The questions were loaded.

1. How much resistance does a 1N4007 diode have when it is forward biased?

Like a LED, you really cannot say for sure what the resistance is.

If you were told what the current flow was then you might use ohms law to calculate an equivalent resistance.

But you weren't, the best answer is as you did, make some assumptions.

Full points for using the data sheet and basing your answer on quoted voltage to current characteristics.

2. When it is reversed biased?

This is a bit tricky.

A reversed biased diode has all the applied voltage dropped across it.

As you pointed out there is a reversed biased leakage value on the data sheet.

This is 5uA at the DC Blocking Voltage, for a 1N4007 this is 1000VDC.

You could say the reversed biased resistance was 1000V/5uA = 200,000,000 ohms = 200Meg

That is, an equivalent resistance of 200Meg at the rated PRV (peak reverse voltage).

In practice, at 5V, a reversed biased 1N4007 has a leakage current of zero amps.

(I know this from experience)

5V/0A = infinite (for all intents and purposes)

3. How much resistance does a fresh 9 volt carbon battery have?

The point here is, we sometimes miss the fact that an electronic device may have resistance.

Ex: a capacitor has an ESR (effective series resistance) .

Note: as a battery is discharged the internal resistance goes up leaving less voltage for the load.

Full points.

4. What is the total circuit resistance in the schematic below? Itot = 106.67ma

Back to the box.

With the applied voltage of 5V and a total current of 106.67mA your equivalent total resistance would be 5/160.67mA = 46.7 ohms.

At best, if all the 150 ohm resistors were in parallel this would give you 150/5 = 30 ohms leaving 16.7 inside the box.

Click the

LINK below the drawing to see what was in the box.

Some conclusions:

- There is a difference between equivalent resistance and resistance.

- Equivalent resistance can vary as circuit conditions change where resistance will not.

- Data sheets are important!

- Measurements may be necessary to dissect a circuit which quite often is a black box to the technician.

- It appears, you have quick mind and you will do well in electronics!

Edit:

Let's say there was a LED in the box instead of the 1.8 volt battery and the LED forward voltage drop was 1.8V.

It would be correct to say the power dissipated by the LED would be W=1.8v X 106.67mA = 190mW.

If this was a 16.7 ohm resistor it would also dissipate the same 190mW.

The power calculation is valid maybe you can considered the equivalent resistance calculation as valid