National 4 Summary Notes

Transcription

1 North Berwick High School Department of Physics National 4 Summary Notes Unit 3 Electricity and Energy Physics N4 Unit 1: Pupil Notes Page 1 of 22

2 Section 1: Generation of Electricity What is electricity? Electricity is a form of energy associated with stationary or moving charges in a material. Electricity is the general name we give to the supply of electrical energy. Electrical energy is carried (or transferred) around a circuit by tiny particles called - electrons. Think about a lamp and a switch connected by wires to a battery. When the switch is closed, the lamp lights up negative charges called electrons from the negative side (terminal) of the battery move through the wires and the lamp to the positive terminal of the battery. This movement, or flow, of negative charges is called an electrical current (or current for short). A current is a movement of electrons. As the electrons move through the lamp, some of their electrical energy is changed into heat and light. Electricity is used to power many devices at home, school, or industry. Generating electricity Fuel burning power stations lose a lot of energy in the form of waste heat. Electrical energy is lost as electricity is transmitted through power lines; the greater the distance we need to send the energy the greater the power loss. Using a transformer we can change the output Voltage from the power station up to 400kV for connection to the super grid. A transformer consists of a primary coil, a core and a secondary coil. By using a higher voltage to transfer the electrical energy from the power stations to the National Grid we lose less energy to heat. Electricity pylons carry the electricity high above us as the high voltages used are very dangerous. Physics N4 Unit 1: Pupil Notes Page 2 of 22

3 Types of Fuel Non - renewable Renewable Uranium and the fossil fuels (coal, oil and gas.) Wind, waves, tidal, hydro, geothermal, bio fuels, etc Energy Advantage Disadvantage Wind Cheap to run clean Not always windy Wind farms use large area of land Turbines can be unslightly Solar Cheap to run Clean Expensive to install Sun is not always shining Wave Cheap to run Clean Wave size variable Suitable sites are distant form user Possible shipping hazard Tidal Cheap to run Clean Expensive to set up Few suitable locations Hydroelectric Cheap to run Clean Expensive to build Reservoirs use a large area of land Environmental damage Geothermal Cheap to run Clean Expensive to set up Few suitable locations Physics N4 Unit 1: Pupil Notes Page 3 of 22

4 Efficiency Not all of the energy we change (chemical, kinetic, heat, potential, etc.) to make electricity in Power Stations ends up as electrical energy. Some of this energy is wasted or lost, usually in the form of heat. Efficiency calculations are shown later in this booklet. Physics N4 Unit 1: Pupil Notes Page 4 of 22

5 Section 2. Practical electrical and electronic circuits Conductors and Insulators Negative charges (electrons) can only move from the negative terminal to the positive terminal of a battery if there is an electrical path between them. Materials that allow negative charges to move through them easily, to form an electrical current, are known as conductors. Materials, which do not allow electrons to move through them easily, are called insulators Conductors are mainly metals, such as copper, silver, gold, however, carbon is also a good conductor. Glass, plastic, wood and air are examples of insulators. Current Electrical current is a flow of negative charges called electrons. Electrons will flow through a conductor if there is a potential difference, i.e. if there is a power supply. Electrical current is given the symbol I and is measured in Amperes (A). Voltage Think about a lamp connected to a battery. The battery changes chemical energy (from the materials inside it) into electrical energy. This electrical energy is carried by the charges (electrons) that move around the circuit and is given up as heat and light as they pass through the wire of the lamp. The voltage supplied to a circuit is a measure of the electrical energy given to each electron in a circuit. When the supply voltage is increased, a larger current flows through the circuit. Voltage is measured in Volts (V). Physics N4 Unit 1: Pupil Notes Page 5 of 22

6 Direct Current When a circuit is connected to a battery, the electrons always flow round the circuit in one direction from negative to positive This is called direct current (d.c.). The battery supplies electrical energy to the electrons? Using Ammeters and Voltmeters We use an Ammeter to measure current in Amperes. We use a Voltmeter to measure voltage in Volts. When an ammeter is connected in a circuit to measure the current through a component, it is connected in series with the component. Remember, we have to break the circuit to insert the ammeter. When a voltmeter is connected in a circuit to measure the voltage across a component, it is connected in parallel with the component. Remember, we do not break the circuit; we simply attach the voltmeter across the component we are measuring. Physics N4 Unit 1: Pupil Notes Page 6 of 22

7 Series Circuits In a series circuit there is only one path for the current to flow. If one of the lamps is unscrewed, the rest go out because the circuit is no longer complete. Adding more lamps reduces the overall current in the circuit and also reduces the voltage available for each lamp. We can write the Series circuit rules as: I s = I 1 = I 2 = I 3 = I 4... Where I s is equal to the total current of the supply in a series circuit, the current is the same at ALL points in the circuit. V s = V 1 + V 2 + V 3... Where V s is equal to the voltage of the supply the Voltage of the supply is split across the different components in the circuit. Parallel Circuits Physics N4 Unit 1: Pupil Notes Page 7 of 22

8 In a parallel circuit there is more than one path (called a branch) for the current to flow. If one lamp is unscrewed, the others remain lit. We can write the Parallel circuit rules as: I s = I 1 + I 2 + I 3 + I 4... Where I s is equal to the total current of the supply in a parallel circuit, the current is split between the branches in the circuit. V s = V 1 = V 2 = V 3... Where V s is equal to the voltage of the supply the voltage across the different branches in the circuit are equal to the supply voltage. Adding more lamps in parallel does not affect the voltage of the individual branches. It will, however, increase the current drawn from the supply. Physics N4 Unit 1: Pupil Notes Page 8 of 22

9 Circuit Symbols Circuit symbols are used in electrical circuits to represent circuit components or devices to make them easier to draw and understand. Some of the circuit symbols you might need to know are shown below. Name Symbol Name Symbol Cell Fuse Battery Light Bulb DC Supply Motor AC Supply Switch (open) Switch (closed) Buzzer Resistor Variable Resistor Light Dependent Resistor Voltmeter Thermistor Ammeter Light Emitting Diode (LED) Diode Physics N4 Unit 1: Pupil Notes Page 9 of 22

10 Photodiode Capacitor Loudspeaker Microphone NOT Gate AND Gate OR Gate Physics N4 Unit 1: Pupil Notes Page 10 of 22

11 Section 3. Ohm's Law Resistance All materials oppose current passing through them. Note that wires / components with low resistance are generally termed conductors; those with high resistance are generally termed insulators. In circuits, the components resistance, the current and the voltage are all linked. To investigate this the following circuit is set up The voltage is varied and readings are taken as follows. Voltage (V) Current (A) Plotting voltage against current for these results gives the following graph: Physics N4 Unit 1: Pupil Notes Page 11 of 22

12 Potential Difference / V y = 50x R² = 1 Current / A The gradient of the line in the graph is 50, which matches the resistance of the resistor in the circuit. This means that V/I = R. We can state, then, that the voltage is equal to the current times the resistance. Symbol Name Unit Unit Symbol V Potential volts V Difference I Current amperes A R Resistance ohms Ω This relationship is known as Ohm s law and can be used for circuits with many types of components. Physics N4 Unit 1: Pupil Notes Page 12 of 22

13 Resistors in series R 1 R 2 R 3 The resistors here are shown in series. If you imagine that each resistor is a tunnel and lots of people want to get from the left to the right, each tunnel is going to slow them down. In other words, the delay at each resistor will add up. In equation form, this is Symbol Name Unit Unit Symbol R total Total Resistance ohms Ω R 1 Resistance 1 ohms Ω R 2 Resistance 2 ohms Ω R 3 etc Other resistors ohms Ω In this equation we put at the end as there can be any number of resistances. Worked examples 1. Calculate the resistance of the following circuit R total R 1 R 2 R 3 A =? = 60 Ω = 35 Ω = 22 Ω R total R total R total = R 1 + R 2 + R 3 = = 117 Ω 2. The total resistance of this circuit is 25 kω. Calculate the value of Resistor 2 V R total R 1 R 2 R 3 = 25 kω = 12 kω =? = 500 Ω R total R 2 R 2 R 2 = R 1 + R 2 + R 3 = R = = = 12.5 kω Physics N4 Unit 1: Pupil Notes Page 13 of 22

14 A variable resistor is a resistor whose resistance can be changed. The resistance is normally changed by altering the length of the wire in the resistor (the longer the wire, the higher the resistance). Variable resistors are often used a volume or brightness controls on televisions or dimmer switches for lights. A variable resistor changes the amount of resistance in a circuit. A resistor converts electrical energy into heat energy. Worked example: A 12 V battery supplies a motor which has a resistance of 18 Ω. What is the current in the circuit? V R I = 12 V = 18 Ω =? V = 12 = I = I = IR I 18 12/ A Physics N4 Unit 1: Pupil Notes Page 14 of 22

15 Section 5 Electrical Components and Electronics Practical series and parallel circuits. Series circuits are often used in lighting circuits, continuity testers, old style Christmas lights: benefits (simple to construct) and drawbacks (one component fails, the whole circuit breaks). Here are some examples of useful series circuits: Stair lighting uses two or more 2-way switches in series: Continuity testers: Simple conductors can be tested for continuity by trying to pass a current through them in series with some indicator device (bulb, buzzer, ammeter) If there is a current, the series circuit is complete If there is no current, there is a break in the circuit. Never use a continuity tester with any equipment connected to the mains supply!!! Physics N4 Unit 1: Pupil Notes Page 15 of 22

16 Parallel circuits These are often used in new style Christmas lights, car lighting circuits, and household ring circuit: benefits (one component fails only that branch is broken) and drawbacks (more complex). Car lighting circuit: If one bulb blows, the rest of the lights must stay on Sidelights should come on first and together; headlamps come on after side lamps Car often uses the car chassis as a return instead of wire Positive is the live and the negative is attached to chassis for return Physics N4 Unit 1: Pupil Notes Page 16 of 22

17 Electronic systems and Digital and Analogue signals. What do electronic systems do? In their simplest form they respond to an input, process this information in some way and then provide a response (an output). We can draw a block diagram like this to represent any electronic system: Input devices generally change some type of physical quantity (light, heat, sound, touch) into an electrical signal. Process devices take these electrical signals and change them in some way. Output devices turn this electrical signal into something that we can use or react to; for instance sound, light, heat, movement, etc. There are two types of signals used by electronic systems: Analogue and Digital. Analogue signals can have a continuous (changing) value: Physics N4 Unit 1: Pupil Notes Page 17 of 22

18 Digital signals can only have two possible values: These are: Maximum (ON): called logic 1 or logic high. Minimum (OFF): called logic 0 or logic low. Digital versus Analogue signals: Digital signals carry more information per second than analogue signals. Digital signals maintain their shape over distances far better than analogue signals Logic Gates Logic gates are digital devices. They are used to combine or change digital electronic signals. There are 3 basic types of logic gate called the NOT gate (sometimes called an inverter), the AND gate and the OR gate They accept inputs of High (1) or Low (0) They produce outputs of High (1) or Low (0) Remember our simple electronic system block diagram: - Inputs and outputs are represented on a truth table. Physics N4 Unit 1: Pupil Notes Page 18 of 22

19 The NOT gate INVERTS its input signal that is it flips a 1 to a zero and a zero to a one! The output is always the opposite of the input. E.g. Input HIGH (1) gives us an Output LOW (0) NOT gate truth table: INPUT OUTPUT Physics N4 Unit 1: Pupil Notes Page 19 of 22

20 Physics N4 Unit 1: Pupil Notes Page 20 of 22

21 The NOT gate only has ONE input! The AND gate has TWO inputs: Input A and B. It requires both inputs (A and B) to be high to give an output of high. If both inputs are low the output will be low. If the inputs are different the output is low. AND gate truth table: Input A Input B Output A simple way to remember this the AND gate is shaped like a D The OR gate has 2 inputs: The output will be logic 1 when either A or B are logic 1 Input A Input B Output Real-life electronic systems Electronic systems are part of our daily lives, so much so that we probably do not even notice them. The following inputs and outputs can be combined with logic gates, for example, how do streetlights turn on at night and off in the morning? Physics N4 Unit 1: Pupil Notes Page 21 of 22

22 A We want the streetlight to turn on at night, but the light sensor switches on when it detects light. A NOT gate can turn it into a darkness sensor. Physics N4 Unit 1: Pupil Notes Page 22 of 22