# Search results

1. ### Basic diode analysis problem

You dont get two answers if you use a better model. In particular the model that you are using of a diode equals a battery is insufficient for this problem. Batteries can both source and sink current. Batteries have finite internal resistance. Adding a perfect diode in series with the battery...
2. ### Basic diode analysis problem

You can model a silicon diode as a 0.7V battery in series with a "perfect" diode. A little more care is required when modelling a germanium diode as a 0.3V battery and a "perfect" diode as germanium diodes leak a lot more than silicon under reverse bias. Leakage won't be an issue for this circuit.
3. ### Fleming's left and Right hand rule

Yes you can use just one hand for both. It is much better to use the right hand to give the direction of a magnetic field around a conductor which you can then visualise interacting with the field from the magnets. It is much better to just use the right hand for the vector cross product...
4. ### Largest resistor value such that BJT remains in active mode

Assume that the magnitude of the base-emitter voltage for the BJT is largely constant. What value might you choose?
5. ### Thermodynamics problem - weight and piston connected by string

What gases will you find in air? What is their composition (monatomic , diatomic, etc)? If one type is predominant then what?
6. ### Thermodynamics problem - weight and piston connected by string

What simplifying assumptions could you make?
7. ### Surface tension question.

Cosine rule. √7 R may be the answer in the book but I'm confident that particular answer is incorrect.
8. ### Surface tension question.

I tried the simpler problem of two bubbles of the same radius which makes the common surface a flat disc. My algebra got messy on that one too, so I did some approximating and ended with each bubble's radius increasing by approx 6%. Which means I'd be offering 1.06R as the distance between the...
9. ### Surface tension question.

Is there a fairly simple equation for the volume of the two bubbles once they have touched? There is a relationship for the three radii of a double bubble 1/Rcommon = 1/Rsmall - 1/Rlarge . Which can give another equation that relates the change in radius of the smaller bubble to the larger...
10. ### Measuring Voltage Across Two Points In A Series Circuit

Your updated circuit in post #3 has a different value for one of the resistors compared to post #1. How can current flow in your circuit? Is the 7V battery connected to one resistor or two?
11. ### Measuring Voltage Across Two Points In A Series Circuit

Each of your components has two nodes. The resistors don't care which way around they are connected as they are symmetrical devices, is this the case with the two batteries? The voltmeter has two nodes too, does it care how it is connected? I suspect the reason you are unsure of the answer is...
12. ### Measuring Voltage Across Two Points In A Series Circuit

Since you want to be precise: Is your circuit a loop or a line? You have described components connected in a line which would result in zero current in both resistors. Where is the "point above the earth" as that is not a named node in your series circuit ? If this is a new node then this...
13. ### Direction of Friction in Rolling Objects

The directions of the forces associated with friction between a pair of surfaces always acts to reduce the relative motion between the two surfaces. To apply this to a situation where there is no slip imagine the motion that would take place if friction between the two surfaces was zero.
14. ### Finding direction using magnetic fields

try working out the direction of the magnetic field in the middle of a loop of wire carrying a current.
15. ### Physics Honors-Falling Bodies

To expand on nasu's hint.. What other equations do you know that might be relevant? While it is great if you instantly know the correct equation to use, when unsure a list of equations that might be useful along with a table of knowns and unknowns for each equation can often illuminate how to...
16. ### Conservation of Energy, PE & KE, block on incline with KE & friction?

It really is KE = PE + Losses to friction have a closer look at the connection between the height above the starting point and how far along the slope the block travels
17. ### Centre of mass Momentum problem-Two blocks

try solving the problem without the wall in place.
18. ### Static stick and cyclinder, did I approach this problem correctly?

I missed that :( so you are correct that μ can be less than tan(θ) === Let the tension in the rod at the top be T Let the friction where the cylinder touches the inclined plane be F Let the normal reaction where the cylinder touches the inclined plane be R T=F (to avoid the cylinder...
19. ### Static stick and cyclinder, did I approach this problem correctly?

It feels wrong :( Imagine that instead of a rod on top of the cylinder, the cylinder is held by a horizontal light string to the top of the cylinder instead. The additional mass of the rod is absorbed into the mass of the cylinder (which is now 3m/2). The only thing special about the string is...
20. ### Very simple circuit of 2 batteries

yes and no. you can split the current however you like but the circuit is still going to sort them all out to give the same answer. Again this is a property of linear systems and is just like the additions: 1+1 = 2 and 3 - 1 = 2 So "yes" you can draw the current loops however you wish, "no"...
21. ### What causes a magnetic field

There is nothing stationary about the movement of charges when a dc current flows in a conductor. It might help if you can post details of your book where it says that it wouldn't be right.
22. ### Very simple circuit of 2 batteries

Superposition is a property of linear systems. The why is because the system is linear. Take any linear circuit of batteries and resistors. If you double all the batteries then all the voltages and currents double too. What is less obvious but still true is that if you just vary one of the...
23. ### Very simple circuit of 2 batteries

Both your examples are linear circuits. These have the wonderful property that just like 1+1 = 2 the circuit can be considered as a superposition of simpler systems. You can turn all batteries off bar one and work out the result for that battery. Then by proceeding in turn with a different...
24. ### Physics of Flight - From where does lift come?

So correct me if I have this wrong: at the point in the model where you have enough information to successfully apply Bernouilli's principle you no longer need to apply Bernouilli's principle?
25. ### Acceleration and weight question

If I have a mass of 180lbs then on the surface of the earth my weight will be 180lbs (mass) x 1 gravity (acceleration) to give 180lbs (force) weight. Gravity on the surface of the moon is 1/6th that on Earth so now my 180lbs mass feels like a _____ lb weight.
26. ### Acceleration and weight question

astronauts weigh less on the surface of the moon than on the earth.
27. ### Physics of Flight - From where does lift come?

I think I can manage one more statement before I'm out of my depth. The lift and drag forces so far discussed aren't real in the sense that you can point to a few molecules of air and say that is where the lift is or that is where the drag is. On a real wing the lift is produced over a large...
28. ### Acceleration and weight question

try it and see, tell us the answer you get.
29. ### Acceleration and weight question

and since the question is unclear how about the 180lb astronaut in the rocket?
30. ### Acceleration and weight question

The equation is f=ma weight = mass x (acceleration due to gravity + additional acceleration from motion) lbs are awkward weight in lbs = mass in lbs x acceleration in earth gravities weight in the lift going up = 180 x(1 + ???? ) lbs === and because the question is unclear there would...