efficiency And how to improve it Objectives To

efficiency And how to improve it

Objectives • To be able to explain in words what the efficiency of a device is • To be able to calculate efficiency using either power or energy data (and to be able to rearrange the equation) • To describe how to increase efficiency by lubrication • To describe how to reduce the heat losses from a house-comparing thermal conductivity and thickness of insulating material

How is energy efficiency calculated? The energy efficiency of a device can be calculated using this formula: useful output energy efficiency = total input energy l Useful output energy is measured in joules (J). l Total input energy is measured in joules (J). l Energy efficiency does not have any units. It is a number between 0 and 1 which can be converted into a percentage by multiplying by 100. 3 of 32 © Boardworks Ltd 2011

Efficiency and power The efficiency of a device can be calculated using power instead of energy. Power and energy are closely related. Power is the rate at which energy is converted. efficiency = useful output power total input power l Useful power is measured in watts (W). l Total power is measured in watts (W). l Efficiency calculations using power are carried out in exactly the same way. As for energy, the efficiency may be expressed as a decimal or a %. 4 of 32 © Boardworks Ltd 2011

Efficiency tells us what proportion of the input energy is transferred to useful output energy. The energy saving lightbulb transfers a higher proportion of the input energy (electrical) into useful output (light) energy and is therefore more efficient.

Why is energy efficiency important? New electrical appliances display information about how energy efficient they are. This helps consumers to identify which appliance will use the least energy and cost the least to run. Energy efficiency is not just about saving money – it’s also about saving the planet. Appliances that are more efficient need less electricity, so less fossil fuel needs to be burnt in fossil fuel power stations. This is important for reducing pollution and the effects of climate change. 6 of 32 © Boardworks Ltd 2011

The efficiency of a light bulb This filament bulb is supplied with 100 J of electrical energy, which it converts to 45 J of light energy. a) How much energy is wasted? 55 J Wasted energy = Total – Useful = 100 J – 45 J = 55 J b) In what form is the energy wasted? heat c) What is the efficiency of the bulb? 0. 45 or 45% Efficiency = Useful Total = 45 J 100 J = 0. 45 or 45%

A radio This radio is supplied with 300 J of electrical energy which it converts to 96 J of sound energy. a) How much energy is wasted? 204 J Wasted energy = Total – Useful = 300 J – 96 J = 204 J b) In what form is the energy wasted? heat c) What is the efficiency of the radio? 0. 32 or 32% Efficiency = Useful Total = 96 J 300 J = 0. 32 or 32%

A television This television converts 2 000 J of electrical energy into useful energy at an efficiency of 65%. a) What useful energy does a television produce? light and sound b) How much useful energy is produced? 1 300 J Efficiency = Useful Total Useful = Efficiency x Total = 0. 65 x 2 000 J = 1 300 J

Appliance Energy Input (J) Useful energy output (J) Wasted Energy (J) Efficiency (%) Light bulb 60 12 48 20 Energy saving light bulb 30 22 8 73 Car 75 000 20 000 55 000 27 TV 300 180 120 60 Laptop 40 10 30 25 Patio heater 2000 200 1800 10 Human being 120 86 34 72

Efficiency, input energy and output energy 11 of 32 © Boardworks Ltd 2011

We can improve the efficiency of machines by reducing the frictional force between surfaces. How can we do this?

ice Wood Add a lubricant such as oil Bench Ice on wooden bench Bench Wood on wooden bench Use these diagrams to explain why it is easy to move a block of ice across a wooden bench, but harder to move a block of wood. Can you think of a way of making the block of wood easier to move?

Using thermal insulation

Finding the best conductor • All the rods have the same length and cross-sectional area. • They are all heated equally at one end with the bunsen burner. • When the other end of a rod reaches a certain temperature the paraffin wax melts and the match stick falls off. • A match stick will fall off the best conductor first. • This should be the rod made of copper.

Investigating conduction in metals 16 of 31 © Boardworks Ltd 2011

Rate of thermal transfer Heat transfer by conduction can increased by: 1. using a better conducting substance 2. decreasing the thickness of the substance 3. increasing the area of the substance 4. increasing the temperature difference across the substance 100°C 60°C glass copper 20°C

Insulating the walls of our houses Choose a material with a low thermal conductivity http: //lab. concord. org/embeddable. html#interactives/energy 2 d/htb/S 3 C 1. json

Also if we keep the inside of our houses just a little bit cooler, and perhaps put on an extra jumper, we will also reduce the rate of heat energy from our houses