Here we are doing a follow up on $textbf{Example 21.10.}$ and calculate how much more energy is dissipated if the voltage difference is increased to the 15 V.

Since voltage difference is proportional to the power and power to the energy, we expect an increase in the dissipated energy. For calculation we follow the same procedure, as in the Example:

We calculate the power:

$$
P=frac{V^{2}}{R} =frac{(15 mathrm{~V})^{2}}{570 Omega}
$$

that gives:

$$
P = 0.395 mathrm{W}
$$

And to have dissipated energy in 65 s, we have:

$$
Delta E=P Delta t
$$

$$
Delta E=0.395 mathrm{W} cdot 65 mathrm{s}
$$

$$
boxed{color{#c34632}Delta E=25.86 mathrm{J}}
$$

$$
Delta E=25.86 mathrm{J}
$$

Here we calculate the power usage of the CD player. When we know the current and voltage, we simply calculate the power with the relation:

$$
P = UI
$$

We know both pieces of information from the problem, we just take into account that current is given in mA, and we should use A. So, we put in the numbers:

$$
P = 4.1 mathrm{V} cdot 22 times 10^{-3} mathrm{A}
$$

which gives the result of:

$$
boxed{color{#c34632}P = 0.0902 mathrm{W}}
$$

$$
P = 0.0902 mathrm{W}
$$

In this problem, we calculate the resistance of the electric heater, if we know that power dissipated is 120 W and the heater is connected to the 120 V outlet.

We use the following relation for the power in the electric circuit:

$$
R=frac{V^{2}}{P}
$$

Putting in the numbers we have:

$$
R=frac{{(120 mathrm{V}})^{2}}{120 mathrm{W}}
$$

The square in numerator and number in denominator do cancel out, so the result is:

$$
boxed{color{#c34632}R= 120 mathrm{Omega}}
$$

$$
R= 120 mathrm{Omega}
$$