# Homework Help: More easy circuits pleaseeee

1. Aug 12, 2009

### Stripe

1. The problem statement, all variables and given/known data
Ok so for Diagram A:
It is desired to set up an alarm to indicate when it is too cold. For this purpose the circuit shown is to be used and it is intended that the globe will go on only when the temperature falls below 18C. The positions A and B in the circuit are to be occupied by a resistor and a thermistor (20 ohms resistance at 100C and 500 ohms resistance 0C.)
Which of the following combinations would be successful?

A is thermistor B is 1 ohm resistor

A is thermistor B is 100 ohm resistor

A is a 1 ohm resistor B is a thermistor

A is a 100 ohm resistor B is a thermistor

AND then diagram B:

It was imagined that the globe should glow but did not. If all components were correctly soldered, which of the following could best explain the fault?

The thermistor is too hot.

The base voltage needs to be lowered

Resistance of the transistor is too low

The 100 Ohm resistance should be bigger.

2. Relevant equations

http://img35.imageshack.us/img35/3010/mathhh.png [Broken]

3. The attempt at a solution

Ok so part a, i thought that the answer was one of the first 2, because i thought that because the electricity would reach a first, it would have to be changed there before it reaches the base.

I thought that if A was a normal resistor, say 100 ohm, regardless of what happens at B, the base will always receive the same voltage because if B was the thermistor in this case, it would only affect the voltage after it???

Along with this could someone explain the importance of A and B??? i thought the base would still work if b was not present? Then i thought maybe B was there to create a parallel circuit so A could work?

For part B, the answer was that the 100 ohm resistor should be bigger. BUT WHY???!?!? isn't the 100 ohm resistor located AFTER the transistor so it makes no difference???

and i was told that voltage never changes but current does????? GOD I AM CONFUSED