energy

• Kinetic energy is the energy that an object possesses due to its motion
• Kinetic energy depends on two factors: speed and mass
• The faster an object is moving, the more kinetic energy it will have
• The more mass an object has, the more kinetic energy it will have, as long as speed is kept equal
• The equation to calculate kinetic energy is: E_k = 1/2 * m * v^2
• In the equation, E represents energy, k represents kinetic energy, m represents mass (measured in kilograms), and v represents velocity or speed (measured in meters per second)
• To calculate kinetic energy, ensure all values are in the correct units (e.g., convert tons to kilograms)
• For the plane with a mass of 20,000 kg and speed of 5 m/s:

  • Kinetic energy = 250,000 joules or 250 kilojoules
• For the particle with a mass of 0.0001 kg and speed of 4000 m/s:

  • Kinetic energy = 800 joules or 0.8 kilojoules
• Gravity is a force of attraction between two objects, dependent on their mass and distance apart
• Gravitational force is stronger for large objects close by, like the Earth or the Moon
• Gravitational field strength is denoted by the letter g, with Earth's at 9.8 newtons per kilo and the Moon's at 1.6
• Weight of an object is calculated by multiplying its mass by the gravitational field strength
• Weight is the force of attraction an object experiences in a gravitational field
• Mass is an intrinsic property of an object, while weight is the force acting on it in a gravitational field
• To lift an object and overcome gravity requires energy, which is transferred to the object's gravitational potential energy store
• Gravitational potential energy formula: ep = mgh (mass times gravitational field strength times height)
• Gravitational potential energy is measured in joules
• Example calculation: An apple with a mass of 0.1 kilos thrown 3 meters up has a gravitational potential energy of 2.94 joules
• When an object is heated, energy is transferred to its thermal energy store
• Heating can take place in three different ways depending on the medium involved:

  • For solids, heat is transferred by conduction
  • For fluids, heat is transferred via convection
  • Heat is transferred through empty space in the form of radiation
• Conduction:

  • Vibrating particles transfer energy to neighboring particles
  • Particles vibrate faster and collide more often with more energy, passing along the heat until it's evenly spread out
  • Mainly occurs in solids due to closely held particles
  • Thermal conductivity determines how well objects transfer energy by conduction
• Convection:

  • Mainly occurs in fluids (liquids and gases)
  • Particles in fluids move around faster by random diffusion when heated
  • Higher energy particles move away from the warmer region towards the cooler region, causing fluid expansion and less density in the warmer region
  • Forms convection currents
  • Seen in oceans, inside buildings with radiators
  • To reduce convection, stop the free flow of fluids (e.g. sleeping under a blanket)
• Conduction and convection both involve particles gaining kinetic energy

  • In conduction, energy is transferred between different particles
  • In convection, particles themselves move
• Heat energy can be transferred without particles through radiation

  • Energy is carried by infrared waves
  • Objects constantly absorb and emit radiation
  • The hotter the object, the more radiation it emits
  • Infrared radiation is absorbed by objects, making them feel hot
  • Infrared radiation is emitted by very hot objects like metal and coal
• To reduce unwanted energy transfers, thermal insulation and lubrication are used
• In order to keep a house warm, heat energy escaping to the outside environment needs to be reduced
• Ways to reduce heat loss in buildings:

  • Ensure the house is sealed closed to prevent air from passing out (convection)
  • Use materials with low thermal conductivity for walls to reduce heat loss by conduction
  • Implement cavity walls with an air gap filled with insulating foam to reduce conduction and convection
  • Use double glazing with a tiny air gap to reduce heat loss by conduction
• Friction is the resistance encountered when an object moves over a solid or through a fluid
• Reducing friction:

  • Adding oil as a lubricant reduces friction, making movement easier
  • Making cars and planes streamlined reduces air resistance, allowing them to use less fuel and be more efficient
• There are two different definitions for power:

  • Power is the rate at which energy is transferred, shown by the equation power equals energy transferred divided by the time it took to transfer that energy
  • Power is the rate at which work is done, written as power equals work done over time
• Work done is a special measure of the energy transferred when a force is used to move an object by a certain distance
• Power is measured in watts, energy transferred and work done are measured in joules, and time is measured in seconds
• To calculate power, use the equation power equals energy transferred divided by time taken or power equals work done divided by time taken
• Comparing two lamps to determine which is more powerful:

  • Left lamp: 1200 joules over 20 seconds = 60 watts
  • Right lamp: 1500 joules over 30 seconds = 50 watts
  • Left lamp is more powerful
• Calculating energy transferred for an 1100 watt microwave on for 3 minutes:

  • Energy transferred = power times time
  • Energy transferred = 1100 watts * 180 seconds = 198,000 joules or 198 kilojoules
• Calculating power used to push a car down the street:

  • Work done = 9 kilojoules or 9,000 joules
  • Time taken = 20 seconds
  • Power = work done divided by time taken = 450 watts
• Efficiency is the proportion of the energy supplied that is actually transferred into the useful energy output
• Efficiency can be calculated as a decimal or a percentage using the equation: efficiency = useful energy output / total energy input
• For power, efficiency can be calculated as: efficiency = useful power output / total power input