How to calculate how much energy in joules?

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I had a lab experiment where you had to burn the food items and place under a boiling tube and record the change of temperature. How do I work out how much energy the food provides or gave off in joules ?

Is it true that if i use 10 g of water I have to multiply it by 4.2 and then by the temperature change?

Answer: 
Amount of energy in the food = amount of energy released. Energy released = mcΔT, where m is the mass of the water, c is the specific heat capacity (for water it is 4200 J•kg^-1•K^-1). and ΔT is the change in temperature of the water. suppose you had 10g of water which changed temperature from 27°C to 100°C. Then the amount of heat transfered to the water is Q = mcΔT = (0.010)(4200)(100 - 27) = 3066 J = 3.066 kJ

How to flow positive charge current to the earth?

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As I know if negative line fails current can be flowed by connecting to earthing wire.But when I connected the positive current direct to the surface current didn't flow. Why?

Answer: 
Maybe your earth connection (wire) has failed also .. if so, you have no electrical safety connection .. so beware. Have you checked for a blown main fuse.

How to solve for ΔH given change in internal energy and expansion work?

in

For a certain reaction at constant pressure, ΔU = -73 kJ, and 43 kJ of expansion work is done by the system. What is ΔH for this process?

Thanks for your help!! :)

Answer: 
Enthalpy and internal energy are related as: H = U + P∙V So their changes are related as ΔH = ΔU + Δ(P∙V) For a constant pressure process: ΔH = ΔU + P∙ΔV Work done by a system is given by the integral W = ∫ P dV form a initial to final volume For constant pressure process this simply fies to: W = P∙ ∫ dV = P∙ΔV Hence, ΔH = ΔU + W = -73 kJ + 43 kJ = 30 kJ

How to determine an expression for the speed of a block on an inclined plane?

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A block of mass M slides from rest down a uniform plane, inclined at an angle theta to the horizontal. The coefficient of kinetic friction between the block and the slope is u. Determinean expression for the speed of the block after it has slid a distance d using:

i) Newton's second law
ii) Work-KE theorem (total work done=change in kinetic energy)

Thanks!

Answer: 
i. Gravitational force down the plane = M g sin(θ) Frictional force down the plane = - M g u cos(θ) using Newtons second law: F = m a M g sin(θ) - M g u cos(θ) = M a a = g sin(θ) - g u cos(θ) constant acceleration, therefore can use v² = u² + 2 a s: v² = 0 + 2 (g sin(θ) - g u cos(θ)) d v = √(2 d g (sin(θ) - u cos(θ)) ii. Work done by gravity = m g h = M g (d sin(θ)) Work done by friction = force*distance = - (M g u cos(θ)) d Total work done = M g d sin(θ) - M g d u cos(θ) Work-KE theorem : total work done = change in KE initial KE = 0.5 M 0² = 0 final KE = 0 + M g d sin(θ) - M g d u cos(θ) = M g d sin(θ) - M g d u cos(θ) 0.5 M v² = M g d sin(θ) - M g d u cos(θ) v² = 2 g d (sin(θ) - u cos(θ)) v = √(2 d g (sin(θ) - u cos(θ))

How to know whether a pinned support reaction has two reactions?

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I'm looking at a simple truss. Say it's been pinned in both positions, one position has a rope attached to it the other is slightly inclined (30 degrees). I know its difficult to imagine, but when should or how do I know whether or not there are less than 2 forces (Fx and Fy) acting on a simple pinned reaction?

Thank you very much

Answer: 
If you are dealing with statics, which I assume you are if you're looking for reaction forces. Always assume that there are forces in the x and y directions. By making the free body diagrams for each part and running a complete analysis of the system, Summing all forces in the x and y directions to zero Summing all moments about a chosen point to zero. the knowing whether Fx = 0 or Fy = 0 will be apparent. If you do enough of these exercises, you will begin to figure out when a force component might be zero just by the geometry of the layout. Until then, do the whole problem or accept a lot of wrong results. Your choice.

How to find total energy of a falling ball when given kinetic and potential energy?

in

I don't think it's just adding the kinetic and the potential energy together, but I am not sure.

Answer: 
Yes, just add them. Assuming the KE and PE are at the same point on the trajectory of the ball. If not, you need to add details.

How to build the quantum model of an atom?

in

For chemistry class, I need to build a quantum model of the Neon atom. I already have the configuration : 1s2 2s2 3p6, but I need to know the materials I need to buy and how I should build it. I appreciate the help :) This atom is pretty easy but a list of materials would be good.

Answer: 
Try this. http://tina.harshbarger.org/photos/atom1.jpg I think the materials they use for this Neon atom model are easy to recognize.

How to find force of friction and the friction coefficient using a graph?

in

If you're given a graph with applied force on then x axis and acceleration on the x axis, how do you work out the force of friction and the friction coefficient?

Answer: 
You no that the net force causes the acceleration, so F - F_friction = m a Isolating a: a = (F - F_fric)/m a = (1/m) * F - F_fric/m So plotting a (along the y-axis) versus F on the x-axis gives a straight linewith slope 1/m and crossing the y-axis at -F_fric/m . So read off the slope, this gives you 1/m, and then the crossing point (which gives you -Ffric/m) so that allows you to calculate F_fric. But you also know that F_fric = mu F_normal = mu m g So mu = F_fric/(m g) = -1/g * (F_fric / m). So mu is -1/g * the y_crossing. This mu is the coefficient of dynamic friction. Actually, if you read of at which F the acceleration is still zero (line crosses the x-axes) you have determined the maximum static friction. Dividing that by mg gives you the coefficient of static friction.

How to prevent an egg from breaking in crash?

in

For a science project we have to fill up a 2 liter bottle with "something" and then put an egg in it. Nothing can be attached to the egg, it can only surround it. Then the bottle will be attached to a cart which will be driven into a wall. What "something" should I put in the 2 liter bottle to keep the egg from breaking?

Answer: 
Jello works really well.

How to find angular acceleration of the wheel?

in

You are holding the axle of a bicycle wheel with radius 23 cm and mass 1.01 kg. You get the wheel spinning at a rate of 57 rpm and then stop it by pressing the tire against the pavement. You notice that it takes 1.91 s for the wheel to come to a complete stop. What is the angular acceleration of the wheel?

Please show step by step so that I can understand the thought process behind this problem.

Answer: 
Initial angular vel = (57/ 60) rev/s x 2π rad/rev .. .. Wi = 5.97 rad/s Final ang vel, Wf = 0, Time = 1.91 s Ang accel* = (Wf - Wi)/ t rad/s² .. .. (0 - 5.97) / 1.91 .. .. .. ►a = - 3.13 rad/s² (deceleration) * angular accel has the same equation form as linear accel .. a = (v-u) / t.

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