# Search results

1. ### I Heat transfer within a laptop computer from CPU and GPU

Hello all I have a practical question regarding heat transfer within my laptop. My laptop uses a shared-heatpipe design, meaning that my CPU and GPU are connected by a common piece of copper, and then each chip goes off to its own heatsink/fan. While playing games, I was monitoring the...
2. ### How do you calculate the energy required via torque when...

Let's say you have a lever arm connected to a load at the end. But the load is attached to a hinge, so no matter where the lever arm is, the load is always pointing down. I know that Energy = [integral] tau d_phi, but does that only work if the angle b/w arm and load is constant? What if the...
3. ### How does the Beer Lambert law work at non-normal incidences?

I want to calculate the intensity of a transmitted wave in a medium, but not at normal incidence. I want to consider the case of 45 degrees. What proportion of the intensity of the wave is lost by reflection? Specifically I am looking at a the reflection off of a thin film of aluminum...
4. ### Have you ever heard of the physicist Alex Abastian (sp?)

I'm not sure how it's spelled, Abastian, Ebastian, etc. He died recently, probably within the past year or two.
5. ### Measuring the electron mobility of a molecule

Hello PF Let's say I have a device with three layers, A, B, and C, where layers A and C are the anode and cathode. I can measure the IV curves from that device easily. Now lets say I added a fourth layer, D, in between A and B. Layer D interacts with layer B such that it increases it's...
6. ### Completely dissolving a gas into a liquid

Hello PF I want to dissolve a gas in a liquid. I figured I could create a closed loop vacuum system. I introduce gas into the lines at atmospheric pressure, and then close off the lines, so the intake and exhaust of the vacuum pump are connected. Then I turn the pump on and the gas just...
7. ### What would cause sudden change in evaporation chamber behavior?

Hello all, I use an evaporation chamber at my lab every day. Basically, put some substance in the chamber, close it, bring it down to vacuum, and pump enough current through it to vaporize it where it deposits on a substrate. I've been using 175A to create a deposition rate of about 1...
8. ### Epicyclic motion and precession

Imagine I am standing but not moving forward/backward, but I am rotating my body about my own axis. Now imagine if I moved in a circular path as I was spinning on my own axis. Is this considered to be precessing (assuming the axis of rotations are always in the z direction)? My friend...
9. ### Problems charging a lead-acid battery

Hello guys, I have a problem charging a lead-acid battery. I have a 0-30V 0-3A power supply. If I hook up the power supply directly to the battery (with a resistor and/or diode) and set the voltage to an appropriate level, the LA battery will only draw about 150 mA. After a few minutes...
10. ### The construction of half silvered mirrors

Hello guys, I have a question about the construction of half silvered mirrors. I read that they are created using aluminum vapor (so some sort of vapor deposition process?). And I read that the thickness of the aluminum coating can affect the reflectance and transmittance properties...
11. ### What will happen with this scale and submerged objects

Homework Statement Hello all, I saw a picture today of a physics problem which is really a head scratcher in my opinion. See the attached photo. You have two columns of water on a scale. In the left column, you have a ping pong ball, attached by a string to the bottom of the container...
12. ### Gas consumption to overcome drag force

Homework Statement A car moving with speed v is subject to a drag force F_{d}=0.5\rho A c_{d}v^{2}, where \rho is the density of air (1.25 kg/m^{3} at STP), A is the cross sectional area of the car, and c_{d} is the drag coefficient. A Honda Civic has A=1.9 m^{2} and c_{d}=0.36. Assuming...
13. ### Confused about the sizes of atomic particles

Hey guys, I was doing some research and I am confused since I can't seem to get a straight answer. I wanted to know what the diameters/radii of protons and electrons are. But no place I go to agrees on the numbers. The general trend for protons is somewhere around 10^-15, but I've seen...
14. ### Acceleration and thrust of a rocket

Hey guys, I want to know if I have an equation on thrust correct. I figured whats the acceleration as a function of time. You have the upward thrust = dm/dt v, assuming the mass flow rate and the gas ejection rate is constant and the downward force = -m(t) g, where m=m(t) since it's...
15. ### Moving a loop of wire away from a current carrying wire

Homework Statement Hey all, I am studying for the GRE and I need help with this question: A rectangular loop of wire with dimensions shown above (see attachment) is coplanar with a long wire carrying current I. The distance between the wire and the left side of the loop is r. The loop is...
16. ### Warming a laser diode using a thermistor

I use a green laser pointer at night to do astronomy related work. I got a cheap \$5 green laser pointer off of ebay, and when it's a warm summer night, the device works fine. However, the second it touches the cold, it fizzles out and gives me almost zero output. My idea was to use a...
17. ### Can I replace an LC circuit with a crystal?

Hello all, I am trying to make an FM transmitter that operates at only 1 frequency. I need the frequency tolerance to be very high, so I'd like to choose a crystal instead of an LC circuit. This is the circuit I'm building: http://cappels.org/dproj/FMdist/fmdis.htm If I were to replace...
18. ### The moment of inertia of a group of seven pennies

Homework Statement See attached photo Homework Equations The Attempt at a Solution I figured I would use the parallel axis theorem. I'm stuck between two different methods of doing the question, both of which are choices in the answers. My gut instinct says to take the...
19. ### The observed frequency of a 1,200 Hz wave

Homework Statement During a hurricane, a 1,200 Hz warning siren on the town hall sounds. The wind is blowing at 55 m/s in a direction from the siren toward a person 1 km away. With what frequency does the sound wave reach the person? (The speed of sound in air is 330 m/s.) (A) 1,000...
20. ### Method of generating 170 VDC to drive nixie tubes

In order to operate, nixie tubes need 170 VDC. I know that the mains voltage is 120 VAC-RMS. So if you account for the RMS value, 120*1.414 is nearly exactly 170 V. So would it be possible to make a full wave rectifier? I decided to simulate the circuit in spice. If I use a 500 uF capacitor...
21. ### Strength of induced magnetic field inside an inductor

I am trying to make a crude coil gun. I remember that if I run current through a coil of wire, it generates a magnetic field according to the right hand rule. Will the inductance of the coil of wire greatly affect the strength of the magnetic field, or is it mainly due to the strength of the...
22. ### Is there such a thing as an inverted zener diode?

Zener diodes act like regular diodes, except that they allow current to flow in the reverse direction if the voltage is high enough. That is, current with voltage ABOVE a certain value will flow. My question is, is there such a diode that only allows current to flow if the voltage is BELOW...
23. ### How would you calculate the amplitude of an electromagnetic wave?

Would it just be the energy of the wave (i.e. E = hf)?
24. ### Direction and signs of unit vectors

If I have a negative quantity pointing in the positive z direction, is that the same as a positive quantity pointing in the negative z direction? e.g. does -A\hat{z}=A\hat{-z}?
25. ### Force on a dipole from a point charge?

Homework Statement A point charge q is situated at the origin. A dipole p is placed at r. The angle θ is defined by \hat{p}\cdot\hat{r}=cos(\theta) Calculate the vector force Fp acting on the dipole by the nonuniform E field of the point charge. Homework Equations The Attempt...
26. ### Potential of two point charges

Homework Statement Two point charges q1 and q2 are located on a straight line normal to a grounded (V=0) conducting plane. Choosing the z axis to go through the charges, q1 is at a distance z1 above the conductor, and q2 is at a distance z2. Find the potential V(x,y,z) everywhere above...
27. ### Potential difference inside a spherical shell

Homework Statement A top half of a spherical shell has radius R and uniform charge density sigma. Find the potential difference V(b)-V(a) between point b at the north pole, and point a at the center of the sphere. Homework Equations The Attempt at a Solution \oint E ds =...
28. ### Charge enclosed in a sphere

Homework Statement A solid insulating sphere of radius R is charged so that the charge density is proportional to the distance to the center, p(r)=kr, where k is a constant. Find Q(r), the charge enclosed by a concentric spherical surface of radius r<= R. What is the total charge of the...
29. ### If the polar ice caps were to melt .

If the polar ice caps were to melt..... wouldn't the sea levels decrease? Water ice is less dense than liquid water, so the volume (and displacement of liquid water) is greater when its frozen. But when you melt that, the volume decreases, and some of the water that was initially displaced...
30. ### How does refrigeration work?

I know I can easily look this up on Wikipedia, but I chose to use my intuition on this one ;) Tell me if my intuition based approach is correct: You have a compressor which takes a refrigerant gas such as Freon and compresses/decompresses it. I'm imagining it works something like a piston in...