Elecricity (Basic Circut Questions)

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  • #1
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Homework Statement


Ok this isn't particularly homework, but just revision needed. I tried asking the teacher but he's making no sense whatsoever. So, Firstly:

1. So here is my very badly drawn diagram. A very simple circuit with an LED.
http://img195.imageshack.us/img195/9484/33374444.png [Broken]
Ok so firstly, (yes that is a transistor at the top) the light doesn't turn if the resistance of the orange resistor is high enough right? Example. If the battery was giving say 9V, the current would increase so that there is 9V going up and 9v going towards the base of the transistor, and the 9V at the orange would get reduced depending on the orange resistance. Saying that the orange one is low, 8V continues on and reaches the base, making the resistance of the transistor low, which means the LED turns on.

But what is the relation between the orange and light blue resistor? Wouldn't the electricity split up to go through the blue resistor and to also go up into the base, which would cancel out the light blue resistor?

And another question, how many Volts does 1 ohm resist?

Sorry if this seems really dumb but i've asked but have gotten no good answers :(
 
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Answers and Replies

  • #2
954
117
Eh, the statement on how many volts 1 ohm resists does not make sense; a resistor is said to have a resistance of 1 ohm if a current of 1A flows through it when a potential difference of 1V is applied across it. There is no "resistance" of a certain potential difference.
 
  • #3
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Wait so resistors are supposed to SLOW down the CURRENT and not lower the VOLTAGE?

Because according to my teacher's example it seems to resist voltage therefore lowering the voltage after it.
 
  • #4
954
117
Yes, resistors impede the motion of electrons (that's why when temperature increases, resistance increases, as the greater vibrations of the lattice atoms impede the motion of the electrons to a greater extent)
They don't "resist voltage". Consider a 1 ohm resistor placed across an emf of say 6V. The p.d. across the resistor is 6V. If I replace it with a 5 ohm resistor, the p.d. across the resistor is still 6V. The drop in potential across the resistor is due to electrical energy being converted into other forms of energy (heat) in the resistor.
 
  • #5
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^^+1

Also I and not sure what the blue resistor is or how that transistor will switch the LED on and off. I presume that is what it is meant to do. You have not labelled the base, collector or emitter. It is difficult to assist at this stage.
 
  • #6
vk6kro
Science Advisor
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I see the second resistor as green.

The voltage divider used on the base bias circuits of transistors was originally introduced to avoid thermal runaway in early transistors. This happened if there was a high resistance between the base and the supply line.
This is not normally a problem with Silicon transistors.

However, if you do it like that (with two resistors ), you can have relatively small resistors and a circuit that works for all transistors of the same type.
The alternative is to set up each transistor individually with a single high value resistor from base to the + power supply. Typical values of 330 K to 820 K may be required.
Amplifiers set up like this are a little more sensitive to ambient temperature than ones with a voltage divider.
Some current is wasted through the green resistor, but the amplifier is more stable and more reproducible.
 
  • #7
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Ok i think i partially understand the role of resistors.

Quick question what is meant when electrons flow from neg to pos whereas current flows pos to neg?

Thanks for the replies so far guys i really appreciate it!
 
  • #8
954
117
By convention, current is defined as the flow of positive charge, whereas in the circuit, it is actually the flow of the negative charges (electrons) that cause the 'current'. Hence, conventional current flows opposite to the direction of electron flow.
 
  • #9
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But how can you have "positive charge"? Isn't it only possible to have negative charge because only electrons flow?

Sorry i am so confused and getting angry my head is going to explode :|
 
  • #10
vk6kro
Science Advisor
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It doesn't matter. Students get really worried about this.

Long ago, before anyone knew what electrons were, someone made a guess and said "something" flowed from positive to negative. They got it wrong. Too bad.

There are only a couple of times where it makes any difference. There are some rules about which way magnetic fields are produced and how motors turn where you have to use conventional current. Otherwise, forget it.

i am so confused and getting angry my head is going to explode
really?
 
  • #11
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yes because so many people are giving different answers but now it's cleared up!!

So basically Conventional current doesn't really physically exist, but is used because it is not really different to the real charge!

right??? :|
 
  • #12
vk6kro
Science Advisor
4,081
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So basically Conventional current doesn't really physically exist, but is used because it is not really different to the real charge!

That's right.
It wouldn't take much to get rid of it altogether, but since it really, really doesn't matter, it isn't worth the trouble.


Electronics is fun. Don't get hung up on the small stuff.
 
  • #13
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i really appreciate the help guys :D!!!!!!!

sorry i know it is a small matter but i don't feel like i can continue unless i know everything >_<
 

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