The vacuum flask
A vacuum flask can keep hot liquids hot and cold liquids cold. To keep water hot the flask must stop heat loss by conduction, convection and radiation.
A vacuum flask has the following features:
- a double walled glass bottle;
- a vacuum between the glass walls;
- both glass walls facing the vacuum are silvered;
- a plastic stopper filled with cork or foam;
- cork or sponge packaging and a plastic case to protect the glass bottle.
| Feature | Job |
| Vacuum between the two glass walls. | Prevents conduction and convection through the sides. |
| Silvered surfaces. | Minimises heat transfer by radiation. |
| Plastic cap. | Minimises conduction and convection through the top of the flask. |
| Outer plastic case and sponge/cork filling. | Protects the fragile glass bottle from becoming damaged by knocks and bumps. |
| Feature | Vacuum between the two glass walls. |
|---|---|
| Job | Prevents conduction and convection through the sides. |
| Feature | Silvered surfaces. |
|---|---|
| Job | Minimises heat transfer by radiation. |
| Feature | Plastic cap. |
|---|---|
| Job | Minimises conduction and convection through the top of the flask. |
| Feature | Outer plastic case and sponge/cork filling. |
|---|---|
| Job | Protects the fragile glass bottle from becoming damaged by knocks and bumps. |
| Feature | Conduction | Convection | Radiation |
| Vacuum. | \(\checkmark\) | \(\checkmark\) | \(\text{X}\) |
| Silvered surfaces. | \(\text{X}\) | \(\text{X}\) | \(\checkmark\) |
| Plastic cap. | \(\checkmark\) | \(\checkmark\) | \(\text{X}\) |
| Feature | Vacuum. |
|---|---|
| Conduction | \(\checkmark\) |
| Convection | \(\checkmark\) |
| Radiation | \(\text{X}\) |
| Feature | Silvered surfaces. |
|---|---|
| Conduction | \(\text{X}\) |
| Convection | \(\text{X}\) |
| Radiation | \(\checkmark\) |
| Feature | Plastic cap. |
|---|---|
| Conduction | \(\checkmark\) |
| Convection | \(\checkmark\) |
| Radiation | \(\text{X}\) |
