Energy and Forces

• microwaves have a greater frequency than radio waves
• Efficiency = useful energy transferred / total energy supplied
• All energy transfers waste some energy, so nothing is 100% efficient
• Gravitational potential energy = mass x gravitational field strength x change in height
• Voltage is another name for the potential difference
• Potential difference = energy / charge
• Potential difference is measured in volts
• The nucleus is made of protons and neutrons
• The rate of flow of electric charge is called current
• Current has the unit amps and can be measured with an ammeter
• If a closed circuit includes a source of potential difference like a cell, then there will be a current in the circuit
• Charge = current x time
• When electric current runs through a resistor, the resistor heats up because there are more collisions between electrons and the ions in the metal
• The more collisions with metal ions there are, the harder it is for the free electrons to travel, so the higher the resistance
• Power is measured in watts (w)
• Power = current x potential difference
• When a liquid changes to a solid it freezes
• Moving objects have kinetic energy.
• This is the equation to calculate the amount of energy in an object's kinetic energy store:

  
• For example, this is how to calculate the kinetic energy stored by a 4 kg dog running at 3 m/s:

  
• Energy companies use a unit called a kilowatt-hour (kWh)
• A kilowatt-hour is the amount of energy transferred in 1 hour by an appliance with a power of 1 kW

  EG:

  Using a 2kW fire for 4 hours would use 8kWh of energy

  energy use = power rating X time

  (kWh) (kW) (hours)
• The payback time tell you how long it will take to save the money that an efficiency measure costs

  
  

  payback time = costs of change ÷ saving per year
• The amount of energy transferred per second is the power of the appliance
• The units for power are watts (W) or kilowatts (kW)
• 1 watt = 1 joule transferred every second

  1000W = 1kW