P3 Exam questions

The following questions are examples of questions taken fro previous exams. If you can answer these and can learn these answers and apply them to similar questions you will do really well in the exam.

1. What is Ultrasound? Ultrasound is a sound that has a frequency above the human limit of hearing i.e. a sound with a frequency above Hz (20 kHz)

2. Distance from reflected sound Distance = speed x time Time to wall and back = 0.06 seconds, so time just to wall = 0.06/2 = 0.03 s Distance = 340 x 0.03 = 10.2 m

3. Uses of ultrasound Describe a non-medical use for ultrasound Ultrasound can be used to clean delicate objects. The object is placed in a liquid and then bombarded with ultrasound that gently cleans it

4. State an ultrasound treatment Ultrasound can be used to break down kidney stones

5. What is an X-Ray and how big are the wavelengths? An X-ray is part of the EM spectrum that has a wavelength about the size of the diameter of an atom

6. Define Time period and frequency How does the time period of a pendulum vary with its length? As the length increases the time period increases A pendulum has a period of 0.5 seconds, what is the frequency? f = 1/T f = 1/0.5 = 2Hz

7. Parts of the eye and functions Describe the function of each of these parts of the eye Part of the eyeFunction cornea refracts light - bends it as it enters the eye retinacontains the light receptors ciliary muscles Controls the shape of the lens suspensory ligament Adjust the shape of the lens to make it more or less curved pupil controls how much light enters the eye Lensfocuses light onto the retina iriscontrols how much light enters the pupil

7. Parts of the eye and functions

8. Which diagram shows long sighted, normal vision and short sightedness?

9. Use data from a graph to calculate focal length from power. Accommodation is another word for power. What is the focal length of an average 40 year old? D = 1/f f = 1/D f = 1/4.4 f = 0.23 m

10. Which direction does centripetal force act? Towards the centre of the circle

11. Name the force that provides the centripetal force in the 3 examples below: Frictiongravitytension

12. State the factors that affect the size of the centripetal force acting on an object If the speed that the object travels in the circular motion increases, centripetal force will increase If the mass of the object travelling in circular motion increases, centripetal force will increase If the radius of the circular motion increases, the centripetal force will decrease

13. How can you make objects more stable? Lower the centre of mass (gravity) Increase the area of the base Both of these will ensure that the line of action from the centre of mass will stay within the base for longer as the object tips ensuring that it remains more stable

14. Why do things topple? (with moments explanation) When the line of action from the centre of mass is outside the area of the base of the object It will create a resultant moment that will topple the object about the corner of the object as the pivot (In the diagram there is an anticlockwise moment that will topple the object)

15. Complete the ray diagram to show how a convex lens produces an image of the object Image is: inverted, larger, virtual

16. Complete ray diagrams to show how a concave lens produces an image of the object Image is: diminished, virtual, right way up

17. What is a switch mode transformer and where would they be used? A transformer that uses high frequencies Used in mobile phone / tablet chargers

18. What is an advantage of using a switch mode transformer? They are very light compared to iron core transformers making them easily moveable They use very little power when no load is applied – this is useful for charging batteries as when they are fully charged the transformer stops using so much power.

19. How is electric current induced? When a magnetic field moves relative to a wire held perpendicular to the field a current is induced in the wire This is called the dynamo effect

20. Use of transformer equation to find voltage induced from primary voltage and numbers of primary and secondary turns in the coil in a diagram A transformer has 20 turns on the primary and 400 on the secondary. What is the output voltage if the input voltage is 500V? Vp/Tp = Vs/Ts 500/20 = Vs/400 Vs = (500/20)x400 Vs = 10000V

21. Explain how potential difference is induced in a coil of wire from ac current flowing through another coil inside the second one (but not connected to it) An AC current in a coil induces a varying magnetic field within it This varying magnetic field will be moving relative to the coil of wire within it Perpendicular to the coil of the wire And so because of the dynamo effect electric current will be induced

22. What property of liquids allows hydraulic systems to work? They are incompressible

23. Calculation of pressure, area or force from pressure equation using data in a diagram P=10/5 P = 2 N/m² or Pa P=F/A P=5/4 P = 1.25 N/m² or Pa

24. Calculation of force in a hydraulic system Pressure remains constant so P A = P B 30/0.2 = F/1 150 = F F = 150N

25. Explain the motor effect When a conductor with a current flowing through it is placed perpendicular to a magnetic field It will feel a force This is the motor effect (sometimes called the catapult effect)

26. Which way does the wire move? How do you know? The wire will move down Because of Fleming’s Left Hand Rule

27. What factors that increase motor effect Increasing the magnetic field Increasing the length of wire in the magnetic field Having more electric current flow

28. Explain how a motor works. Electric current flows through the coil so that it flows in one direction down one side and the other direction down the other side This means that, because of Flemings Left Hand rule, one side is forced up whilst the other is forced down This causes the coil to rotate Until the coil has turned 90° When this happens, the split ring commutator ensures that no current flows through the coil for a split second, allowing the coils momentum to carry it forward When the current starts to flow again, it starts to force one side up and the other down so it continues to rotate

29. What is the total clockwise moment below? Moment = Force x perpendicular distance Total clockwise moment = (5x2.5) + (7x5) Total clockwise moment = 12.5Nm + 35 Nm Total clockwise moment = 47.5 Nm

30. The anticlockwise moment is 25Nm. What distance is the force away from the pivot? Moment = Force x perpendicular distance Perpendicular distance = Moment / force Perpendicular distance = 25/5 Perpendicular distance = 5m

31. Explain what the principle of moments is The principle of moments is when there is no resultant moment: the sum of the clockwise moments equals the sum of the antclockwise moments Use it to find the force in the example below: Sum of Anticlockwise moments = sum of clockwise moments F x 0.25 = 5 x 0.50 F = 2.5 / 0.25 F = 10 N

32. Calculate refracted angle from refractive index and incident angle n = sin  1 /sin  = sin50/sin  2 sin  2 = sin50/1.33 sin  2 =  2 = 35.2°

33. Calculation of refractive index from critical angle n = 1/sin  c Total internal reflection n = 1/sin  c 1.5 = 1/sin  c sin  c = 1/1.5 = 0.67  c = 41.8°

34. What conditions are needed for total internal reflection to occur The light must be with an transparent object that has a higher refractive index than the external object The angle the travels at must be greater than the critical angle