P3 Physics. Further Physics. Expected Knowledge. Version #8

Transcription

1 P3 Physics P3 Further Physics Expected Knowledge Version #8

2 GCSE Physics for teaching from September 2011 onwards (version 1.0) Unit 3 Physics 3 P3.1 Medical applications of physics Physics has many applications in the field of medicine. These include the uses of X-rays and ultrasound for scanning, and of light for image formation with lenses and endoscopes P3.1.1 X-rays a) X-rays are part of the electromagnetic spectrum. They have a very short wavelength and cause ionisation. Knowledge Properties of X-rays include: they affect a photographic film in the same way as light they are absorbed by metal and bone they are transmitted by healthy tissue their wavelength is of the same order of magnitude as the diameter of an atom. Revised b) X-rays can be used to diagnose and treat some medical conditions. Examples include CT scans, bone fractures, dental problems and killing cancer cells. The use of charge-coupled devices (CCDs) allows images to be formed electronically. c) Precautions to be taken when X-ray machines and CT scanners are in use. P3.1.2 Ultrasound a) Electronic systems can be used to produce ultrasound waves, which have a frequency higher than the upper limit of hearing for humans. You must know that the range of human hearing is about 20 Hz to Hz. b) Ultrasound waves are partially reflected when they meet a boundary between two different media. The time taken for the reflections to reach a detector can be used to determine how far away such a boundary is. c) Calculation of the distance between interfaces in various media. You may be required to use data from diagrams of oscilloscope traces. s = v x t s is distance in metres, m v is speed in metres per second, m/s t is time in seconds, s d) Ultrasound waves can be used in medicine. Examples include pre-natal scanning and the removal of kidney stones.

3 GCSE Physics for teaching from September 2011 onwards (version 1.0) Unit 3 Physics 3 P3.1 Medical applications of physics Physics has many applications in the field of medicine. These include the uses of X-rays and ultrasound for scanning, and of light for image formation with lenses and endoscopes Knowledge P3.1.3 Lenses a) Refraction is the change of direction of light as it passes from one medium to another. - When light travels into a more dense medium it refract towards the normal - When light travels into a less dense medium it refract away from the normal b) A lens forms an image by refracting light. c) In a convex or converging lens, parallel rays of light are brought to a focus at the principal focus. Revised The distance from the lens to the principal focus is called the focal length. refractive index = sin i i is the angle of incidence sin r r is the angle of refraction d) The nature of an image is defined by its size relative to the object, whether it is upright or inverted relative to the object and whether it is real or virtual. e) The nature of the image produced by a converging lens for an object placed at different distances from the lens. - see sheet: Images formed by a converging lens f) The use of a converging lens as a magnifying glass. g) The nature of the image produced by a concave or diverging lens. h) The construction of ray diagrams to show the formation of images by converging and diverging lenses. You must be able to complete ray diagrams drawn on graph paper. i) The magnification produced by a lens is calculated using the equation: magnification = image height object height

4 P3.1.4 The eye a) The structure of the eye. You should know the function of these parts of the eye: retina lens cornea pupil / iris ciliary muscle suspensory ligaments. You should understand how the action of the ciliary muscle causes changes in the shape of the lens, which allows the light to be focused at varying distances. b) Correction of vision using convex and concave lenses to produce an image on the retina: long sight, is caused by the eyeball being too short, or the eye lens being unable to focus short sight, is caused by the eyeball being too long, or the eye lens being unable to focus. c) Range of vision. The eye can focus on objects between the near point and the far point. The near point is approximately 25 cm and the far point is infinity. d) Comparison between the structure of the eye and the camera. The film in a camera or the CCDs in a digital camera is the equivalent of the retina in the eye. Cameras and the eye both contain a converging lens that focuses real rays onto the film / CCD/ retina e) The power of a lens is given by the equation: P = 1 f P is power in dioptres, D f is focal length in metres, m the power of a converging lens is positive and the power of a diverging lens is negative. f) The focal length of a lens is determined by: the refractive index of the material from which the lens is made, and the curvature of the two surfaces of the lens. g) For a given focal length, the greater the refractive index, the flatter the lens. This means that the lens can be manufactured thinner. P3.1.5 Other applications using light a) Total internal reflection and critical angle. refractive index = 1 sin c c is the critical angle

5 b) Visible light can be sent along optical fibres. Examples of uses of optical fibres include the endoscope for internal imaging. c) The laser as an energy source for cutting, cauterising and burning. Applications include use in eye surgery.

6 GCSE Physics for teaching from September 2011 onwards (version 1.0) Unit 3 Physics 3 P3.2 Using physics to make things work Many things, from simple toys to complex fairground rides, are constructed from basic machines such as the lever. Knowledge P3.2.1 Centre of mass a) The centre of mass of an object is that point at which the mass of the object may be thought to be concentrated. Revised -- You will be expected to be able to describe how to find the centre of mass of a thin, irregular sheet of a material. b) If freely suspended, an object will come to rest with its centre of mass directly below the point of suspension. c) The centre of mass of a symmetrical object is along the axis of symmetry. d) For a simple pendulum: T = 1 f T is periodic time in seconds, s f is frequency in hertz, Hz e) The time period depends on the length of a pendulum. -- To know applications of the pendulum should include simple fairground and playground rides. P3.2.2 Moments a) The turning effect of a force is called the moment. b) The size of the moment is given by the equation: M = F x d M is the moment of the force in newton-metres, Nm F is the force in newtons, N d is the perpendicular distance from the line of action of the force to the pivot in metres, m c) If an object is not turning, the total clockwise moment must be exactly balanced by the total anticlockwise moment about any pivot. d) The calculation of the size of a force, or its distance from pivot, acting on an object that is balanced. e) Ideas of simple levers as force multipliers. f) If the line of action of the weight of an object lies outside the base of the object there will be a resultant moment and the body will tend to topple. Applications should include vehicles and simple balancing toys.

7 P3.2.3 Hydraulics a) Liquids are virtually incompressible, and the pressure in a liquid is transmitted equally in all directions.this means that a force exerted at one point on a liquid will be transmitted to other points in the liquid. b) The use of different cross-sectional areas on the effort and load side of a hydraulic system enables the system to be used as a force multiplier. c) The pressure in different parts of a hydraulic system is given by: P = F A P is the pressure in pascals, Pa F is the force in newtons, N A is the cross-sectional area in metres squared, m2 P3.2.4 Circular motion a) When an object moves in a circle it continuously accelerates towards the centre of the circle. This acceleration changes the direction of motion of the body, not its speed. b) The resultant force causing this acceleration is called the centripetal force and is always directed towards the centre of the circle. You need to be able to identify which force(s) provide(s) the centripetal force in a given situation. (planet in orbit = gravity, bus coing around a corner = friction between tyre and road, bicycle wheel = tension in spokes). c) 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 the radius of the circle decreases.

8 GCSE Physics for teaching from September 2011 onwards (version 1.0) Unit 3 Physics 3 P3.3 Keeping things moving Electric currents produce magnetic fields. Forces produced in magnetic fields can be used to make things move. This is called the motor effect and is how appliances such as the electric motor create movement. Many appliances do not use 230 volts mains electricity. Transformers are used to provide the required potential difference. Candidates should use their skills, knowledge and understanding to: interpret diagrams of electromagnetic appliances in order to explain how they work compare the use of different types of transformer for a particular application. Additional guidance: Examples might include some mobile phone chargers and power supplies for lap top computers. Knowledge P3.3.1 The motor effect a) When a current flows through a wire a magnetic field is produced around the wire. Applications of electromagnets could include their use on cranes for lifting iron/ steel. b) The motor effect and its use. Revised The Motor Effect: "When the wire carrying a current is placed across a magnetic field, there is a force on it which makes it move." This effect is used in: - electric motors, and - loudspeakers. c) The size of the force can be increased by: increasing the strength of the magnetic field increasing the size of the current. d) The conductor will not experience a force if it is parallel to the magnetic field. e) The direction of the force is reversed if either the direction of the current or the direction of the magnetic field is reversed. Candidates will be expected to identify the direction of the force using Fleming s left-hand rule. Expected Knowledge P3#8_Being Revised

9 P3.3.2 Transformers a) If an electrical conductor cuts through a magnetic field a potential difference is induced across the ends of the conductor. b) If a magnet is moved into a coil of wire a potential difference is induced across the ends of the coil. c) The basic structure of the transformer. d) An alternating current in the primary coil produces a changing magnetic field in the iron core and hence in the secondary coil. This induces an alternating potential difference across the ends of the secondary coil. e) In a step-up transformer the potential difference across the secondary coil is greater than the potential difference across the primary coil. f) In a step-down transformer the potential difference across the secondary coil is less than the potential difference across the primary coil. g) The potential difference across the primary and secondary coils of a transformer are related by the equation: V p = n p V s n s V p is the potential difference across the primary coil in volts, V V s is the potential difference across the secondary coil in volts, V n p is the number of turns on the primary coil n s is the number of turns on the secondary coil h) If transformers are assumed to be 100% efficient, the electrical power output would equal the electrical power input. (Remember power = current x voltage so Power in = Power out = V p x I p = V s x I s ) The input to a transformer is determined by the required output. V p = Is V s Ip V p is the potential difference across the primary coil in volts, V I p is the current in the primary coil in amperes (amps), A V s is the potential difference across the secondary coil in volts, V I s is the current in the secondary coil in amperes (amps), A i) Switch mode transformers operate at a high frequency, often between 50 khz and 200 khz. j) Switch mode transformers are much lighter and smaller than traditional transformers working from a 50 Hz mains supply. This makes them useful for applications such as mobile phone chargers. k) Switch mode transformers use very little power when they are switched on but no load is applied. Expected Knowledge P3#8_Being Revised

10 5/16/2013 Refraction Waves change speed when they enter a denser medium, and this may mean a change of direction. A ray travelling along the normal passes straight through without changing direction Glass to Air the ray bends AWAY FROM the normal Lenses Converging lenses make parallel rays converge to a focus. Parallel rays converge at the principal focus (or focal point). Diverging lenses make parallel rays diverge (spread out). The point the rays appear to come from is the principal focus (or focal point). Principal Axis Focal point / Focus Air to Glass the ray bends TOWARDS the normal Images formed by a diverging lens Real Images A real image is formed when the light rays really pass a point. They can be projected onto a screen Virtual Images A virtual image is formed when the light rays never actually met at the image. They cannot be projected onto a screen 1

11 Real Images A real image is formed when the light rays really pass a point. They can be projected onto a screen Virtual Images A virtual image is formed when the light rays never actually met at the image. They cannot be projected onto a screen

12 5/16/2013 The cornea is the transparent front part of the eye. It also refracts light to help produce a clear image. Suspensory ligaments hold the lens in place. The pupil is the gap in the centre of the iris. The retina contains special cells that detect light intensity and colour. The convex lens is made of a jelly-like substance that allows it to change shape. The optic nerve sends the signals to the brain. The iris is the coloured part. It expands or contracts to allow different amounts of light to pass through the pupil. This helps you see in lower light conditions such as night-time. Ciliary muscles help the lens change shape. The ciliary muscles contract to pull on the suspensory ligaments and quickly change the shape of the lens in the eye. This changes its focal length to keep the focussed image at the retina. When looking at distant objects, the lens is stretched to become thinner. When looking at nearer objects the lens springs back to a fatter shape. A concave lens is used to correct short-sightedness A convex lens is used to correct long-sightedness The Critical Angle Below the critical angle most light is refracted, but some is reflected back. Optical fibres use total internal reflection to transmit light or infrared very quickly. At the critical angle some light is directed along the boundary, with some reflection. Above the critical angle all the light is reflected back. This is called Total Internal Reflection Pulses travel along the fibre, reflecting off the inside of the glass until they reach the end. 2

13 5/16/2013 What is a centre of mass? The mass of an object is spread throughout the object. There is a point where this mass appears to be concentrated. This is called its centre of mass. The centre of mass is where the axes of symmetry intersect, Method 1 Use the lines of symmetry to find the centre of mass. Method 2 Learn This 1. Suspend the shape so it can swing freely. 2. Find the vertical using a plumb line. 3. Draw a line along the line of the string 4. Hang the shape from another point and use the plumb line to find the vertical again. 5. The centre of mass is where the two lines cross. turning forces Moment = force perpendicular distance (or Turning Force) (from force to pivot) (Nm) (N) (m) The principle of moments: In equilibrium, total anti-clockwise moments = total clockwise moments. Hydraulics Hydraulic systems use the principle that pressure is transmitted throughout a liquid. They are used to transfer movement from one part of a machine to another without linking the parts mechanically. All hydraulic systems use two pistons linked via a pipe carrying a special oil called hydraulic fluid. In a car, the braking system is hydraulic. The driver pushes the brake pedal, which applies pressure to the liquid in the system. This pressure is transmitted throughout the liquid and a much larger force is applied to the brake pads at the wheels. P3.2.4 Circular motion When an object moves in a circle it continuously accelerates towards the centre of the circle. This acceleration changes the direction of motion of the body, not its speed. The resultant force causing this acceleration is called the centripetal force and is always directed towards the centre of the circle. Force applied here Force transferred here Pressure inside all parts of the hydraulic system is the same Tension in string Gravity Friction between tyre W and = gravity road 3

14 5/16/2013 When a current flows through a wire a magnetic field is produced around the wire. FLEMMINGS LEFT HAND RULE LEARN THIS: MOTORS QUESTION Explain how a motor works ANSWER The coil is inside the magnetic field produced by the magnets. (1 mark) When a current flows through the coil it experiences a force due to the magnetic field. (1 mark) There is a downward force on one side of the coil. (1 mark) And an upward force on the other side of the coil (1 mark) so the coil turns around the axle. (1 mark) What is the role of the commutator and brushes? To allow the coil to rotate without the wire getting tangled and ensuring the current flowing in the same direction around the coil Transformers Key Facts Step up transformers increase voltage and decrease current Step down transformers decrease voltage and increase current transformers have three parts: primary coil secondary coil a soft iron core Soft iron is used in the core because it is a temporary magnet. You must use an alternating current so that you get a changing magnetic field. The wires are insulated to prevent short circuits 4

15 Physics Equations Sheet Physics (PH3) s = v t refractive index = sin i sin r magnification = P = 1 f refractive index = T = 1 f M = F d P = F A V p n p V = s n s V p l p = V s l s image height object height 1 sin c s v t i r P f distance speed time angle of incidence angle of refraction power focal length c critical angle (Higher Tier only) T f M F d P F A V p V s n p n s V p l p V s l s periodic time frequency moment of the force force perpendicular distance from the line of action of the force to the pivot pressure force cross-sectional area potential difference across the primary coil potential difference across the secondary coil number of turns on the primary coil number of turns on the secondary coil potential difference across the primary coil current in the primary coil potential difference across the secondary coil current in the secondary coil Insert for GCSE Physics (PH3) Question Papers v1.0

16 PH3 - Equations Sheet - Key Units s distance meters (m) v speed meters per second (m/s) t time time (s) refractive index no units i angle of incidence degrees ( o ) r angle of refraction degrees ( o ) Magnification image height object height no units does not matter as long as same units used for image and object P power dioptres (D) f focal length meters (m) refractive index no units c critical angle degrees ( o ) T periodic time seconds (s) f frequency Hertz (Hz) M moment of the force Newton Meters (Nm) F force Newtons (N) d perpendicular distance from the line of action of the force to the pivot Meters (m) P pressure pascals (Pa) F force Newtons (N) A cross-sectional area Meters squared (m 2 ) Vp potential difference across the primary coil volts (V) Vs potential difference across the secondary coil volts (V) np ns number of turns on the primary coil number of turns on the secondary coil Vp potential difference across the primary coil volts (V) Ip current in the primary coil amperes (amps) (A) Vs potential difference across the secondary volts (V) Is current in the secondary coil amperes (amps) (A)