by Chris Woodford. Last updated: March 22, 2020.
Cars run on gasoline... and people run on tea and coffee (at least in my house)!
If you drink coffee or tea by the bucket, you'll be glad someone once
had the gumption to invent a super-efficient way of turning cold
water into hot—namely, the electric kettle (also known as
an electric tea kettle). Fill it with water, plug it in, switch it on,
and in a couple of minutes you have piping hot water ready for
drinking or cooking. How exactly does a kettle work? Why does it take
so long to boil? And how does it know when to switch itself off?
Let's take a closer look!
Photo: An electric kettle is a convenient way of making heat energy from electricity. It's a water heater, but it's also an energy conversion device that illustrates one of the most basic laws of physics: the conservation of energy (discussed further below).
What is an electric kettle?
Kettles are among the simplest of household appliances. Lift the lid and peer inside and
you'll see, at the very bottom of the water container, a coil of
thick metal called the heating element. When you plug the kettle into an electrical outlet, a
large electric current flows into the heating element. The element's
resistance (the tendency any material has to stop electricity
flowing through it) turns the electrical energy into heat. In other
words, the element gets hot. Since it's in direct contact with cold water, the heat passes to the water by conduction and rapidly warms
it up too.
Photo: Top: The heating element inside the base of the electric kettle shown in our upper photo. Bottom: In some kettles, the element is concealed from view, under the inside floor, to stop it from "furring up" with limescale. This is a neater design, but it does make for a much noisier kettle.
How long does a kettle take to boil?
You can boil water in all kinds of ways—even in a simple pan on an open fire or stove—though an enclosed kettle is usually much faster: it stops heat escaping, allows the pressure to rise faster
(remember that water boils when its saturated vapor pressure equals atmospheric pressure), and helps the water to boil more quickly. But do you ever get frustrated at how long it takes your kettle to boil? Don't! The amazing thing is that your kettle boils as quickly as it does—and here's
If you keep pumping heat energy into the bottom of a kettle (faster than heat is escaping
through the top and the sides), sooner or later the water inside it
will boil. A basic law of physics called the
conservation of energy tells us that if you need to boil a liter of water,
starting from the same temperature, you'll always have to add the same amount of energy to do it.
Whether you use a camp fire or a kettle, a microwave or some amazing
stirring device in the manner of James Prescott Joule (see box below), the amount of energy you have to put in to boil the water is exactly the same.
Let's say you start with 1 liter (roughly 1 kilogram, 2.2 lbs) of cold water
at about 10°C (50°F) and you want to raise it 90°C to its boiling point
(100°C or 212°F). The amount of energy you need is 4.2 × 1000 grams × 90
degrees = 378,000 joules or 378 kJ.
The mysterious "4.2" is a constant value called the specific heat capacity of water.
Every material has a different specific heat capacity, which is simply the amount of
energy you have to put in to raise the temperature of one gram of the
material by one degree centigrade. You need to add 4.2 joules of
energy to raise the temperature of 1 gram of water by 1°C, so
water's specific heat capacity is 4.2 J/g/°C.
378kJ to boil a liter of water is rather more energy than you might think.
An energy-efficient lamp rated at 10
watts uses 10 joules of energy every second (because 1 watt means using one joule per second),
so it would take it 37,800 seconds—about 10.5 hours—to use
as much energy as our kettle uses in a single boil!
Artwork: Kettles use a lot of energy to boil water, but do the job quickly (in about
2.5 minutes) because they work at high power. With the same amount of energy, you could power a microwave oven for about 8 minutes, a laptop computer for an hour and 20 minutes, or an energy-saving lamp for about 10.5 hours.
If you're using an electric kettle rated at 2400 watts, that means it's consuming 2400
joules of electrical energy per second and putting (roughly) the same
amount of energy into the water as heat each second as well. Divide
378,000 by 2400 and you find it takes the kettle about 160 seconds
to do the job, which sounds about right—an
electric kettle generally does take about 2–3 minutes to boil. An old proverb
says a watched pot (kettle) never boils, but that dates from the time
when most people used to boil water on hideously inefficient open
coal fires. An electric kettle can boil water in just a
couple of minutes because it can add heat
energy to the water much more quickly and efficiently than an open
fire (which allows heat to escape in all directions).
If your kettle were rated at something like 2400 watts (W) and you were using a UK power
supply of 240 volts (V), that means the current passing through the
element would be 2400 / 240 or 10 amps (A). By household standards, that's a hefty current:
in comparison, the little charger I have for my iPod draws a maximum current
of 0.67 amps—the kettle is using 15 times more! So the answer to
how an electric kettle works so quickly is by using a relatively
large electric current. The amount of heat produced is
proportional to the current (a 10 amp current would produce twice as much heat as a 5 amp current
passing through the same heating element if the voltage were constant), so bigger currents
produce more heat—and heat things much more quickly—than smaller ones.
Photo: The concealed heating element from a typical modern kettle, looking from below. The element is sealed in the light gray central part and (if you look closely) you can just see its two terminals poking out on the bottom right. The darker gray rim (the bit my thumb is touching) is a rubbery-plastic gasket that seals the heating element inside the bottom of the kettle and stops water seeping through. The long tube at the top carries steam from inside the kettle down to a thermostat that switches the element off at the right time (as explained below).
How do instant hot water boilers work?
If you're tired of waiting and you want your kettle to boil faster, there are only two things you can do. One is
to use more electric current—in other words, buy a more powerful
kettle; the other use is to use less water.
"Instant" water boilers/dispensers (such as the Breville Hot Cup and the Morphy
Richards Meno), which can boil as little as a cupful of water really
quickly, combine these methods. They use a more powerful heating
element than an ordinary kettle (typically 3000 watts or more) and
they're designed so the element can operate safely in contact with
only a small amount of water. If you're boiling only (say) a
quarter of a liter of water, you need only a quarter as much energy—say 100,000 joules.
And if you're supplying that energy with a 3000 watt element, do the math and you'll find you can do it in about 30 seconds instead of
2.5 minutes. Can you see another great benefit here? If you're
boiling an entire kettle to make just a single hot drink, you're
effectively wasting three quarters of the energy you're consuming.
Boiling only as much water as you need saves you a significant amount
of money—and helps the environment too.
How does a kettle know when to switch off?
Artwork: How an electric jug kettle switches off. There's a steam vent and tube (yellow, 43 and 44) leading down from the top of the water chamber (gray, 38) to a bimetallic thermostat and switch (orange and red, 1 and 2). When the kettle boils, steam whooshes down this tube, heats the thermostat, and makes it flip open, switching off the heating element (green, 39) and stopping the water from boiling any more. Artwork from US Patent 4,357,520: Electric water-boiling container having switch-on dry and stream sensitive thermally responsive control units by John C. Taylor, courtesy of US Patent and Trademark Office.
Early electric kettles came with built-in danger: it was relatively easy to switch them on,
go off and do a chore or two, and then forget about them. If you were
lucky, when you came back a few minutes later, you'd find your
kitchen filled with clouds of steam. If you were unlucky, your kettle
element might burn out, blow a fuse, or even start a fire.
Thankfully, virtually all modern kettles switch themselves off
automatically using thermostats (mechanical, electrical, or
electronic devices that respond to changes in temperature). Many are
based on designs developed by English inventor
John C. Taylor, whose
companies Otter Controls and Strix Ltd have developed more than a
billion thermostats of this kind worldwide.
How do they work? The simplest ones are mechanical and use a bimetallic
thermostat (described in our main article on thermostats) integrated into the
element unit at the bottom of the kettle. It consists of a disc of
two different metals bonded tightly together, one of which expands
faster than the other as the temperature rises. Normally the thermostat is curved
in one direction, but when the hot water reaches boiling point, the
steam produced hits the bimetallic thermostat and makes it suddenly
snap and flex in the opposite direction, a bit like an
umbrella turning inside out in the wind. When the thermostat snaps open, it pushes a lever that trips
the circuit, cuts off the electric current, and safely switches off
the kettle. More sophisticated kettle thermostats (used in systems
such as the fashionable Marco Über coffee boiler) are entirely electronic and allow water to be heated to precise temperatures and maintained there indefinitely by repeatedly switching the current on
Photo: Here's what a typical Strix thermostat-switch looks like in reality. I've used dots the same color as in the artwork above to show the key parts in this old, disassembled kettle. The steam tube (yellow) carries steam down to the bimetallic thermostat. The thermostat (orange) switches the kettle off. A switching unit (red) and several wires connect the thermostat, the power on switch (pink), and the cordless connector (dark blue) to the heating element's two terminals (green). The thermostat and switch are screwed to the bottom of the light gray, concealed heating element (shown in the photo higher up this page).
Photo: A closeup of the bimetallic thermostat (shown by the orange dot in the other photo).
"The Mechanical Equivalent of Heat"
Artwork: The experiment Joule carried out to find the mechanical equivalent of heat.
Electric kettles might seem horribly mundane, but they're worth reading and writing
about because they illustrate brilliantly one of the most
fundamental physical laws of our universe: you can convert one kind of energy into another,
but you can't create energy out of thin air or make it vanish into nothing. This hugely important idea is called the conservation of energy and English physicist James Prescott Joule (1818–1889) was one of the first people to get to the heart of it.
Joule devised a brilliant experiment. He attached a heavy weight (1) to a rope looped over a pulley (2), so as the weight fell, the rope turned an axle (3) and stirred a paddle wheel inside a container full of water (4). He reasoned that the "mechanical" energy he added to the water in this way would turn into heat energy, warming the water up very slightly. After repeated experiments, he successfully proved that the energy (or, as he called it, vis viva) lost by the falling weight was exactly equal to the energy gained by the warming water. In this way, Joule confirmed that mechanical energy
(or work) and heat energy were interchangeable, publishing the result in a famous paper called The Mechanical Equivalent of Heat, which is still considered one of the most important confirmations of the
theory of conservation of energy.
Joule believed he could find evidence to support his ideas in the real world. All he had to do was find
a waterfall and measure the temperature at the top and bottom; the falling water would convert potential
energy into heat, producing a temperature difference that would, he believed, confirm
his theory. According to his calculations, the mighty Niagara Falls would be a fifth of a degree warmer
at the bottom than the top, though measuring that would have been quite hard!
In an attempt to settle the matter, Joule took some thermometers on his honeymoon
to Chamonix, France in 1847 and tried to measure a waterfall there, though he couldn't do it accurately enough
to prove his point.
Find out more
Find out more
On this website
You might like these other articles on our site covering similar topics:
- Burning Desire for Efficiency by Tom Murphy. As I explained above, it takes the same amount of energy to heat a certain quantity of water by the same temperature, however you choose to do it. But some methods are more efficient than others. As Tom Murphy explains in this great blog post, electric kettles are significantly more efficient than stove-top kettles and microwaves.
- Which is more energy efficient—boiling water using an electric kettle, a kettle on a gas hob or a microwave oven?: The Guardian, Notes & Queries, 2011. Guardian readers offer their various opinions on the efficiency of different water-boiling methods.
- Fiddly, Fussy or Just Plain Ugly Kettles by Alice Rawsthorn. The New York Times, August 9, 2009. Why do kettles seem to be so badly designed? This writer wonders about aesthetics, but maybe she would have been better off thinking about how science and engineering constrain the design of a machine that can boil water quickly and efficiently?
If you're interested in real technical nitty gritty, why not take a look at a few of the many
patents covering how kettle-like appliances work? Here are four I've picked out, but you'll
find lots more in the records.
- Fuse by Maurice Lee Warner: A modified fuse to stop electric percolators from boiling dry. US patent 1794045, Feb 24, 1931.
- Electric coffee pot by Ambrose Olds. An electric coffee percolator that maintains your brew at a set temperature. US patent 1998732. April 23, 1935
- Electric water-boiling container having switch-on dry and stream sensitive thermally responsive control units by John C. Taylor. US patent 4,357,520, November 2, 1982.
- Thermally-sensitive control arrangement for containers provided with electric immersion heaters by John C. Taylor et al. US patent 4,621,186. November 4, 1986.