AQA Physics 3

AQA Physics 3

1. Medical applications of physics

   • 1.1 X-rays
     • X-rays are used in hospitals to make images and CT scans
     • X-rays are used to destroy tumours at or near the body surface
     • X-rays can damage living tissue when they pass through it
     • X-rays are absorbed more by bones and teeth than by soft tissue
     • Keywords: X-ray, charge-coupled device, CT scanner
   • 1.2 Ultrasound
     • Keywords: Ultrasound wave
     • Ultrasound waves are sound waves of frequency above 20,000Hz
     • Ultrasound can be used for diagnosis and treatment
     • Ultrasound waves are partly reflected at a boundary between 2 different types of body tissue
     • An ultrasound scan is non-ionising so it is safer than an X-ray
   • 1.3 Refractive index
     • Refractive index, n, is a measure of how much a substance can refract a light ray
     • n = sin(i) / sin(r)
     • Keywords: Refraction, refractive index
   • 1.4 The Endoscope
     • The critical angle is the angle of incidence of a light ray in a transparent substance which produces refraction along the boundary
     • n = 1 / sin C
     • Total internal reflection occurs when the angle of incidence of a light ray in a transparent is greater than the critical angle
     • An endoscope is used to see inside the human body directly
     • Keywords: Critical angle, total internal reflection, endoscope, optical fibre
   • 1.5 Lenses
     • A converging lens focuses parallel rays to a point called the principal focus
     • A diverging lens makes parallel rays spread out as if they came from a point called the principal focus
     • A real image is formed by a converging lens if the object is further away than the principal focus
     • A virtual image is formed by a diverging lens, and by a converging lens if an object is nearer to the lens than the principal focus
     • Keywords: Converging lens, principal focus, focal length, real image, virtual image, magnifying glass, magnification, diverging lens
   • 1.6 Using lenses
     • A ray diagram can be drawn to find the position and nature of an image formed by a lens
     • When an object is placed between a converging lens and F, the image formed is virtual, upright, magnified and on the same side of the lens as the object
     • A camera contains a converging lens that is used to form a real image of an object
     • Keywords: Principal axis
   • 1.7 The eye
     • Light is focused on to the retina by the cornea and the eye lens, which is a variable focus lens
     • The normal human eye has a range of vision from 25cm to ∞
     • P = 1 / f
     • Keywords: Near point, far point, range of vision, power of a lens, dioptre
   • 1.8 More about the eye
     • A short-sighted eye is an eye that can only see near objects clearly. We use a diverging (concave) lens to correct it
     • A long-sighted eye is an eye that can only see distant objects clearly. We can use a converging (convex) lens to correct it
     • The higher the refractive index of the glass used to make a corrective lens, the flatter and thinner the lens can be
     • Keywords: Short sight, long sight

2. Using physics to make things work

   • 2.1 Moments
     • The moment of a force is a measure of the turning effect of the force on an object
     • M = F * d
     • To increase the moment of a force F, increase F or increase d
     • Keywords: Moment, line of action, pivot, load, effort
   • 2.2 Centre of mass
     • The centre of mass of an object is that point where its mass can be thought to be concentrated
     • When a suspended object is in equilibrium, 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
     • Keywords: Centre of mass, equilibrium
   • 2.3 Moments in balance
     • For an object in equilibrium, the sum of the anticlockwise moments about any point = the sum of the clockwise moments around that point
     • To calculate the force needed to stop an object we use the equation
     • Keywords: Principle of moments
   • 2.4 Stability