Why do some appliances consume more power than others?

By MUNGAI KIHANYA

The Sunday Nation

Nairobi,

07 September 2008

Perhaps because of the recent hikes in power charges, Edwin Nduhiu, has been wondering about electromagnets. He says, “Someone told me that an electromagnet (for example the ones used to lift those heavy metallic boxes at the coast) ONLY uses up electricity because of the resistance in its coils, and not because of the magnetic force it produces. Is this true?”

Now, Edwin, suppose you plug an electromagnet (or any other device, for that matter) into an electrical socket but you don’t switch it on. How much power will it consume? Zero, of course! Next question: what will be the resistance in this electrical connection?

It is not zero! Resistance, as the word implies, is the ability of a connection (what the technicians call a circuit) to resist the flow of electricity (or power, for that matter). Thus if no electricity is able to flow (zero power consumption), then it means that the resistance is extremely high – it approaches infinity.

Clearly then, the higher the resistance of an appliance, the lower the power it will consume, and vice versa. Thus it is wrong to say that an electromagnet consumes electricity “because of the resistance …” That resistance actually does the reverse – it reduces the amount of power consumed.

Now, the coils of the electromagnet are made from a very good electrical conductor – usually high purity copper wires. Thus the coil has very little resistance… but that is before the power is switched on.

When powered, the presence of the magnetic force slows down the flow of electricity through the coil. This makes the coiled wires consume less electricity than they would if they were straight.

***

“Why does a person sink when drowning but floats to the surface when dead?” that is the straightforward question from Martin Kamanda. The answer is equally straightforward: a live person is slightly denser that water, but after death the body’s density decreases. The question is; why?

The process of decomposition produces a lot of gasses that get trapped inside the body and causes it to swell (and smell!). The swelling increases the volume occupied by the dead body thereby reducing its density.

This is why the body floats after a few days. This process can be demonstrated using drinking straws as follows:

Pour some carbonated drink in a glass and then put a straw inside it. You will notice that the straw will rise up in a few seconds and fall of the glass.

The reason for this is that carbon dioxide bubbles form on the surface of the straw. This reduces the density of the straw causing it to float.

Go on; try it out – it’s a good reason to buy a drink!