Energy Basics & Calculations
Energy is the capacity to do this, and it is measured in this unit.
Work, measured in joules (J)
This form of energy is stored in the bonds between atoms in food, batteries and fuels.
Chemical energy
Sound or light: which can travel through empty space?
Light. Sound needs a medium to travel through
Name the three ways heat can be transferred.
Conduction, convection and radiation
Energy cannot be created or destroyed, only transferred or transformed. Name this law.
The Law of Conservation of Energy
Convert 3.8 kJ into J.
3,800 J
A stretched rubber band or compressed spring stores this form of energy.
Elastic energy
The splitting of white light into the colours of the rainbow is called this.
Dispersion
A metal pot handle becomes hot because particles pass heat along by direct contact. Name this method of heat transfer.
Conduction
In a light globe, electrical energy is transformed into light and heat. Which is the useful energy and which is wasted?
Light is useful energy. Heat is wasted energy
A worker pushes a trolley with a force of 50 N for 10 m. Calculate the work done.
W = F × d
W = 50 × 10
W = 500 J
Sort these into kinetic and potential: thermal, chemical, gravitational, elastic, electrical, sound, nuclear, light.
Kinetic: thermal, electrical, sound, light. Potential: chemical, gravitational, elastic, nuclear
What does a high amplitude produce, what does a high frequency produce, and through which type of material does sound travel fastest?
High amplitude: a loud sound. High frequency: a high pitch. Sound travels fastest through solids
Which is measured in joules and which in degrees Celsius: heat or temperature? Which one is the average kinetic energy of the particles?
Heat is measured in joules. Temperature is measured in °C and is the average kinetic energy of the particles
Compare energy transfer with energy transformation.
Transfer: energy moves from one object to another (for example by conduction). Transformation: energy changes from one form to another (for example chemical to electrical)
A battery stores 2.5 MJ of energy. Convert this to kJ and to J.
2,500 kJ and 2,500,000 J
Nuclear energy powers the Sun and nuclear power stations. Name the process in each.
The Sun: nuclear fusion. Power station: nuclear fission
Describe the difference between transparent, translucent and opaque materials.
Transparent: light passes through and a clear image is seen. Translucent: some light is scattered, so the image is fuzzy. Opaque: no light passes through (it is reflected or absorbed) and no image is seen
Why can heat from the Sun reach Earth, and why do dark coloured objects get hotter than light coloured ones?
Radiation does not need a medium, so heat travels through space as waves. Dark colours absorb more heat, while light and shiny colours reflect more
Describe the energy changes in a swinging pendulum.
At the highest point it has maximum potential energy and no kinetic energy. At the lowest point it has maximum kinetic energy. Energy changes back and forth, and some is lost as heat and sound due to friction
A 10 kg box sits on a shelf 3 m above the ground. Using g = 10 N/kg, calculate its gravitational potential energy (GPE).
GPE = m × g × h
GPE = 10 × 10 × 3
GPE = 300 J
A roller coaster car is at the top of a hill. Which energy is greatest there, how does its energy change on the way down, and what does mechanical energy equal?
Gravitational potential energy is greatest at the top. It changes into kinetic energy as the car falls. Mechanical energy = kinetic energy + potential energy, and the total stays the same if there is no friction
What is refraction, why does an object in water look disjointed, and does a convex or concave lens bend light towards the centre?
Refraction is the bending of light when it enters a different material because it travels slower. This bends the light path so the object looks disjointed. A convex lens bends light towards the centre
Explain how a convection current forms in a heated liquid, using particles and density.
Heated particles move apart, so the fluid becomes less dense and rises. Cooler fluid moves in to replace it. As the heated fluid moves away it cools, becomes denser and sinks, and the cycle repeats
A kettle uses 2000 J of electrical energy and 1500 J of it heats the water. Calculate its efficiency and say where the remaining energy goes.
Efficiency = useful energy ÷ total energy × 100
= 1500 ÷ 2000 × 100
= 75%.
The remaining 500 J is wasted, mostly as heat lost to the surroundings