Joule
James Prescott Joule ran the family brewery near Manchester and did his experiments at home. In 1845 he lowered a paddle wheel into a vessel of water. Two weights on cords turned it. The weights went down, the paddles turned, and the water warmed up.
The warming was tiny, a fraction of a degree. Joule measured it with thermometers on which he could read hundredths of a degree. The experiment showed that work and heat are the same thing in different forms. Today work, heat and electrical energy share one unit, the joule. One joule is the work done by a force of one newton over a distance of one metre. That is what it takes to lift an apple by one metre.
| Notation | J |
| Definition | newton metre · N·m · kg·m²·s⁻² · watt-second |
| Reference values | 1 cal — 4.184 J · 1 kWh — 3.6 MJ · 1 eV — 1.602 176 634·10⁻¹⁹ J |
| Introduced | 1889, International Electrical Congress · for heat, CGPM 1948 |
| Standards | ISO 80000-4 · ISO 80000-5 · SI Brochure 9th ed. |
| Typical values | 1 J — an apple lifted one metre · 400 kJ — eating an apple · 4.184 GJ — a tonne of TNT |
01 · Definition
A newton over a metre of path
The joule is the work done by a force of one newton when its point of application moves one metre along the force. In SI base units it is a kilogram metre squared per second squared.
The same unit measures any energy: kinetic, thermal, electrical, chemical. A watt for one second is also a joule. Energy passes from one form to another, and the total stays the same. That is exactly what Joule’s experiment showed.
How many joules a lift takes
An object is lifted from the floor to the height set by the slider. The violet fill on the ruler is the distance covered. The dashed line is one metre. The work equals the weight times the height and becomes the object’s stored energy. If you let it go, the same joules are released when it hits the floor.
One quantity in three habits. Chemists and dietitians count in calories: a calorie warms a gram of water by one degree and equals 4.184 J. Power engineers count in kilowatt-hours: a one-kilowatt appliance uses 3.6 MJ in an hour. Physicists write joules. Food labels often carry both figures, kcal and kJ.
How many drops of the weights warm the water by one degree
Two weights hang on cords passed over pulleys. The cords are wound on a drum that turns a paddle in a copper vessel of water. The weights descend 1.6 m, the paddles churn the water, and it warms up. Then the weights are wound back up with the paddle uncoupled. On the right is a thermometer under a magnifier, graduated in 0.01 K. Below it are two bars: violet is the work of the weights, orange is the heat that went into the water. The grey part is the loss to friction in the pulleys and the heat lost to the room. The “Drop” button releases the weights once, “20 times” in a row, as Joule did.
Drop the weights a few times and see how much the water warms up.
02 · Conversion
| Unit | Value | Where it is met |
|---|---|---|
| {n} | {v} | {note} |
The food “calorie” on a label is a kilocalorie, 4.184 kJ.
A kilowatt-hour is 3.6 million joules. A kettle uses about that much boiling a litre of water ten times.
An electronvolt is the energy of an electron that has passed through a potential difference of one volt. It is used for atoms and photons.
03 · Orders of magnitude
04 · Measuring instruments
05 · Writing rules
06 · Neighbouring units
07 · Historical section
There are several calories, and they all differ. The thermochemical one is exactly 4.184 J. The international one, from the steam tables, is exactly 4.1868 J. The fifteen-degree calorie, which warms a gram of water from 14.5 to 15.5 °C, is about 4.1855 J. The difference is under one per cent, but it shows in a precise calculation.
That is why in 1948 the General Conference on Weights and Measures decided to measure heat in joules and leave the calorie in the past. The joule is defined through the kilogram, the metre and the second and does not depend on the properties of water. Joule’s own experiment gave 772 foot-pounds per British thermal unit, about 4.15 J per calorie. That is less than one per cent off today’s value.
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