Jawapan fizik kertas 3 pertengahan tahun 2011

Marking Schemes Mid Term 2011 P3

Jawapan fizik kertas 2 tingkatan 5 ( pertengahan tahun )

sila semak jawapan anda dan jumpa saya esok untuk sebarang pembetulan .

answer phyiscs paper 2 form 5 ( midterm 2011)

Free fall

Objects fall because they are pulled towards the Earth by the force of gravity.

This force is known as the pull of gravity or the earth’s gravitational force.

The earth’s gravitational force tends to pull everything towards its centre.

Free fall 
An object is falling freely when it is falling under the  force of gravity  only. 
Objects and people will experience weightlessness during free fall.

Examples of objects not in free fall:

1. A piece of paper does not fall freely because its fall is affected by air resistance.
2. Flying birds because there is also an additional force of lift.

Examples of objects in free  fall:

1. An object falls freely only in vacuum. The absence of air means there is no air resistance to oppose the motion of the object. In vacuum, both light and heavy objects fall freely. They fall with the same acceleration ie. The acceleration due to gravity, g.
2. In a falling elevator. 

(gravitational acceleration,g  =  9.8 ms^-2, in SPM often approximated g as 10 ms^-2)

s.wg of conductor and resistance

Diagram  6.1  and  Diagram  6.2  show  circuits  used  to  investigate  the  relationship between resistance and cross sectional area of a wire. Constantan wire of s.w.g. 36 and s.w.g 24 with same length are used. 
Rajah 6.1 dan Rajah 6.2 menunjukkan  litar yang digunakan untuk menyiasat   hubungan antara rintangan dan luas keratan rentas dawai. Dawai konstantan s.w.g. 36 dan s.w.g 24 dengan panjang yang sama digunakan.

 

(a)  Using Diagram 6.1 and Diagram 6.2;

      Dengan menggunakan  Rajah 6.1 dan Rajah 6.2; 

(i)  Compare the cross sectional area of the constantan wires .

       Bandingkan luas keratan rentas dawai konstantan.

(ii) Compare the potential difference across the constantan wires.

      Bandingkan beza keupayaan melalui dawai konstantan.

        

(iii) Compare the magnitude of the current that flow  along the constantan wires.

       Bandingkan magnitud  arus yang mengalir melalui dawai-dawai  konstantan tersebut.

(b)  Using your answers in 6(b);

      Dengan menggunakan jawapan anda dalam 6(b); 

(i)  Compare the resistance of the  constantan wires.

       Bandingkan rintangan dawai konstantan. 

(ii) Relate the cross sectional area of constantan wire to the resistance of the wire.

      Hubungkait luas keratan rentas dawai konstantan dengan  rintangan dawai.

(c)   What will happen  to the  reading of voltmeter when  another constantan wire s.w.g  36 of the same length is connected parallel to the wire in circuit Diagram 6.1?  Explain your answer.          

Apakah yang berlaku kepada bacaan voltmeter apabila dawai konstantan s.w.g 36 yang sama panjang dipasang selari dengan dawai dalam litar Rajah 6.1?  Jelaskan jawapan anda. 

*sewaktu menjawab soalan sebegini, gunakan maklumat yang diberi dan memerhati gambarajah yang sediakan.
*Jawab dengan ayat ringkas .
* tambahan :

Apakah maksud s.w.g ?

http://en.wikipedia.org/wiki/Standard_wire_gauge

Suggested answer. 

a. 

i)   Cross sectional area in Diagram 6.2 is larger than in Diagram 6.1. 

            ii)  Potential difference 6.1 > 6.2

            iii)  Current for both circuits are the same

            

             b.

             i)   Resistance 6.1 > 6.2

             ii)  As the cross sectional area increases, resistance decreases.

             

            c) Reading of voltmeter decreases because total/effective resistance decreases.

Light bulb and resistance

Diagram shows a bulb labelled 12 V, 24 W.
a. What is meant be 12 V, 24 W.
b. Why filament is made of high resistance conductor.
c. State 2 factors that affected the resistance of the conductor (filament).

Suggested answer :


a. 12 V, 24 W means that the bulb will consume 24 J of electrical energy every second if it is connected to 12 V mains supply.

b. To give out more heat energy and the bulb will become brighter. 
(If you connect a bulb to a constant current source, then its brightness increases with increasing resistance, because the power I² R is proportional to the resistance)
c.
i) Length of  conductor
The resistance of a conductor increases with length.


ii) Cross sectional area ( thickness) of  conductor
The thicker the conductor, the lower the resistance.

iii) Temperature of conductor 
Filament light bulbs are non Ohmic conductors,it resistance increases with temperature.
It is a known fact that as an filament light bulb gets hotter, it gets brighter as well.

iv) Material
For fixed length and thickness of a conductor, its resistance is affected by the type of material.
Example resistivity of tungsten is higher than aluminium.

more info : http://en.wikipedia.org/wiki/Electrical_resistivity_and_conductivity