector is used to calculate how far away this is, and an image is produced by a computer.
Name three uses of ultrasound. It is non ionising and safer than Xrays, so can be used to produce images of an unbborn baby. It can be used to clean delicate mechanisms, by putting them in a tank of water, and vibrating the dirt off. Cracks in metal casting can be detected.
What is the motor effect? The force a wire carrying an electric current experiences when placed in a magnetic field
When is the force at a maximum and when is it at zero? It is at maximum when the wire is at an angle of 90* to the magnetic field, and zero if the wire is parallel.
How can the size of the force be increased? Increasing the strength of the magnetic field, or increasing the size of the current.
How can the direction of the force on the wire be reversed? If the direction of the current or direction of the magnetic field are reversed.
Why does the coil spin when a current passes through it? A force acts on each side of the coil due to the motor effect. The force on one side of the coil is in the opposite direction to the force on the other.
How can you speed up an electric motor? More current, more turns on the coil, stronger magnetic field, a soft iron core in the coil
What is the role of the split ring commutator? It reverses the direction of current around the coil every half turn - because the sides also swap over every half turn, this keeps the coil rotating in the same direction.
What happens when a wire or coil of wire cuts through magnetic field lines? A potential difference is induced across the ends of the wire/coil. If they are aprt of a complete circuit a potential difference is induced.
What happens if you reverse the direction of movement of the wire or the polarity of the magnet? The direction of the induced pd changes - it is only induced while there is movement.
How can the size of the induced pd be increased? By increasing speed of movement, strength of the magnetic field, the number of turns on the coil, or the area of the coil.
What is the difference between a dynamo and a generator? In a dynamo a magnet spins, causing magnetic field lines to continuously cut across a coil, inducing an alternating pd. In a generator, a coil is rotated inside a stationary magnetic field.
What happens if the coil is rotated more quickly? The peak value of the alternating current increases, the frequency of the alternating current increases.
Why do transformers only work for alternating current? The direction of the current needs to constantly change to produce a constantlyt changing magnetic field around the core, which induces potential difference in the secondary coil
Why are the wires insulated? So that the current doesn't short circuit across the iron core.
What is the difference between the coils in step up and step down transformers? In a step up transfomer the secondary coil has more turns, whereas in a step down transformer the primary coil has more turns.
Why does the National Grid use transformers? The higher the pd at which electrical energy is transmitted, the lower the energy loss as heat. The pd needs to be stepped down for safety, and so it works with appliances.
How long ago do scientists think the Big Bang took place? 13 thousand million years ago
When did the nuclei of the lightest elements form? In the first few minutes
When did uncharged atoms form? As the universe expanded over millions of years, and its temperature fell.
What are very large groups of stars called? Galaxies
How do stars emit visible light? Nuclear fusion reactions, caused by intense heat
How is a star first formed? Dust and gas are pulled together by gravitational attraction.
Name all of the stages of a star's life, in order. Dust and gas, protostar, main sequence star, red giant...white dwarf, black dwarf OR supernova, neutron star, black hole.
Describe a protostar Gravitational forces pull dust and gas together, and the cloud becomes increasing dense, forming a protostar. As it becomes denser it gets hotter, and hydrogen and other light elements are formed.
Describe a main sequence star It is undergoing hydrogen fusion in the core, which makes it radiate energy and radiation - this balances out the gravity which is pulling the dust and gas inwards, and makes the star stable for billions of years until the hydrogen runs out.
Describe a red giant When a star runs out of hydrogen nuclei, it swells into a red giant because the surface has cooled.
Describe a white dwarf/black dwarf A small red giant will contract to form a white dwarf, and then eventually a black dwarf when no more light is emitted.
Describe a supernova/neutron star A large red giant will continue to collapse and then explode into a supernova, throwing debris of its outer layers into space. The core is left as a neutron star.
What is a black hole? If a neutron star is big enough it will become a black hole - with a gravitational field so strong that even light can't escape from it.
Where are elements heavier than iron formed, and why? In the final stages of the life of a big star - the process requires a lot of input energy
How do we know that the sun is a second generation star? Heavy elemnts, that couldn't have been fused by the sun, are present in our universe.
Created by: Rayrayy