Yearly Archives: 2012

The grapefruit juice effect

Mixing medication and grapefruit juice isn’t something that jumps to mind as being dangerous, but depending on the drug involved, it can be lethal.

Grapefruit contains bergamottin, dihydroxybergamottin, bergapten and bergaptol, chemicals that act as a natural defence mechanism against pests. These chemicals, part of a group of chemicals called furanocoumarins, interfere with an enzyme called CYP3A4 found in the liver and intestine. This enzyme plays a critical role in how the body metabolises xenobiotics, chemicals that are not naturally occurring in the body and which aren’t normally found there.

The action of grapefruit juice on CYP3A4 affects a large variety of drugs, including sedatives, painkillers, immunosuppresants, statins and artificial hormones. The effect of grapefruit juice can be either an overdose or an insufficient dose, depending on the action of the CYP3A4 enzyme on the particular drug.

The discovery that grapefruit juice could interact with drugs was made by mistake when researchers investigating how alcohol interacted with a drug called felodipine used grapefruit juice to mask the taste of the alcohol. Their results did not tie in with other trials involving felodipine and led to their suggestion that this was “possibly due to a pharmacokinetic interaction with the grapefruit juice vehicle”.1 Luckily for those of us that enjoy grapefruit, scientists at the University of Florida are already working on breeding a low-furanocoumarin grapefruit.2

1 D.B. Bailey et al, “Ethanol enhances the hemodynamic effects of felodipine”, Clinical and Investigative Medicine 12(6) (1989): 357-362. PMID: 2612087.

2 Chunxian Chen et al, “Characterization of Furanocoumarin Profile and Inheritance Toward Selection of Low Furanocoumarin Seedless Grapefruit Cultivars”, Journal of the American Society for Horticultural Science 136(5) (2011): 358-363. Link.

Specific heat, latent heat and scalds

Why is being scalded by boiling steam so much worse for you than being scalded or burnt by a liquid or a solid at the same temperature?

The specific heat capacity of a material measures how much energy is required to change the temperature of that material. The specific heat capacity of water is 4180 joules per kilogram per kelvin, meaning that it requires 4180 joules of energy to raise the temperature of one kilogram of water by one kelvin.

The latent heat of a material is the energy required to change the state of a material without changing the material’s temperature. There are therefore two latent heats: the latent heat of fusion is the energy required to turn a solid to a liquid or vice versa, and the latent heat of vaporisation is the energy required to turn a liquid to a gas or vice versa. For water the latent heat of fusion is 334 000 joules per kilogram and the latent heat of vaporisation is 2 260 000 J/kg.

If a one gram drop of boiling water (at 100°C) falls on skin at a temperature of 35°C then the temperature of the water quickly falls by 65°C. To drop the temperature of one gram of water by 65°C requires a change in energy of 272 joules. Because heat always flows from a hotter body to a colder one* this heat flows into the skin, damaging skin cells as it does.

The situation is different if one gram of boiling steam (still at 100°C) hits the skin. First it has to change state into water, and then cool down just as above. In the process of changing state from a gas at 100°C to water at 100°C it releases a huge amount of energy: an additional 2260 joules when compared with the 272 joules released as it cools. If we assume that the severity of the scald is proportional to the energy released (which is a very reasonable assumption) then a scald with boiling steam does 931% of the damage that a scald with boiling water does.

The graph above shows how the temperature of a 1kg block of ice at −100°C changes as energy is supplied to it. The horizontal sections occur when energy is being absorbed but the temperature of the substance is not changing; this is because the energy is being used to weaken bonds between molecules as the state changes first from solid to liquid and then from liquid to gas. The longer horizontal section in the liquid-gas state change indicates that more energy is required to turn water into steam than is required to turn ice into water. This is reflective of the relative strengths of the intermolecular bonds in solids, liquids and gases. The differing gradients of the sloped sections reflects the fact that the specific heat capacity of water varies with state.†

* More accurately, the net flow of heat is always from a hotter body to a colder one.

† For the sake of simplicity, the specific heat capacity of water in each state has been assumed not to vary.

Why using a number twice in your PIN might be a good idea

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Modern smartphones use large glass touchscreen panels that show the presence of grease from hands and faces very easily. Should your phone be stolen it could be possible for the thief to discern the PIN required to unlock it by analysing these grease patterns.

If your pin was “1234” then the thief would only have to try all 24 permutations in order to guarantee being able to unlock the phone:

  • {1,2,3,4} {1,2,4,3} {1,3,2,4} {1,3,4,2} {1,4,2,3} {1,4,3,2} {2,1,3,4} {2,1,4,3} {2,3,1,4} {2,3,4,1} {2,4,1,3} {2,4,3,1} {3,1,2,4} {3,1,4,2} {3,2,1,4} {3,2,4,1} {3,4,1,2} {3,4,2,1} {4,1,2,3} {4,1,3,2} {4,2,1,3} {4,2,3,1} {4,3,1,2} {4,3,2,1}

But if you had chosen “1233” then the thief would not know which number had been used twice, and would have more permutations to check:

  • {1,2,3,3} {1,3,2,3} {1,3,3,2} {2,1,3,3} {2,3,1,3} {2,3,3,1} {3,1,2,3} {3,1,3,2} {3,2,1,3} {3,2,3,1} {3,3,1,2} {3,3,2,1}
  • {1,2,2,3} {1,2,3,2} {1,3,2,2} {2,1,2,3} {2,1,3,2} {2,2,1,3} {2,2,3,1} {2,3,1,2} {2,3,2,1} {3,1,2,2} {3,2,1,2} {3,2,2,1}
  • {1,1,2,3} {1,1,3,2} {1,2,1,3} {1,2,3,1} {1,3,1,2} {1,3,2,1} {2,1,1,3} {2,1,3,1} {2,3,1,1} {3,1,1,2} {3,1,2,1} {3,2,1,1}

By choosing a PIN with a repeating digit you have made it 50% harder for the thief. It is also possible that the thief might not realise that a digit had been repeated, and then have to guess at the fourth digit, which would make life much harder. If this idea is extended to a 5-digit PIN then the increase in difficulty becomes 100% – it is twice as difficult with a repeated digit as without one. For every digit added it becomes fifty percentage points more difficult.

Demonstrating refractive index

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The refractive index of a material governs how much light bends as the light moves into it. You’ve probably seen this bending effect when looking at the surface of a swimming pool: the bottom of the pool looks closer to the surface than it actually is because light rays bend as they travel from water to air.

But if the refractive index of two materials is the same, as is the case for sunflower oil and Pyrex, then light doesn’t bend at all, and you end up with the nice effect demonstrated below.

To say that this demonstration impressed my pupils would be an understatement.

My favourite photograph from the 2012 Olympics

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During the 2012 Olympics, the underwater cameras in the swimming pool have been tweeting regularly. On Sunday, the PoolCam sent out my favourite image of the whole London 2012 Olympic Games: a magnificent demonstration of total internal reflection.

Light refracts as it travels from one medium to another. Total internal reflection occurs when light travels from a medium with a high refractive index to one with a low refractive index at an angle above the critical angle for those two media.

In the photograph above you can see out of the pool at the top of the image because the angle of incidence is less than the critical angle. Beyond the critical angle, light is totally internally reflected and the bottom of the pool is reflected back towards the camera. Because the angle is the same in all directions this creates a semicircle, which is visible at the top of the image.

If the camera is on the bottom of the pool looking up you therefore see a perfect “see-through” circle looking at the roof, surrounded by water reflecting the bottom of the pool. This effect can be seen in a previous @L2012PoolCam photograph: