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Resistance & Electrical Circuits 
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#1
Feb1514, 04:25 PM

P: 127

Hi, I am massively confused with what resistance is and how it applies to circuits and would be very grateful if someone could help. I know this sounds like a basic question but I have been reading for hours and the more I read the less it makes sense. My book says that resistance is 'the property of a material which makes moving charges dissipate their energy'. It talks about the electrons in a wire interacting with the atomic lattice of the material resulting in electrons transferring their energy to the lattice.
If you now consider two very simple circuits each of which has the same source voltage of 6V; one has a 10 ohm resistor and the other has a 30 ohm resistor. According to my book the 30 ohm resistor has a greater tendency to dissipate the energy of the charges. However, the voltage drop across each resistor is the same (6V) therefore each coulomb of charge will dissipate 6J of energy. So if each unit of charge has no choice except to dissipate 6J of energy then how can this be a definition of resistance as each unit of charge is dissipating the same amount of energy? My book also says electrons gain kinetic energy from voltage source and transfer this energy by colliding with the lattice. But wouldn't this mean that when the electrons pass the resistor they would have zero kinetic energy? Finally, I know that the current in the circuit with the 30 ohm resistor will be less but what causes this specifically as I don't see how you can explain the slowing down of current AND the transfer of energy with the same information  i.e. interactions with the atomic lattice. Any help is much appreciated! 


#2
Feb1514, 04:44 PM

Mentor
P: 16,987

Resistance is not that complicated, but your book seems to be doing a bad job of it.
Some materials, known as resistors, have the property where the amount of current through the material is proportional to the voltage across the material. Resistance is simply the constant of proportionality between voltage and current: V=IR. That is it. I would take the above as the definition of resistance. From that definition you can easily derive the fact that resistors dissipate energy, but I would not take the dissipation of energy as the definition. I would take the proportionality between voltage and current as the definition of resistance. To show the relationship between energy dissipation and resistance simply note that the power dissipated is the product of the current and the voltage, so P=IV. Then substitute in the definition of a resistance to get P=IČR=VČ/R. Note that R can appear on the top or the bottom of the power dissipation equation depending on whether the current or the voltage is fixed, so using energy dissipation as the definition is problematic at best. 


#3
Feb1514, 07:43 PM

P: 9

Note that the power dissipation of a resistor is given off as heat, and is measured in terms of watts.
You can compare the current through your 30Ω and 10Ω resistors to water flowing from a single pipe that branches into two pipes, one of which is narrower (the 30Ω one) than the other. Naturally, there is more current through the wider pipe. "Resistance" should therefore be interpreted literally. 


#4
Feb1514, 08:35 PM

PF Gold
P: 372

Resistance & Electrical Circuits
Not sure if this is helping, maybe not.
maybe think of it like this. You know voltage is the measure of energy (joules) per unit charge (coulombs). Amperes is a measure of charge (coulombs) per unit time (seconds). The product of the two gives Joules per second, or watts (power dissipation). In the 10 ohm circuit, the resistor limits the current flow to 6/10 amps and in doing so, drops the entire 6 volts. It took 6/10 amps to drop it. in the 30 ohm circuit, LESS current (6/30) drops the entire voltage. It means that charge is more susceptible to losing the energy it carries when moving through a higher resistance. Make any sense? 


#5
Feb1514, 10:58 PM

P: 474

That's resistivity. Property of a material which makes moving charges dissipate their energy is known as resistivity. Let's say we have a big slab of some material with some resistivity. If we cut some bar shaped objects out of that slab, those bars will have some resistance, which depends on the resistivity of the material and the dimensions of the bars. 


#6
Feb1614, 04:02 AM

P: 127




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