Q: Given a $$1.50\ \mathrm{m}^2$$ window with two $$0.80\ \mathrm{cm}$$ glass panes and a $$1.00\ \mathrm{cm}$$ air gap, inside temperature $$15^\circ \mathrm{C}$$, outside $$-10^\circ \mathrm{C}$$, calculate the rate of heat conduction through the glass only. Use $$k_{glass} = 0.8\ \mathrm{W}/\mathrm{m}\cdot\mathrm{K}.$$

$$Q = \frac{kA\Delta T}{d} = \frac{0.8 \times 1.5 \times 25}{0.016} = 1875\ \mathrm{W}$$

Q: If the rate of heat transfer through a window is $$20.0\ \mathrm{J/s}$$, how much energy is transferred in $$10$$ minutes?

$$E = 20.0 \times 600 = 12,000\ \mathrm{J}$$

Question 1

Which of the following best describes a non-thermal equilibrium relationship between three objects A, B, and C, as discussed in the study materials?

Accepted Answer

A is in thermal equilibrium with B, B is in thermal equilibrium with C, but A is not in thermal equilibrium with C.

Question 2

A person reads the temperature in a freezer in Celsius, omits the negative sign, and then converts the value to Fahrenheit using $$F = C 	imes \frac{9}{5} + 32. $$Oddly, the result matches the correct Fahrenheit temperature. What was the original Celsius reading?

Accepted Answer

$$-40^\circ \mathrm{C}$$

Question 3

Why is the bimetallic strip in a thermometer typically shaped as a spiral?

Accepted Answer

To maximize the length of the strip in a compact space, increasing sensitivity to temperature changes.

Question 4

At what temperature is the length of a bimetallic strip in a thermometer expected to be smallest?

Accepted Answer

At the lowest temperature measured, because both metals contract.

Question 5

Why are two different metals used in a bimetallic strip thermometer instead of just one?

Accepted Answer

Because different metals have different coefficients of thermal expansion, causing the strip to bend with temperature changes.

Question 6

A double-paned window has two glass panes, each $$0.80\ \mathrm{cm}$$ thick, separated by a $$1.00\ \mathrm{cm}$$ air gap. The inside surface temperature is $$15^\circ \mathrm{C}$$, and the outside is $$-10^\circ \mathrm{C}. $$What is the main reason for increased heat transfer in the air gap compared to the glass?

Accepted Answer

Convection currents in the air gap increase heat transfer.

Question 7

Given a $$1.50\ \mathrm{m}^2$$ window with two $$0.80\ \mathrm{cm}$$ glass panes and a $$1.00\ \mathrm{cm}$$ air gap, inside temperature $$15^\circ \mathrm{C}$$, outside $$-10^\circ \mathrm{C}$$, calculate the rate of heat conduction through the glass only. Use $$k_{glass} = 0.8\ \mathrm{W}/\mathrm{m}\cdot\mathrm{K}.$$

Accepted Answer

$$Q = \frac{kA\Delta T}{d} = \frac{0.8 	imes 1.5 	imes 25}{0.016} = 1875\ \mathrm{W}$$

Question 8

A $$70\ \mathrm{kg}$$ ice skater exerts pressure on ice with blades $$0.20\ \mathrm{cm}$$ wide and $$25\ \mathrm{cm}$$ long. If the pressure is $$1.1\ \mathrm{atm}$$ and the temperature is $$-0.5^\circ \mathrm{C}$$, will the ice melt under the skater? Use the phase diagram provided.

Accepted Answer

No, because the pressure is not high enough to lower the melting point below $$-0.5^\circ \mathrm{C}.$$

Question 9

Which equation best describes the rate of heat conduction through a material?

Accepted Answer

$$Q = \frac{kA\Delta T}{d}$$

Question 10

If a window is made of a single $$1.60\ \mathrm{cm}$$ thick glass pane with the same area and temperature difference as the double-paned window, how does the rate of heat conduction compare to the double-paned window?

Accepted Answer

It is lower, because the thicker glass increases resistance to heat flow.

Question 11

A bimetallic strip bends when heated because:

Accepted Answer

The two metals expand at different rates, causing the strip to curve.

Question 12

If the rate of heat transfer through a window is $$20.0\ \mathrm{J/s}$$, how much energy is transferred in $$10$$ minutes?

Accepted Answer

$$E = 20.0 	imes 600 = 12,000\ \mathrm{J}$$

Question 13

Which of the following is a plausible reason for using a bimetallic strip in a thermometer instead of a liquid-in-glass design?

Accepted Answer

Bimetallic strips are more durable and can be used in rugged environments.

Question 14

If the temperature difference across a window doubles, what happens to the rate of heat conduction?

Accepted Answer

It doubles, because $$Q \propto \Delta T.$$

Question 15

A student claims that the pressure exerted by an ice skater is enough to melt ice at $$-5^\circ \mathrm{C}. $$Using the phase diagram, is this claim correct?

Accepted Answer

No, because the melting point of ice cannot be lowered to $$-5^\circ \mathrm{C} by $$pressure alone.

Question 16

Which physical property is most important for the function of a bimetallic strip thermometer?

Accepted Answer

Coefficient of thermal expansion

Question 17

If the area of a window doubles but the thickness remains the same, how does the rate of heat conduction change?

Accepted Answer

It doubles, because $$Q \propto A.$$

Question 18

Which of the following best explains why the bimetallic strip bends toward one metal when heated?

Accepted Answer

The metal with the higher coefficient of thermal expansion expands more, causing the strip to bend toward the metal with the lower coefficient.

Thermal Physics: Temperature, Heat Transfer, and Phase Changes – Study Notes

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