Physics: Post your doubts here!

[Boss201]

In a simple electrical circuit , the current in a resistor is measured as (2.50 ± 0.05) mA. The resistor is marked as having a value of (4.7 Ω ± 2%).

If these values were used to calculate power dissipated in the resistor , what would be the percentage uncertainty in the value obtained ?

A.2%
B.4%
C.6%
D.8%

From marking scheme (Ans :C 6%)[Jun 2004 paper 1 number 6]

 

[Boss201]

u = 10 , v = 20 , s = 100
using v^2 = u^2 + 2as
a = 1.5

Thanks

 

[Sarosh Jameel]

PLZ HELP !

 

[nehaoscar]

Why cant't A be in equilibrium??

 

[eliyeap]

View attachment 54688
Why cant't A be in equilibrium??

For an object in equilibrium, the forces acting on it has to have a concurrent point which means they have to have a common point of intersection. If you extend the forces they should intersect at a common point. For A there is no concurrent point

 

[eliyeap]

PLZ HELP !

Is it C?

 

[eliyeap]

In a simple electrical circuit , the current in a resistor is measured as (2.50 ± 0.05) mA. The resistor is marked as having a value of (4.7 Ω ± 2%).

If these values were used to calculate power dissipated in the resistor , what would be the percentage uncertainty in the value obtained ?

A.2%
B.4%
C.6%
D.8%

From marking scheme (Ans :C 6%)[Jun 2004 paper 1 number 6]

Percentage uncertainty of current= 0.05/2.5 * 100=2%
P= (I^2)R
Percentage uncertainty of P= 2(2)+2=6%

 

[nehaoscar]

How D??

 

[nehaoscar]

Why not C??

 

[nehaoscar]

How??

 

[nehaoscar]

Why not B??

 

[nehaoscar]

Someone please explain why C??

 

[Ahmed Aqdam]

View attachment 54690
How D??

Area is reduced four times so so length must increase four times as volume remains same.
Hence resistance will be 0.2*4*4=3.2

 

[Mahnoorfatima]

Just asking, How do you guys post the screenshot thingy? I don't usually get answers if I post the whole paper and ask for a certain question.

 

[Mahnoorfatima]

Someone please explain why C??

Current is unchanged because it's connected with the fixed resistor which will have a different reading as compared to the one with the variable one as they're connected in parallel and the reading of the voltmeter decreases because there's an increase in resisitance leading to a decrease in the current in this part of the circuit. thus reducing the Pd as well.

 

[Ahmed Aqdam]

View attachment 54691
Why not C??

The reason of D + it shouldn't be I/R, IR is correct.

Just asking, How do you guys post the screenshot thingy? I don't usually get answers if I post the whole paper and ask for a certain question.

Upload a file then full image.

 

[Boss201]

(ANS : D )

How ?

 

[Mahnoorfatima]

I don't get one thing, forexample in Torque when an object is horizontal, shouldn't the resultant turning moment be zero?

 

[Mahnoorfatima]

Four resistors of resistance R, 2R, 3R and 4R are connected to form network.
Battery of negligible internal resistance and voltmeter are connected to the resistor network as shown.


Voltmeter reading is 2 V.
What is electromotive force (e.m.f.) of the battery?
A 2 V B 4 V C 6 V D 10 V

The solution given on that physics website goes like:
Ohm’s law: V = IR
At the 2R resistor, the voltmeter reading is 2V. So, current I through it = 1 A and current I is constant through any specific loop (since the resistors are connected in series).

So, through the 2nd loop, total resistance = 3 + 2 + 1 = 6R
e.m.f. = total p.d. = IR = 1 x 6 = 6V
They've ignored the 4R which is parallel to the other three R's. WHY?????

 

[Ahmed Aqdam]

Four resistors of resistance R, 2R, 3R and 4R are connected to form network.
Battery of negligible internal resistance and voltmeter are connected to the resistor network as shown.


Voltmeter reading is 2 V.
What is electromotive force (e.m.f.) of the battery?
A 2 V B 4 V C 6 V D 10 V

The solution given on that physics website goes like:
Ohm’s law: V = IR
At the 2R resistor, the voltmeter reading is 2V. So, current I through it = 1 A and current I is constant through any specific loop (since the resistors are connected in series).

So, through the 2nd loop, total resistance = 3 + 2 + 1 = 6R
e.m.f. = total p.d. = IR = 1 x 6 = 6V
They've ignored the 4R which is parallel to the other three R's. WHY?????

It is in parallel with the other 3 resistors so it will have the same p.d. as the series combination.