Energy

• What happens to energy when it is used?

  Energy is transferred between different energy stores and objects.
• What occurs when energy is transferred to an object?

  The energy is stored in one of the object's energy stores.
• What are the eight energy stores you need to know?

  1. Thermal energy stores
  2. Kinetic energy stores
  3. Gravitational potential energy stores
  4. Elastic potential energy stores
  5. Chemical energy stores
  6. Magnetic energy stores
  7. Electrostatic energy stores
  8. Nuclear energy stores
• How is energy transferred mechanically?

  By a force doing work.
• What is one way energy can be transferred electrically?

  By moving charges doing work.
• What is a closed system?

  A system where neither matter nor energy can enter or leave.
• What is the net change in total energy of a closed system?

  The net change is always zero.
• How does energy transfer occur when boiling water in a kettle?

  Energy is transferred to the water's thermal energy store by heating.
• What does work done mean in terms of energy transfer?

  Work done is another way of saying energy transferred.
• How is energy transferred when a person throws a ball upwards?

  Energy is transferred from the chemical energy store of the person's arm to the kinetic energy store of the ball and arm.
• What happens to a ball dropped from a height?

  The gravitational force does work, transferring energy from the ball's gravitational potential energy store to its kinetic energy store.
• What does friction do in a car's braking system?

  Friction does work, transferring energy from the wheels' kinetic energy stores to the thermal energy store of the surroundings.
• What occurs during a collision between a car and a stationary object?

  Energy is transferred from the car's kinetic energy store to other energy stores, such as elastic potential and thermal energy stores.
• What is the formula for kinetic energy?

  Kinetic energy (J):

  $E_k =$$\frac{1}{2}mv^2$

  • Mass (kg)
  • Speed (m/s)
• Calculate the kinetic energy of a car with a mass of 2500 kg traveling at 20 m/s.

  $E_k =$$\frac{1}{2} \times 2500 \times 20^2 =$$500,000 \text{ J}$
• What is the formula for gravitational potential energy?

  Gravitational potential energy (J):

  $E_p =$$mgh$

  • Mass (kg)
  • Height (m)
  • Gravitational field strength (N/kg)
• What happens when an object falls without air resistance?

  Energy lost from the gravitational potential energy store equals energy gained in the kinetic energy store.
• How does air resistance affect falling objects?

  Air resistance causes some energy to be transferred to other energy stores, such as thermal energy stores.
• What is the formula for elastic potential energy?

  Elastic potential energy (J):

  $E_e =$$\frac{1}{2}ke^2$

  • Spring constant (N/m)
  • Extension (m)
• What is specific heat capacity?

  Specific heat capacity is the amount of energy needed to raise the temperature of 1 kg of a substance by 1 °C.
• What is the equation for change in thermal energy?

  Change in thermal energy (J):

  $\Delta E =$$mc\Delta \theta$

  • Mass (kg)
  • Specific heat capacity (J/kg°C)
  • Temperature change (°C)
• How can you investigate specific heat capacities?

  By measuring the temperature change of a material as energy is transferred to it.
• What is the conservation of energy principle?

  Energy can be transferred, stored, or dissipated, but it can never be created or destroyed.
• What is dissipated energy?

  Dissipated energy is energy stored in a way that is not useful, often as thermal energy.
• What happens in a closed system with a cold spoon and hot soup?

  Energy is transferred from the thermal energy store of the soup to the thermal energy store of the spoon, but no energy leaves the system.
• How is power defined?

  Power is the rate of energy transfer or the rate of doing work.
• What is the unit of power?

  Power is measured in watts.
• How can you calculate power?

  Power can be calculated using the equations \( P = \frac{E}{t} \) or \( P = \frac{W}{t} \).
• If a motor lifts a stunt performer with 8000 J of work in 50 s, what is the power of the motor?

  $P =$$\frac{8000}{50} =$$160 \text{ W}$
• What is conduction?

  • Conduction is the process where vibrating particles transfer energy to neighboring particles.
  • Occurs mainly in solids.
• What is convection?

  • Convection is where energetic particles move away from hotter to cooler regions.
  • Occurs only in liquids and gases.
• How does thermal conductivity affect energy transfer?

  Materials with high thermal conductivity transfer energy quickly between their particles.
• What happens to the density of a liquid or gas when it is heated?

  The density decreases as the particles move faster and the space between them increases.
• What occurs in a constant heat source with liquids and gases?

  The warmer, less dense region will rise above denser, cooler regions.
• What is thermal conductivity a measure of?

  How quickly energy is transferred through a material
• In which states of matter does convection occur?

  In liquids and gases