# A cell phone charger uses 4.83 joules per second when plugged into an outlet, but only 1.31 joules per second actually goes into the cell phone battery. The remaining joules are lost as heat. That’s why the battery feels warm after it has been charging for a while. How efficient is the charger?

Answer: Well, that depends on how you look at it.

Efficiency =

(amount of power that does what you want) / (total power supplied).

Power to charge battery . . . 1.31 watts
Power changed to heat . . .  3.52 watts
Total power supplied . . . . . . 4.83 watts

Using the cell-phone as a heater,
efficiency of the charger in supplying heat = 3.52 / 4.83 =  72.9 % .

Using the cell-phone as cell-phone,
efficiency of the charger in charging the battery = 1.31 / 4.83 =  27.1 % .

The charger is more efficient as a heater than it is as a battery charger.

## Related Questions

A car or truck moving down the highway has what energy by virtue of its motion

Kinetic energy because it is moving

Which of the following electromagnetic waves is not used for communication? A gamma rays
B microwaves
C infrared waves

Microwaves and radio waves are employed in radio and satellite communications while infrared waves are used in remote controls and infrared features of new phones and other electrons. However, gamma rays have far too much energy and cause damage to the body. They are not used in communication.

The equation r (t )=(2t + 4)⋅i + (√ 7 )t⋅ j + 3t ²⋅k the position of a particle in space at time t. Find the angle between the velocity and acceleration vectors at time t equals 0.

Explanation:

We should first find the velocity and acceleration functions. The velocity function is the derivative of the position function with respect to time, and the acceleration function is the derivative of the velocity function with respect to time.

Similarly,

Now, the angle between velocity and acceleration vectors can be found.

The angle between any two vectors can be found by scalar product of them:

So,

At time t = 0, this equation becomes

awhite billiard ball with mass mw = 1.47 kg is moving directly to the right with a speed of v = 3.01 m/s and collides elastically with a black billiard ball with the same mass mb = 1.47 kg that is initially at rest. The two collide elastically and the white ball ends up moving at an angle above the horizontal of θw = 68° and the black ball ends up moving at an angle below the horizontal of θb = 22°.

speed of white ball is 1.13 m/s and speed of black ball is 2.78 m/s

initial kinetic energy = final kinetic energy

Explanation:

Since there is no external force on the system of two balls so here total momentum of two balls initially must be equal to the total momentum of two balls after collision

So we will have

momentum conservation along x direction

now plug in all values in it

so we have

similarly in Y direction we have

now plug in all values in it

so we have

now from 1st equation we have

so speed of white ball is 1.13 m/s and speed of black ball is 2.78 m/s

Also we know that since this is an elastic collision so here kinetic energy is always conserved to

initial kinetic energy = final kinetic energy