used to diagnose and treat medical conditions. State the precautions necessary when X ray machines and CT scanners are used.

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Dominick Banks
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1 Page 1 State the properties of X rays. Describe how X rays can be used to diagnose and treat medical conditions. State the precautions necessary when X ray machines and CT scanners are used. What is meant by ultrasound? How are ultrasound waves used to visualise internal structures? State two examples of the use of ultrasound in medicine. State an advantage and disadvantage of ultrasound. What is meant by refraction? What is meant by the principal focus of a lens? State the nature of an image produced from a convex (converging) lens when the object is closer than the focal length. State the nature of an image produced from a convex (converging) lens when the object is at the focal length. State the nature of an image produced from a convex (converging) lens when the object is between the focal length (F) and twice the focal length (2F).

2 Page 2 CT scans use X rays to diagnose tumours. Dental X rays diagnose dental problems. Charge-coupled devices allow X ray images to be analysed electronically. X rays can be used to kill cancer cells. X rays have short wavelength (around the same as the diameter of an atom). X rays are ionising. X rays pass through soft tissue but are absorbed by metal and bone. X rays affect a photographic film the same way as light. Ultrasound is sound waves which have a higher frequency than humans can hear (greater than 20,000 Hz). X rays are ionising and mutate DNA, increasing the risk of cancer. We need to minimise exposure to X rays (eg limit the number of X ray scans a patient can have in a year, ensure that X ray technicians are protected with lead screens etc). Ultrasound is used to scan pregnant women to check the health of a foetus. It can also be used to break up kidney stones so they can be passed out of the body. Ultrasound waves are reflected at density boundaries (eg between soft tissue and bone). The time taken for reflection can be used to calculate distance from the ultrasound probe. Refraction is the change in direction of light when it passes from one medium to another (slows down or speeds up). Ultrasound is safe as it is not ionising. However, it produces low-resolution images. With a convex (converging) lens, when the object is closer than the focal length, the image is magnified, upright and virtual. This is a magnifying glass. Parallel rays of light are brought to a focus at the principal focus. The distance from the lens to the principal focus is called the focal length (F). With a convex (converging) lens when the object is between the focal length (F) and twice the focal length (2F), the image is magnified, inverted (upside down) and real. With a convex (converging) lens, when the object is at the focal length, there is no image.

3 Page 3 State the nature of an image produced from a convex (converging) lens when the object is at twice the focal length (2F). State the nature of an image produced from a convex (converging) lens when the object is further than twice the focal length (2F). What can we say about the image produced by a concave (diverging) lens? What is the equation to calculate the magnification produced by a lens? In the eye, what is the function of the retina? In the eye, what is the function of the lens. In the eye, what is the function of the cornea? In the eye, what is the function of the pupil/iris. In the eye, what is the function of the ciliary muscle? In the eye, what is the function of the suspensory ligaments? What is the range of vision of the human eye? What is meant by long sight? How is this corrected?

4 Page 4 With a convex (converging) lens when the object is further than twice the focal length (2F), the image is smaller than the object, inverted (upside down) and real. With a convex (converging) lens when the object is at twice the focal length (2F), the image is the same size as the object, inverted (upside down) and real. magnification = image height object height The image produced by a concave (diverging) lens is always upright, diminished (smaller than the object) and virtual. The image is also on the same side of the lens as the object. The lens carries out the final stage of focussing the light onto the retina. The shape of the lens can be adjusted to focus on near or distant objects. The retina contains lightsensitive cells (photoreceptors). The light is focused here and electrical impulses are sent to the brain. The pupil and iris work together to control the intensity of the light pass into the eye. If the light is too bright, the iris makes the pupil smaller. The cornea is the transparent front of the eye. Light rays are partially focused as they pass through the cornea. The suspensory ligaments attach the lens to the ciliary muscle. The ciliary muscle contracts or relaxes to control the shape of the lens during focusing. In long sight, the eyeball is too short, or the lens is unable to focus the light strongly enough. This means that the light is focused behind the retina. This is corrected using a convex (converging) lens. The range of vision is between 25cm (the near point) and infinity (the far point).

5 Page 5 What is meant by short sight? How is this corrected? In a camera, what is the equivalent of the retina in the eye? How do we calculate the power of a lens? What can we say about the power of a converging lens and a diverging lens? What two factors determine the focal length of a lens? Why are lenses made of a high refractive index material so useful? How do we calculate the refractive index of a material? What is meant by total internal reflection. State an example of total internal reflection in medicine. State an example of total internal reflection in technology. Describe a medical use for lasers. What is meant by the centre of mass of an object?

6 Page 6 In a camera, the film (or the charge-coupled device) is the equivalent of the retina in the eye. In short sight, the eyeball is too long, or the lens focuses the light too strongly. This means that the light is focused in front of the retina. This is corrected using a concave (diverging) lens. The power of a converging lens is positive. The power of a diverging lens is negative. P = 1 f The unit of power is the dioptre (D) Lenses with a high refractive index can be manufactured thinner. So for example, glasses can have thinner, lighter lenses. The focal length of a lens is determined by the refractive index of the material and the curvature of the two surfaces of the lens. If a beam of light strikes a boundary at greater than the critical angle, the beam reflects back into the medium. This is used in optical fibres. refractive = 1 index sin c c = critical angle for that material Total internal reflection is used to send broadband signals down optical fibres to homes and businesses. Total internal reflection is used in endoscopes to look inside the body. The centre of mass of an object is the point at which the mass may be thought to be concentrated. Lasers are used in laser eye surgery to correct vision defects. They can also be used to cauterise (seal) a wound.

7 Page 7 How can we determine the centre of mass of an object. What is meant by the time period of a pendulum. How do we calculate the time period for a pendulum? Which type of objects are most stable (less likely to topple)? What does the time period of a pendulum depend on? What do we call the turning effect of a force? How do we calculate the size of a moment? What is the unit of moment? If an object (eg a seesaw) is not turning, what can we say about the moments? Why do objects topple? What can we say about pressure in a liquid? How do we calculate the pressure in a hydraulic system?

8 Page 8 The time period of a pendulum is the time taken to swing from one side to the other and back again. Objects which are more stable have a wide base and a low centre of mass (such as a gokart). Allow the object to hang from a pin. Use a weight on a string (plumline) to draw a vertical line down the object. Reposition the object and repeat. Where the two lines cross is the centre of mass. T = 1 f T = time period (s) f = frequency (Hz) The turning effect of a force is called the moment. The time period of a pendulum is only dependent on the length of the pendulum. The unit of moment is Nm. Moment (Nm) = force (N) x distance (m) An object topples if the line of action of the weight lies outside the base of the object. This will result in a moment. If an object is not turning then the total clockwise moment is balanced by the total anticlockwise moment. P = F A P = pressure (Pa) F = force (N) A = cross-sectional area (m 2 ) Liquids are virtually incompressible. This means that the pressure in a liquid is transmitted in all directions.

9 Page 9 In circular motion, where is the direction of acceleration? In circular motion, what is the effect of the acceleration? What do we call the resultant force causing the acceleration in circular motion? What is the direction of centripetal force in circular motion? State three ways we can increase the centripetal force needed to make an object perform circular motion. What happens when a current flows through a wire? State two ways that we can increase the force experienced by a current-carrying wire placed in a magnetic field. If a current-carrying wire is parallel to a magnetic field, what can we say about the force? In Fleming s left hand rule, what is shown by: The first finger The second finger The thumb How does a transformer work? What is the purpose of a stepup transformer? What is the purpose of a stepdown transformer?

10 Page 10 In circular motion, the effect of the acceleration is to change the direction but not the speed of the object. In circular motion, the direction of acceleration is towards the centre of the circle. Centripetal force is always directed towards the centre of the circle. In circular motion, the resultant force causing the acceleration is called the centripetal force. When a current flows through a wire, a magnetic field is produced around the wire. We can increase the centripetal force needed to make an object perform circular motion by: Increasing the mass of the object Increasing the speed of the object Decreasing the radius of the circle If a current-carrying wire is parallel to a magnetic field, it will not experience a force. We can increase the force experienced by a current-carrying wire placed in a magnetic field by increasing the strength of the magnetic field or increasing the size of the current. In a transformer, an alternating current in the primary coil produces a changing magnetic field in the secondary coil. An alternating potential difference is induced across the ends of the secondary coil. In Fleming s left hand rule, The first finger = field (N to S) The second finger = current (+ to -) The thumb = direction of motion In a step-down transformer, the potential difference across the secondary coil is less than that across the primary coil. In a step-up transformer, the potential difference across the secondary coil is greater than that across the primary coil.

11 Page 11 What is the transformer equation? What can we say about the power input and output for a transformer? What is the operating frequency of a switch-mode transformer? What are the advantages of a switch-mode transformer compared to a traditional 50 Hz transformer?

12 Page 12 If a transformer is 100% efficient, the output power would be equal to the input power. However, remember that transformers are never 100% efficient. Vp = np Vs ns Vp = potential difference (primary coil) Vs = potential difference (secondary coil) np = number of turns in primary coil ns = number of turns in secondary coil Switch-mode transformers are much lighter and smaller than traditional transformers. They also use very little power if they are switched on but no load is applied. Switch-mode transformers work at a very high frequency (50,000 Hz - 200,000 Hz).

Learning Intentions: P3 Revision. Basically everything in the unit of Physics 3

Learning Intentions: P3 Revision Basically everything in the unit of Physics 3 P3.1 Medical applications of physics Physics has many applications in the field of medicine. These include the uses of X-rays