Advanced level physics voltage help? Watch

Montenzuma
Badges: 8
Rep:
?
#1
Report Thread starter 4 weeks ago
#1
It is necessary to maintain a p.d. of at least 3.4 volts across a device of resistance 0.5 ohms. What is the least number of cells, each of emf 2.0 V and internal resistance 0.20 ohms, which must be used in series?
(answer is 6 cells calculated at 5.3)

First I calculated the current with the pd 3.4 volts and the resistance 0.5 ohms, giving a current of 6.8 amps. Because they are asking in series that means the current will stay the same.
Then I worked out 1/Rtotal 1/0.2 which is 5
but im not sure what to do next :/

Help is much-appreciated thanks
0
reply
RogerOxon
Badges: 21
Rep:
?
#2
Report 4 weeks ago
#2
(Original post by Montenzuma)
It is necessary to maintain a p.d. of at least 3.4 volts across a device of resistance 0.5 ohms. What is the least number of cells, each of emf 2.0 V and internal resistance 0.20 ohms, which must be used in series?
(answer is 6 cells calculated at 5.3)

First I calculated the current with the pd 3.4 volts and the resistance 0.5 ohms, giving a current of 6.8 amps. Because they are asking in series that means the current will stay the same.
Then I worked out 1/Rtotal 1/0.2 which is 5
but im not sure what to do next :/

Help is much-appreciated thanks
At the required current, what is the terminal voltage of each (series) cell? How many are required to reach 3.4V?
0
reply
Montenzuma
Badges: 8
Rep:
?
#3
Report Thread starter 4 weeks ago
#3
(Original post by RogerOxon)
At the required current, what is the terminal voltage of each (series) cell? How many are required to reach 3.4V?
I'm not sure because the number of cells is what we have to figure out
0
reply
RogerOxon
Badges: 21
Rep:
?
#4
Report 4 weeks ago
#4
(Original post by Montenzuma)
I'm not sure because the number of cells is what we have to figure out
Each cell has an internal resistance of 0.2 Ohm, so how much of it's 2V EMF is lost at the required current?
0
reply
Montenzuma
Badges: 8
Rep:
?
#5
Report Thread starter 4 weeks ago
#5
(Original post by RogerOxon)
Each cell has an internal resistance of 0.2 Ohm, so how much of it's 2V EMF is lost at the required current?
that would be v/r so 2/0.2 = 10 amps?
0
reply
RogerOxon
Badges: 21
Rep:
?
#6
Report 4 weeks ago
#6
(Original post by Montenzuma)
that would be v/r so 2/0.2 = 10 amps?
No. You calculated the minimum current required to maintain 3.4V across the load. Now calculate how much of each cell's EMF will be lost at that minimum current.
1
reply
Montenzuma
Badges: 8
Rep:
?
#7
Report Thread starter 4 weeks ago
#7
(Original post by RogerOxon)
No. You calculated the minimum current required to maintain 3.4V across the load. Now calculate how much of each cell's EMF will be lost at that minimum current.
ahh so V=IR = 2 x 10 = 20 volts?
0
reply
RogerOxon
Badges: 21
Rep:
?
#8
Report 4 weeks ago
#8
(Original post by Montenzuma)
It is necessary to maintain a p.d. of at least 3.4 volts across a device of resistance 0.5 ohms. What is the least number of cells, each of emf 2.0 V and internal resistance 0.20 ohms, which must be used in series?
(answer is 6 cells calculated at 5.3)

First I calculated the current with the pd 3.4 volts and the resistance 0.5 ohms, giving a current of 6.8 amps. Because they are asking in series that means the current will stay the same.
(Original post by Montenzuma)
ahh so V=IR = 2 x 10 = 20 volts?
No.

To maintain a voltage of 3.4V across the 0.5 Ohm load, you calculated that a minimum current of 6.8A is required. That current flows through all the cells in series, so each cell loses the same amount of its EMF.

Each cell has an internal resistance of 0.2 Ohm, so how much of its EMF is lost when 6.8A is flowing through it?
0
reply
Montenzuma
Badges: 8
Rep:
?
#9
Report Thread starter 4 weeks ago
#9
(Original post by RogerOxon)
No.

To maintain a voltage of 3.4V across the 0.5 Ohm load, you calculated that a minimum current of 6.8A is required. That current flows through all the cells in series, so each cell loses the same amount of its EMF.

Each cell has an internal resistance of 0.2 Ohm, so how much of its EMF is lost when 6.8A is flowing through it?
oh
0.2 ohms, 6.8A so voltage is IR = 0.2 x 6.8 = 1.36V ? is that the emf lost or the total emf:confused:
0
reply
RogerOxon
Badges: 21
Rep:
?
#10
Report 4 weeks ago
#10
(Original post by Montenzuma)
oh
0.2 ohms, 6.8A so voltage is IR = 0.2 x 6.8 = 1.36V ? is that the emf lost or the total emf:confused:
That's what is lost of each cell's 2V EMF. Now calculate what's left, per cell. Finally, find how many cells you need in series to get 3.4V.
0
reply
Montenzuma
Badges: 8
Rep:
?
#11
Report Thread starter 4 weeks ago
#11
(Original post by rogeroxon)
that's what is lost of each cell's 2v emf. Now calculate what's left, per cell. Finally, find how many cells you need in series to get 3.4v.
aaaa thank u sm i got it now
0
reply
X

Quick Reply

Attached files
Write a reply...
Reply
new posts
Back
to top
Latest
My Feed

See more of what you like on
The Student Room

You can personalise what you see on TSR. Tell us a little about yourself to get started.

Personalise

University open days

  • Sheffield Hallam University
    Get into Teaching in South Yorkshire Undergraduate
    Wed, 26 Feb '20
  • The University of Law
    Solicitor Series: Assessing Trainee Skills – LPC, GDL and MA Law - London Moorgate campus Postgraduate
    Wed, 26 Feb '20
  • University of East Anglia
    PGCE Open day Postgraduate
    Sat, 29 Feb '20

People at uni: do initiations (like heavy drinking) put you off joining sports societies?

Yes (532)
66.67%
No (266)
33.33%

Watched Threads

View All