Physics HL/SL Help

[HiggsHunter]

Hello! i have a question, the class test I gave has a question that goes " The magnitude of the electrostatic force between the proton and electron in a hydrogen atom is FE. The magnitude of the gravitational force between them is FG."

Answer the teacher gave me to the Fe part: identifies Fe or (1.6 x 10^-19)²/ epsilon₀ x 4 pi r²

I'm really confused by this as I'm an SL student and this seems a BIT MUCH to ask stupid IBO, Also this atleast at seemingly bears no semblance to any formula we've been taught!

The formula (Coulomb's Law) for the electrostatic force between two charges q1 and q2 is given on page 8 of the Physics Data Booklet that I uploaded to the "Other IBO documents" part of the Files section. It is in the core part of the syllabus. In this case, q1 = q2 = 1.6 x 10ˆ-19, so the formula for Fe given by your teacher is correct.

[DeBrogliez]

Also, it is given in the question that the power dissipated in each of the two resistors is the same.

Thanks for the additional information about the question, DeBrogliez. Perhaps there is a lesson here? To be able to solve the problems it's really important to take note of all the information given in the questions!

Yes! Absolutely

We should also remember that the questions are made short as possible; meaning that almost every word is important.

This is also true for multpile choice questions

So take your time to read the question carefully before you attempt to answer

Haha! I already repped you, but! Here is the circuit diagram.

Yea the diagram circuit is important here.

Because if they were connected in parallel there will be a different story.

Assuming that they are connected in series; then the current throughout the circuit must be the same.

Also, it is given in the question that the power dissipated in each of the two resistors is the same.

Since Power = I*V, or Power = I^2*R. And now we know that both power & current are the same. Then resistance and voltage must be the same as well (according to the equation).

That's why you take the intersection of the graph, because you wanna know that "same voltage" occurs at what current?

Which happens to be 0.8A in this case

Then, things are easier.

ϵ = I(R1+R2) = 0.8*(5+5) = 8V

Power = I^2*(R1+R2) = 0.8^2*(5+5) = 6.4W

Anyways, hope I helped

You lost me at Power and Current are the same, care to elucidate?

I think HiggsHunter already clarified my point, thanks!

Hello! i have a question, the class test I gave has a question that goes " The magnitude of the electrostatic force between the proton and electron in a hydrogen atom is FE. The magnitude of the gravitational force between them is FG."

Answer the teacher gave me to the Fe part: identifies Fe or (1.6 x 10^-19)²/ epsilon₀ x 4 pi r²

I'm really confused by this as I'm an SL student and this seems a BIT MUCH to ask stupid IBO, Also this atleast at seemingly bears no semblance to any formula we've been taught!

I'm not sure if I actually got your question

But you want to know from where did he get this equation?

Well, In topic 6, you have to be aware of the equation F = (k*q1*q2)/(r^2)

Where k is the Coulomb constant (given in the data booklet)

You also have to know that both the proton and the electron have the same charge (elementary charge)

Except that one is positive, the other is negative

This elementary chrage charge is 1.6*10^-19

By substituiting in the equation, F = (k*1.6*10^-19*1.6*10^-19)/(r^2) = (k*(1.6*10^-19)^2)/(r^2)

As for the equation you posted, you didn't put k

You assumed that k = 1/(epsilon₀ x 4 pi)

Which is the same thing!

You can try that in your calculator

K = 8.99*10^9

ϵ = 8.85*10^-12

So, 1/(4*pi*8.85*10^-12) = 8.99*10^9 = K

I hope I could answer your question!

[DropBoite]

Hello! i have a question, the class test I gave has a question that goes " The magnitude of the electrostatic force between the proton and electron in a hydrogen atom is FE. The magnitude of the gravitational force between them is FG."

Answer the teacher gave me to the Fe part: identifies Fe or (1.6 x 10^-19)²/ epsilon₀ x 4 pi r²

I'm really confused by this as I'm an SL student and this seems a BIT MUCH to ask stupid IBO, Also this atleast at seemingly bears no semblance to any formula we've been taught!

The formula (Coulomb's Law) for the electrostatic force between two charges q1 and q2 is given on page 8 of the Physics Data Booklet that I uploaded to the "Other IBO documents" part of the Files section. It is in the core part of the syllabus. In this case, q1 = q2 = 1.6 x 10ˆ-19, so the formula for Fe given by your teacher is correct.

Peculiar, because I know Fe= k q1 x q2/r²

^{The reason I was puzzled is because I still don't know what the epsilon business is.}However this has been cleared by the end bit of the prev post! WOOP! thanks !(yesss I repped you! )

Edited April 20, 2012 by DropBoite

[HiggsHunter]

Hello! i have a question, the class test I gave has a question that goes " The magnitude of the electrostatic force between the proton and electron in a hydrogen atom is FE. The magnitude of the gravitational force between them is FG."

Answer the teacher gave me to the Fe part: identifies Fe or (1.6 x 10^-19)²/ epsilon₀ x 4 pi r²

I'm really confused by this as I'm an SL student and this seems a BIT MUCH to ask stupid IBO, Also this atleast at seemingly bears no semblance to any formula we've been taught!

The formula (Coulomb's Law) for the electrostatic force between two charges q1 and q2 is given on page 8 of the Physics Data Booklet that I uploaded to the "Other IBO documents" part of the Files section. It is in the core part of the syllabus. In this case, q1 = q2 = 1.6 x 10ˆ-19, so the formula for Fe given by your teacher is correct.

Peculiar, because I know Fe= k q1 x q2/r²

^{The reason I was puzzled is because I still don't know what the epsilon business is!}

Yes, and Coulomb's constant k = 1/(4 pi epsilon0), which gives you the formula I cited.

Epsilon0 is the permittivity of free space, a constant that is related to the permeability of free space and the speed of light. (Both of which have defined values in the SI system of units). The values of all of these constants are given on page 1 of the Physics Data Booklet.

Edited April 20, 2012 by HiggsHunter

[DropBoite]

Hello! i have a question, the class test I gave has a question that goes " The magnitude of the electrostatic force between the proton and electron in a hydrogen atom is FE. The magnitude of the gravitational force between them is FG."

Answer the teacher gave me to the Fe part: identifies Fe or (1.6 x 10^-19)²/ epsilon₀ x 4 pi r²

I'm really confused by this as I'm an SL student and this seems a BIT MUCH to ask stupid IBO, Also this atleast at seemingly bears no semblance to any formula we've been taught!

The formula (Coulomb's Law) for the electrostatic force between two charges q1 and q2 is given on page 8 of the Physics Data Booklet that I uploaded to the "Other IBO documents" part of the Files section. It is in the core part of the syllabus. In this case, q1 = q2 = 1.6 x 10ˆ-19, so the formula for Fe given by your teacher is correct.

Peculiar, because I know Fe= k q1 x q2/r²

^{The reason I was puzzled is because I still don't know what the epsilon business is!}

Yes, and Coulomb's constant k = 1/(4 pi epsilon0), which gives you the formula I cited.

Epsilon0 is the permittivity of free space, a constant that is related to the permeability of free space and the speed of light. (Both of which have defined values in the SI system of units). The values of all of these constants are given on page 1 of the Physics Data Booklet.

I am aware of that Higgs, thank you though.

Question: IF a positive charge is placed in a magnetic field, what will happen to it? Far as I remember nothing, but my memory is getting a bit rusty

[DeBrogliez]

It won't be affected only if it was parallel to the magnetic field

Otherwise, there will be a force acting on it

Use F = qVB*sin(theta) to calculate that force

[HiggsHunter]

I am aware of that Higgs, thank you though.

Great! I thought that you were puzzled by the epsilon0.

Question: IF a positive charge is placed in a magnetic field, what will happen to it? Far as I remember nothing, but my memory is getting a bit rusty

In the question that you cited, the positive charge is not just placed in the field, but is initially moving normal to the field direction. Hence it experiences a force normal to the direction of the field.

[DropBoite]

what formula relates: height to velocity and acceleration? like if a girl throws a ball from atop a hill what height will be its maximum?

[DeBrogliez]

what formula relates: height to velocity and acceleration? like if a girl throws a ball from atop a hill what height will be its maximum?

v^2 = u^2 - 2ax

@ max height, V=0 m/s

So, u^2 = 2ax

U: Initial Velocity

A: Acceleration

X: Max. Height

[HiggsHunter]

what formula relates: height to velocity and acceleration? like if a girl throws a ball from atop a hill what height will be its maximum?

v^2 = u^2 - 2ax

@ max height, V=0 m/s

So, u^2 = 2ax

U: Initial Velocity

A: Acceleration

X: Max. Height

Since the question states that the girl throws the ball from the top of a hill, I presume that she is not throwing it vertically upwards? In that case, you will first have to calculate the vertical component of the initial velocity u sin(theta) before applying the equation of motion as indicated by DeBrogliez.

[DropBoite]

Yes, unfortunately that stupid girl decided to throw it upward , Now what? Vertical component how? I'm sorry I'm doing EM waves right now I'm a total zero on the Core syllabus, wrapping up the options!

[DeBrogliez]

what formula relates: height to velocity and acceleration? like if a girl throws a ball from atop a hill what height will be its maximum?

v^2 = u^2 - 2ax

@ max height, V=0 m/s

So, u^2 = 2ax

U: Initial Velocity

A: Acceleration

X: Max. Height

Since the question states that the girl throws the ball from the top of a hill, I presume that she is not throwing it vertically upwards? In that case, you will first have to calculate the vertical component of the initial velocity u sin(theta) before applying the equation of motion as indicated by DeBrogliez.

Yes, of course, I was assuming that the ball is thrown vertically upwards.

Otherwise, you have to resolve the velocity vector into vertical and horizontal components before using this equation.

Also, I would like to add the energy approach. Initially, the ball has kinetic energy that is converted into potential energy at maximum height.

Kinetic Energy = Potential Energy

0.5*m*v^2 = mgh

By canceling the mass and rearranging, you end up with the same equation as stated above.

Edited April 23, 2012 by DeBrogliez

[HiggsHunter]

The vertical component of the initial velocity is u sin(theta), where theta is the angle of elevation, so the maximum height of the ball is

h = (uˆ2 x sinˆ2 (theta))/2g. But did the girl throw the ball *vertically* upwards? If so, what is the significance of the hill?

[DeBrogliez]

Yes, unfortunately that stupid girl decided to throw it upward , Now what? Vertical component how? I'm sorry I'm doing EM waves right now I'm a total zero on the Core syllabus, wrapping up the options!

Did the question mention anything about the angle at which the ball was thrown?

For example, if it is 40 degrees above the horizontal

Then, the vertical component of velocity = v*sin(40) ... as HiggsHunter said

And then use that equation to find the maximum height

[DropBoite]

No my friend... I'll get back to you on that tomrrow when I get to mechanics .

As of now I have some questions on electricity, I hope it's not a problem !

1)Ref. circuit.jpg "Three identical electrical heaters each provide power P when connected separately to a supply S which has zero internal resistance. On the diagram below, complete the circuit by drawing two switches so that the power provided by the heaters may be either P or 2P or 3P."

Could someone explain to me how the attached switches bring about the effect the question desires?

2)Refer circuit 2.jpg.

2a) A resistor of constant resistance 1.0 Ω is connected in series with the cell in (b) and with X. Use the graph to deduce that the current in the circuit is 1.3 A.

I'm not getting this answer because the at the outset the marking scheme says Total pd across 1 ohm resistor = 1.3 V. I just can't see how they got this value. Info: X- 0.83 ohm. Cell emf= 2 V, Internal resistance is negligible. I've even attached a graph of the Current voltage characteristics of component X.

3) A question relevant to 2) is what does neglegible internal resistance imply? In series: e= v1+v2 and paralle: e= V ? I think I asked this before

Thank you for your patience and all the best is you're May 2012 !

[HiggsHunter]

1)Ref. circuit.jpg "Three identical electrical heaters each provide power P when connected separately to a supply S which has zero internal resistance. On the diagram below, complete the circuit by drawing two switches so that the power provided by the heaters may be either P or 2P or 3P."

Could someone explain to me how the attached switches bring about the effect the question desires?

For P: Upper switch only closed

For 2P: Lower switch only closed

For 3P: Both switches closed

[HiggsHunter]

3) A question relevant to 2) is what does neglegible internal resistance imply? In series: e= v1+v2 and paralle: e= V ? I think I asked this before

Negligible internal resistance implies that the output voltage of the cell is equal to its emf (which is given as 2v), and is independent of the current that it supplies to the external circuit.

[HiggsHunter]

As of now I have some questions on electricity, I hope it's not a problem !

No problem! I'm happy to help if I can.

2)Refer circuit 2.jpg.

2a) A resistor of constant resistance 1.0 Ω is connected in series with the cell in (b) and with X. Use the graph to deduce that the current in the circuit is 1.3 A.

I'm not getting this answer because the at the outset the marking scheme says Total pd across 1 ohm resistor = 1.3 V. I just can't see how they got this value. Info: X- 0.83 ohm. Cell emf= 2 V, Internal resistance is negligible. I've even attached a graph of the Current voltage characteristics of component X.

The potential divider shown in the circuit is used for adjusting the voltage across the component X to plot its V-i characteristic. It concerns the first part of the question, not the part that you cite.

When the component X is connected directly to the 2v battery, the graph shows that the current is 2.4A, so its resistance is 2/2.4 = 0.83 ohms. The 1 ohm resistor is not present at this time.

When the 1 ohm resistor is inserted in series with the circuit, the voltage across it is equal to i x R = i volts.

Hence if V is the voltage across the component X, we have i + V = 2.

Referring to the graph showing the voltage/current characteristic for component X, you see that this occurs at i = 1.3A, V = 0.7v.

Hence the current in the 1 ohm resistor is 1.3A and the voltage across it is 1.3v.

[DropBoite]

As of now I have some questions on electricity, I hope it's not a problem !

No problem! I'm happy to help if I can.

2)Refer circuit 2.jpg.

2a) A resistor of constant resistance 1.0 Ω is connected in series with the cell in (b) and with X. Use the graph to deduce that the current in the circuit is 1.3 A.

I'm not getting this answer because the at the outset the marking scheme says Total pd across 1 ohm resistor = 1.3 V. I just can't see how they got this value. Info: X- 0.83 ohm. Cell emf= 2 V, Internal resistance is negligible. I've even attached a graph of the Current voltage characteristics of component X.

The potential divider shown in the circuit is used for adjusting the voltage across the component X to plot its V-i characteristic. It concerns the first part of the question, not the part that you cite.

When the component X is connected directly to the 2v battery, the graph shows that the current is 2.4A, so its resistance is 2/2.4 = 0.83 ohms. The 1 ohm resistor is not present at this time.

When the 1 ohm resistor is inserted in series with the circuit, the voltage across it is equal to i x R = i volts.

Hence if V is the voltage across the component X, we have i + V = 2.

Referring to the graph showing the voltage/current characteristic for component X, you see that this occurs at i = 1.3A, V = 0.7v.

Hence the current in the 1 ohm resistor is 1.3A and the voltage across it is 1.3v.

Okay! the first reply i understood but in the second reply can you please tell me if what assumptions i posted were correct, series parallel stuff.

Secondly, great attempt but i dont understand certain things yet. If. for the voltage of the 1ohm resistor, why insert 'i' next to v and call it i volts?

moving on, i +v =2 i get as its. total voltage and the components are in series, but this assumes that i is the voltage, but im dumbstruck when you use this equation for the graph! because what i understood tells me that i is voltage but then you use it as current ? please explain! thanks!

[HiggsHunter]

Okay! the first reply i understood but in the second reply can you please tell me if what assumptions i posted were correct, series parallel stuff.

Could you clarify your question about the "series parallel stuff"?

Secondly, great attempt but i dont understand certain things yet. If. for the voltage of the 1ohm resistor, why insert 'i' next to v and call it i volts?

moving on, i +v =2 i get as its. total voltage and the components are in series, but this assumes that i is the voltage, but im dumbstruck when you use this equation for the graph! because what i understood tells me that i is voltage but then you use it as current ? please explain! thanks!

The voltage across a resistor R ohms carrying current i amps is ( i x R) volts.

Since in this case R = 1 ohm, when the current through the resistor is i amps, the voltage across it is i volts.

The voltage/current characteristic shows us that when the voltage across the component X is 0.7v, the current through it is 1.3A. For this current, the voltage across the 1 ohm resistor is 1.3v. Hence the voltage across both components is (0.7 + 1.3) = 2v, the voltage supplied by the cell.