Biology; Chemistry; Physics: Post your doubts here!

[Saad Mughal]

http://papers.xtremepapers.com/CIE/Cambridge International O Level/Physics (5054)/5054_s08_qp_2.pdf
Q no 11 (a) (i)

Equipment: Source A, Source B, Source C, A Piece of Card, Aluminium 2-3 mm, Lead, Geiger Muller Tube.
Diagram: The piece of paper, the aluminium and lead are placed in parallel (about 1 cm apart when drawing), the Geiger Muller tube is placed behind the paper, aluminium and lead (all three), the source is placed (about 1 cm on diagram) away from the paper.
The materials used for showing the penetrating power: Thick piece of card, Aluminium (2-3 mm) and Lead (25mm).
Observations: When source A is placed, the geiger muller tube (placed beyond the card) shows no reading at all, this shows that alpha particles have been completely stopped by the card. When source B is placed, the geiger muller tube shows reading beyond the paper but it does not show reading when placed beyond the aluminium, this shows that beta has been stopped by the aluminium. When source C is placed, the geiger muller tube shows reading beyond the aluminium but this reading decreases when the G-M tube is placed after the lead, this shows that gamma rays have partially been stopped by the lead.
Summarize (or if you want then write) this, it should give you 5/5.
Hope that helps you understand.

 

[Hskk]

i guess there is a mistake in a markhing scheme... for this paper http://papers.xtremepapers.com/CIE/Cambridge International O Level/Physics (5054)/5054_s12_qp_21.pdf question 11 part b sub part 2, the marking scheme http://papers.xtremepapers.com/CIE/Cambridge International O Level/Physics (5054)/5054_s12_ms_21.pdf says DOWN towards POSITIVE plate. whereas the positive plate is on top. is it really a mistake or is it supposed to be this way?

Yeah, it should've been towards negative.

 

[Saad Mughal]

i guess there is a mistake in a markhing scheme... for this paper http://papers.xtremepapers.com/CIE/Cambridge International O Level/Physics (5054)/5054_s12_qp_21.pdf question 11 part b sub part 2, the marking scheme http://papers.xtremepapers.com/CIE/Cambridge International O Level/Physics (5054)/5054_s12_ms_21.pdf says DOWN towards POSITIVE plate. whereas the positive plate is on top. is it really a mistake or is it supposed to be this way?

Yeah that's a mistake.

 

[AhsanAfzal]

http://papers.xtremepapers.com/CIE/Cambridge International O Level/Physics (5054)/5054_s12_qp_22.pdf
Q9 (b) ii- why does it deflects to left?

 

[usama321]

Physics Matters pg 374. One of the reasons of choosing tungsten as the filament's metal in a lamp is its high resistivity. Higher resistance = higher temperature thus more light generated. According to the book, Y should be brighter.

well if the book states higher resistivity, than X has higher resistivity. resistance = resitivity*l/a. So resistivity = resistance *a/l. The filament X has more area and less length, so its resistivity should be higher. Furthermore, due to the high resistance, the overall current in the circuit would be reduced too, you have to consider this too

 

[usama321]

RADIOACTIVITY!
In a magnetic field, the beta particles are deflectd more than the alpha particles because they are lighter. But s marking scheme states, the deflection of radioactive particles between two opositly charged plates is due to the the attraction and repulsion from both the plates. So does that mean that here alpha particles will be deflected more because they have a greater charge of +2 as compared to -1 of the beta particles.

actually, the beta particle would deflect more, as it is the LIGHTEST. you have to consider that the alpha particle is a much heavier particle than beta, which is a single electron

 

[Saad Mughal]

http://papers.xtremepapers.com/CIE/Cambridge International O Level/Physics (5054)/5054_s12_qp_22.pdf
Q9 (b) ii- why does it deflects to left?

Lenz's law states that the direction of the induced current acts opposite to the magnetic field that induces it.
When the electromagnet is magnetized, the deflection is towards the right (then zero), hence when the switch is open (electromagnet demagnetized), the deflection is towards the left (then zero).

 

[Saad Mughal]

actually, the beta particle would deflect more, as it is the LIGHTEST. you have to consider that the alpha particle is a much heavier particle than beta, which is a single electron

RADIOACTIVITY!
In a magnetic field, the beta particles are deflectd more than the alpha particles because they are lighter. But s marking scheme states, the deflection of radioactive particles between two opositly charged plates is due to the the attraction and repulsion from both the plates. So does that mean that here alpha particles will be deflected more because they have a greater charge of +2 as compared to -1 of the beta particles.

usama321 is correct. Alpha does have more charge but the deflection of beta particles is more because beta have almost zero mass when compared to alpha (which is quite heavy when compared to the beta particles).

 

[usama321]

Lenz's law states that the direction of the induced current acts opposite to the magnetic field that induces it.
When the electromagnet is magnetized, the deflection is towards the right (then zero), hence when the switch is open (electromagnet demagnetized), the deflection is towards the left (then zero).

actually i am a bit confused about this too. When the deflection is to the right, current is actually travelling from the top of coil B to the bottom, and then through the circuit? i don't understand how we decide the direction of the deflection

 

[Saad Mughal]

actually i am a bit confused about this too. When the deflection is to the right, current is actually travelling from the top of coil B to the bottom, and then through the circuit? i don't understand how we decide the direction of the deflection

It passes from the top to the circuit and then back from the bottom (I guess ).

 

[AhsanAfzal]

when switch is opened again the poles/direction of current are reversed?

 

[TheStallion-Reborn]

It passes from the top to the circuit and then back from the bottom (I guess ).

actually i am a bit confused about this too. When the deflection is to the right, current is actually travelling from the top of coil B to the bottom, and then through the circuit? i don't understand how we decide the direction of the deflection

Im quite doubtful about this, give a read to transformers. Im gona do it too

 

[HDIWAN]

in determining brightness of a bulb we look at the voltage but in case from mains the voltage is same so bulb will more brightness will be the one with more current through it right???

well if the book states higher resistivity, than X has higher resistivity. resistance = resitivity*l/a. So resistivity = resistance *a/l. The filament X has more area and less length, so its resistivity should be higher. Furthermore, due to the high resistance, the overall current in the circuit would be reduced too, you have to consider this too

 

[Hskk]

well if the book states higher resistivity, than X has higher resistivity. resistance = resitivity*l/a. So resistivity = resistance *a/l. The filament X has more area and less length, so its resistivity should be higher. Furthermore, due to the high resistance, the overall current in the circuit would be reduced too, you have to consider this too

Higher resistance= brighter lamp. Is this statement correct?

 

[Saad Mughal]

when switch is opened again the poles/direction of current are reversed?

The magnetism in the coil fades away, this means that the opposed induced current will oppose the magnetic field (in this case attract it), this will cause a slight jitter to the left as before.

 

[usama321]

Higher resistance= brighter lamp. Is this statement correct?

this statement is correct, but you don't consider only this factor. The fact is when there is higher resistance, the overall current flow would become less. This would lead to less voltage, and therefore less energy would be dessipated per charge while going through the bulb. As a result, overall less heat would be produced. (just read some of it from google to be sure )

 

[usama321]

in determining brightness of a bulb we look at the voltage but in case from mains the voltage is same so bulb will more brightness will be the one with more current through it right???

yup, thats correct

 

[Saad Mughal]

Im quite doubtful about this, give a read to transformers. Im gona do it too

Is it even relevant to the syllabus?

 

[AhsanAfzal]

got it

 

[Saad Mughal]

got it

Ok