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## Class 12 Physics (India)

### Course: Class 12 Physics (India) > Unit 3

Lesson 3: Resistivity, and Ohm's law- Introduction to circuits and Ohm's law
- Resistivity and conductivity
- Current, resistance, and resistivity review
- Electric potential difference and Ohm's law review
- Factors affecting resistance (basic)
- Calculating resistance, voltage, and current using Ohm's law
- Ohm's law: vector form
- Conductivity and resistivity: Ohm's law
- Temperature dependence of resistivity
- Resistivity vs. temperature: conductors and semiconductors

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# Current, resistance, and resistivity review

Review the key terms, equations, and skills related to current, resistance, and resistivity, including how to find the current direction and what resistance depends on.

## Key terms

Term (symbol) | Meaning | |
---|---|---|

Current (I) | Measure of how much charge passes through a given area over time. SI units of start text, A, m, p, e, r, e, space, left parenthesis, A, right parenthesis, end text. | |

Ampere (start text, A, end text) | Current equivalent to transferring 1, start text, c, o, u, l, o, m, b, end text of charge per second. SI units of start fraction, start text, C, end text, divided by, start text, s, end text, end fraction. | |

Direct current (DC) | Constant flow of charge in one direction. | |

Resistor | Device used to reduce current flow. | |

Resistance (R) | Measure of how much an object resists current flow. Depends on material, length, and cross sectional area. SI units of Ohms (\Omega). | |

Resistivity (rho) | Measure of how much a specific material resists current flow. SI units of \Omega, dot, start text, m, end text. | |

Ohm (\Omega) | The unit of electrical resistance. SI units of start fraction, start text, k, g, end text, dot, start text, m, end text, squared, divided by, start text, s, end text, cubed, dot, start text, A, end text, squared, end fraction. |

## Equations

Equation | Symbols | Meaning in words |
---|---|---|

I, equals, start fraction, delta, q, divided by, delta, t, end fraction | I is current, delta, q is net charge, and delta, t is change in time. | Current is the change in charge over the change in time. |

R, equals, start fraction, rho, l, divided by, A, end fraction | R is resistance, rho is resistivity, l is length, and A is cross sectional area | Resistance is proportional to resistivity and length, and inversely proportional to cross sectional area. |

## How to visualize the current

Current measures the flow of charges through an area over time. Figure 1 shows a wire with charges q moving to the left through the wire, which has a cross sectional area A. Imagine we counted how many charges passed through the cross sectional area in one second. This rate is the current.

## Finding the current direction

Current direction is designated by the symbol I along with an arrow and always refers to the flow of positive charge as shown in Figure 2A. This is sometimes called

**conventional current**.In conductors such as wires, the electrons are the only charge that move. The electrons flow opposite to I (see Figure 2A). The direction of the flow of electrons is called

**electron current**, and its direction is opposite to I (see Figure 2B). The convention of I representing the flow of positive charge is a historical convention that is equivalent to negative charge flowing in the opposite direction.## What does resistance depend on?

Resistance depends on an object’s size, shape, and material. In Figure 3 below, the cylinder's resistance is directly proportional to its length l. The longer the cylinder, the higher the resistance.

Additionally, the resistance is inversely proportional to the cross sectional area A. If the diameter of the cylinder is doubled, the cross-sectional area increases by a factor of 4. Therefore, resistance decreases by a factor of 4.

The resistivity rho of a material depends on the molecular and atomic structure, and is temperature-dependent. For most conductors, resistivity increases with increasing temperature.

## Learn more

For deeper explanations of current and resistance, see our video on resistivity and conductivity.

## Want to join the conversation?

- Can there be a resistance in a vacuum even if the liquid hits that walls or other objects within the vacuum?(5 votes)
- It's important to remember that water through a tube is only an analogy. A tool to help describe physical interactions. Our senses were not designed to observe "subatomic particle-wave electron thingys" but by connecting them to real life phenom we're familiar with we can better learn about the universe... Hopefully you keep that in mind. In terms of your question, yes, if there is a pressure difference and a tube connecting the ends, water will flow; if your tube is filled of stone there will be more "resistance".(16 votes)

- If the electrons are the ones who move in a current,then wouldn't the atoms become unstable? I mean as we know that the electrons move in orbital shells around the positively charged atomic nucleus then how are they able to seperate from the atom and move freely in a current....please help me to have a clearer image of this....( sorry if my question was found to be inappropriate but I just had to polish my basics).(3 votes)
- Only the valence electrons in a conductor metal atom usually are involved in the flow of current(1 to 3 out of the many electrons a metal atom has. Copper has 29). These valence electrons too are loosely bound to the nucleus and can be used for current flow easily, this is because on losing a certain amount of electrons these metal atoms actually gain stability in some ways.

Also even when they participate in current flow the electrons are still there near every metal atom(a bit like water flowing in a pipe).

So as a combination of all these factors it doesn't lead to unstability. But yes, if you tried to extract say 10, or even 5 electrons from Copper atom got them away then the copper atom would be quite unstable.

Feel free to ask a follow up!

Hope that helps!(4 votes)

- The electric current has magnitude as well as direction but its not a vector quantity,why?(4 votes)
- Current follows the direction of the wire it is placed in. If you bend the wire, the strength of the current does not change. The arrow is simply indicating the direction of positive charge.

So though current has magnitude, it's direction is not similar to that of a vectors 'direction'. If you connect a wire carrying 3A of current at a 90 degree angle with a wire carrying 4A, and allow the current to flow into a separate wire, the total current in the new wire will be 7A and not 5A (which you would have gotten if you added the current vectorially).(1 vote)

- Would the same principles and formulas apply for proticity as they do for electricity?(3 votes)
- sorry about this question..

how can i rearrange the formula to get Length?

is L = RA/P??

thanks(4 votes)- yes. you multiply both sides of resistance formula R=ρL/A with A, and formula becomes RA=ρL, then you divide both sides with ρ, and formula becomes RA/ρ=L. same principle is valid for any kind of formulas rearrangements(1 vote)

- What is the use rheostat in verifying the Ohm's law through an electric circuit?

Question from the chapter Current Electricity from ICSE Board in India.(2 votes) - Why is the sigma lowercase?(1 vote)
- why cant an electron flow in the positive direction?(1 vote)
- I'm assuming you mean in the direction of current.

Well, that's simply because of convention. It*is*electron flow that constitutes a current, but electrons flow from a lower potential (a negative charge) towards a higher potential (a positive charge).

Now it's convention that states that positive charges have higher potential. And we like to keep things uniform. For example, water always flows from a higher potential(high up in a mountain, for example) to a lower potential(down the mountain, as in a waterfall). Same thing for a ball rolling down a hill.

So, for uniformity, we consider current to flow from higher potential(the positive charge) to the lower potential (the negative charge) even though the electrons that constitute the current in reality flow in the**opposite**direction.(2 votes)

- When using the current equation in order to find the current, how do you find the net charge?(1 vote)
- As you mentioned, I assume this simple equation only holds for ohmic materials and only to the point of destruction.

As it is an abstraction, as soon as one takes temperature and other factors into account it will

become about semiconductors or nonlinearly behaving conductors - it is a simplified model but it suffices.(1 vote)