Formal assessment section 3

Question 1

theextension of an elastic object is ___________ to the force applied , provided that the ___________ is not exceeded

Answer 1

directly proportional

limit of proportionality

Question 2

what must happen to change the shape of an object?

Answer 2

more than one force must be applied

Question 3

what does elastically deformed mean?

Answer 3

when an object returns to its original shape after the forces are removed

Question 4

what does inelastically deformed mean?

Answer 4

when an object does not return to its original shape

Question 5

what happens when the limit of proportionality for the extension of an elastic object and applied force has been exceeded? (3)

Answer 5

• doubling the force will no longer exactly double the extension
• the relationship becomes non-linear
• a force-extension graph will stop being a straight line

Question 6

what is the equation for , extension , force, and spring constant

Answer 6

force= spring constant x extension

Question 7

what is spring constant?

Answer 7

it indicates how easy it is to stretch or compress a spring- the higher the spring constant, the stiffer the spring.

Question 8

when is the work done on the spring and the elastic potential energy stored not equal

Answer 8

when the spring is inelastically deformed

Question 9

How do Loudspeakers work?

Answer 9

1) when a current flows through the coil, both the coil and the speaker core experience a force due to the magnetic field. The speaker core moves
2) Because the current is alternating when the direction of the current through the coil reverses, so too does the direction of the force exerted on it and the speaker core
3) this causes the speaker core to vibrate back and forth, producing a sound wave
4) the air around the core also vibrates creating variations in pressure that cause a sound wave

Question 10

How do scrapyards work?

Answer 10

Scrap vehicles are lifted in a scrap yard using powerful electromagnets attached to cranes

when the current is turned on, the electromagnet attracts magnetic material below it

when the current is turned off, there is no more magnetic force

Question 11

How do circuit breakers work?

Answer 11

this is a switch in series with an electromagnet, held by a spring

when the current is too large, the switch is pulled open by the electromagnet and it stays open until it is reset manually

Question 12

How do electric bells work?

Answer 12

1) When the switch is pushed, the electromagnet is magnetised
2) The electromagnet attracts the armature
3) The hammer strikes the gong and breaks the circuit
4) the armature springs back, completing the circuit again and remagnetising the electromagnet
5) the cycle repeats for as long the button remains pushed

Question 13

How do relays work?

Answer 13

Relays are used to switch an electrical machine on or off

1) a small current through the coil of the electromagnet magnetises the iron core
2) this pulls the armature onto the electromagnet, whose other end pushes the contacts together, completing the circuit.
3) When the current is switched off, the contacts open again, switching the circuit off.

Question 14

How do microphones work?

Answer 14

1) Sound waves hit a flexible diaphragm that is attached to a coil of wire, wrapped around a magnet

2) This causes the coil of wire to move in the magnetic field, which generates current
* the diaphragm oscillates due to the changes in air pressure related to the sound wave

3) The microphone diagram and coil vibrate at the same frequency as the incoming sound. The bigger the amplitude of the sound, the bigger the amplitude of the vibration
4) this induces a potential difference and current in the coil with the same frequency of the incoming and wave and an amplitude dependent on the amplitude of the incoming wave

Question 15

What is a convex lens?

What happens in a convex lens?

Answer 15

lens that curves outwards

rays of light are brought to a focus at the principal focus.

Question 16

What is the focal length?

Answer 16

The distance from the lens to the principal focus

Question 17

Why doesn’t the middle ray get refracted?

Answer 17

because it is normally incident onto its surface

Question 18

What is a concave lens?

What happens in a concave lens?

Answer 18

lens that curve inwards

parallel rays of light entering the lens spread out, this makes the rays of light appear to have come from the principal focus on the same side of the lens they have originated

Question 19

What type of images do convex lenses?

What type of images do concave lenses?

Answer 19

real or virtual

only virtual

Question 20

What is a real image?

Answer 20

on the opposite side of the lens to the object can be projected onto a screen

lens come together

Question 21

What is a virtual image?

Answer 21

on the same side as the object and can only be seen by looking through the lens

lens dont come together

Question 22

equation for magnification

Answer 22

image height/ object height

Question 23

How to make a lens more powerful?

Answer 23

• making the focal length more powerful
• make the lens more curved
• use different material that naturally refract light strongly

Question 24

What happens to the image when an object is more than 2 focal lengths from the convex lens?

Answer 24

• image is diminished
• inverted
• the image is real

Question 25

What happens to the image when an object is between 1-2 focal lengths from the convex lens?

Answer 25

• magnified
• inverted
• real

Question 26

describe what happens to light when it passes through a lens

Answer 26

it refracts :

the light bends towards the normal when it passes into the lens

then away when it passes out of the lens

Question 27

What happens to the image when an object is reflected by a concave lens?

Answer 27

• diminished
• right way up
• virtual ( rays do not meet at the principal focus)

Question 28

What happens to the image when an object is closer to the lens than the focal length for a convex lens?

Answer 28

• virtual image
• magnified
• right way up

this is why convex lenses are used as magnifying glasses

Question 29

required practical 9 light box

Answer 29

1) Set up a ray box, slit and lens so that a narrow ray of light is produced.
2) Take an A3 paper and draw a straight line down the centre using a rule.
3) Use a protractor to draw a second line at right angles to this line. Label this line with an ‘N’ for ‘normal’.
4) Place the longest side of a rectangular acrylic polymer block against the first line. With the normal near the middle of the block, carefully draw around the block without moving it.
5) Turn off all the lights in a ray and. Use the ray box to shine a ray of light at the point where the normal meets the block. This is the incident ray.

The angle between the normal and the incident ray is called the angle of incidence.

6) Using a pencil on the paper, mark the path of:
the incident ray, the reflected ray and transmitted ray with a cross

7) Remove the block. Join the crosses to show the paths of the light rays.

8) Repeat steps 2 to 7 for a rectangular glass block.
Measure the angle of incidence, angle of refraction and angle of reflection for each block.