• Got a question on how to use our new website? Check out our user guide here!

Preliminary Past Paper Question (1 Viewer)

Ambility

Active Member
Joined
Dec 22, 2014
Messages
337
Gender
Male
HSC
2016
I was just doing a past paper and my answer to a question didn't match the answer in the answers. Here's the question.

A magician decides to use an electric field to levitate (float) a light plastic ball in the air. The ball has a mass of 15 grams and can be given a charge of +0.5 coulombs. Determine the strength and direction of the external electric field that the magician would require to levitate the ball.

I got where the answers say .

Who's right?
 

InteGrand

Well-Known Member
Joined
Dec 11, 2014
Messages
6,090
Gender
Male
HSC
N/A
I was just doing a past paper and my answer to a question didn't match the answer in the answers. Here's the question.

A magician decides to use an electric field to levitate (float) a light plastic ball in the air. The ball has a mass of 15 grams and can be given a charge of +0.5 coulombs. Determine the strength and direction of the external electric field that the magician would require to levitate the ball.

I got where the answers say .

Who's right?
Since your answer and the given answers differ by some orders of magnitude, the error may be due to units: did you make sure to convert the mass of the ball to SI units of kg (i.e. use 0.015 as the value for m in any equations)?

Edit: just did the Q, and got 0.294 using 0.015 kg for mass. So the answers probably made the mistake with units somewhere (though they are out by two orders of magnitude; if they had used m = 15, they would have been out by three. Maybe they just used m = 1.5 or something).
 
Last edited:

MilkyCat_

Active Member
Joined
Oct 3, 2014
Messages
604
Gender
Female
HSC
2016
Can you give some working out for this question as I remember I was stuck on a similar one
thanks
 

Crisium

Pew Pew
Joined
Feb 17, 2014
Messages
2,022
Location
Australia
Gender
Male
HSC
2015
Can you give some working out for this question as I remember I was stuck on a similar one
thanks
Sure thing

Using the formula E = F / q

You know the value of q but not the value of E and F

Remember that F = mg and since they have given you the value of m (0.015 kg) and you know the value of g (9.8) you can find F and hence find E

F = 0.015 x 9.8

F = 0.147

Therefore

E = 0.147 / 0.5

E = 0.294 NC^-1

Remember to convert everything to the right units!
 

MilkyCat_

Active Member
Joined
Oct 3, 2014
Messages
604
Gender
Female
HSC
2016
Sure thing

Using the formula E = F / q

You know the value of q but not the value of E and F

Remember that F = mg and since they have given you the value of m (0.015 kg) and you know the value of g (9.8) you can find F and hence find E

F = 0.015 x 9.8

F = 0.147

Therefore

E = 0.147 / 0.5

E = 0.294 NC^-1

Remember to convert everything to the right units!
ah wow it was that easy haha
cheers man
 

Users Who Are Viewing This Thread (Users: 0, Guests: 1)

Top