National 4 Physics - Electricity and Energy Summary Notes

Transcription

1 Electromagnetism Magnetic fields Magnetic fields are found around any permanent or electromagnet. They are normally invisible but can be shown up by placing a sheet of paper over the magnet and sprinkling iron filings on top. These line up along the magnetic field lines. The magnetic field lines for a permanent bar magnet is shown opposite. N S If two magnets are placed close to one another the magnetic field lines interact to produce a force. Two unlike poles placed near each other produce an attracting force. The field lines are as shown below. N S N S Two like poles will produce a repulsive force e.g. north and north pole or south and south pole. S N N S 1

2 An electromagnet or solenoid is created by wrapping a current carrying wire around a soft iron core such as a nail. The magnetic field produced is similar to the one for a single bar magnet. current carrying wire Applications of Magnets Magnets can be used in a range of applications. Electric motors and loudspeakers. Permanent magnets are used along with electromagnets to produce the movement in d.c. motors and loudspeakers. Refrigerator door seals. Flexible magnets in the rubber door seals of refrigerators ensure that the door seal fits tightly against the steel body of the refrigerator. Pickups on electric guitars are magnetic and the vibrating strings produces a small current which can be amplified. Audio and video tapes, although not used as much these days, use a coating on a plastic strip which is magnetised when passed over a record head. The magnetised strips can then produce small currents when passed over the playback head. Powerful electromagnets are used to pick up iron and steel in scrap yards. Turning off the current to the magnet switches off the magnet and the metal is released. Electromagnets are used in automated door locks. Relays, like the one shown opposite, can be used to switch on or off large currents or voltages with a small current or voltage. Activating the relay coil by passing a current through it attracts the pivot bar, so closing the switch contacts. relay coil with iron core coil connections iron pivot arm switch contacts switch connections 2

3 Transformers A transformer is a device which can convert a voltage from one value to another. Step-up transformers increase the voltage and step-down transformers decrease the voltage. A transformer consists of two coils of wire wound around an iron core. iron core input voltage primary coil secondary coil output voltage Transformers will only work with an alternating current (a.c.) supply. The voltage in the secondary coil will change depending on the turns ratio of the secondary coil to the primary coil. A higher number of turns of wire in the secondary means a greater voltage out of the secondary coil. Practical electrical and electronic circuits Electrical circuits Symbols are used to represent components in electrical circuits. Some of these are shown below. switch linked wires cell + battery + voltmeter V ammeter A ohmmeter resistor variable resistor light bulb 3

4 Series and parallel circuits Circuits can be series or parallel or a combination of the two. In a series circuit there is only one path through the components and back to the cell or battery. In a parallel circuit there are two or more paths around the circuit. There is at least one point in the circuit where there is a choice of paths. Voltage and Current Voltmeters are used to measure the voltage in a circuit. Voltage is measured in volts. Voltmeters are always placed in parallel or across a component. Ammeters are used to measure the current in a circuit. Current is measured in amperes. Ammeters are always placed in series or in line with a component. It does not matter if the ammeter is before or after the component the current is the same. V A Current in a circuit is the flow of electrons through the wires and components In the series circuit below the current is the same through all components. The readings on ammeters A1 and A2 will be the same. The supply voltage, V supply, is split between the components in the circuit i.e. V 1, V 2 and V 3. Vsupply A1 A2 V1 V2 V3 4

5 Ohmmeters and Resistance A resistor is a device which opposes the flow of current through a circuit. Its resistance is measured in ohms. An ohmmeter can be used to measure the resistance of a component. A length of wire will have a certain resistance. The size of the resistance will depend upon: what material the wire is made from; the length of the wire the longer it is, the more its resistance; its cross sectional area (thickness) the thinner the wire the more its resistance; its temperature the hotter the wire the higher its resistance. The resistance of a conductor can also be found by using Ohm s Law. This involves measuring the voltage across the component and the current through it using a circuit like the one opposite. A The current is measured at different voltages. The value of the resistor is the voltage divided by the current. i.e. I V will give a constant value equivalent to the resistance of the circuit. V Thus voltage, current and resistance are related in the following formula: voltage = current resistance V = I R where V = voltage measured in volts I = current measured in amperes R = resistance measured in ohms Worked example A 12 V supply is connected to lamp which draws a current of 0 5 A. Calculate the resistance of the lamp. voltage = current resistance V = I R 12 = 0 5 R 12 R = 0 5 = 24 ohms 5

6 Electronic Circuits Electronic components Electronic circuits consist of an input, a process and an output e.g. a calculator has a keyboard as the input, a microprocessor carries out the process part and the liquid crystal display is the output. Symbols are used to represent electronic components. Some of these are shown below. Input devices Output devices switch loudspeaker thermistor light emitting diode light dependent resistor + lamp microphone motor M Switches:- switches provide an on or off signal to a circuit. Thermistor:- a thermistor is a device which changes its resistance with temperature. Its resistance can decrease or increase with a rise in temperature but the commonest thermistors decrease their resistance as temperatures increase. Light dependent resistors (LDR):- the resistance of an LDR alters as the light falling on it changes. Increasing the light level decreases the resistance. Microphone:- a microphone converts sound energy into electrical energy. Loudspeaker:- a loudspeaker converts electrical energy into sound energy. Light Emitting Diode (LED):- an LED converts electrical energy into light energy. It works from a low voltage using a small current. It will only operate in a circuit if connected the right way round i.e. its negative connection must be connected to the negative terminal of the power supply. 6

7 Lamp:- a lamp or bulb converts electrical energy into light energy. Motor:- an electric motor converts electrical energy into kinetic energy. Logic gates Logic gates are used in electronic circuits. Logic gates have one or more inputs and an output. The signals in and out of a logic gate are digital and described as being ON or OFF, HIGH or LOW or 1 or 0. Three common logic gates are shown below. A TRUTH TABLE can be completed for each one showing what the output will be for a given input. NOT gate Sometimes called an INVERTER, the output from the not gate is the opposite of the input. input output INPUT OUTPUT AND gate An AND gate has two inputs and both must be HIGH for the output to be HIGH. input A input B output INPUT A INPUT B OUTPUT OR gate An OR gate has two inputs and the output will be HIGH if either of the inputs is HIGH. input A input B output INPUT A INPUT B OUTPUT

8 Logic gates are used to control a circuit e.g. a lamp is switched on when the switch is ON and the light sensor detects that it is dark. light sensor switch AND gate lamp Light sensor gives a HIGH output when it is dark and the switch gives a HIGH output when switched on. When both inputs are HIGH the lamp is switched on. Logic gates can be combined together to provide new functions e.g. an AND gate and NOT gate can be combined to give a completely new truth table as shown. input A input B C output INPUT A INPUT B C OUTPUT Electrical Power Energy consumption Appliances in the home convert electrical energy into other forms. A television converts electrical energy into light and sound energy. A washing machine converts electrical energy into kinetic energy and heat energy. The rate at which appliances use energy is termed their power and is measured in watts. If an appliance has a power of 1 watt it uses 1 joule of energy every second. Appliances with high power ratings usually produce a lot of heat. Toasters, electric irons, kettles etc. usually have power ratings of over 1000 watts. 8

9 Calculating power The power of an appliance can be calculated using the formula below. power = energy time P = E t where P = power measured in watts E = energy measured in joules t = time measured in seconds Worked example An electric heater with an output power of 1 kw is switched on for 5 minutes. Calculate the energy output from the heater. power = P = 1000 = E = = energy time E t E joules Energy waste and efficiency The biggest usage of energy in the home is for heating the house and heating water. This may be by electricity or by using gas or oil fired boilers. Sometimes coal or wood may be used. There are ways of improving the efficiency of heating our homes. Insulate walls and loft as most heat escapes through these areas. Loft insulation should be at least 200 mm thick and is most often glass fibre or mineral wool. Air trapped in the wool stops heat escaping. Wall insulation if fitted when a house is built consists of thick foam panels with a foil backing to reflect the heat back into the house. Fit double glazing to stop heat escaping through windows. Fit insulation around the hot water storage tank even if it is already covered in foam. The money saved quickly repays the cost. Turn down room thermostats. Even a small reduction in temperature will save money. The bigger the temperature difference between the inside and outside of a house, the more quickly heat will be lost. Reduce draughts by fitting draught excluders on external doors, letterboxes etc. and around opening windows. 9

10 There are other ways in which energy can be saved such as: Turn out the lights when not in the room. Don't fill your kettle with more water than you need otherwise water is being heated which will then just cool down. Only use a dishwasher when full as every use of the dishwasher can cost up to 50 p. Don t leave appliances on standby as even the one or two watts of power consumed, if repeated for several appliances, 24 hours a day, can soon mount up. Choose energy efficient appliances. All new appliances have a sticker to show how energy efficient they are. It gives the appliance a rating from A (very efficient) to G (inefficient). Have showers instead of baths as showers use a lot less hot water. Electrical appliances have a rating plate attached which gives information on the voltage it uses and its power in watts. This is the rate at which it uses energy but the useful energy or power output will be less due to energy losses. An electric blender has a power rating of 150 watts but the useful kinetic energy it gives out is only 75 watts. 230 volts a.c. 50 hertz 150 watt Model No. N413 Made in Scotland. Its efficiency can be calculated from the formula: power out % efficiency = power in 100 In the case of the blender it is 50 % efficient. The amount of energy an appliance loses varies the greater the loss of energy, the less efficient is the appliance. Worked example A light bulb is rated at 60 watts but only produces 3 watts of useful light. Calculate the efficiency of the light bulb. power out efficiency = 100 power in 3 = = 5% 10

11 The energy input to the appliance and the energy output can also be used to calculate efficiency using the formula: energy out % efficiency = energy in 100 Worked example An electric drill is 40 % efficient. Calculate the useful energy output if uses joules in drilling a hole. % efficiency = energy out energy in 100 energy out 40 % energy out joules Sources of Energy At the present time our main sources of energy are fossil fuels coal, oil and gas. The reserves of these fuels, i.e. the quantity of them left, is limited. These fuels take millions of years to form naturally and so are not being replaced so are referred to as non-renewable energy sources. Oil and gas reserves in the North Sea will last for about another 40 years. There is a lot more coal under the UK but it is often in deep mines so harder to get at. Coal also produces more pollution when burned. Renewable sources of energy are constantly being replaced due to natural processes such as the wind, tides, rainfall etc. Renewable energy sources include wave power, solar power, geothermal power, hydroelectric power, wind power, tidal power and bio fuel. All renewable energy sources have the advantage that they are always being replaced. However, it can be expensive to gather the energy from these sources and they are not always available eg. if its not windy then wind farms are not much use. 11

12 Generation of Electricity A voltage can be generated across a wire by moving it between the poles of a magnet. The kinetic energy of the moving wire is converted into electrical energy. If the wire is stationary then no voltage will be generated. magnets wire bar magnet S V The same effect can be created by keeping the wire stationary and moving the magnet e.g. a bar magnet can be pushed in and out of a coil of wire. V N coil of wire The voltage generated can be increased by having more coils of wire, a stronger magnet or increasing the kinetic energy of the magnet by moving it faster. Power stations use generators which are rotated by turbines. The turbines are rotated by high pressure steam. A fuel, such as coal, oil or gas, is burned to create heat to turn water into steam. The block diagram below shows a typical fossil fuel powered power station. BOILER fossil fuel burned - water turned into steam TURBINE steam at pressure causes turbine blades to rotate GENERATOR turbine turns generator which produces electricity In a conventional power station you will often see cooling towers. Water vapour or steam rises from these as hot water is cooled. 12

13 In hydro electric power stations, the turbines are rotated by water which runs downhill from behind a dam. The water has potential energy when stored behind the dam and this is changed into kinetic energy as it flows downhill. dam generating station Electricity which is generated by the power station must be transmitted to the consumers who may live many kilometres away. The output voltage from the generator is increased to hundreds of thousands of volts. This has the effect of reducing the current of the transmitted electricity which means that much smaller cables can be used. The energy losses are also much reduced. Once the electricity reaches the houses it supplies, the voltage is reduced back to a lower value by transformers in a sub-station. Gas Laws and the Kinetic Model Kinetic Model of Gases The particles of a gas can be thought of as being in constant motion. When they collide with the walls of a container they provide a small outwards force. The many, many collisions of the gas particles with the walls of a container which causes the pressure of a gas. Pressure and temperature At a high temperature the gas particles have a lot of kinetic energy and move quickly. There are many collisions with the container walls, each collision producing more force on the wall. If the temperature of the gas is reduced the gas particles move more slowly with less kinetic energy. There are fewer collisions with the container walls and with less force than before so the pressure is less. 13

14 Pressure and volume If a container holding a gas is sqeezed or compressed the pressure inside increases e.g. placing your finger over the end of a bicycle pump and pushing down the handle. In a container with a large volume there are relatively fewer collisions with the container walls as the gas particles have further to travel between walls. If the volume of the container is reduced there are more collisions with the container and so the pressure is greater. Volume and temperature Heating a container of gas will make it expand till the pressure inside is equal to the pressure outside the container. The gas particles collide with the walls of the container pushing it outwards. Air particles outside push the walls inwards so a balance is achieved. If the temperature of the gas is raised they have more kinetic energy and hit the walls more often and with more force. This pushes the container walls outwards causing till the pressure inside the container equals the pressure outside once more. 14