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• # Common Questions Asked in Post UTME Physics Exams Day4

Physics

Chima Sunday (Tutor)
17-06-2016 17:41:00 +0000

Try your best to solve these questions. Show working on paper when necessary and include a picture of your hand written Solution in your response. Wish you the best!

1. An object is placed 18cm in front of a convex mirror produced an image 6cm behind the mirror. Find the focal length of the mirror.
A. 9.0cm
B. 7.5cm
C. -6.5cm
D. -9.0cm
(uniben)

Updated Solution
here object distance u = 18cm, image distance v = -6cm(negative because it is formed behind the mirror, hence it is virtual)
so from mirrors formular 1/f = 1/u +1/v
= 1/18 + 1/(-6)
looking for LCM
1/f = 18 + (-6)/(-6)×18
inverting the equation to obtain
f = (-6)×18/(18 - 6)
factoring 6 from the denominator
f = (-6)(18)/6(3-1)
f = -18/2 = -9cm
This is really expected since the mirror is a convex one, thus must have a corresponding negative focal length.

2. A landlord has eight 40W electric light bulbs, four 60W bulbs and two 100W bulbs in his house. If he has all the points on for five hours daily, and if PHCN charges 5kobo per unit, his bill for 30days is
A. #570
B. #723
C. #805
D. #365
E. #4050
(unilag)

Updated Solution
8 × 40W = 320W
4 × 60W = 240W
2 × 100W = 200W
Thus the total power P = 320 + 240 +200 = 760W = 0.76KW
for a day, W = Pt = 0.76 × 5 = 3.8KWH
so for 30days W = 3.8 × 30 = 114KWH
Since 1KWH = 5kobo, thus 114KWH = 114 × 5 = 570kobo = 5.7 naira
So if the # stands for kobo, the correct option should be Option A

3. The purpose of dielectric material in a parallel plate capacitor is to
A. Increase the capcitance
B. Decrease it capacitance
C. Insulate the plate from each other
D. Increase the magnetic field between the plates
(unizik)

Updated Solution
Dielectrics in a simple word is Insulators.
However, dielectrics in capacitors serve three purposes
1. To keep the conducting plates separate from each other
2. To increase the effective capacitance of the capacitor by reducing the electric field strength
3. To reduce or prevent the possibility of sparking by the two plates, via a process we call dielectric breakdown.
So correct Option is A

• Shegx01@gmail.com Shegx01

decrease the capacitance (electric field)

5 17-06-2016 20:43:00 +0000

• Shegx01@gmail.com Shegx01

I've tried to solve others the first question I kept on getting 4.5cm the second in not near it at all I'll be glad is their is a solution provided by the tutor

5 17-06-2016 20:48:00 +0000

• Dorisusifoh Dorisusifoh

1a, 2d, 3d

5 17-06-2016 21:23:00 +0000

• Chima Sunday (Tutor)

you are correct for number 1 can you include a hand solutions for post. thanks sheg. your effort is really commendable.

5 17-06-2016 21:34:00 +0000

• Chima Sunday (Tutor)

@ Dorisusifuh thanks for attempting, but can include a hand written solution for your post?

5 17-06-2016 21:37:00 +0000

• Chima Sunday (Tutor)

*you

5 17-06-2016 21:38:00 +0000

• Chima Sunday (Tutor)

Thanks to every one @Sheg and Doris for your efforts, and those that peeped and shied away, well i appreciate. Thumb up to Sheg for your relentlessness, making sure you attempt almost all the questions in here, thats the spirit, keep it up. Please @ all do know that this post has been updated with its solution, so try and check it up. You are free to ask question if you have. thanks

5 20-06-2016 15:14:00 +0000