Warming an iron(Exposition)

A simple view of an iron

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Many domestic appliances switch from an electrical pathway to warm things up (and even cool them down as well). The heating by particles pathway and the heating by radiation pathway are both used, depending on what needs to be warmed.

Consider which of these pathways might be used by the following:

• An electric bar heater
• A night storage radiator
• An electric hob
• A toaster
• Imagine using an iron. Again, there are two theoretical perspectives: the engineer's detailed view and the physicist's energy view. These are less distinct than in the case of the motor but they are still separate. The engineer will concentrate on the resistive element that provides the heating: on its dimensions (both cross-sectional area, and length), on how effectively the plate next to it is warmed, and on how to insulate the operator from this warming. The pathways view is much simpler, at least for the perfect iron: all of the power in the electrical pathway is switched to the heating by particles pathway.

Perfect switching–and less than perfect switching

Real-world irons don't only warm clothes: the surroundings are also warmed, so you might model the process by including two heating by particles pathways, one filling the intended thermal store of energy (perhaps associated with a blouse warming up), the other filling thermal stores associated with the surroundings.

The better the design of the iron, the more power in the pathway directed to the energy store associated with the blouse.

How to calculate accumulations in stores as a result of warming

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The power in the electrical pathway (set by the current and the potential difference, as ever) and the time both set the accumulation. As the iron is a mains appliance, the current will in practice be set by choosing the resistance of the element in the iron.

The energy accumulating in the thermal store is set by the power in the heating by particles pathway and the time. But there is no simple way of finding the power in this pathway from measurements made with commonly available laboratory instruments, so watch the energy accumulating in the store directly, as the temperature rises (more on this in the SPT: Energy topic).

The number of joule kelvin-1  × temperature rise in kelvin gives this accumulation. The more mass there is, the greater the number of joule kelvin-1, so we often factor this out, writing

mass  × a constant for the material.

So the energy accumulated in this store, as a result of the temperature rise can be calulated by mass  × constant  × temperature rise.

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