That's pretty cool. What other materials could you use to get the same effect?

in general, most of the conductors possess a linear variation of resistance with length at particular physical conditions..however the proportionality constant varies with any changes made in the physical conditions like temperature, material, etc.

Okay, so now I know that resistance is proportional to Resistivity*length/area. I was recently reading a Science Olympiad question paper. There was a question that baffled me: If the length of the copper wire is doubled, what will be the effect on its resistance. I should think the answer is 2R. But apparently the answer is 4R. Why?

This is an interesting question, but it's a trick. This question assumes you *stretch* the wire, rather than simply double the amount of wire. This decreases the cross sectional area as well. (will need to do these experiments next)

From what I observe, the metal clips are the best conductors and provide the highest deflection. As the length of the graphite increases, the deflection decreases. Does this prove a linear relationship between length of conductor and current. Maybe more of the conductor opposes the flow of current?

yes ! resistance is equal to resistivity ( characteristic property of a material ) times length divided by cross sectional area . here area and resistivity being constant..as we alter the length..due to the linear variation of resistance..we end up getting lesser deflection !

Main content

Physics library

Course: Physics library > Unit 18

Lesson 6: Discovery of resistors

Variable resistor (pencil)

Name: Variable resistor (pencil)
Uploaded: 2013-04-23T00:46:05Z
Description: We use the inside of a pencil (graphite) to apply different "lengths of pencil" to the circuit. What relationship do you observe between length of graphite and deflection angle?

Google Classroom

We use the inside of a pencil (graphite) to apply different "lengths of pencil" to the circuit. What relationship do you observe between length of graphite and deflection angle? Created by Brit Cruise.

Want to join the conversation?

Sort by:

Bruno Morais
Posted 10 years ago. Direct link to Bruno Morais's post “That's pretty cool. What ...”
That's pretty cool. What other materials could you use to get the same effect?
Button navigates to signup pageButton navigates to signup page
(7 votes)
Answer
- sanketdoshi
  Posted 10 years ago. Direct link to sanketdoshi's post “in general, most of the c...”
  in general, most of the conductors possess a linear variation of resistance with length at particular physical conditions..however the proportionality constant varies with any changes made in the physical conditions like temperature, material, etc.
  Comment on sanketdoshi's post “in general, most of the c...”
  (4 votes)
eightsquare
Posted 10 years ago. Direct link to eightsquare's post “Okay, so now I know that ...”
Okay, so now I know that resistance is proportional to Resistivity*length/area. I was recently reading a Science Olympiad question paper. There was a question that baffled me: If the length of the copper wire is doubled, what will be the effect on its resistance. I should think the answer is 2R. But apparently the answer is 4R. Why?
Button navigates to signup pageComment on eightsquare's post “Okay, so now I know that ...”
(5 votes)
Answer
- brit cruise
  Posted 10 years ago. Direct link to brit cruise's post “This is an interesting qu...”
  This is an interesting question, but it's a trick. This question assumes you stretch the wire, rather than simply double the amount of wire. This decreases the cross sectional area as well. (will need to do these experiments next)
  Comment on brit cruise's post “This is an interesting qu...”
  (6 votes)
suniltyagi
Posted 8 years ago. Direct link to suniltyagi's post “Why does he use the water...”
Why does he use the water cup ? what is the use of it ?
Button navigates to signup pageButton navigates to signup page
(2 votes)
Answer
- Milinium Otaku
  Posted 5 years ago. Direct link to Milinium Otaku's post “If you watch previous vid...”
  If you watch previous videos, you'll see why. As a short explanation, he uses the cup of water because it's the water and the zinc of the battery that cause a reaction, which is what is being measured.
  Button navigates to signup page
  (1 vote)
adinsx0001
Posted 9 years ago. Direct link to adinsx0001's post “For how long can you disc...”
For how long can you discharge one of those home made batteries?
Button navigates to signup pageButton navigates to signup page
(3 votes)
Answer
eightsquare
Posted 10 years ago. Direct link to eightsquare's post “From what I observe, the ...”
From what I observe, the metal clips are the best conductors and provide the highest deflection. As the length of the graphite increases, the deflection decreases. Does this prove a linear relationship between length of conductor and current. Maybe more of the conductor opposes the flow of current?
Button navigates to signup pageButton navigates to signup page
(2 votes)
Answer
- sanketdoshi
  Posted 10 years ago. Direct link to sanketdoshi's post “yes ! resistance is equal...”
  yes ! resistance is equal to resistivity ( characteristic property of a material ) times length divided by cross sectional area . here area and resistivity being constant..as we alter the length..due to the linear variation of resistance..we end up getting lesser deflection !
  Button navigates to signup page
  (3 votes)
Lewis Bowyer
Posted 8 years ago. Direct link to Lewis Bowyer's post “is this pencilly apparatu...”
is this pencilly apparatus also called a potentiometer? and if so are potentiometers just resistors with a variable output?#
Button navigates to signup pageButton navigates to signup page
(1 vote)
Answer
Ria Golez
Posted 8 years ago. Direct link to Ria Golez's post “Is there any other ways y...”
Is there any other ways you can use graphite for besides using it for the inside of a pencil?
Button navigates to signup pageButton navigates to signup page
(1 vote)
Answer
- Milinium Otaku
  Posted 5 years ago. Direct link to Milinium Otaku's post “I'm sure there are, most ...”
  I'm sure there are, most likely in factories somewhere. If you want details you could try looking it up on Google.
  Button navigates to signup page
  (1 vote)
Sophie Foster
Posted a year ago. Direct link to Sophie Foster's post “What materials did you us...”
What materials did you use?
Button navigates to signup pageButton navigates to signup page
(1 vote)
Answer
Jameson Ross
Posted 8 years ago. Direct link to Jameson Ross's post “what is in graphite that...”
what is in graphite that makes it do that
Button navigates to signup pageButton navigates to signup page
(1 vote)
Answer
lucjustme
Posted 7 years ago. Direct link to lucjustme's post “Why do you use hot water ...”
Why do you use hot water here?
Button navigates to signup pageButton navigates to signup page
(1 vote)
Answer

Video transcript

(pencil scratching) (water splashing in cup)