# Physics

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An electromagnet has a steel core (κM≈ 2500) with an approximately circular cross sectional area of 7.0 cm2. The radius of the magnet is 12.0 cm; there is a small air gap of only 2.2 mm (see sketch). The current through the magnet's N= 120-turn coil is 24 A.

What will the magnetic field strength be inside the air gap? Express your answer in Tesla. Assume that the magnetic field is azimuthal (i.e. points around the circle formed by the steel) everywhere and d≪ the radius of the electromagnet.

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ans=1.44
for people with other values use this

B=(meu not*N*I)/((2*pi*R/km) +d)

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Thanx hmmmm, Other problems? (1,2,3,5 & 6)

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Problem 3:
a)
r = 0
theta = 0
k = (Q_0*(1+2*T/T)) / (Pi*R^2*epsilon_0)

b)
r = 0
theta = mu_0*I*r/(2*Pi*R^2)
k = 0

c)
r = - 1/mu_0 * E X B <- note the sign.
theta = 0
k = 0

d), e) -Q*I*D/(Pi*R^2*epsilon_0)

Note that Q = Q_0*(1+2T/T) and I = dQ/dt

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Thanks, ^_ ^

What about probs. 1, 2 and 5 ?

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Can somebody tell me what the value of R is?
thanks

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the parentheses are confusing me for (Q_0*(1+2*T/T))

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WHat is value of R please?

cannot figure out.

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Please tell what the value for R is?

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I=dQ/dt
and the value of R is a i.e. radius of the circular plate of the capacitor.

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formula for question d and e?? answer is wrong while using formula given by ^_^. can u recheck the formula??

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Thanks Anonymous. sorry for question but which values are dQ and dt please in the question?
and dt=distance separated=2.2mm?

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Can someone explain the formula:

""for people with other values use this

B=(meu not*N*I)/((2*pi*R/km) +d)""

what is I and "not"

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I is the current dQ/dt and meu not is 4*pi*1E-7

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I got it ...

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sorry it is meu_0 and I is given in question
meu=4*pi*1E-7 as mentioned by other friends

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RADIATION PRESSURE ON THE EARTH? Can comeone help with this?

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@TIA: Q is given in the question. Differentiate this Q(t) with respect to time an find the current.

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a) 5.823e8
b) 3.56e22

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Thanks guys, did somebody have formula for problem 1 and 2?

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Thanks Anonymous, I still have issues identifying I=dQ/dt

could you plug in the numbers from the above example please so I can see?
From the above example it seem like this is this correct?

Q= Q_0*(1+2T/T)
d=separated distance in my case 6mm
t=time 2

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Can someone explain with values for I = dQ/dt please?

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@TIA: Q=Q_0*(1+t/T)
differentiate with respect to t,
dQ/dt=Q_0/T=I
Simply plugin the values of Q_0 and T and you get the current.

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Many thanks Anonymous!
Could you tell me what the values for E, X and B stand for in:
r = - 1/mu_0 * E X B

and the D value for e) -Q*I*D/(Pi*R^2*epsilon_0) ?

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what is D and r? any idea . also EXB does it come out negative or positive

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formula for question 2
a)B=meu_0*N*I/l
b)L=meu_0*N^2*A//l
c)I=V/R
d)U=1/2(LI^2)

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formula for question 2
a)B=meu_0*N*I/l
b)L=meu_0*N^2*A//l
c)I=V/R
d)U=1/2(LI^2)

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Thanks guys, what is value A please?

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pi*r^2

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and in neumerator I its current and in denominator its l length

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THanks hmmmm, r is the radius b=0.2 cm right?

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What is value for 'r' please?

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By the way guys, is this an error?
L=meu_0*N^2*A//l

Does it not mean:
L=meu_0*N^2*A*I/l or
L=meu_0*N^2*A/l ?

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Only c) worked for me, I am surely making some wrong with a), c) and d).

Did someone figure out with above formula please?

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a), b) and d) I meant.
Help appreciated

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any hint on 2f? 6?

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Consider a charging capacitor made out of two identical circular conducting plates of radius a=11 cm. The plates are separated by a distance d=2 mm (see figure below, note that d≪a). The bottom plate carries a positive charge

Q(t)=+Q0(1+tT)
with Q0=4e-06 C and T=0.002 sec, and the top plate carries a negative charge −Q(t) . The current through the wire is in the positive kˆ-direction. You may neglect all edge effects.

(a) Calculate the components of the electric field (in V/m) inside the capacitor for t=2T

(b) Calculate the components of the magnetic field B(r,t) (in Teslas) at time t=2T inside the capacitor at a distance r=1.1 cm from the central axis of the capacitor.

(c) Calculate the components of the Poynting vector S⃗ (r,t) (in W/m2) at time t=2T between the plates at a distance r=1.1 cm from the central axis of the capacitor.

(d) What is the flow of energy (in W) into the capacitor at time t=2T ?

(e) How fast is the energy stored in the electric field changing (i.e. what is the rate of change in W) within the capacitor at time t=2T ?

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Regarding Q2, e and f -> The energy is 1/2*Q^2*C. We have Q at this oment of time and C comes out from the geometry of the capacitor. Now, guess what - it's wrong. Any ideas?

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Q2 e) :
W(c) = W(l) |=>

(q^2)/(2C) = (L*(I^2))/2 |=>

q = sqrt[C*L*(I^2)]

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sorry i meant Q3, d and e

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Q2 (f):

t = (pi*sqrt(LC))/2

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thanks BoobleGum. Any hint for Q6?

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Q3 (e), (d):
(Q_0*(1+t/T)*I*d)/(pi*(R^2)*E_0),

E_0 = 8.85 * 10^(-12)
I = Q_0/T

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fa, I only half made it

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fa, how Q1 d) is solved?

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Booblegum, this was mentioned in another thread:
d) t=-ln(0.25)*2*R*C

could you help with question 2 a, b, c please as I think there is something wrong with the formula or I am making a mistake.

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a)B=meu_0*N*I/l
b)L=meu_0*N^2*A//l
c)I=V/R
d)U=1/2(LI^2)

I get only c) right
could somebody help me with the values, mine is as following:
A solenoid has N=2070.0 turns, length d=40 cm , and radius b= 0.2 cm. The solenoid is connected via a switch, S1, to an ideal voltage source with electromotive force e= 9 V and a resistor with resistance R= 39 Ohm . Assume all the self-inductance in the circuit is due to the solenoid. At time t=0, S1 is closed while S2 remains open.

a)When a current I=0.138 A is flowing through the outer loop of the circuit (i.e. S1 is still closed and S2 is still open), what is the magnitude of the magnetic field inside the solenoid (in Tesla)?

Thanks

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t=-ln(0.25)*2*R*C -this formula at me isn't accepted

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FLu, at me so:
Q2:
a) B = mu_0*I*N/(l), here l = length
b) L = mu_0*((N/L)^2)*V, V = (pi*R^2)*d
c) I = V/R
D) U = (L*I^2)/2

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Give me the correct formula for Q1 d)

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THanks Booblegum, will try it out again and report back.

Booblegum I did not try the first question out with the formula, this was mentioned by another one and I thought it works, just look at my profile under:
Thanks lo, can you help with Problem 2?
This were you can find what was posted and maybe also ask Io. Otherwise, can somebody give formula for question 1)d) please or was somebody successful with abvove formula?

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thanks)

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Booblegum, what is L value please? I cannot identify.
thanks

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In b) mu_0*((N/L)^2)*V

I don't know which value is L?

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length (l);

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Booblegum thanks, I get a) but b) and d) not.

b) L = mu_0*((N/L)^2)*V, V = (pi*R^2)*d
Is V the epsilon value or calculated by
V = (pi*R^2)*d?
I put my values in but it does not give right answer.
1.25663706*10^-6*((2079/0.42)^2)*9

V = (pi*R^2)*d
does d=length?

d)U = (L*I^2)/2
Is L=length
and I=current or
am I missing something again?

thanks again and sorry for questions

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)) òàê òû ðóññêèé?) ß æ íèõåðà ïî àíãëèéñêè íå ïîíèìàþ, è íå çíàþ êàê òåáå îòâåòèòü. Äàâàé çàâòðà ìîæåò åñëè ïîëó÷èòñÿ îáúÿñíþ.

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Sor! tomorrow I will explain

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THanks BoobleGum, see you tomnorrow!

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how to find R in problem 3?

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Guys how do we go about the Electromagnetic wave question???

Can anybody help??????

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BoobleGum if you have time, could you help with this one below please?

b) L = mu_0*((N/L)^2)*V, V = (pi*R^2)*d
Is V the epsilon value or calculated by
V = (pi*R^2)*d?
I put my values in but it does not give right answer.
1.25663706*10^-6*((2079/0.42)^2)*9

V = (pi*R^2)*d
does d=length?

d)U = (L*I^2)/2
Is L=length
and I=current or
am I missing something again?

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FLu,
b) I think it`s rigth, but

in formula mu_0*((N/L)^2)*V,
value V is volume = (circle area) * (length solenoid),

d=length, pi*R*R = circle.

I have V = pi*0.001*0.001*0.14 = 4.39822971503e-07

R = radius = 0.1sm = 0.001m
d = length = 14sm = 0.14m

L = 1.26*10^(-6)*[(609/0.14)^2]*4.39822971503e-07
= 0.00001048109706

PS Lewin swears therefore more I won't explain, sor)

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yes FLu you are doing it wrong
L=(mu_0*A*N^2)/L

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hmmmm, you me certainly forgive, but you are a little fool

mu_0*((N/L)^2)*pi*(R^2)*d

equally

mu_0*A*N^2)/L

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FLu your ans of 2b is
1.69e-4

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Thanks BoobleGum and hmmmm
b) worked now with BoobleGum's great explaination
Hoever, still have problem with d.

d)U = (L*I^2)/2

Here L is the length described as d?
and I is the current described in question a)?

The result comes wrong with that formula or the value I understand is not the one to put in.

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Tatar, ìîæåò òû çíàåøü êàê íàéòè âðåìÿ â ïåðâîì âîïðîñå?

Ó ìåíÿ ýòî ïîñëåäíåå çàäàíèå îñòàëîñü, òîëüêî âîò íå ðåøàåòñÿ.

È êñòàòè çàìåòèë ó ïèíäîñîâ òîëüêî îäèí ìàòþê, òîëüêî Ëåâèí åãî ñ ðàçíûìè îêîí÷àíèÿìè ïðèìåíÿåò.

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Flu, the energy is for a long time. That's means that is the I maximun.

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Do you have the rigth answaer for 2e) about the capacitor? The I max is for the ans of 2c.

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And, in this formula :
(mu_0*N*I)/((2*pi*R/km) +d)
what d means? the volume? so, i mean Croos section x air grap?

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SyLaR no sorry, have not figured d) e and f) out.

Could you tell me quick with values assocated with I and L?

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I thought this is formula for d)
d)U = (L*I^2)/2 ?

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Ah, ok. You have the inductance for answer 2b. And the time for 2c.
2b*2c^2/2.
And for 2f is sqrt(2c*capacitor)*pi/2

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Yes flu, that the formula. L*i^2/2

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SyLaR it does not deliver right answer for d and f.
does it mean 2* the answer of b and c?

or maybe a bracket problem, could you check please?

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can anyone help me with question 6?

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For me didn't work SyLaR. did work for someone the formula for d) and f)?

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Tatar,
ïÿòîå

à, íàïðÿæåíèå â êâàäðàòå ïîäåëåííîå íà äâà ñîïðîòèâëåíèÿ. íàïðÿæåíèå ýòî ÷èñëî ïåðåä ñèíóñîì
á, òîò æå îòâåò òîëüêî äåëåííûé åùå íà äâà.

øåñòîå

à, ëÿìáäà ýòî äâà ïè óìíîæåííîå íà ñêîðîñòü ñâåòà è ïîäåëåííîå íà ÷èñëî ïåðåä ò.

á, ýòî ÷èñëî ïåðåä ò ïîäåëåííîå íà äâà ïè

À íóëåâîå ýòî á (ìàãíèòíîå ïîëå) óìíîæåííîå íà ìèíóñ îäèí è óìíîæåííîå íà ñêîðîñòü ñâåòà

ê èêñ è ê èãðèê ðàâíû íóëþ ê çýä ðàâíî ïåðâîìó àðãóìåíòó âíóòðè ñèíóñà

îìåãà ýòî ÷èñëî ïåðåä ò ñ ïëþñîì

ïîëèíã âåêòîð ýòî Á óìíîæåííîå íà å (åëåêòðè÷ ïîëå) è äåëåííîå íà äâà ìþ

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In Q4 what is Km Is it kM=2500

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hmmmm pls. find ans of the below. An electromagnet has a steel core (κM≈ 2500) with an approximately circular cross sectional area of 5.0 cm2. The radius of the magnet is 10.0 cm; there is a small air gap of only 2.6 mm (see sketch). The current through the magnet's N= 120-turn coil is 34 A.

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hmmmm pls. post ans for this question pls I have got only one chance left.
A solenoid has N=1863.0 turns, length d=46 cm , and radius b=0.4 cm, (b<<d) . The solenoid is connected via a switch, S1 , to an ideal voltage source with electromotive force ϵ=3 V and a resistor with resistance R=41 Ohm . Assume all the self-inductance in the circuit is due to the solenoid. At time t=0 , S1 is closed while S2 remains open.

(a) When a current I=4.39e-2 A is flowing through the outer loop of the circuit (i.e. S1 is still closed and S2 is still open), what is the magnitude of the magnetic field inside the solenoid (in Tesla)?

(b) What is the self-inductance L of the solenoid (in H)?

(c) What is the current (in A) in the circuit a very long time (t>>L/R) after S1 is closed? .

(d) How much energy (in J) is stored in the magnetic field of the coil a very long time (t>>L/R) after S1 is closed?

For the next part, assume that a very long time (t>>L/R) after the switch S1 was closed, the voltage source is disconnected from the circuit by opening switch S1. Simultaneously, the solenoid is connected to a capacitor of capacitance C=651 μF by closing switch S2. Assume there is negligible resistance in this new circuit.

(e) What is the maximum amount of charge (in Coulombs) that will appear on the capacitor?

(f) How long does it take (in s) after S1 is opened and S2 is closed before the capacitor first reaches its maximum charge?

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The circuit below consists of three identical resistors each with resistance R=37 Ohm, two identical batteries with emfs E=25 V, and a capacitor with capacitance C=401 μF. The capacitor is initially uncharged at t = 0.

(a) After a very very long time, t≫RC, what is the current i1 (in A)?
i1=

(b) After a very very long time, t≫RC, what are the currents i2 and i3 (in A)?

i2=

i3=

(c) After a very very long time, t≫RC, what is the magnitude of the voltage (in V ) across the capacitor ?
VC=

(d) Assuming that the capacitor starts uncharged, how long will it take (in seconds) for the voltage across the capacitor to reach 3/4 of its maximum value?
t=

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Can any one pls put the correct formulas in order.

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Q6 ans can be solved by HW7 And HW8

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In problem 2:
1/2*L*I^2= Total Energy.
And, 1/2*C*V^2 = Total energy. (Same of the first)
Here you get V.
And the energy in coulumb = C * V
Thats the answer of the p2. e)

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question 7 anyone?

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sorry i mean question 6, question 6 anyone?

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Q1 last part.....neyone ??? I knw the formula -ln(0.25)*2*R*C, where R= Rth.....what is the formula for Rth ???

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For q1 (d):

find the general resistance of the scheme on the condenser, and there will be to you a happiness

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And naturally set up him in a formula given above

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A circuit consists of a resistor of R=5 Ω, a capacitor of C=7 μF, and an ideal self-inductor of L=0.08 H. All three are in series with a power supply that generates an EMF of 6sin(ωt) Volt. The internal resistance of both the power supply and the inductor are negligibly small. The system is at resonance.

part b)

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anyone get #6

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What would be the radius (φmax, in degrees) of a glass bow? The glass
What would be the radius (φmax, in degrees) of a glass bow? The glass beads have an index of refraction n=1.5. We spread them out on the ground and we observe a glass bow as the sun is high in the sky.