Energy. E d. Energy Power = time. E t P = E t = P

Transcription

1 Energy Forms of energy Energy can never be created or destroyed. It can only be transformed from one type to another (or other types). here are many different forms of energy: Kinetic (movement) Energy Gravitational Potential Energy Chemical Potential Energy Nuclear Potential Energy Elastic Potential Energy Heat Energy Light Energy Sound Energy Electrical Energy Work Done Work done is a measure of the energy transferred during an energy transformation. Work done has the symbol Ew and is measured in joules (J). Work Done, Force & Distance work done= force distance E W = Fd F = d = E d W E F W Power Power is the rate at which energy is transferred during an energy transformation. Power has the symbol P and is measured in watts (W). Power, Energy and time Energy Power = time P = E t E = Pt E t = P

2 as National 5 Physics Summary Notes Mechanical Energy Gravitational Potential Energy he change in gravitational potential energy, Ep, of an object is the work done against gravity (lifting) or by gravity (falling). Gravitational potential energy is measured in joules (J). Gravitational Potential Energy, mass and height E P = mgh EP m = gh g = EP mh EP h = mg Kinetic Energy Kinetic energy, Ek, is the energy an object has due to its motion. Kinetic energy is measured in joules (J) he kinetic energy of an object depends on both its mass and speed. Increasing the mass and/or the speed of the object increases its kinetic energy. Kinetic Energy, mass and speed = E = K v mv E = m E m = v K K Conservation of Energy During any energy transformation the total amount of energy stays the same. his is known as the principle of conservation of energy. he principle of conservation allows numerical problems involving different forms of energy to be solved. For example, when a vehicle rolls down a slope, gravitational potential energy is converted into kinetic energy and heat (work done by friction). Calculations can be performed on each of the spate forms of energy (EP=mgh, EK=½mv, and EW=Fd) but it is also known that the total amount of energy stays the same and so (in this case) EP = EK + EW. Energy Loss In some situations it may appear that energy has been lost. In these cases it will have been turned into less useful (or sometimes less obvious) forms. he most common example is when energy is lost as heat, due to friction.

3 Electrical Charge Carriers and Electric Fields Electrical Charges here are types of charges, positive and negative. Electric fields exist around charges. hese fields cause other charges to experience a force, either attraction or repulsion. Opposite charges attract. Like charges repel. In electricity we follow the flow of the negative charges. hese are known as electrons. Current An electrical current is the quantity of charge that flows past a fixed point every second. current = I = charge time Q t Current is measured in amperes (A), charge is measured in coulombs (C) and time is measured in seconds (s). Potential Difference (Voltage) When a charge is moved by an electric field, the charge is given energy. his energy is then used elsewhere in the circuit. he energy per unit charge is known as the voltage. voltage = energy charge Direct and Alternating Current A direct current is one which flows in one direction. A good example of this is current from a battery or cell. he flow is always from the negative terminal to the positive terminal. An alternating current is when the negative and positive terminals reverse polarity at regular intervals. his causes the current to flow in one direction and then the opposite direction. he mains electrical supply is an alternating current. It has a frequency of 50 Hz. Circuits A current will only flow in a circuit which is complete. here must be at least one continuous connection between the two terminals of the power supply. Energy is supplied by the power supply and is converted into other forms of energy by the components in the circuit. V = E Q Voltage is measured in volts (V), energy is measured in Joules (J) and charge is measured in coulombs (C). In this circuit the electrons flow in a clockwise direction. 3

4 Components Circuits may consist of many components. In a component, an energy change normally takes place. For example, in a lamp electrical energy is converted into light (and heat). Component Function and use Most common energy source in an electric circuit. Cells have chemically stored energy. Energy is given to the electrons which pass through the cell. his energy is then passed on to components A combination of two or more cells connected in series. he voltage supplied by a battery is the sum of the voltages of each of the cells. A lamp converts electrical energy into light energy (and some heat too). It is an example of an output device. A switch is used to break the circuit. his creates an open circuit and stops electrons from flowing. A motor converts electrical energy into rotational kinetic energy. It does this using magnets and coils of current carrying wire. A motor is used as an output device. A microphone converts sound energy into electrical energy. It is an example of an input device. A loudspeaker converts electrical energy into sound energy. It is an example of an output device. A photovoltaic cell converts light energy into electrical energy. It may be used as an input device. It is the basis for the solar cells used in solar panels. A fuse is a safety device. It contains a piece of wire which melts when the current reaches a certain level. his isolates an appliance from the mains and protects the appliance and the user. Appliances usually have a 3A (under 700W) or 3A (over 700W) fuse. A diode is an electrical one-way-street. It only allows the current to flow in one direction. In the case of the one shown, electrons would only be able to flow from right to left. A capacitor is able to store charge. It is initially neutral, however when connected to a DC power supply, like a cell, one plate becomes positively charged while the other becomes negatively charged. If it is then connected across a lamp, the capacitor will discharge and the lamp will come on for a short period. It is often used in electronics for timing circuits, since its charge/discharge time can be controlled by its capacitance and the resistance of a resistor. A relay is an electrical switch. It allows a low voltage (6V) primary control circuit (see electronics section) to control a high voltage (30V) secondary circuit. 4

5 Series Circuits A series circuit is one in which the component are connected in a single loop. Parallel Circuits In a parallel circuit, current can take more than one path. In a series circuit: the current is the same at all points the voltages across the separate components add up to the supply voltage. In a parallel circuit: the currents in the branches add up to the current from the supply the voltage is the same across each branch and is equal to the supply voltage Ammeters and Voltmeters Each meter is responsible for measuring a different quantity and each meter is connected in a different way. Ammeters are connected in series (in line with) with components, while voltmeters are connected in parallel (across) components. Resistance Resistors reduce the current (flow of charge) in a circuit. Resistance is measured in ohms (Ω). An ohm meter is used to measure the resistance of a component directly. he ohm meter does not require a separate power supply and so is connected as shown in the diagram. Ohm's Law Ohm's Law describes the relationship between voltage, current and resistance. For a fixed value resistor there is a direct proportion relationship between the current through the resistor and the voltage across it. 0 his relationship is known as Ohm's Law. V = IR 5

6 Resistors is Series In a series circuit, the total resistance is the sum of all the individual resistances. Resistors is parallel he total resistance in parallel circuits is more complicated. he following equation can be used to calculate the total resistance. R = R + R + R R = R + R + R Electronics Electronic Systems All electronic systems consist of 3 main parts. Input Devices Input devices convert one form of energy into electrical energy. his allows an external signal to be converted into an electrical one, for use in the circuit. Examples include: solar cell light energy electrical energy thermocouple heat energy electrical energy 6

7 Dependant resistors as Input Devices Light Dependant Resistors Light Dependant Resistors, often abbreviated to LDR. hese resistors have a varying resistance which depends on light level. For an LDR as light level increases, resistance decreases. his is often abbreviated to L.U.R.D. to help you remember. hermistors hese resistors have a varying resistance which depends on temperature. For a thermistor as temperature level increases, resistance decreases. his is often abbreviated to.u.r.d. to help you remember. Variable resistors Variable resistors which can be controlled by the user. Analogue Systems An analogue system is often described as continuously variable. he signal can exist at any state between its maximum and minimum value. It is often described as a wave: Digital Systems A digital system is one in which the signal can only exist at one of two states, off and on, also often described as and 0: 0 he Light Emitting Diode he LED is an example of an output device. It converts electrical energy into light energy. Unlike a lamp it produces very little heat energy. he LED only allows a current to flow through it in one direction and so it must be connected the right way round. he LED should always be connected in series with a resistor. he purpose of the resistor is to reduce the current passing through the LED since too large a current will damage the LED. 7

8 LED Protective Resistor Calculation Consider an LED rated at V and 0mA connected to a 6V supply. his circuit is like a voltage divider. If there needs to be V across the LED then there must be 4V across the resistor. In a series circuit the current is the same through each of the component. We can now do a calculation using Ohm's Law. V = IR 4 = 0.0 R 4 R = 0.0 R = 400Ω Voltage Dividers In a circuit with more than one resistor the voltage of the supply is split across the resistors in that circuit. here are equations that can be helpful in finding missing values in these types of circuits. he ratio of the resistances is the same as the ratio of the voltages across them: V = V R R he voltage across the second resistor can be calculated using: R V = Vs R + R 8

9 Alternative Diagrams In these circuits the reading on the voltmeter is determined by the ratio of the resistors. he ratio can be controlled by the use of variable resistors including thermistors and LDRs. his makes them excellent as input devices. Voltage Dividers as Input Devices Light Sensing emperature Sensing As light/temperature increases the resistance of the LDR/thermistor decreases. As a result the voltage across the LDR/thermistor decreases. he reading on the voltmeter decreases. Connected in reverse they carry out the opposite function: As light/temperature increases the resistance of the LDR/thermistor decreases. As a result the voltage across the LDR/thermistor decreases. he share of the voltage across the fixed resistor increases and the reading on the voltmeter increases. 9

10 he npn transistors and the MOSFE ransistors are electronic switches. When connected in a circuit with a voltage divider they are controlled by the voltage across the lower resistor. here are two main types: npn transistor MOSFE switches "on" at 0.7V switches "on" at V Control Circuits Control circuits use all of the pieces we have looked at so far. hey include a sensing section, dependant on light or temperature, an electronic switch and an LED (or other output device). his is a temperature dependant circuit. As temperature increases the resistance of the thermistor decreases. As the resistance of the thermistor decreases the voltage across it also decreases. As the voltage across the thermistor decreases the voltage across resistor increases. When the voltage across resistor rises above 0.7V the transistor starts conducting and the LED goes on. he control circuit can be reversed by switching the locations of the resistors. he circuit can be calibrated by varying the resistance of the variable resistor. Capacitors as time delays In this type of circuit, the capacitor will take a set time to charge. As it charges, the circuit current decreases, the voltage across the resistor decreases and the voltage across the capacitor increases (eventually reaching the 6V of the power supply). When the voltage across the capacitor reaches 0.7V the transistor will conduct, allowing the LED to light. Swapping the locations of the resistor and capacitor will cause the LED to go off after a delay. 0

11 Electrical Power Power Power is the quantity of energy transformed from one form into another form (or other forms) every second. Power = P = Energy time Power is measured in watts (W), energy measured in joules (J) and time measured in seconds (s). E t watt = joule per second Electrical Power In electric circuits, the power transferred by a component can be calculated from the voltage across the component and the current passing through it. Power = Current Voltage P = IV If resistance is known, then the following two equations can also be used to determine power. hey are produced by combining Ohm's Law with P=IV. P V = I R P = R

12 Heat Heat Energy Heat is a form of energy. Heat energy can be produced from another form of energy or changed into another form, for example: - kinetic energy heat energy (through friction) - electrical energy heat energy (in a resistor) - heat energy electrical energy (thermocouple) As it is a form of energy heat is measured in Joules, J. emperature emperature is a measure of how hot or cold an object or substance is. emperature is measured in degrees Celsius, C. emperature can also be measured in Kelvin, K, (see Gas Law and Pressure). emperature is a measure of the average kinetic energy of the particles in a substance. his means that as the temperature of a substance increases, the kinetic energy (and therefore the speed) of the particles in that substance increases too. Changing emperature he temperature of a substance can be changed by supplying it with energy or removing energy from the substance (heating or cooling it). he temperature change depends on the energy supplied, the mass of the substance being heated and the material itself. Different materials have different specific heat capacities. his is the energy required for kg of a substance to achieve a temperature change of C. E H = cm EH - energy supplied/removed J c - specific heat capacity J kg - C - m - mass of the substance kg Δ - temperature change C

13 States of Matter here are 3 states of matter, solid, liquid and gas. A substance can change between states if it receives or loses energy. Changes of State Increasing energy solid liquid melting liquid gas evaporating Decreasing energy gas liquid condensation liquid solid freezing Changing state also requires an increase or decrease in energy. When a substance is changing state it does not experience a change in temperature. Cooling graph of a substance 3

14 Specific Latent Heat of Fusion (solid liquid) Energy is required to change the state of a substance from a solid to a liquid. he energy required depends on the mass of the substance and the substance itself. he property of the substance is known as specific latent heat of fusion. EH - energy supplied/removed J m - mass of the substance kg lf - specific latent heat of fusion J kg - E H = ml f Specific Latent Heat of Vaporisation (liquid gas) Energy is required to change the state of a substance from a liquid to a gas. he energy required depends on the mass of the substance and the substance itself. he property of the substance is known as specific latent heat of vaporisation. E H = ml v EH - energy supplied/removed J m - mass of the substance kg lv - specific latent heat of vaporisation J kg - Heat ransfer Heat energy can be transferred by three methods: Conduction - he vibrations of the hot particles are passed to their neighbours. his takes place in solids. Convection - Hot liquids or gases are less dense than colds ones and so they will rise and the cold gases and liquids will fall. his sets up a convection current. his takes place in liquids or gases. Radiation - A form of electromagnetic radiation (infrared) Energy is transferred as rays. his is the only method by which heat can be transferred through a vacuum. he greater the temperature difference the greater the rate of heat transfer. 4

15 Heat Loss Heat always moves from hot objects to cold objects. he rate of heat loss depends on the temperature difference between the two objects. Reducing Heat Loss Heat loss from the home can be reduced in the following ways: Loft insulation - Reduces heat loss through by conduction through the ceiling. Double Glazing - A vacuum between the panes of glass reduces heat loss by conduction. Draught Excluders - Reduce heat loss by convection. Foil-backed Plasterboard - Reduces heat loss by radiation from the walls. Foil-backed Radiators - Reflects heat back into room and so reduces heat loss by radiation. Cavity Wall Insulation - A foam filling between the walls prevents heat loss due to convection or conduction. Carpets - Reduces heat loss by conduction through floor. Pressure Defining Pressure Pressure is defined as the force exerted per unit area by one medium on another. his includes solids, liquids and gases. hat's right, gases exert pressure too. Calculating pressure Pressure = Force Area Pressure is measured in pascals (Pa) or newtons per square metre (N m - ) Force is measured in newtons (N) Area is measured in square metres (m ) pascal = newton per square metre 5

16 Comparing Pressures Pressure allows us to compare the effect that very different objects may have on their surroundings. ake the example of an elephant and the stiletto heel of a shoe. Which would you rather have step on your foot? Elephant Force exerted by elephant Force is weight so: W = mg = = N Stiletto Force exerted by stiletto Force is weight so: W = mg = = 490 N Area of elephants foot ~0.m Area of stiletto heel ~0.000m F F P = P = A A = = = Pa = Pa 6

17 Gas Laws Kinetic Model of a Gas Physicists often use models to describe situations that are difficult to visualise. his includes the very large and the very small. he kinetic model of a gas describes the motion of the particles in the gas and uses their behaviour to explain our observations of the gas itself. here are 3 important quantities about the gas that we can observe, which can be explained by visualising the particles within the gas. Volume - he volume of a gas is the 3 dimensional space the gas occupies. his space is usually determined by the container that the gas is in. We do not concern ourselves with the volume of the particles, only the volume of the gas as a whole. emperature - he temperature of a gas is a measure of the average kinetic energy of the particles in that gas. he greater the temperature, the greater the average kinetic energy of the particles and therefore the greater their speed. (Note that it is the temperature of the gas and the speed of the particles!) Pressure - he pressure of the gas is caused by the particles colliding with the walls of the container. he more frequent or violent the collisions, the more force is exerted on the walls of the container and therefore the greater the pressure. Relationship between Volume and Pressure of a Gas For a fixed mass of gas at a constant temperature, the pressure of a gas is inversely proportional to its volume: 0 0 P P V = constant V P V = P V 7

18 Relationship between Pressure and emperature of a Gas For a fixed mass of gas at a constant volume, the pressure of a gas increases as the temperature increases: 0 his is not a proportional relationship, as the line does not go through the origin. If we extend the graph into the negative temperature scale we see the following: 0 his is where we introduce the kelvin scale. -73 C is now equivalent to zero kelvin 0 We find that pressure is directly proportional to temperature, when temperature is measured in kelvin. P P = P P = constant 8

19 Converting between Celsius and Kelvin o convert between the two temperature scales we either add or subtract 73: Convert: C to K add 73 K to C subtract 73 Relationship between Volume and emperature of a Gas For a fixed mass of gas at a constant pressure, the volume of a gas is directly proportional to its temperature, when measured in kelvin elvin: 0 V V = constant V = V General Gas Equation All three gas laws combine to form the general gas equation: PV = constant P V = P V Remember: temperatures must be converted to kelvin to work in this equation! 9