Energy – Learning Outcomes Define energy. Recognise different forms of energy. State the Principle of Conservation of Energy. Give examples of converting between different forms of energy. Demonstrate energy conversion. Solve problems about energy conversion. Give renewable and non-renewable sources of energy. Explain the Sun’s method of energy conversion.

Energy Energy is the ability to do work. The main forms of energy are: potential – energy due to position, kinetic – energy due to motion, light – energy stored in photons, sound – energy stored in moving pressure waves, heat – vibrational and translational energy of particles, chemical – energy stored in / released from chemical bonds, nuclear – energy stored in / released from atomic bonds, electrical – energy of charged particles in a circuit, mass – energy condensed into particle form.

Conservation of Energy The Principle of Conservation of Energy states that energy can be neither created nor destroyed, but can be converted from one form to another. e.g. listening to music: electrical -> sound e.g. driving a car chemical -> kinetic e.g. making toast: electrical -> heat and light e.g. a falling object: potential -> kinetic

Potential Energy Potential energy appears in a number of forms on our course, but for now, potential energy usually refers to gravitational potential energy – the energy that an object has due to gravity trying to pull it downwards. This is equal to the work required to lift the object to that height, so 𝑊=𝐹×𝑠 becomes: Formula: 𝐸 𝑃 =𝑚𝑔ℎ 𝑚𝑔=𝑤𝑒𝑖𝑔ℎ𝑡, ℎ=ℎ𝑒𝑖𝑔ℎ𝑡 e.g. the potential energy that Rebecca gave that barbell is:

Kinetic Energy Kinetic energy is the energy that an object has while it is moving. Formula: 𝐸 𝐾 = 1 2 𝑚 𝑣 2 𝑚=𝑚𝑎𝑠𝑠, 𝑣=𝑣𝑒𝑙𝑜𝑐𝑖𝑡𝑦 e.g. a body with mass 1000 kg is moving with a velocity of 40 m s-1. Calculate its kinetic energy.

Kinetic ⇔ Potential Energy Loss in potential energy = gain in kinetic energy for a free-falling object. e.g. A 20kg rock is rolled off a cliff face, falling 100 m before hitting the ground. What is its velocity as it hits the ground? e.g. A 5 m long pendulum is allowed to swing back and forth. If its greatest angle of swing is 20o, how much vertical height does the pendulum gain at the top of its swing? What is its maximum velocity?

Kinetic ⇔ Potential Energy Brick A is dropped from a height 2h, while an identical brick B is dropped from a height h. When brick A hits the ground it has: twice as much kinetic energy as brick B the same kinetic energy KE as brick B four times as much KE as brick B None of the above A B 2h h

Kinetic ⇔ Potential Energy A truck initially at rest at the top of a hill is allowed to roll down. At the bottom its speed is 4 m/s. Next, the truck is again rolled down the hill, but this time it does not start from rest. It has an initial speed of 3 m/s at the top even before it starts going down the hill. How fast is it going when it gets to the bottom? 3 m/s 4 m/s 5 m/s 6 m/s 7 m/s

Collisions We know momentum is conserved in collisions. Kinetic energy is usually not conserved, as some is lost to heat and sound energy. Collisions where kinetic energy is conserved are called elastic collisions. Collisions where kinetic energy is not conserved are called inelastic collisions.

Collisions e.g. A mass of 5 kg travelling at 20 m s-1 collides with and sticks to a mass of 2 kg which is at rest. Find the velocity of the combined mass after the collision. Find the loss in kinetic energy. e.g. A small mass of 5 kg is suspended from a fixed point by a light string 2 m long. Another mass of 6 g moving horizontally at 100 m s-1 strikes the 5 kg mass and sticks to it. The combined mass then swings in a vertical plane. Find the greatest height it reaches.

Sources of Energy – Renewable Wind energy – kinetic energy of the wind. Solar energy – light energy emitted by the Sun. Hydroelectric energy – potential energy of falling water. Wave / tidal energy – kinetic energy of water pulled by the Moon. Biomass energy – chemical energy stored in food.

Sources of Energy – Non-Renewable Coal Oil Peat Gas – each of these are burned, converting stored chemical energy. Nuclear fission – splitting atoms to release nuclear energy. Nuclear fusion – fusing atoms to release nuclear energy.

Solar Energy The primary energy conversion in our Sun is the nuclear fusion of hydrogen isotope nuclei to form helium nuclei. Deuterium (hydrogen-2) fusion: 1 2 𝐻 + 1 2 𝐻 ⇒ 2 3 𝐻𝑒 + 0 1 𝑛 Deuterium-tritium (hydrogen-3) fusion: 1 2 𝐻 + 1 3 𝐻 ⇒ 2 4 𝐻𝑒 + 0 1 𝑛 One helium nucleus is less massive than two hydrogen nuclei, the excess mass is converted to energy according to 𝐸=𝑚 𝑐 2