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# EMF and PD

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1. Emf - is the amount of energy of any form that is changed into electrical energy per coulomb of charge.

pd - is the amount of electrical energy that is changed into other forms of energy per coulomb of charge.
u only have a PD when the electrical energy is actually converted?

So you don't have the PD (voltage?) until the electrical energy reaches a component such as a bulb (where its converted)?
2. EMF is the voltage coming from the battery, whereas the PD is the voltage measured over a particular component
3. (Original post by KBenson)
EMF is the voltage coming from the battery, whereas the PD is the voltage measured over a particular component
OMG THANK YOU! I was looking for a simple explanation like this!!

So if a voltmeter was connected across just the wire (so not across a component) it would be measuring the EMF? (as below)

http://oi44.tinypic.com/in7e4z.jpg
4. (Original post by sabre2th1)
OMG THANK YOU! I was looking for a simple explanation like this!!

So if a voltmeter was connected across just the wire (so not across a component) it would be measuring the EMF? (as below)

http://oi44.tinypic.com/in7e4z.jpg
Yeah surely it must be?
Although generally when you're measuring the EMF you connect around the cell/battery, I think? From what I remember anyway ahah!
5. (Original post by KBenson)
Yeah surely it must be?
Although generally when you're measuring the EMF you connect around the cell/battery, I think? From what I remember anyway ahah!
Thanks.. one last question, do you happen to know anything about the relationship between resistivity and temperature?
6. (Original post by sabre2th1)
Thanks.. one last question, do you happen to know anything about the relationship between resistivity and temperature?

Yes.... think about it in terms of electrons. When you heat the wire up, electrons gain more kinetic energy, and because they are moving around faster and making more collisions with one another and the wire walls, this makes it harder for them to get through the wire (resistance is higher)
7. (Original post by KBenson)
Yes.... think about it in terms of electrons. When you heat the wire up, electrons gain more kinetic energy, and because they are moving around faster and making more collisions with one another and the wire walls, this makes it harder for them to get through the wire (resistance is higher)
Oh, and since resistance increases so does resistivity?
8. (Original post by sabre2th1)
Oh, and since resistance increases so does resistivity?
Yes
9. (Original post by sabre2th1)
Oh, and since resistance increases so does resistivity?

(Original post by spocckka)
Yes
No, resistivity is a constant for a particular material regardless of its size or dimensions.

So, resistivity does not change with [strikethrough]temperature[/strikethrough] resistance.
10. Actually resistivity does change with temperature!

For a metal , it increases.
11. (Original post by Ari Ben Canaan)
No, resistivity is a constant for a particular material regardless of its size or dimensions.

So, resistivity does not change with [strikethrough]temperature[/strikethrough] resistance.

(Original post by teachercol)
Actually resistivity does change with temperature!

For a metal , it increases.
So resistivity only increases with temperature (not resistance), and this is only the case for metals?
12. If resistivity increases, then so does resistance. Thats why the resistance of a filament buld increasew ti htemp. (Its length and CSA dont change)

For metals, the increased lattice vibrations mean more enrgy is lost from eelctrons in collisions so the resistivity increases.

For semiconductors, an increase in temp measn that bonds 'break' releasing charge carries and the resistivity and resistance decrease.
13. (Original post by Ari Ben Canaan)
No, resistivity is a constant for a particular material regardless of its size or dimensions.

So, resistivity does not change with [strikethrough]temperature[/strikethrough] resistance.
Yep, you're very right. Apologies OP.
14. (Original post by teachercol)
If resistivity increases, then so does resistance. Thats why the resistance of a filament buld increasew ti htemp. (Its length and CSA dont change)

For metals, the increased lattice vibrations mean more enrgy is lost from eelctrons in collisions so the resistivity increases.

For semiconductors, an increase in temp measn that bonds 'break' releasing charge carries and the resistivity and resistance decrease.

(Original post by Ari Ben Canaan)
No, resistivity is a constant for a particular material regardless of its size or dimensions.

So, resistivity does not change with [strikethrough]temperature[/strikethrough] resistance.

(Original post by spocckka)
Yep, you're very right. Apologies OP.
Thanks for your replies , but don't they contradict? I don't know which one to take as the right one..
15. (Original post by sabre2th1)
Thanks for your replies , but don't they contradict? I don't know which one to take as the right one..
Hold on, I was right in what I said the first time, resistivity does increase with increasing temperature, due to the metal ions vibrating with a greater amplitude, blocking the electrons' path, as stated by teachercol.
16. Trust me. I'm a Physics teacher .....
17. (Original post by teachercol)
Trust me. I'm a Physics teacher .....
Trusted

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