When do we use I=P/a or I=P/4pi(r)^2?

for example question 1:

30 seconds of exposure to 115 dB sound can damage your hearing, but a much quieter 94 dB may begin to cause damage after 1 hour of continuous exposure.You are going to an outdoor concert, and you'll be standing near a speaker that emits 50W of acoustic power as a spherical wave. What minimum distance should you be from the speaker to keep the sound intensity level below 94 dB?

- we used I=P/4pir^2

Question 2:

A woman wearing an in-ear hearing aid listens to a television set at a normal volume of approximately 60 dB. To hear it, she requires an amplification of 30 dB, so the hearing aid supplies sound at 90 dB to the ear canal, which we assume to be circular with a diameter of 7 mm. What is the output power of the hearing aid?

why did we use I=P/a not I=P/4pir^2?

A kettle is filled with tap water and placed on a stove. The stove element is turned on. Explain why the water starts to boil after a while.

Derive the Maxwell construction by considerations involving the minimization of the Gibbs potential instead of the Helmholtz free energy.

Please help me...

12. How is constructed the deformation tensor?

Start with an ideal gas of volume 4m^3, at a pressure of 1200Pa, and temperature 289K. Construct a cycle using 3 or more processes (of your choice). For each process in your cycle calculate the Q, ?U, and W. Calculate the net change in each of these quantities for the entire cycle, and determine whether the work as a result of the cycle is done ON or BY the system.

You are pushing against the ground as you begin to leap into the air (but you are still in contact with the ground). (a) Are you accelerating as you begin your leap? Choose one answer only. No, you are not accelerating. Yes, you are accelerating upward. Yes, you are accelerating downward. Incorrect: Your answer is incorrect. (b) What is the net force acting on you as you begin your leap? Choose one answer only. The net force acting on you is zero. The net force acting on you is upward. The net force acting on you is downward. Incorrect: Your answer is incorrect. (c) Compare the normal force exerted on you by the ground to the gravitational force exerted on you by the earth (your weight). Choose one answer only. The normal force from the ground is smaller than your weight. The normal force from the ground is greater than your weight. The normal force from the ground is equal to your weight. Incorrect: Your answer is incorrect.

A small city requires about 10 MW of power. Suppose that instead of using high-voltage lines to supply the power, the power is delivered at 120 V.

Assuming a two-wire line of 0.48?cm -diameter copper wire, estimate the cost of the energy lost to heat per hour per meter. Assume the cost of electricity is about 10 cents per kWh.

My son has been wondering about this question: when we freeze milk it changes colour � from white to yellow. Why is this?

In the Olympic shot-put event, an athlete throws the shot with an initial speed of 13.6 m/sm/s at a 40.0 ∘∘ angle from the horizontal. The shot leaves her hand at a height of 1.80 mm above the ground. (Figure 1)

1. How far does the shot travel horizontally before striking the ground when shot with initial angle 40.0 ∘∘ ?

2. Repeat the calculation of the first part for angle 42.5 ∘

3. Repeat the calculation of the first part for angle 45.0 ∘

4. Repeat the calculation of the first part for angle 47.5 ∘

5. With what angle of release does she throw the farthest?

A point charge of -1.5 µC is located at the origin. A second point charge of 10 µC is at x = 1 m, y = 0.5 m. Find the x and y coordinates of the position at which an electron would be in equilibrium.

1.) Your friend is catching a falling basketball after it has passed through the basket. Her hands move straight down while catching the ball. Take that it takes about 0.10 s for the player to lower her hands to stop the ball. Assume the mass of the ball to be 0.60 kg, and that it has fallen a vertical distance of 1.2 m before reaching the player's hand. Determine the average force that her hands exert on the ball while catching it.

2.) On a frictionless horizontal air table, puck A (with mass 0.248kg ) is moving toward puck B (with mass 0.372kg ), which is initially at rest. After the collision, puck A has velocity 0.125m/s to the left, and puck B has velocity 0.653m/s to the right.

What was the speed vAi of puck A before the collision?

Calculate ?K, the change in the total kinetic energy of the system that occurs during the collision.

If the radius of the equipotential surface of point charge is 15.2 m at a potential of 2.30 kV, what is the magnitude of the point charge creating the potential?

Q.1) Projectile motion:A stone is thrown vertically into the air off the edge of a bridge 150ft above a river. If the initial velocity of the rock is 64ft/sec (upward0 and the accleration is constant -32ft/sec² (downward), find the following:

a. the velocity v(t) after t seconds.

b. The position s(t) of the stone above the river after t seconds.

c. The velocity at which the stone strikes the water.

1. The electric field generated by a point charge has strength of 180000n/c at a point 1.50 cm from the point charge what is the charge if the field points towards the charge?

2.An electron was accelerated from rest through a potential difference of 9900V. what is the speed at the end?

3.A 100.0pF capacitor consists of two circular plates of radius 0.40mm. how far apart are the plates?

4. A 4.0m m length of gold wire is connected to a 1.5 V battery, and a current of 4.0mA flows through it. What is the diameter of the wire? (the resistivity of gold is 2.24x10^-8)

5.Two parallel plates that are initially uncharged are separated by 1.2mm. What charge must be transferred from one plate to the other if 11.0kj of energy are to be stored in the plates, if area of each plate is 19.0mm?

Two small metallic spheres, each of mass m = 0.210 g, are suspended as pendulums by light strings of length L as shown in the figure below. The spheres are given the same electric charge of 6.8 nC, and they come to equilibrium when each string is at an angle of ? = 4.85