#1
I have a couple of questions I haven't finished for my physics assignment, would greatly appreciate any help.

1. Two parallel oppositely charged metal plates have an electric field between them. The magnitude is... (either zero or uniform throughout the region)

2. A current is flowing west along a power line. Neglecting Earth's magnetic field, the direction of the magnetic field above it is:

(either N,S,E, or W)

3. What is the total charge on 1 gram of electrons?

4. Do electric field lines point in the direction of increasing or decreasing electric potential?

5. In a 10,000V power line, how many units of energy is carried by each unit of charge making up the current?

6. Describe the characteristics of the force required to create and maintain circular motion.

7. Discuss examples that illustrate how each of the following fields can provide such a force on an object/charged particle that will cause circular motion: Gravitational field, electric field, magnetic field.

8. Calculate the gravitational field intensity at a height of 300km from the Earth's surface.

9. A long loop of copper wire is rotated in a magnetic field around an axis along its diameter. Why does the loop resist this type of motion? Would and aluminium loop make any difference in the resistance of rotation of one of these loops? Explain.
#3
Why do you think we'd do your homework for you?

Also, there's a thread for these types of questions.
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#4
i'm taking physics too this year, and while it is hard, and i rarely complete the homework, i dont think the pit is gonna jump aboard doing it for you


pay attention in class.
#5
1. Two parallel oppositely charged metal plates have an electric field between them. The magnitude is... (either zero or uniform throughout the region)

parallel oppositely charged plates = capacitor = uniform field


2. A current is flowing west along a power line. Neglecting Earth's magnetic field, the direction of the magnetic field above it is:

north
right hand rule

3. What is the total charge on 1 gram of electrons?

a lot
http://www.google.com/search?hl=en&safe=off&client=firefox-a&rls=org.mozilla%3Aen-GB%3Aofficial&hs=0Vn&q=1+gram%2Felectron+mass+*+electron+charge&btnG=Search

4. Do electric field lines point in the direction of increasing or decreasing electric potential?
http://upload.wikimedia.org/math/4/c/9/4c9a08bf067d2abb88531a6a699db07b.png
its the negative gradient..... they point from plus to minus..... in the direction of decreasing electric gradient

5. In a 10,000V power line, how many units of energy is carried by each unit of charge making up the current?
sorry, too much thinking on this one....
but....each electron has an energy of 10000 eV would be the obvious answer

6. Describe the characteristics of the force required to create and maintain circular motion.
always perpendicular to the direction of movement

7. Discuss examples that illustrate how each of the following fields can provide such a force on an object/charged particle that will cause circular motion: Gravitational field, electric field, magnetic field.
gravity = planets orbiting
electric force = electrons orbiting the nucleus in an atom
magnetic field = always perpendicular to the direction of movement, particles in the atmosphere and stuff........the LHC

8. Calculate the gravitational field intensity at a height of 300km from the Earth's surface.
about 9.8m/s/s

9. A long loop of copper wire is rotated in a magnetic field around an axis along its diameter. Why does the loop resist this type of motion? Would and aluminium loop make any difference in the resistance of rotation of one of these loops? Explain.
changing flux through the loop -> induced current in the loop -> magnetic field affecting the loop
no difference between metals


now pay me you lazy freeloader
Last edited by seljer at Oct 5, 2008,
#6
Quote by seljer
1. Two parallel oppositely charged metal plates have an electric field between them. The magnitude is... (either zero or uniform throughout the region)

parallel oppositely charged plates = capacitor = uniform field


2. A current is flowing west along a power line. Neglecting Earth's magnetic field, the direction of the magnetic field above it is:

north
right hand rule

3. What is the total charge on 1 gram of electrons?

a lot
http://www.google.com/search?hl=en&safe=off&client=firefox-a&rls=org.mozilla%3Aen-GB%3Aofficial&hs=0Vn&q=1+gram%2Felectron+mass+*+electron+charge&btnG=Search

4. Do electric field lines point in the direction of increasing or decreasing electric potential?
http://upload.wikimedia.org/math/4/c/9/4c9a08bf067d2abb88531a6a699db07b.png
its the negative gradient..... they point from plus to minus..... in the direction of decreasing electric gradient

5. In a 10,000V power line, how many units of energy is carried by each unit of charge making up the current?
sorry, too much thinking on this one....
but....each electron has an energy of 10000 eV would be the obvious answer

6. Describe the characteristics of the force required to create and maintain circular motion.
always perpendicular to the direction of movement


8. Calculate the gravitational field intensity at a height of 300km from the Earth's surface.
about 9.8m/s/s

9. A long loop of copper wire is rotated in a magnetic field around an axis along its diameter. Why does the loop resist this type of motion? Would and aluminium loop make any difference in the resistance of rotation of one of these loops? Explain.
changing flux through the loop -> induced current in the loop -> magnetic field affecting the loop
no difference between metals


now pay me you lazy freeloader


Thanks.

I've actually answered most of those questions, just needed confirmation cos I don't know if I'm doing 'em right. Also, for #8, I need to show the calculations somehow.

Thanks once again for taking the time.
#7
Quote by Time Seller
Thanks.

I've actually answered most of those questions, just needed confirmation cos I don't know if I'm doing 'em right. Also, for #8, I need to show the calculations somehow.

Thanks once again for taking the time.


refer to newtons law of gravitation

put in the mass of earth and the radius of the earth+300km
#9
1. Two parallel oppositely charged metal plates have an electric field between them. The magnitude is... (either zero or uniform throughout the region)

Uniform.

2. A current is flowing west along a power line. Neglecting Earth's magnetic field, the direction of the magnetic field above it is:

North.

3. What is the total charge on 1 gram of electrons?

Given the mass of an electron is 9.11 * 10^-28 g, one gram of electrons will contain 1.098 * 10^27 electrons. Given that an electron has a charge of 1.60 * 10^-19 coulomb, a gram of electrons will have a charge of 1.76 * 10^8 coulomb.

4. Do electric field lines point in the direction of increasing or decreasing electric potential?

Decreasing.

5. In a 10,000V power line, how many units of energy is carried by each unit of charge making up the current?

Each coulomb of electrons carries 10,000 joules of energy.

6. Describe the characteristics of the force required to create and maintain circular motion.

This force, the centripetal force, needs to act at right angles to the direction of motion and at a constant magnitude.

7. Discuss examples that illustrate how each of the following fields can provide such a force on an object/charged particle that will cause circular motion: Gravitational field, electric field, magnetic field.

Gravitational field: objects in orbit. Electric field: I'm not sure of an example tbh, we did this topic a year ago. Magnetic field: a charged particle entering a uniform magnetic field.

8. Calculate the gravitational field intensity at a height of 300km from the Earth's surface.
If big G is 6.67 * 10^-11 m^3 kg^-1 s^-2, mass of the earth is 5.97 * 10^24 kg and the radius of the earth is 6.37 * 10^6 m
g=GM/(r^2) = (6.67 * 10^-11) * (5.97 * 10^24) / [(6.37 * 10^6 + 30000)^2]
=9.72 N kg^-1

9. A long loop of copper wire is rotated in a magnetic field around an axis along its diameter. Why does the loop resist this type of motion? Would and aluminium loop make any difference in the resistance of rotation of one of these loops? Explain.
Due to lenz' law, which states that a current will be induced in a loop in such a way as to create magnetic flux which opposes the change which caused the current. This has the effect of stopping the loop's motion. An aluminium loop would not make much difference to this phenomenon, perhaps having a slightly lower effect due to a higher resistivity.
#10
Quote by shigidab0p
1. Two parallel oppositely charged metal plates have an electric field between them. The magnitude is... (either zero or uniform throughout the region)

Uniform.

2. A current is flowing west along a power line. Neglecting Earth's magnetic field, the direction of the magnetic field above it is:

North.

3. What is the total charge on 1 gram of electrons?

Given the mass of an electron is 9.11 * 10^-28 g, one gram of electrons will contain 1.098 * 10^27 electrons. Given that an electron has a charge of 1.60 * 10^-19 coulomb, a gram of electrons will have a charge of 1.76 * 10^8 coulomb.

4. Do electric field lines point in the direction of increasing or decreasing electric potential?

Decreasing.

5. In a 10,000V power line, how many units of energy is carried by each unit of charge making up the current?

Each coulomb of electrons carries 10,000 joules of energy.

6. Describe the characteristics of the force required to create and maintain circular motion.

This force, the centripetal force, needs to act at right angles to the direction of motion and at a constant magnitude.

7. Discuss examples that illustrate how each of the following fields can provide such a force on an object/charged particle that will cause circular motion: Gravitational field, electric field, magnetic field.

Gravitational field: objects in orbit. Electric field: I'm not sure of an example tbh, we did this topic a year ago. Magnetic field: a charged particle entering a uniform magnetic field.

8. Calculate the gravitational field intensity at a height of 300km from the Earth's surface.
If big G is 6.67 * 10^-11 m^3 kg^-1 s^-2, mass of the earth is 5.97 * 10^24 kg and the radius of the earth is 6.37 * 10^6 m
g=GM/(r^2) = (6.67 * 10^-11) * (5.97 * 10^24) / [(6.37 * 10^6 + 30000)^2]
=9.72 N kg^-1

9. A long loop of copper wire is rotated in a magnetic field around an axis along its diameter. Why does the loop resist this type of motion? Would and aluminium loop make any difference in the resistance of rotation of one of these loops? Explain.
Due to lenz' law, which states that a current will be induced in a loop in such a way as to create magnetic flux which opposes the change which caused the current. This has the effect of stopping the loop's motion. An aluminium loop would not make much difference to this phenomenon, perhaps having a slightly lower effect due to a higher resistivity.


Woa. Thanks.

I was wondering whether I need to indicate that the charge is negative. Should I?
#11
Each coulomb of electrons carries 10,000 joules of energy.

a bit less
fficial&client=firefox-a">fficial&client=firefox-a">http://www.google.com/search?q=10000+volts+*+electron+charge&ie=utf-8&oe=utf-8&aq=t&rls=org.mozilla:en-GBfficial&client=firefox-a

Quote by Time Seller
Okay. But don't I need the mass of the object at 300km for that?

F = m * g
F = G*m*M/R^2

m * g = G*m*M/R^2

you cancel out m

acceleration due to gravity is independent of the smaller objects in cases like this
Last edited by seljer at Oct 5, 2008,
#12
Quote by Time Seller
Woa. Thanks.

I was wondering whether I need to indicate that the charge is negative. Should I?

Yes, i forgot to put that in. Also, for the power line question you could also put each electron has 10,000 electron volts.
#13
Quote by seljer
Each coulomb of electrons carries 10,000 joules of energy.

a bit less
fficial&client=firefox-a">fficial&client=firefox-a">http://www.google.com/search?q=10000+volts+*+electron+charge&ie=utf-8&oe=utf-8&aq=t&rls=org.mozilla:en-GBfficial&client=firefox-a


F = m * g
F = G*m*M/R^2

m * g = G*m*M/R^2

you cancel out m

acceleration due to gravity is independent of the smaller objects in cases like this

Ah, okay. Thanks man.

I tried using this formula at first, actually:
gr = R^2/r^2 x g

Then I realized it was only for calculating the percentage of gravitational force intensity compared to the intensity on Earth's surface. Lol.
#17
Quote by seljer
hmm, so much for my engineering intuition

newton = kg*m/s^2
so
N/kg = kg*m/s^2/kg = m/s^2

the units are right

and the numbers are right too

Ah yes, I was wrong. I put 30km instead of 300km into my calculator. Using my measly 3 significant figures it now comes out to 8.95 N kg^-1

Edit: I'm fairly sure I'm right about each coulomb carrying 10 000 joules of energy, considering the definition of a volt is a joule per coulomb.
Last edited by shigidab0p at Oct 5, 2008,
#18
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#19
Quote by shigidab0p
Ah yes, I was wrong. I put 30km instead of 300km into my calculator. Using my measly 3 significant figures it now comes out to 8.95 N kg^-1

Edit: I'm fairly sure I'm right about each coulomb carrying 10 000 joules of energy, considering the definition of a volt is a joule per coulomb.


ah, I've figured out where we aren't on the same frequency

each coulomb of charge is carrying 10 kilojoules

but the charge of an electron isn't 1 coulomb but 1.6*10^-19 coulomb
so each electron is carrying 10 kiloelectronvolts of energy, which is 1.6*10^15 joules


edit:
to be pedantic
the electrons aren't carrying any energy
all the energy that is being transported is being transported through the electric and magnetic fields which are in the air around the wire
http://en.wikipedia.org/wiki/Poynting_vector
Last edited by seljer at Oct 5, 2008,
#20
Yeah, I noticed the 30km. Lol.

Also, the question asked: "how many units of energy is carried by each unit of charge making up the current"?

So technically 10kJ should be correct, right? But then again it says 'making up the current', so assuming that I should be calculating the energy carried by electrons, it's 1.6*10^15 joules.

Also, thank for the link.
#21
Quote by Time Seller
Yeah, I noticed the 30km. Lol.

Also, the question asked: "how many units of energy is carried by each unit of charge making up the current"?

So technically 10kJ should be correct, right? But then again it says 'making up the current', so assuming that I should be calculating the energy carried by electrons, it's 1.6*10^15 joules.

Also, thank for the link.


well charge is quantized down to the elementary charge so I assume when they said "how much energy is carried by each unit of charge" they mean each electron in the wire
#22
Quote by seljer
well charge is quantized down to the elementary charge so I assume when they said "how much energy is carried by each unit of charge" they mean each electron in the wire


Fair enough. Regarding the Ponyting Vector, what does S stand for? Charge?