Direction of voltage and electric field

  • Thread starter BassTea
  • Start date
  • #1
2
0
Hi guys,

in a physics book I found the equation E = - grad(U) and in another one E=U/d with E:electric field, U:voltage, d:distance. The minus sign confuses me. Can anybody explain that to me, please?

I am thinking of a room with a positively charged ceiling and a negatively charged floor. So a positive charge will move towards the floor to reduce the potential energy. In that case, is the current I positive or negative?

Thanks a lot.
Basti
 

Answers and Replies

  • #2
ZapperZ
Staff Emeritus
Science Advisor
Education Advisor
Insights Author
35,847
4,676
Hi guys,

in a physics book I found the equation E = - grad(U) and in another one E=U/d with E:electric field, U:voltage, d:distance. The minus sign confuses me. Can anybody explain that to me, please?

The former is the correct general form. The latter is probably a simplification where one is only interested in the magnitude of a constant E-field. It is a simplification of the former.

Zz.
 
  • #3
uart
Science Advisor
2,776
9
Yes that's correct, the negative sign tells us the the E vector points in the direction of decreasing voltage.
 
  • #4
jtbell
Mentor
15,763
4,001
I am thinking of a room with a positively charged ceiling and a negatively charged floor. So a positive charge will move towards the floor to reduce the potential energy. In that case, is the current I positive or negative?

It depends on which direction you call "positive" or "negative." If you call the direction from the floor to the ceiling "positive," then the current is negative in this case. If you call the direction from the ceiling to the floor "positive," then the current is positive in this case.

When there is any chance of confusion in a situation like this, I prefer to specify the direction explicitly, in this case "downward," instead of saying "positive" or "negative."
 
  • #5
2
0
Thank a lot. You're great help.
 

Related Threads on Direction of voltage and electric field

  • Last Post
Replies
2
Views
1K
  • Last Post
Replies
4
Views
2K
  • Last Post
Replies
4
Views
2K
Replies
6
Views
1K
  • Last Post
Replies
2
Views
2K
Replies
2
Views
2K
Replies
1
Views
789
Replies
16
Views
2K
Replies
5
Views
2K
Top