## Problem 7.31- Energy Per Photon & Amount of Energy Needed to

Hannah Pablo 2H
Posts: 10
Joined: Fri Sep 26, 2014 2:02 pm

### Problem 7.31- Energy Per Photon & Amount of Energy Needed to

Problem:
In a microwave oven, radiation is absorbed by water in the food and the food is heated. How many photons of wavelength 4.50 mm are required to heat 350. g of water from 25.0°C to 100.0°C, assuming all the energy is used to raise the temperature?

How do I figure out the energy needed to heat water?

Erika Monasch 4I
Posts: 5
Joined: Fri Sep 26, 2014 2:02 pm

### Re: Problem 7.31- Energy Per Photon & Amount of Energy Neede

You would use the equation q=mCdeltaT to find the heat required to heat the water. For the equation, C is 4.184 J/(degrees C*g), which is the specific heat capacity of liquid water. Did that help?

Niharika Reddy 1D
Posts: 127
Joined: Fri Sep 26, 2014 2:02 pm

### Re: Problem 7.31- Energy Per Photon & Amount of Energy Neede

The energy required to heat the water can be found by using the equation q=mCΔT. The mass is 350.g, the specific heat of liquid water is 4.184 J/(g°C), and ΔT is 100.0°C-25.0°C=75.0°C. Multiplying these values will give the energy required to heat the water: 1.098x10^5 J.

I have attached a picture of my work. I carried multiple decimal places and just accounted for significant figures at the very end for accuracy. Hope this helps!
Attachments
7.31

lilyhui3I
Posts: 8
Joined: Tue Nov 25, 2014 3:00 am

### Re: Problem 7.31- Energy Per Photon & Amount of Energy Neede

Where did the equation E=(hc)/ λ come from. I assumed it to be KE=hv-ϕ where KE=0 and v=c/λ. Is that correct?

Maria Davila 2I
Posts: 12
Joined: Fri Sep 26, 2014 2:02 pm

### Re: Problem 7.31- Energy Per Photon & Amount of Energy Neede

When you are finding the energy of a photon you make use of two equations:
$c=\lambda *v$ and $E_p_h_o_t_o_n = hv$
you then substitute v (in the second equation) for $\frac{c}{\lambda }$
which another way of writing equation 1. Now you can use the wavelength which you will have to turn into meters to cancel out the meter unit in 'c' - speed of light constant.

Hope this helps in understanding why we use these two equations.