# need some help on an electronics course

#### Robert_80004

Joined May 17, 2015
1
I have a few questions i missed and scored a 69 on my lesson. this is one point below passing grade of 70. i need help with the questions i missed. i have been rereading this lesson over and over but cannot seem to locate the correct answers. i am going to post the questions here, i hope someone can help me out.

Question 26 (which i did get right) is

*A step voltage of 15 volts is applied to a 100 ohm resistor in series with a 2-H inductor. The initial current rise (near time 0) equals.

(1) 0.133 A/s

(2) 0.15 A/s

(3) 6.67 A/s

(5) none of these

Questions 27 to 32 refer to the circuit of question 26 (the following questions are the ones i am having issue with)

Question 28) the steady state value of voltage across the inductor is (if i could figure this one out i think the rest will fall into place)

(1) 0 V (i think steady state voltage should be "0" across the inductor at least that is what the math appears comes out to) (only thing is the circuit specifies a step voltage which is confusing me a little when they ask for steady state here.)

(2) 0.15 V

(3) 6.67 V

(4) 7.5 V

Question 29) the initial value of voltage across the inductor is

(2) 0.150 V

(3) 6.67 V

(4) 7.50 V

(5) 15 V (i think this might be the correct answer on this one)

Question 31) After one time constant the voltage across the inductor is.

(1) 5.5 V

(2) 6.3 V

(4) 9.5 V

(5) 15 V

question 32) after one time constant the current through the inductor is.

(1) 37 mA

(2) 63 mA

(3) 55 mA

(4) 95 mA (i think this should have been the correct answer)

Question 33) A 1000 V step is applied across a 400 k ohm resistor in series with a 2uf capacitor. the initial rate of rise of voltage across the capacitor is found by V divided by T, where T= R x C. the initial rate of rise is. (i did get this one right)

(1) 0.25 v/s

(2) 500 V/s

(3) 1000 V/s

(4) 1250 V/s (my correct answer)

question 34) the steady state value of voltage across the resistor, question 33, is

(1) 0 V

(2) 500 V

(4) 1250 V

any help would be greatly appreciated.

thanks,

#### dl324

Joined Mar 30, 2015
9,595
Technically this isn't homework, so Posters can just give you the answers. But it would be more instructional for you if you explained your reasoning for why you thought your answers were correct.

#### MikeML

Joined Oct 2, 2009
5,444

#### crutschow

Joined Mar 14, 2008
24,093
I have a few questions i missed and scored a 69 on my lesson. this is one point below passing grade of 70. i need help with the questions i missed. i have been rereading this lesson over and over but cannot seem to locate the correct answers. i am going to post the questions here, i hope someone can help me out.

Question 26 (which i did get right) is

*A step voltage of 15 volts is applied to a 100 ohm resistor in series with a 2-H inductor. The initial current rise (near time 0) equals.

(1) 0.133 A/s

(2) 0.15 A/s

(3) 6.67 A/s

(5) none of these

Questions 27 to 32 refer to the circuit of question 26 (the following questions are the ones i am having issue with)

Question 28) the steady state value of voltage across the inductor is (if i could figure this one out i think the rest will fall into place)

(1) 0 V (i think steady state voltage should be "0" across the inductor at least that is what the math appears comes out to) (only thing is the circuit specifies a step voltage which is confusing me a little when they ask for steady state here.)

(2) 0.15 V

(3) 6.67 V

(4) 7.5 V

What is the confusion?
The steady-state voltage is the voltage after a long period of time.
So what is the steady-state voltage of the input after the initial step?

Question 29) the initial value of voltage across the inductor is

(2) 0.150 V

(3) 6.67 V

(4) 7.50 V

(5) 15 V (i think this might be the correct answer on this one)

(5) is correct.
Look at the resistor and inductor as a voltage divider.
What is the current at the instant the voltage goes high?
What is voltage drop across the resistor from this current?

Question 31) After one time constant the voltage across the inductor is.

(1) 5.5 V

(2) 6.3 V

(4) 9.5 V

(5) 15 V

That's not correct.
How did you arrive at that answer?

question 32) after one time constant the current through the inductor is.

(1) 37 mA

(2) 63 mA

(3) 55 mA

(4) 95 mA (i think this should have been the correct answer)

Yes, (4) is correct.
Do you know why?
From that you can derive the correct answer to Question 31.

Question 33) A 1000 V step is applied across a 400 k ohm resistor in series with a 2uf capacitor. the initial rate of rise of voltage across the capacitor is found by V divided by T, where T= R x C. the initial rate of rise is. (i did get this one right)

(1) 0.25 v/s

(2) 500 V/s

(3) 1000 V/s

(4) 1250 V/s (my correct answer)

question 34) the steady state value of voltage across the resistor, question 33, is

(1) 0 V

(2) 500 V

(4) 1250 V

How much steady-state current is flowing in this circuit?
What is the voltage drop across the resistor from that current?

any help would be greatly appreciated.

thanks,
My answers are in red above.

Some of your confusion seems to be in determining the voltage across a resistor. Note that the voltage across a resistor is always I*R, thus no current, no voltage.

Last edited:

#### JoeJester

Joined Apr 26, 2005
4,130

#### WBahn

Joined Mar 31, 2012
25,090
You need to go through and show us how you arrived at your answers. That will let use see where you are going right and where you are going wrong in order to help correct the misconceptions you have.