The simplest household appliance is the kettle. 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, which is the tendency of any material to prevent electricity from flowing through it, turns the electrical energy into heat.
The element becomes hot. Because it is in direct contact with cold waters, heat conduction occurs and the element quickly warms up.
The heating element in the base of an electric kettle. Bottom: Some kettles have the heating element hidden from view under the floor to prevent limescale from building up. Although it is a more elegant design, it makes for a louder kettle.
What time does it take for a kettle 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 it saturated vapor pressure equals atmospheric pressure), and helps the water to boil more quickly.
Do you ever find yourself frustrated by the time it takes for your kettle to boil? Do not get frustrated by how long it takes for your kettle to boil. It’s amazing that the kettle boils so quickly. Here’s why.
Keep pumping heat into the kettle’s bottom (faster than heat escapes through the sides and top), the water will eventually boil. The conservation of energy is a fundamental law of physics that states that to boil a liter (at the same temperature) of water you must add the same amount of energy.
The amount of energy required to boil water, regardless of whether you are using a stove or a kettle, or a microwave (see box below), is the same.
Let’s suppose you have 1 liter (roughly 1 kg, 2.2 lbs), of cold water at 10degC (50degF), and that you want to heat it to 90degC (100degC) to reach its boiling point (100degC/212degF). You need 4.2×1000 grams x 90° = 378,000 joules, or 378 kJ.
The constant value of the specific heat capacity of water, or “4.2”, is what you are looking for. Each material has a specific heat capacity. This is the amount of energy required to raise one gram of material’s temperature by one degree Celsius.
To raise 1 gram of water to 1 degC, you need to add 4.2 Joules of energy. Water’s specific heat capability is 4.2 J/g/degC.
It takes 378kJ for a liter to boil water. This is a lot 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: While kettles take a lot of energy in order to boil water, they do the job quickly (in 2.5 minutes), because they are high-powered. You could run a microwave oven for 8 minutes, a laptop for 1 hour, and a lamp that saves energy for around 10.5 hours with the same amount of power.
An electric kettle with a rating of 2400 Watts will consume 2400 joules per second. It also puts roughly the same amount into the water and heat as a 2400 watt kettle.
Divide 378,000 times 2400 to find that the kettle takes about 160 seconds to complete the task. An electric kettle usually boils in about 2-3 minutes. A watchful pot (or kettle) is said to never boil, however, this proverb dates back to the days when water was heated on inefficient open fires. An electric kettle can boil water in a matter of minutes, as it can heat the water more efficiently than an open fire.
If your kettle was rated at around 2400 watts (W), and you used a UK power supply with 240 volts, (V), the current flowing through the element would be 2400/240 or 10amps (A).
This is a huge current by household standards. The current flowing through the kettle would be 2400 / 240 or 10 amps (A) compared to my little charger for my iPod which draws 0.67 amps.
The kettle uses 15 times as much! The answer to the question of how electric kettles work so fast is that they use a large amount of electric current. Because the amount of heat produced is proportional with the current squared (which means that larger currents will produce more heat and heat things more quickly than smaller ones),
Photo: Looking down, the hidden heating element of a modern kettle. The light gray central portion seals the element and you can see the two terminals on the bottom right.
The rubbery-plastic gasket on the darker gray rim (the part my thumb touches) seals the heating element within the kettle’s bottom and prevents water from seeping in.
The tube at the top transports steam from the kettle to the thermostat, which switches off the element at the correct time. (See explanation below).
How do instant hot water boilers function?
There are two options if you don’t want to wait and want your kettle boiling faster. The first is to use more electricity or to buy a stronger kettle. The second is to use less water.
Combining these methods creates “instant” water boilers/dispensers, such as the Breville Hot Cup or the Morphy Richards Meno. They can boil water as quickly as one cupful.
These kettles use a stronger heating element than an average kettle (3000 watts or greater) and are designed to work safely with a very small amount of water.
You only need a quarter the amount of energy to boil (say, 25%) a liter of water. That’s about 100,000 joules. If you supply that energy with a 3000-watt element, you will find it takes only 30 seconds rather than 2.5 minutes.
Another great benefit is this: To boil a whole kettle of water to make a single hot beverage, you are effectively wasting three-quarters of the energy that you consume. Boiling only as much water as you need saves you a significant amount of money–and helps the environment too.
How does a kettle tell when it is time to turn off?
Artwork: How an electrical jug kettle turns off. The steam vent and tube (yellow 43 and 44) lead down from the top (gray 38) to the bimetallic thermostat (orange, red, 1, and 2) and then to the switch and switch (red, 1, and 2).
The steam from the kettle boils causes the tube to heat up, causing the thermostat to flip open. This turns off the heating element (green 39), and stops the water from boiling.
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.
The danger of early electric kettles was that they were easy to turn on, then go off to do some chores, then forget about them. If you were lucky enough, your kitchen would be steaming when you returned a few minutes later.
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.
They work in a certain way. 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. The thermostat is normally curved in one direction.
However, when hot water reaches boiling point the steam generated hits the bimetallic thermometer and causes it to snap and flex in opposite direction. It’s a bit like an umbrella that turns in the wind.
The thermostat opens by pushing a lever. This trips the circuit and cuts off the electric current. It then safely turns off the kettle.
More sophisticated kettle thermostats (used in systems such as the fashionable Marco Uber coffee boiler) are entirely electronic and allow water to be heated to precise temperatures and maintained there indefinitely by repeatedly switching the current on and off.