# P3 physics AQA GCSE

• P3 AQA physics
• X-rays
• absorbed by metal, bone and teeth
• have a high frequency
• transmitted by healthy tissues
• Charged Coupler devices form electronic images of X-rays
• organs and tissues need to be stained with a contrast medium before they can be seen
• operators go behind a lead screen because they are ionising and can cause damage to cells
• they can be used to treat cancerous tumours near the surface of the body.
• ultrasound
• an ultrasound wave is a sound wave with a frequency above 20000 Hz
• because of their high frequency they cannot be heard by humans
• an ultrasound will hit the boundary between two surfaces and bounce back
• they are non- ionising, so are safer than X-rays
• used for diagnosis and treatment (scanning eye, pre natal scanning, shattering kidney stones.)
• lenses
• ray diagram video convex lenses
• ray diagram video concave lenses
• Endoscopes
• endoscopes are used to look inside a patients body.
• Laser light may be used as an energy source in an endoscope to carry our surgical procedures (cutting, cauterizing, burning)
• Laser light can be used in eye surgery (the colour of the laser matches the type of tissue for maximum absorbtion
• Laser light passes through the cornea and only affects the retina
• refractive index
• refraction is the change in direction of light as passes from one transparent substance into another
• refraction happens because of waves change speed when they cross a boundary. the change in speed of the waves cause a change in direction, unless the waves are travelling along the normal
• a light ray will refract when it crosses from air into glass. it is refracted towards the normal
• the refractive index of a substance is a measure of how much the substance can refract a light ray
• it is found with the equation n=  sin I / sin r
• a light ray that travels along the normal is not refracted
• the eye
• iris - coloured ring of muscle that controls the amount of light entering the eye.
• Cornea- transparent layer that protects the eye and helps it to focus light onto the retina
• pupil- the central hole formed by the iris. light enters through the pupil.
• ciliary muscles - attached to the lens by suspensory ligaments. the muscle changes the thickness of the eye lens.
• eye lens- focuses light onto the retina
• retina- the light sensitive cells around the inside of the eye
• blind spot- region where the retina is not sensitive to light
• optic nerve- carries nerve impulses from retina to brain
• moments
• the turning effect of a force is called it's moment
• the size of the moment is given by the equation        M = F x D
• to increase the moment  either the force must increase or the distance from the pivot must increase.
• it is much easier to undo a nut by pushing on the end of a long spanner than the end of a short one, this is because the long spanner increases the distance between the line of action and the pivot.
• when using a lever the force we are trying to move is called the load and the force applied to the lever is called the effort
• centre of mass
• to find the centre of mass of a irregular sheet of card :         1) suspend the sheet from a pin held by a clamp stand so it can turn freely. 2) when it comes to rest hang a plumb line from the same pin 3) mark the position of the plumbline against the sheet 4) hang the sheet at another point and repeat the procedure 5) the centre of mass is where the lines that marked the position of the plumbline cross
• the centre of mass of a symmetrical shape is where the lines of symmetry cross
• although an object is made up of many particles it can be thought of as being concentrated at one single point. this is called the centre of mass
• any object that is freely suspended will come to rest with its centre of mass directly below the point of suspension. the object is then in equilibrium
• moments in balance
• if an object is in equilibrium, it is balanced not turning. we can take the moments about any point and will find that the total clockwise moment and the total anticlockwise moment are equal
• there are lots of everyday examples of the principle of moments such as seesaws and scales.
• stability
• the line of action of the weight of an object acts through its centre of mass
• the wider the base of an object and the lower its centre of mass, the further it has to tilt before the line of action and weight move outside the base.
• the stability of an object is increased by making it's base wider and it's centre of mass lower.
• hydraulics
• pressure is given by the equation       P = F / A
• liquids cannot be compressed, so the pressure in a liquid is transmitted equally in all direction
• this property  is made use of in hydraulic pressure systems
• the force exerted by a hydraulic pressure system depends on: the force exerted on the system, the area of the cylinder on which the force acts,the area of the cylinder that exerts the force
• the use of different cross sectional areas on the effort and load sides of a hydraulic system means that the system can be used as a force multiplier, therefore a small effort can be applied to move a large load.
• circular motion
• when an object moves in a circle it is continuously changing direction, so it is continuously changing velocity. in other words, it is accelerating. this acceleration is called the centripetal acceleration.
• an object only accelerates if a resultant force is acting on it this is the centripetal force and always acts towards the centre of the circle
• if the centripetal force stops acting , the object will continue to move in a straight line at a tangent to the circle
• the centripetal force needed to make an object perform circular motion increases as : the mass of the object increases, the speed of the object increases or the radius of the circle decreases.
• pendulums
• a pendulum moves to and fro along the same line. this is an example of oscillating movement
• a simple pendulum consists of mass, called a bob, suspended on the end of a string. when the bob is displaced to one side and let go the pendulum oscillates back and forth through the equilibrium position.
• the equilibrium position is the place where the bob is when it stops moving.
• the amplitude of the oscillation is the distance from the equilibrium position to the highest point on either side.
• the time period  of the oscillation is the time taken for one complete cycle.
• the time period depends only on the length of the pendulum and increases as its length increases.
• the frequency of the oscillations is the number of complete cycles per second.
• electromagnet
• the ends of a magnet are called the magnetic poles, there is a south pole at one end and a north pole at the other.
• if two magnets are placed same poles together then they will repel each other.
• if unlike poles are together the two magnets will attract to each other
• when a current flows through a wire a magnetic field is produced. an electromagnet is made by wrapping insulated wire around an iron core.
• when a current flows through the wire it will become strongly magnetised. when the current is switched off the iron looses its magnetism.
• electromagnet are used in devices such as scrapyard cranes, circuit breakers , electric bells and relays.
• the motor effect
• when we place a wire carrying an electric current in a magnetic field, it may experience a force
• the force is maximum if the wire is at 90 degrees to the magnetic field. and is zero if it is parallel.
• Flemings left hand rule is used to determine the direction of the force: the thumb , first two fingers are all held at right angles to each other
• the first finger represents the magnetic field pointing north to south, the second finger represents the current, the thumb represents the the direction of the force.
• the size of the force can be increased by increasing the strength of the magnetic field, increasing the current
• the direction of the force on the wire is reversed if either the direction of the current or the direction of the magnetic field is reversed.
• in an electric motor the speed of the motor is increased by the size of the current. the direction of the motor can be reversed by reversing the direction of the current
• when a current passes through the coil the coil spins because : a force acts on each side of the coil due to the motor effect, the force on e coil is in opposite direction to the force on the other side.
• the split ring commutator reverses the direction of the current around the coil every half turn because the sides swap over each half turn the coil is always pushed in the same direction.