What is the basic unit of energy? What is the relationship between work, heat, and energy?
The SI unit of energy is the joule, symbol J, named after British physicist James Joule. 1J=1N·m=1W·s=1kg·m²/s² (work done by 1 newton force moving 1 meter, or energy released by 1 watt power in 1 second). Work, heat, and energy are fundamentally the same physical quantity, manifesting differently in contexts: called work in mechanics (W=Fs), electrical energy in electricity (W=Pt=UIt), heat in thermodynamics (Q=cmΔT), calories in nutrition—all measured in joules or calories.
How many joules in 1 kWh? How many kilocalories?
1 kWh = 1 kilowatt-hour = 1000W×3600s = 3,600,000J = 3600kJ ≈ 860 kcal. This is the most commonly used conversion for electricity billing and metering. 1 kWh can run a 1 HP air conditioner for about 1 hour, a 10W LED light for 100 hours, charge a phone about 80 times, or power an electric vehicle for 5-8 km. 1 kWh thermal value ≈ 860 kcal; heating 1 liter of water from 20°C to 100°C requires 80 kcal ≈ 0.093 kWh.
What is the relationship between cal, kcal, and Calories? Which 'calorie' is on food labels?
1 cal (small calorie) = heat needed to raise 1 gram of water 1°C ≈ 4.184J; 1 kcal (kilocalorie) = 1000 cal = 4184J = 4.184kJ = 1 Calorie (large calorie). 'Calories' (capital C) on nutrition labels actually means kcal: basal metabolic rate is about 1200-1400 kcal/day for adult women, 1500-1800 kcal/day for men; a bowl of rice ≈ 200 kcal, an egg ≈ 70 kcal, running 1 hour burns about 500-700 kcal. 'Calories 250' on food packaging means 250 kcal = 250,000 small calories!
How do you convert kJ to kcal? Why do food labels show both?
1 kcal ≈ 4.184 kJ, conversely 1 kJ ≈ 0.239 kcal. Chinese and EU food labels require both kJ and kcal; US labels show Calories (kcal). For example, a package labeled 'Energy 2000kJ/100g' ≈ 2000÷4.184 ≈ 478 kcal/100g; a can of cola ≈ 140 kcal ≈ 586 kJ. For nutrition calculations: protein 4kcal/g, carbohydrates 4kcal/g, fat 9kcal/g, alcohol 7kcal/g—multiply by 4.184 to get kJ.
What is BTU? Why is BTU commonly used in HVAC?
BTU (British Thermal Unit) is defined as the heat needed to raise 1 pound of water 1°F; 1 BTU ≈ 1055J ≈ 0.252 kcal ≈ 0.293 Wh. Air conditioners, furnaces, fireplaces, water heaters and other HVAC equipment commonly use BTU/h (BTU per hour) for cooling/heating capacity in North America and Commonwealth countries: 1 HP air conditioner ≈ 9000 BTU/h ≈ 2.5kW cooling capacity; 1 Ton AC = 12000 BTU/h ≈ 3.5kW; residential gas stoves typically 10000-15000 BTU per burner.
What is the relationship between Wh and mAh? How do you convert battery capacity to energy?
Wh (Watt-hour) = Voltage(V) × Capacity(Ah) = Power × Time, representing total energy stored in a battery. Phone batteries typically 10-15 Wh (3.8V × 3000-4000 mAh), laptops 45-90 Wh, electric vehicles 40-100 kWh. Note that mAh is a charge unit, not energy—you must multiply by voltage to get energy: Wh = V × mAh/1000. For example, a 10000 mAh power bank (3.7V) = 37 Wh; the flight limit is 100 Wh (≈ 27000 mAh @ 3.7V).
What are PS·h and HP·h? Which is metric and which is imperial?
PS (Pferdestärke, metric horsepower) = 75 kgf·m/s ≈ 735.5W; HP (Horsepower, imperial/mechanical horsepower) = 550 ft·lbf/s ≈ 745.7W. 1 PS·h = 735.5W × 3600s ≈ 2.648×10⁶J ≈ 2.648MJ ≈ 0.7355 kWh; 1 HP·h ≈ 2.685MJ ≈ 0.7457 kWh. China, Europe, and Japan commonly use metric PS; UK and US use imperial HP—the difference is about 1.4%. Automotive engine power is often labeled in kW or PS/HP, fuel consumption in L/100km or MPG (miles per gallon); energy conversion can be used for engine efficiency calculations.
What are the energy orders of magnitude for kinetic, potential, electrical, chemical, and nuclear energy?
1 J is about the energy needed to lift an apple (100g) 1 meter; 1 kJ is the potential energy to lift 1 kg about 102 meters; 1 kcal can raise 1 kg of water 1°C; 1 kWh can light a 10W LED for 100 hours; 1 liter of gasoline ≈ 32MJ ≈ 8.9kWh ≈ 7700 kcal; 1 kg TNT explosion ≈ 4.2MJ; 1 kg uranium-235 fission ≈ 83TJ ≈ 23 million kWh (equivalent to over 2000 tons of coal); 1 gram of mass fully converted E=mc² ≈ 9×10¹³J ≈ 25 million kWh.
How do you estimate calories burned during exercise?
Approximate estimates (for 60-70 kg adults): brisk walking (5 km/h) ≈ 300 kcal/hour, jogging (8 km/h) ≈ 500-600 kcal/hour, running (12 km/h) ≈ 700-850 kcal/hour, cycling (15 km/h) ≈ 350 kcal/hour, swimming ≈ 500-700 kcal/hour, jump rope ≈ 700 kcal/hour, yoga ≈ 200 kcal/hour, housework ≈ 150-250 kcal/hour. Human energy efficiency is about 20%-25%; mechanical output of 100-200W corresponds to burning 500-800W of heat.
What are the calorie contents (kcal) of common foods?
Reference values (per 100g): vegetables 15-50 kcal (cucumber 15, tomato 18, carrot 37, potato 77); fruits 30-90 kcal (apple 52, banana 89, avocado 160); staples: rice 116, noodles 110, steamed bun 223, bread 265; meat/eggs: chicken breast 165, beef 190, pork (lean) 143, (belly) 395, egg 144, salmon 208; snacks: potato chips 540, chocolate 550, cookies 450-500; beverages: cola 43/100ml, beer 43/100ml, liquor 230-300/100ml; oils ≈ 900 kcal/100g. A bowl of rice (150g cooked) ≈ 175 kcal.
How do you calculate electricity bills from kWh?
Electricity bill = Consumption (kWh) × Electricity rate (currency/kWh). Residential tiered rates in China are about 0.5-0.8 yuan/kWh, commercial/industrial 0.6-1.5 yuan/kWh. Appliance consumption = Power (kW) × Usage time (h). Example: 1.5 HP air conditioner cooling power ≈ 1.2kW, running 8 hours consumes 1.2×8 ≈ 9.6 kWh ≈ 5-8 yuan; 2000W water heater heating 1 hour uses 2 kWh ≈ 1-1.6 yuan; 10W router monthly usage = 0.01×24×30 ≈ 7.2 kWh ≈ 4 yuan; 50-inch TV 100W watching 2 hours uses 0.2 kWh. Beware standby power consumption—many appliances draw 1-5W on standby, consuming several kWh per year.
Which energy units does this tool support? Why no eV, erg, ft·lb?
This tool supports 9 energy units: J, kJ, Wh, kWh, cal, kcal, BTU, PS·h (metric horsepower-hour), HP·h (imperial horsepower-hour), covering over 90% of daily scenarios in physics, electricity, nutrition, HVAC, automotive engineering. eV (electron volt = 1.6×10⁻¹⁹J) is used in particle physics and semiconductor band theory, a specialized physics unit; erg (10⁻⁷J) is an old CGS unit rarely used in engineering; ft·lb (foot-pound = 1.356J) is an imperial work unit—BTU and HP·h already cover the main imperial needs for daily energy/heat scenarios. Other units may be added based on future demand.
