Stored Magnetic Energy - Inductor Circuit


by Bryon
Tags: circuit, energy, inductor, magnetic, stored
Bryon
Bryon is offline
#1
Mar28-11, 12:09 PM
P: 99
1. The problem statement, all variables and given/known data
In the above circuit, the EMF from the battery is 12 V and the resistor has a resistance is 6.6 Ω. The inductor consists of a long, thin cylindrical coil of wire with 30000 turns, a radius of 5 cm and a length of 61 cm.

Answer the following questions for a time 1.4 seconds after the battery has been connected.

(c) How much energy has been delivered by the battery up to this point?


2. Relevant equations
U = 0.5LI^2

∫Udt from 0 to 1.4s

3. The attempt at a solution

I found the inductance and the current for the 1st two parts of the problem:

(a) What is the inductance of the solenoid? 14.56171141 H
(b) What is the current through the battery? 0.8542201 A

I plugged in the numbers to find the energy: U =0.5*14.56171141*(0.8542201^2) = 10.62556402 J

Then I think that you would have to integrate over time from 0 to 1.4s which gave me a result of 7.4378948 J.

It did not like the answer. Any suggestions?
Phys.Org News Partner Science news on Phys.org
Better thermal-imaging lens from waste sulfur
Hackathon team's GoogolPlex gives Siri extra powers
Bright points in Sun's atmosphere mark patterns deep in its interior
cupid.callin
cupid.callin is offline
#2
Mar28-11, 12:34 PM
P: 1,135
you know the eqn of current in circuit at some time t

use it to find the charge flown through battery ... q = ∫i dt
where t changes from 0 to 1.4

now work done by battery = Q(EMF) = energy delivered
Bryon
Bryon is offline
#3
Mar29-11, 07:52 AM
P: 99
Do I just inetrate (V/R)*(1-e^(Rt/L))? Its the only thing i can think of now that will be of any help. I tried to figure this one out from the hints you gave me but no luck.

gneill
gneill is offline
#4
Mar29-11, 08:27 AM
Mentor
P: 11,416

Stored Magnetic Energy - Inductor Circuit


Quote Quote by Bryon View Post
Do I just inetrate (V/R)*(1-e^(Rt/L))? Its the only thing i can think of now that will be of any help. I tried to figure this one out from the hints you gave me but no luck.
You have the expression for the current with respect to time. That's the current that is being delivered by the battery to the circuit. Do you know what the instantaneous power (watts) delivered by the battery is?
Bryon
Bryon is offline
#5
Mar29-11, 08:39 AM
P: 99
Yes, it is P=V*I(t). So, when I find the total current over time I can just plug it in to that!
gneill
gneill is offline
#6
Mar29-11, 08:54 AM
Mentor
P: 11,416
Quote Quote by Bryon View Post
Yes, it is P=V*I(t). So, when I find the total current over time I can just plug it in to that!
Yup. Integrate the power to find the total energy delivered.
cupid.callin
cupid.callin is offline
#7
Mar29-11, 08:58 AM
P: 1,135
Quote Quote by gneill View Post
Yup. Integrate the power to find the total energy delivered.
But same is integrate current and then multiply by V

both methods seems different but are same, right?
Bryon
Bryon is offline
#8
Mar29-11, 09:01 AM
P: 99
Thanks for the help! I see if i can get it in a bit!
gneill
gneill is offline
#9
Mar29-11, 09:02 AM
Mentor
P: 11,416
Yes; in this case the voltage is constant, so it can be "pulled out of" the integral.


Register to reply

Related Discussions
The energy stored in an inductor Electrical Engineering 6
Energy Stored In a Magnetic Coupled Circuit Engineering, Comp Sci, & Technology Homework 0
energy stored in an inductor Introductory Physics Homework 7
How much of this energy is stored in the magnetic field of the inductor? Introductory Physics Homework 2
Magnetic field stored in an inductor? Classical Physics 0