Test yourself - Moments & Hydraulics

The diagram shows a mobile crane. The tower is raised and lowered hydraulically.

(a)   What two properties of a liquid are used by a hydraulic machine?      (2 marks)

(b)   The clockwise moment about point P, caused by the weight W, is 8 ´ 10⁶ Nm.

       Calculate, in newtons, the size of the weight W.

       Write down the equation you need to use and show how you work out your answer.  (2 marks)

(c)   The maximum weight that the crane can be used to lift is 5 ´ 10⁵ newtons.

       Explain why this maximum weight must not be exceeded.                    (2 marks)

P3.2 Using physics to make things work - Summary Sheet

I have now uploaded the summary sheets for Using physics and Keeping things moving.  

Summary Sheets

Motor Effect Video

A video about the motor effect from mygcsescience.com:

http://www.my-gcsescience.com/revision/separate/the-motor-effect

P3.3 Keeping things moving - The motor effect

What you need to know:

The motor effect

•When a current flows through a wire a magnetic field is produced around the wire.

•When a current carrying wire is placed in a magnetic field it experiences a force.  This is called the motor effect.

•The size of the force can be increased by:

  - increasing the strength of the magnetic field

  -  increasing the size of the current.

•The conductor will not experience a force if it is parallel to the magnetic field.

•The direction of the force is reversed if either the direction of the current or the direction of the magnetic field is reversed.

• We can use Fleming’s LEFT hand  rule to determine the direction of  movement, the current or magnetic field

Fleming’s left hand rule:

Is used to work out the direction of the force (movement)experienced by the wire:

Applications:

•Electromagnets are used on cranes in scrap metal yards for lifting iron/steel

•The motor effect is used in electric motors. Electric motors are found in washing machines, hair dryers etc…

Extended writing questions - lenses, refraction etc...

Some ideas for possible questions...

1) Describe an experiment you could do, including the measurements you would need to take, to determine the refractive index of a transparent block.

2) Compare the similarities and differences between images formed by a magnifying glass and a diverging lens.

3) Describe the factors which affect the focal length of a lens and explain how the lenses in modern glasses can be manufactured to be very thin.

4) Describe an experiment that could be carried out to determine the critical angle of a glass block.

5) Explain how total internal reflection occurs. Why is total internal not possible if light is travelling in air towards a glass surface?

6) Explain how an endoscope works and it's uses in medicine.

Answers to follow shortly.

Test yourself markscheme - Refraction, refractive index and total internal reflection

Mark scheme for exam questions - How did you do???

(a)   (i)    Critical angle

       (ii)   The light is totally internally reflected. 

       It would be insufficient to simply say that ‘the light is reflected’ or even 

      ‘reflected back into the block’.

       (iii)   refractive index =  1/ sin c 

                                        = 1/ sin 42

                                        = 1/ 0.669 = 1.5

       Alternatively the equation refractive index = sin i/sin r could have been used.      

       However, if you use this equation you must imagine the light to be travelling from

       air into glass and not the other way around. So take the angle of incidence as 

       either 31° or 49° with the associated angle of refraction either 20° or 30°.

(b)

• An endoscope has two bundles of optical fibres. 
• Light that is directed into one bundle travels from one end to the other by being totally internally reflected many times. 
• The light is then reflected back through the second bundle of optical fibres.
• The image is viewed through a lens, 
• or if a miniature camera is attached to the optical fibres the image can be displayed on a computer screen.

    It is perfectly okay to answer a ‘prose’ type question using bullet points. However, if 

    you do, make sure each bullet point is a complete sentence.

Test yourself - Refraction, Refractive Index and Total Internal Reflection

Exam style question about applications of refraction and total internal reflection

A student investigates the refraction of light as it passes out from a glass block into the air.

  

Each time the student changes the angle of incidence (i) in the glass block he measures the angle of refraction (r) in the air. The student’s results are given in the table.

Angle of incidence (i)	Angle of refraction (r)
20°	31°
30°	49°
42°	90°
50°	no refracted ray
70°	no refracted ray

(a)   (i)    What name is given to the angle of incidence that produces an angle of refraction equal to 90°?                                      (1 mark)

       (ii)   What happens to the light once the angle of incidence exceeds 42°? (1 mark)

       (iii)  Use data from the table to calculate the refractive index of the glass block.

              Write down the equation you need to use and show how you work out your answer.      (2 marks)

(b)   A doctor may use an endoscope to look directly inside a patient’s body. Explain how an endoscope works.  (4 marks)