Potential Difference in Electronvolts?

Watch
Tom Osborne
Badges: 10
Rep:
?
#1
Report Thread starter 1 year ago
#1
I am stuck on this question from the 2016 PAT test where the potential difference is given in electron volts. How do you solve this? The electrons are accelerated through a potential difference of 5.0 keV towards a fluorescent screen. What is the final speed of the electrons when they hit the screen? [You may assume that Planck’s constant is 6.0×10−34 J s, the speed of light is 3.0×108 m/s, the mass of the electron is 1.0×10−30 kg, and the charge on the electron is 1.6 × 10−19 C.]
0
reply
Eimmanuel
Badges: 13
Rep:
?
#2
Report 1 year ago
#2
(Original post by Tom Osborne)
I am stuck on this question from the 2016 PAT test where the potential difference is given in electron volts. How do you solve this? The electrons are accelerated through a potential difference of 5.0 keV towards a fluorescent screen. What is the final speed of the electrons when they hit the screen? [You may assume that Planck’s constant is 6.0×10−34 J s, the speed of light is 3.0×108 m/s, the mass of the electron is 1.0×10−30 kg, and the charge on the electron is 1.6 × 10−19 C.]

When the electron moves through a potential difference of 5.0 keV, the change in potential energy is


5.0 kV × (1.6 × 10−19 C)

Using conservation of energy


Change in potential energy = Gain in KE

would allow you to find the speed.
Last edited by Eimmanuel; 1 year ago
1
reply
Tom Osborne
Badges: 10
Rep:
?
#3
Report Thread starter 1 year ago
#3
Ok thank you, I understand the conservation of energy part. But why can electronvolts be used as a unit for potential difference, when it is usually a unit of energy, isn't it? And how did you know that corresponds to 5kV?
0
reply
Joinedup
Badges: 20
Rep:
?
#4
Report 1 year ago
#4
I'd guess it was a typo and 5 kV was meant
0
reply
Tom Osborne
Badges: 10
Rep:
?
#5
Report Thread starter 1 year ago
#5
Yeah I guess so. It just seems unlikely from an Oxford PAT exam.
0
reply
Joinedup
Badges: 20
Rep:
?
#6
Report 1 year ago
#6
Probably everyone at Oxford thinks they're too important to proof read an exam paper tbh.
0
reply
MagnumKoishi
Badges: 21
Rep:
?
#7
Report 1 year ago
#7
Could be referring to the *potential energy* difference for those electrons, which is indeed a quantity of energy (such as eV), and not energy/charge (such as V).

In that case, take it that the elections gain 5keV of energy. Then 1/2 mv^2 for v.

But then again, it's more likely that it's a typo. It is odd for a uni entrance exam
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

Poll: What factors affect your mental health most right now? Post-lockdown edition

Anxiousness about restrictions easing (30)
5.32%
Uncertainty around my education (65)
11.52%
Uncertainty around my future career prospects (66)
11.7%
Lack of purpose or motivation (75)
13.3%
Lack of support system (eg. teachers, counsellors, delays in care) (32)
5.67%
Impact lockdown had on physical health (28)
4.96%
Social worries (incl. loneliness/making friends) (60)
10.64%
Financial worries (35)
6.21%
Concern about myself or my loves ones getting/having been ill (22)
3.9%
Exposure to negative news/social media (34)
6.03%
Difficulty accessing real life entertainment (15)
2.66%
Lack of confidence in making big life decisions (56)
9.93%
Worry about missed opportunities during the pandemic (46)
8.16%

Watched Threads

View All
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