find the voltage across each diode. assume practical model.

please help me get started.

 

Odd. Didn't you buy textbook for the course?

 

I have a text but I don't understand the Diagram here.please help.

 

What don't you understand about the diagram? We are not mind readers?

What is the "practical model" that you are supposed to assume for your diodes?

Could you show your best attempt to work the problem?

 

In the diagram on the left. I can say that the diode is reverse bias because the cathode is connected to a higher potential 8v. now if it is reverse biased it is like an open switch. The voltage across it would be the sum of the batteries.

but I don't understand what to use 8-v =3v or 5-8=-3v?

 

In the diagram on the left. I can say that the diode is reverse bias because the cathode is connected to a higher potential 8v. now if it is reverse biased it is like an open switch. The voltage across it would be the sum of the batteries.

but I don't understand what to use 8-v =3v or 5-8=-3v?

I would go with 5-8=-3 volts.
Or.
You can say turn the circuit up side down and you have: -8-(-5)=-8+5=-3 volts.

 

but why is that? why it is not the other way?

 

The first thing you have to do is be clear about what you mean by "the voltage across it". Do you mean the voltage of the cathode relative to the anode or do you mean the voltage of the anode relative to the cathode. Usually we talk about the voltage across a diode as being the voltage at the anode relative to the cathode, which is expressed mathematically as

Vdiode = Vanode - Vcathode.

So, if no current is flowing in the resistor, what is the voltage at the anode? What is the voltage at the cathode? What is the diode voltage?

 

but why is that? why it is not the other way?

You said it yourself, the anode of the diode is at lower potential than the cathode of the diode.

Let look at what is going on there.
We start on the left, we have 5 volts. We move to the right and we run into a resistor. The resistor will have some voltage drop across it, it is small resistor so lets say the drop is 0.5 volts (just so we have a number to work with). Then we leave the resistor and arrive to the anode of the diode, what is the voltage at the anode? 5-0.5=4.5 volts. Then we move to the cathode of the diode and what do we have there? 8 volts. So. What is the voltage drop from anode to cathode? 4.5-8=-3.5 volts. What does it mean? It means, like you said, that the diode is reversed biased. What does that mean? It means that diode is OFF, that no current in flowing through it. What does that mean? It means that diode is acting as an open circuit, like you said. What does that mean? It means that there is no current though the resistor and my earlier assumption/guess that there is some small voltage drop across the resistor is wrong, what I though was 0.5 volts is really 0 volts. What does that mean? It means that voltage at the anode is not 4.5 volts, it is really 5 volts, and the voltage drop across the diode is 5-8=-3 volts.

 

Is it always the case that when we say "voltage across a diode" means voltage at the anode relative to the cathode?

 

Is it always the case that when we say "voltage across a diode" means voltage at the anode relative to the cathode?

Usually, but not always. The most common situation in which you see the other convention used is with Zener diodes.

 

Is it always the case that when we say "voltage across a diode" means voltage at the anode relative to the cathode?

Yes. Same with resistor.

 

Yes. Same with resistor.

????

I didn't know that a resistor had an anode and a cathode? How do you tell which end is which?

 

????

I didn't know that a resistor had an anode and a cathode? How do you tell which end is which?

I was referring to the voltage drop. Anode and cathode refer to the internal structure of the diode.