# Diode paralell with a resistor problem.

1. Feb 24, 2012

### lionely

Today my teacher asked us a question

Basically he said a match box was taped shut and a current was passed through the circuit a reading came up on the Ammeter, then the cell was switched around, and a current ran through the circuit but the Ammeter read a smaller reading than before. Why?

2. Feb 24, 2012

### PeterO

A lower current would indicate a higher resistance. What do you know about the effective resistance of a diode in forward bias and reverse bias.

3. Feb 24, 2012

### lionely

Reverse bias resistance higher umm the thing is I know all of that stuff. But I forgot to say My teacher said we don't know what's in the box... after sometime of guessing he told us that it was a diode and resistor in parallel.

4. Feb 24, 2012

### PeterO

Lets suppose that a diode has a resistance of 1Ω in forward bias, and 100Ω in reverse bias [you can investigate more reasonable figures - I just made them up]

If the diode was in parallel with a 100Ω resistor you can calculate the effective resistance.

When both are 100Ω we get 50Ω
When 1 is 100Ω and the other only 1Ω we know the effective resistance is < 1Ω
[when resistors are in parallel, the effective resistance is always less than the smallest resistor involved]

< 1Ω would allow a much higher current that 50Ω.

5. Feb 24, 2012

### lionely

Oh, I understand. But i'm not really sure how a diode works, in the question it said only the cell was altered and not like the position of the match box so the diode inside would always be forward bias?

6. Feb 24, 2012

### PeterO

Forward and reverse bias refers to "which end" is connected to which terminal of the power supply.

If you swap the battery round, you alter the bias!

7. Feb 24, 2012

### lionely

OH!!! Sorry I didn't know I didn't do my homework on this part of electricity so I don't know it well :( Sorry. I need to do it but thank you so much this made it so much clearer!