A medical freezer is used to keep vaccines cool. (a) (i) Calculate the energy required by the freezer to cool 0.50 kg of solid vaccines from -5 °C to -35 °C. Assume the specific heat capacity of the vaccines is 1900 J/kg °C. Use the Equation Sheet. Energy = ................................ (ii) It takes 5 minutes for the freezer to cool the vaccines. Calculate the power of the freezer. Use the Equation Sheet. Power = ................................. W (iii) Suggest two reasons why the actual power of the freezer will be greater than that calculated in part (a)(ii). 1 2 (b) (i) When the vaccine is used by doctors, it has to be changed back into a liquid. The temperature of the vaccine is first raised to its melting point but it remains as a solid. Explain two reasons why more energy is needed to change the solid vaccine into a liquid at its melting point. 1 2 (ii) Calculate the number of 5 mg vaccine doses which can be melted using 6800 J of energy. Assume the specific latent heat of fusion for the vaccines is 340,000 J/kg. Use the Equation Sheet. Number of vaccine doses = ..............................

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A medical freezer is used to keep vaccines cool - OCR Gateway - GCSE Physics - Question 23 - 2023 - Paper 3

Question 23

A medical freezer is used to keep vaccines cool. (a) (i) Calculate the energy required by the freezer to cool 0.50 kg of solid vaccines from -5 °C to -35 °C. Assume... show full transcript

Worked Solution & Example Answer:A medical freezer is used to keep vaccines cool - OCR Gateway - GCSE Physics - Question 23 - 2023 - Paper 3

Step 1

(a) (i) Calculate the energy required by the freezer to cool 0.50 kg of solid vaccines from -5 °C to -35 °C.

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Answer

To calculate the energy required, we use the formula:

$Q = mc heta$

Where:

$Q$ = energy (in joules)
$m$ = mass (0.50 kg)
$c$ = specific heat capacity (1900 J/kg °C)
$heta$ = change in temperature = final temperature - initial temperature = (-35 °C) - (-5 °C) = -30 °C.

Substituting the values, we have:

$Q = 0.50 imes 1900 imes (-30)$ $Q = 0.50 imes 1900 imes 30 = 28500 ext{ J}$

Therefore, the energy required is 28500 J.

Step 2

(a) (ii) Calculate the power of the freezer.

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Answer

Power can be calculated using the formula:

$P = \frac{E}{t}$

Where:

$P$ = power (in watts)
$E$ = energy (28500 J)
$t$ = time (in seconds). Since 5 minutes = 5 × 60 = 300 seconds, we have:

$P = \frac{28500}{300} = 95 ext{ W}$

Thus, the power of the freezer is 95 W.

Step 3

(a) (iii) Suggest two reasons why the actual power of the freezer will be greater than that calculated in part (a)(ii).

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Answer

The freezer may have heat losses to the environment, which requires additional power to maintain the cooling.
The initial temperature of the vaccines may be higher than -5 °C, leading to greater energy requirements than calculated.

Step 4

(b) (i) Explain two reasons why more energy is needed to change the solid vaccine into a liquid at its melting point.

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Answer

At the melting point, the vaccine particles need to overcome the intermolecular forces holding them in a solid structure, requiring additional energy.
The energy is also needed to facilitate the phase change from solid to liquid, which is not accounted for by temperature change alone.

Step 5

(b) (ii) Calculate the number of 5 mg vaccine doses which can be melted using 6800 J of energy.

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Answer

To find the number of doses, we use the latent heat formula:

$Q = mL$

Where:

$Q$ = energy (6800 J)
$m$ = mass of the vaccine
$L$ = specific latent heat of fusion (340000 J/kg).

Rearranging gives:

$m = \frac{Q}{L} = \frac{6800}{340000} = 0.02 ext{ kg}$

Since 1 dose = 5 mg = 0.005 kg,

$ext{Number of doses} = \frac{0.02}{0.005} = 4$

Thus, 4 doses of 5 mg can be melted using 6800 J of energy.

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A medical freezer is used to keep vaccines cool - OCR Gateway - GCSE Physics - Question 23 - 2023 - Paper 3

Worked Solution & Example Answer:A medical freezer is used to keep vaccines cool - OCR Gateway - GCSE Physics - Question 23 - 2023 - Paper 3

(a) (i) Calculate the energy required by the freezer to cool 0.50 kg of solid vaccines from -5 °C to -35 °C.

(a) (ii) Calculate the power of the freezer.

(a) (iii) Suggest two reasons why the actual power of the freezer will be greater than that calculated in part (a)(ii).

(b) (i) Explain two reasons why more energy is needed to change the solid vaccine into a liquid at its melting point.

(b) (ii) Calculate the number of 5 mg vaccine doses which can be melted using 6800 J of energy.

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