P1* - Efficiency, Sankey Diagrams, Wave Basics, Wave Properties, Diffraction and Refraction

Question 1

Why are useful machines “useful”?

Answer 1

Useful machines are only useful because they convert energy from one form to another.
E.g. Cars, you put in chemical energy ( Petrol or diesel ) and the engine converts it into kinetic ( movement ) energy.

Question 2

Why is only some of the output energy useful?

Answer 2

• The total energy output is always the same as energy input, but only some of the output energy is useful, e.g. for every joule of chemical energy you put into your car you will only get a fraction of it converted back into useful kinetic energy.
• This is because some of the input energy is always lost or wasted often as heat.

Question 3

How is efficiency gauged?

Answer 3

The less energy that is wasted, the more efficient the device is.

Question 4

What do Sankey diagrams show?

Answer 4

Sankey diagrams are energy transformation diagrams, they make it easy to see at a glance how much of the input energy is being usefully employed compared to how much is being wasted.

Question 5

What is the amplitude?

Answer 5

Amplitude is the displacement from the rest position to the crest. ( Not the trough to the crest ).

Question 6

What is the wavelength?

Answer 6

Wavelength is the length of a full cycle of the wave.

Question 7

What is the frequency?

Answer 7

Frequency is the number of cycles or oscillations passing a certain point per second.

Question 8

What is frequency measured in?

Answer 8

Frequency is measured in Hz.

1Hz is 1 Wave per second.

Question 9

How do waves travel?

Answer 9

Waves travel in a straight line through whatever substance they are travelling in.

Question 10

What happens when waves arrive at an obstacle?

Answer 10

When waves arrive at an obstacle ( or meet a new material ), their direction of travel is changed, however they are always travelling in a straight line.

Question 11

How can a wave’s direction of travel be changed?

Answer 11

A waves direction of travel can be changed by:
Reflection
Refraction
Diffraction

Question 12

What allows us to see objects?

Answer 12

The reflection of light is what allows us to see objects, the light bounces off of them into our eyes.

Question 13

What happens when light reflects from an uneven surface?

Answer 13

When a beam of light reflects from an uneven surface such as a piece of paper, the light reflects off at different angles.

Question 14

What happens when light reflects from an even surface?

Answer 14

When it reflects from an even surface ( smooth and shiny like a plane mirror ) then it is all reflected at the same angle and you get a clear reflection.

Question 15

What is the law of reflection?

Answer 15

The law of reflection applies to every reflected ray.

Angle of incidence = angle of reflection.

Question 16

When does an object experience total internal reflection?

Answer 16

• A wave hitting a surface can experience total internal reflection.
• When the light ravels through a dense material like glass or water, towards a less dense substance like air.
• If the angle of incidence is big enough the ray doesn’t come out at all, but reflects back into the material.
• Big enough means bigger than the critical angle for that particular material
• Every material has its own, different critical angle.

Question 17

What happens when waves pass through a gap or pass an object?

Answer 17

All waves spread out ( diffract ) at the edges when they pass a gap or pass an object.

Question 18

What does the amount of diffraction depend upon?

Answer 18

The amount of diffraction depends on the size of the gap relative to the wavelength of the wave.
The narrower the gap, or the longer the wavelength the more the waves spread out.

Question 19

What happens when a wave crosses a boundary between two substances?

Answer 19

Waves travel at different speeds in substances which have different densities
When a wave travels between boundary between two directions it changes speed, which causes it to change directions.

Question 20

When are waves refracted?

Answer 20

Waves are only refracted if they meet a new medium at an angle, if they are travelling along the normal they will change speed , but are not refracted - they don’t change direction.

Question 21

How is energy lost in the car?

Answer 21

• The chemical energy is converted mostly into heat and sound energy, this is wasted energy.

Question 22

What happens if the angle of incidence is less than the critical angle?

Answer 22

• If the angle of incidence is less than the critical angle:

- Most of the light is refracted into the outer layer, but some of it is internally reflected.

Question 23

What happens if the angle of incidence is equal to the critical angle?

Answer 23

• The ray would go along the surface, with a bit of internal reflection.

Question 24

What happens if the angle of incidence is greater than the critical angle?

Answer 24

No light will come out

- All of it is internally reflected , i.e. total internal reflection.