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## Class 10 Physics (India)

### Unit 3: Lesson 3

Circuits, Ohm's law & resistance

# Introduction to circuits and Ohm's law

Ohm’s Law is V = IR, where V = voltage, I = current, and R = resistance. Ohm’s Law allows you to determine characteristics of a circuit, such as how much current is flowing through it, if you know the voltage of the battery in the circuit and how much resistance is in the circuit. Created by Sal Khan.

## Want to join the conversation?

• what happen when there is no resistor in a circuit ?
• It is impossible to have a zero resistance wire because there are factors of the wire to provide resistance. If the resistance is very low, however, then it would probably trip a circuit breaker or blown a fuse. If you didn't have a circuit breaker, which is not likely, then the wire would melt and you would probably start a fire.
• Isn't the source of voltage shown in this video a cell, not a battery?
• Hello Mihir,

Maybe. Some sources would identify the schematic symbol as a cell. In practice it makes little difference and the two are used interchangeably.

The important part is to look for the voltage as it changes based on the battery a.k.a. cell chemistry.

Regards,

APD
• Around , Sal says "the more things there are for the electrons to bump into, or the less space there is for the electrons to move, the more they slow down'. So shouldn't this mean that the best conductors would be gases (because there are really few atoms or molecules floating around) and not metals like silver or copper which are densely packed with atoms?
• not quite, because you need "free" electrons for good conductivity.
The best conductors are superconductors. There the electrons are in a quantummechanical state that somehow prevent them of bumping into atoms around them.
• Can electrons move at the speed of light ?
• In reality, the net movement of electrons is extremely slow: about 10^-4 m/s. But there are millions of electrons in even a tiny section of wire. What matters is the lump sum flow of charge, and that's why things turn on in the blink of an eye.
• I've been taught that current flows from positive to negative, and you are saying that electrons flow from negative to positive. Is this just like a two lane highway with stuff flowing in each direction? Also, what about LEDS? In order for them to light up, the current has to flow from positive to negative.

Is this just semantics?
• All you have to remember is that current flow is opposite to electron flow...
• Sal says that no matter how many charges flow from one side of the battery to the other, the potential difference remains the same. Why is it so? I thought the potential difference decreases, making the battery/ cell dead.
• When we draw a symbol for a battery in a circuit, we are indicating an imaginary battery whose voltage never changes. A real battery will maintain fairly steady voltage until near the end of its life.
• At ,V=I*R, so as voltage across the circuit increases, current in the circuit increases.
But in "High tension wires" high voltage is kept so that there is less flow of current. In this case current is inversely proportional to the voltage.Why so?
• V is directly proportional to I
to remove the above sign we introduce a constant of proportionality R(resistance)
which gives us,
V=IR WHERE
v
(1 vote)
• I have heard that electrons can travel through a vacuum
(hoping this is not a misconception made by me),
consider an insulating hollow tube where the 'hollowness' is 'filled'(not the appropriate word ;)) with vacuum. Is there any resistance ?
• Yes, electrons can travel through vacuum. The electrons won't have any resistance from direct interactions with matter but being a charged particle there can be interactions with electric and/or magnetic fields.
• when we have resistors in series, the current through all the resistors is same and the voltage drop (or simply voltage) at each resistor is different.

Question 1: it is fine that voltage drop (potential drop) is different because each resistor offers different resistance (suppose). but how is the current through each resistor same? if we have resistors of different resistance, shouldn't the current be different through each resistor?

similarly, when we have resistors in parallel, the current through each resistor is different but the voltage drop at each resistor is same.

Question 2: current is through each resistor is different because resistance of each resistor is different (suppose). but how is the voltage drop at each resistor same here? shouldn't the voltage drop at each resistor be different because each resistor offers different resistor?
• ok lets take resistor as blockages in pipe....so won't the water flow be slowed down by the blockages in pipe? similarly, won't the electron flow be slowed down by the resistor. slowing down electron flow mean less Q/t and this means less a current. doesn't it?
• I think that Kirchhoff's junction rule is a bit unclear about the fact that while the charge is flowing through the resistor, the current is less than before, but the current after the resistor is equal to before.

I was just confused for a while, because they seemed to be contradicting themselves using the formula I = V/R to get less current but then said it was the same. Maybe it should be more clear.

I don't really have a question I guess, but I don't now where else to put this.