P5 Electric Circuits Route map Over the next 12 lessons you will study : Friday 21 October 2011 P5.1 Static electricity P5.2 Electric Charge & Circuit symbols P5.3 Simple Circuits P5.4 Electric Current End of module test P5.5 Controlling Current P5.6 Measuring resistance P5.7 Resistor combinations P5.8 Measuring Voltage P5.9 Electrical Power P5.10 Domestic Appliances P5.11 Generating Electricity P5.12 Distributing Electricity
P5.1 Static Electricity Decide whether the following statements are true or false:
Understand that static electricity can be produced when one material is rubbed against another material
Understand that there are two types of charge: positive and negative and to know that ‘like’ charge attracts and ‘unlike’ charge repels
Friday 21 October 2011 First activity: Describe three examples of static electricity occurring in in everyday life, for example static charges creating when taking of a woolly jumper. Literacy: Static electricity, electric charge, repel, repulsion, attract, attractions, friction, rubbing, positive, negative, electron, electric field and electric current. Numeracy: A Lightning bolt travels at about 14,000mph bringing 300,000 volts of electricity to the ground in just a few milliseconds, and heating up the air to 30,000°C - 5 times hotter than the sun’s surface. PLTS Independent enquirers Creative thinkers Reflective learners We will focus on Team workers Effective participators Self managers
P5.1 Static Electricity Extension questions: 1: Explain what happens when a charged plastic comb is brought close to a stream of water coming from a tap ? 2: Explain why sometimes a TV screen becomes quickly dusty again when it is wiped with a cloth ? 3: The moving blades of a helicopter can charge up a helicopter when it flies. Explain why this happens ? 4: Before someone is lowered to the ground from a helicopter they touch the ground with an insulated metal lead. Why do they do this ? Know this: a: Know that static charges arise as the result of friction between two insulating materials. b: Know that opposite charges attract and like charges repel. c: Know that electrons are negatively charged. Friday 21 October 2011 Introduction: Electric current is the flow of electric charge. Some insulating materials become electrically charged when they are rubbed together. A substance that gains electrons becomes negatively charged, while a substance that loses electrons becomes positively charged. Charges that are the same (eg positive and positive) repel, while unlike charges (eg positive and negative) attract. Charged objects are able to attract small uncharged objects towards them, such as pieces of paper. Only by rubbing different insulating materials against each other do they become electrically charged, because conducting material direct the charge flow to earth.
P5.1 a Look at the photograph and information and answer all the questions: Count the and sum the charges for each balloon in diagrams one to three and work out whether they will attract or repel one another ? Explain why DVD or blue-ray players attract dust ? Counting charges and working out whether an object is negatively or positively charged is simple. Remember, only electrons are lost or gained from materials because of friction. When an object gains electrons it becomes negative, and when it loses electrons it becomes positive. In the example below, simply count off the positive and negative charges and determine the overall charge on the balloon. If the two charges are opposite they will attract, if they are the same they will repel. Counting static charges Rubbing a balloon against your hair can give it a charge. We can count charges by summing negative and positive charges for each balloon. Work out for each pair, whether they will attract, repel or do nothing ? Key concepts + _ _ _ _ + + _ + _ + + + _ _ + + _ _ _ + + + + + + + + + + + _ + _ + + _ 1 2 3
P5.1 b Look at the photograph and information and answer all the questions: Explain how the toner follows the pattern or text of the image or writing that is being photocopied ? What would happen to the photocopied image if the toner was not heated during the final step ? All photocopies use static charges. All photocopiers work by doing the following: The drum is positively charged. Light from the image to be copied, when it hits the charged drum allows electrical charge to flow to Earth, leaving these areas without charge and the dark areas with a positive charge. Negative toner is then attracted to the positively charged areas of the drum. The toner is then transferred and fixed onto the paper by heating. Key concepts + + + + + + light image positive charge toner plain paper paper copy heater ++ ++ + + + Roller image transfer 1 2 3 4 5 +
P5.1 Plenary Lesson summary: opposite same equal repel Friday 21 October 2011 When paint is sprayed from a paint gun, the painter normally needs to use a fair amount of skill to ensure the paint goes on evenly. By connecting the spray nozzle to a negative electrode, it is possible to charge each droplet of paint. If the car part is then given the opposite charge, the paint droplets will be attracted to the car body part. So there is less mess and less wastage. How Science Works: Research into what is a moving charge or current and look into simple circuits including the similarities and difference between parallel and series circuits Preparing for the next lesson: If two identical rods are rubbed with a cloth they will become charged. The charge on each of the rods is the _______, so they will _______ one another. The charge on the cloth will be _______ and ________ to the charge on the rod. Decide whether the following statements are true or false : False True 3: It is dangerous to walk around in a lightning storm with a spiked umbrella ? False True 2: Rubber soles on your shoes will help you stay charged ? False True 1: Two positive charges will attract one another ?
P5.2 Electric charge and circuit symbols Decide whether the following statements are true or false:
Understand what is an electric charge and how a Van de Graaff Generator can produce high voltage sparks
Understand that when charge flows it behaves like an electric current
Understand how to interpret and recognise circuit diagrams and symbols
Friday 21 October 2011 First activity: Give four examples of an electrical insulator that will not let current flow and four examples of an electrical conductor that will allow current to flow. Give the four components required to make a simple circuit that will allow a current to flow to a bulb ? Literacy: Circuits, circuit diagrams, symbols, ammeter, voltmeter, battery, cell, resistance, variable resistance, switch, bulb, diode, fuse, connecting wire, insulator and conductor Numeracy: Hundreds of thousands of volts can be generated with a school Van de Graaff generator! However the sparks from the Van de Graaff are not life threatening because the current flowing is so small . PLTS Independent enquirers Creative thinkers Reflective learners We will focus on Team workers Effective participators Self managers
P5.2 Electric charge and circuit symbols Extension questions: 1: If you attach a ‘head of hair’ to the top of the Van de Graaff and switch the Van de Graaff on, what would you see and why ? 2: There are 2 cells & 2 lamps in a one loop (series) circuit. When the switch is closed do they light separately or together ? Explain why. 3: If one of the lamps ‘blows’, what happens then ? Explain why. 4: If you forget to touch the metal end of the mains testing screwdriver when testing how charges flow, what will happen and why ? Know this: a: Know that when a charge flows it behave like a current c: Know how to represent simple circuits using circuit symbols and diagrams. A battery is a collection of cells. c: Know that a current will only flow in a circuit if the circuit is complete. Friday 21 October 2011 Introduction: When a Van de Graaff is running, charge collects on the dome. As the charge builds up, the electric field around the dome gets stronger and the charge may jump to a nearby object in the form of a spark. If a mains testing screwdriver is held close to the dome and its metal end is touched the indicator lamp inside the screwdriver lights up So there is an electric current flowing thorough the lamp making it light. This is because charge is escaping from the dome and travelling across the air gap, through the screwdriver and indicator lamp, and through the person touching the screwdriver’s metal end and finally to ‘earth’. In an electrical circuit, all the components have charges that are free to move. When a circuit is made, the battery causes these free charges to move. They then move in a continuous loop around the circuit.
P5.2 a Look at the photograph and information and answer all the questions: Explain why the boy’s hair look as it does in the photograph above left ? Explain what happens during the discharge or the charged dome when you bring a earthed conducting object like a metal rod close to the van der Graaf metal dome ? Friction between objects can cause a build up of an electrical charge. The Van de Graaff generator demonstrates the nature of static charges. Electrons are deposited on the metal dome causing the build up of a negative charge. If you’re touching the generator, your hair may spread out with individual hair repelling one another. Discharging the dome is easy. If you bring another metal object close to the dome, it will discharge safely to Earth with a spark jumping across the gap. The van der Graaf generator Key concepts 1 2 3
P5.2 b Look at the photograph and information and answer all the questions: Simple devices like torches make use of series circuits that connect a single device like a bulb to a switch and a power supply, for example a battery. A series circuit, unlike parallel circuits, only provide one route for the electrons to flow. The drawback of using series circuits is that if one component fails, the circuit is broken causing total circuit failure. Look at the three series circuits opposite. Explain what happens to the brightness of the bulbs as you add bulbs in a series circuit ? In a circuit what carries energy around the circuit from the power supply to the device like a bulb or buzzer ? Look at the circuit symbols opposite, try and name all of them ? A simple series circuit V A Circuit symbols Key concepts + - M
P5.2 Plenary Lesson summary: charge high motor object Friday 21 October 2011 Smoke contains lots of tiny bits of dirt. If people breathe in the dirt it can damage their lungs. The particles of dirt can be removed by (a) passing the smoke through a positively charged metal grid so that the bits of dirt pick up a positive charge and then (b) attracting the positive bits of dirt to negatively charged pieces of metal further up the chimney. How Science Works: Research into current models which help explain about charges in a electric circuit and how negatively charged electrons flow around a simple circuit. Preparing for the next lesson: The _______ voltage sparks from a Van de Graaff show the path being taken by ________ as it travels through the air. The charge then flows through the ______ or the person and down to the earth. A eclectic current is a flow of charge. A current can do useful work like light a bulb or turn an electric _______. Decide whether the following statements are true or false : False True 3: This is a the circuit symbol for a voltmeter ? False True 2: A voltmeter is used to measure how much current is flowing in the circuit ? False True 1: The sparks from a Van de Graaff are not dangerous to the average person ?
P5.3 Simple Circuits Decide whether the following statements are true or false:
Understand how to use symbols and circuit diagrams to represent a simple circuit diagrams
Understand how charges flow in a electric circuit and know that direction of conventional current is from positive to negative
Friday 21 October 2011 First activity: In the ‘peas in a pipe model’ of an electric circuit on page 154, what models the part of the battery, the moving charge and a resistance ? Literacy: Circuit, current, charge, flow, electrons, positive, negative, , battery, circuit diagrams, cell, bulbs, battery, cells, wires, components and switches. Numeracy: A 500 milliamp-hour battery could produce 5 milliamps for 100 hours, or 10 milliamps for 50 hours, or 25 milliamps for 20 hours, or (theoretically) 500 milliamps for 1 hour. PLTS Independent enquirers Creative thinkers Reflective learners We will focus on Team workers Effective participators Self managers
P5.3 Simple Circuits Extension questions: 1: The moving rope model of an electric circuit has one person moving a loop of rope through the hands of a number of other people who are holding the rope loosely. For this model what plays the part of the battery, the charge flowing and the resistance? 2: If the people supporting the loop of rope held it slightly tighter what effects would be noticed and how do they relate to an electrical circuit ? 3: In the same model how could you illustrate the effect of putting a second battery in the circuit – first pointing in the same direction as the first battery and then in the opposite direction ? Know this: a: Know that conventional current flows from +ve to -ve. b: Know that the current flow along a metal wire is due to the drift of electrons in the opposite direction to the marked current flow. Friday 21 October 2011 Introduction: The chemical reactions inside the battery have the effect of separating electric charges, so that positive charge collects on one terminal of the battery and negative charge on the other. If the battery is connected into the circuit the charges on the battery terminals sets up an electric field in the wires of the circuit which makes the free charges in the wires drift slowly along the wires. Although the charges move slowly they all begin to move at once, starting at the moment the battery is connected. Scientists developed this model of how a battery worked before they knew much about the structure of the atom and at that time they decided that the direction of the current should be from positive to negative. We now know that it is the drift/flow of the negative electrons that makes the current and that the charges actually move from the negative terminal to the positive terminal. Although we understand what really happens, conventional current is still defined as flowing from positive to negative.
P5.3 a Look at the photograph and information and answer all the questions: The key ides of any model of an electric circuit is that a) charges are present throughout the circuits at all time b) When the circuit is completed, the battery provide the energy to move the charge in a current flow c) all the charges move around together d) the charges flow through the battery itself and e) the moving charge can do useful work when is travels through a component. In the simple circuit shown opposite left, explain what the voltage would be across the bulb ? Charges are present in all material but only free to move in conductors. Explain why conducts like copper metal are used to carry a moving charge in a circuit ? What would happen to the moving charge is the electrical switch was open ? Modelling what happens in a simple circuit Bulb Cell Switch components Switch Key concepts + - 6V V pump + - 6V V + - 6V
P5.3 b Look at the photograph and information and answer all the questions: Any model is a way of thinking or describing how something works. In the model left, the charges are shown by the delivery van moving away from the battery which is shown by the factory. The delivery van moves around the circuit (track) where is passes through a component shown by the shop offloading its goods which represent energy.
Describe how you could change the delivery truck model to include ?
more than one battery
Modelling what happens in a simple circuit
For the delivery truck model opposite write down what corresponds to ?
the battery in the circuit
the electric current
a flat battery
the component that is using the energy
how energy is transferred around the circuit
Bulb Cell Switch Key concepts + - 6V V + - 6V
P5.3 Plenary Lesson summary: current drift negative positive Friday 21 October 2011 If you unscrew one of the bulbs in a Christmas tree circuit all the lights go out. This is because all the bulbs are connected in series and when one is unscrewed it leaves a break in the circuit – so the current stops flowing. If two of more bulbs are broken it is almost impossible to find the faulty bulbs therefore making the tree lights useless ! How Science Works: Research into how current flows around a circuit and typical current use for different appliances. Look into series and parallel circuits. Preparing for the next lesson: Conventional current is still defined as flowing from the ________ to the negative terminal. Even though we now know that it is the ______/flow of the negative electrons that makes the _______ and that the charges actually move from the ________ terminal to the positive terminal. Decide whether the following statements are true or false : False True 3: When electrons flow through a cell or battery they gain energy ? False True 2: When electrons flow through a component they lose some of their energy ? False True 1: The electron carries a positive charge as it flows around a complete circuit ?
P5.4 Electric current Decide whether the following statements are true or false:
Understand that current is a flow of charge and in a simple series circuit it is the same everywhere.
Understand the in parallel or branched circuits, the current in the separate branches of a branched circuit add up to the current just before it divided (or after it joined)
Friday 21 October 2011 First activity: On your mini white board or in your exercise - draw a one loop circuit with 2 batteries (cells) facing the same direction, one switch on the positive side of the battery, two light bulbs, a resistor and an ammeter ? Literacy: Series circuits, parallel circuits, ammeter, flow, electrons, direction, terminal, current, in series, in parallel, coulomb and charge Numeracy: One amp of current flowing around a completed circuit is one coulomb of charge flowing for one second. You need just over 6 million million million electrons flowing inside a wire, if you are to get one coulomb of charge. PLTS Independent enquirers Creative thinkers Reflective learners We will focus on Team workers Effective participators Self managers
P5.4 Electric current Extension questions: 1: Explain what happens to a current in a simple circuit when a) the switch is open b) it goes through a battery and c) goes through a bulb ? 2: Match the following currents with the following devices a) 0.1 Amp b) 100 Amp c) 13 Amp i) electric chair ii) hairdryer iii) laptop ? 3: Convert the following currents into milliamps (mA) a) 1 amp b) 0.1 amp c) 13 amps 4: A current in an a) electric kettle is 8000mA. What is it in Amps and b) the current in a digital watch is 0.000 04A what is it in Amps Friday 21 October 2011 Introduction: An electric current is a flow of charge. The current through a torch bulb makes the fine wire of the filament heat up and glow. The bigger the current through the bulb the brighter it glows. A better way of measuring the size of electric current is to use an ammeter. The reading on the ammeter in amperes or amps (A) for short tells you how much charge is flowing through the ammeter every second. In a simple electric circuit made up of one single loop (called a series circuit) – the current is the same every where. In a circuit with branches (a parallel circuit), the current splits at the junctions with some charge flowing through one branch and the rest flowing through the other branch. The current in the two branches must add up to the total current in the wire either before or after the circuit branches Know this: a: Know that current is a flow of charge and behaves differently in series and parallel circuits b: Know that currents smaller than one amp are often measured in either milliamps (mA) or microamps ( : 1A = 1000mA or 1000 000 A
Key concepts P5.4 a Look at the photograph and information and answer all the questions: Explain why a light bulb only draw a current of 0.1 amps whereas a kettle requires a current 130 times larges at 13 amps ? A student was overheard saying that a taser gun wasn’t fatal because although it used a very high voltage (10,000 V) it used only a very small current…is he right or wrong ? Current (I) is measured in amps. The larger the current, the greater the flow of charge or electrons along a circuit. Current is measured with an ammeter, connected into a circuit in series. Current tells us how many energy carrying electrons there are flowing around the circuit. A two amp current will have twice the number of electrons when compared to a one amp current. The higher the current, the more energy a circuit supplies a device. Current use by different appliances A light bulb draws a current of 0.1 amps because it require very few energy carrying electrons to produce light. A kettle draws a current of 13 amps because it requires lots of energy carrying electrons to heat water to make tea or coffee. A cooker draws a current of 30 amps because it requires lots of energy carrying electrons to heat food by using the oven.
P5.4 b Look at the photograph and information and answer all the questions: Draw a parallel circuit with 4 bulbs, 1 power supply (6V cell), 4 switches and 4 branches...use the correct circuit symbols ! Look at the parallel circuit opposite. If the current at A1 is 0.6 amps and at A2 0.2 amps and A3 0.12 amps, work out the current at A4 and A5 ? Current in parallel circuits In a simple one loop series circuit the current is the same anywhere in that circuit, for example A1 A2 and A3. As you add components like bulbs, each bulb becomes dimmer because they share the voltage. Complex devices use parallel circuits which offer many different routes for electrons to flow. In parallel circuits, bulb brightness remains the same, despite adding more and more bulbs, because the current can divide itself between the bulbs. Also, if one bulb breaks, the other stays lit, because there is more than one route for the electrons to flow. Current in a series circuits Key concepts + - 6V + - 6V A1 A2 A3 + - 6V + - 6V A1 A5 A2 A3 A4
Key concepts P5.4 c Look at the photograph and information and answer all the questions: In circuit two A2 has a value of 0.6 amps and A3 has a value of 0.5 amps. What is the value of A1 and A4 ? In circuit three A1 has a value of 2 amps, A2 has a value of 0.3 amps and A3 has a value of 1.1 amps. Work out the value of A4 and A5 ? Current (I) is a measure of moving charge passing a point in a closed circuit every second. At the junctions of parallel circuits, the current splits with some charges flowing through one branch and some charges flowing through the over branch. The amount of charge flowing through the braches of any parallel circuit must equal th total amount of charge in a single wire before of after the branching point in a circuit. Parallel circuits circuit one circuit two circuit three + - 6V V + - 6V + - 6V + - 6V + - 6V + - 6V A1 A3 A2 A4 A1 A3 A2 A4 A5
P5.4 Plenary Lesson summary: series current equal branches Friday 21 October 2011 The electric light bulbs in a house are connected in a parallel circuit, with each light bulb on a separate branch with its own switch so that the light can be switched on and off without affecting the other lights. Also if one bulb fails the other are still able to work in a parallel circuit. How Science Works: Research into how batteries work and what different voltages battery have and look also into resistance, what causes it and using resistance. Preparing for the next lesson: The _________ which is measured using an ammeter in a simple ________ circuit (one loop) is the same everywhere in the circuit. The total current in the two minor ________ of a parallel circuit is _________ to the current in the circuit before the circuit branched. Decide whether the following statements are true or false : False True 3: A milliamp is bigger than a microamp ? False True 2: A radio has 0. 2A flowing through it and this is equal to 2mA ? False True 1: The greater the current through a bulb – the brighter it is ?
P5.5 Controlling current Friday 21 October 2011 Literacy: Battery, cell, power, chemical energy, electrical energy, voltage, voltmeter, current, ammeter, resistance, ohms, Ohm’s law, circuit and components Numeracy: Imagine a torch with a bulb that requires a voltage of 9 volts to shine brightly. A torch designer can either uses a single 9 volt battery or six 1.5 volt batteries all connected in series. PLTS Independent enquirers Creative thinkers Reflective learners We will focus on Team workers Effective participators Self managers
Understand that a larger battery size (eg size D instead of a size AA) contains more chemical and will therefore produce electrical energy for longer
Understand that a higher battery voltage will produce more push and a greater current.
Understand what is resistance and the factors that affect resistance in a circuit.
First activity: On your mini white board - draw a two loop circuit with 3 batteries (cells) facing the same direction and one switch on the negative side of the battery on the top line, one light bulb and a switch on the second line, and two bulbs and a resistor on the bottom line ?
P5.5 Controlling current Extension questions: 1: Explain why a watch only requires a very small low voltage battery compared to a car which uses a large high voltage battery ? 2: A torch bulb requires a voltage of 6 volts to work, how many 1.5. volt AA batteries would be needed to produce this voltage and how should they be connected ? 3: Suggest two different ways in which you could change a simple electric circuit to make the electric current bigger ? 4: Explain why resistance can be both useful and also unwanted ? Friday 21 October 2011 Introduction: A battery changes chemical energy into electrical energy. Larger batteries hold more chemicals and so can produce more electrical energy and can last for longer before they become used up. Batteries can also have different voltages – a 4.5V battery is actually made from three 1.5V batteries joined in series. All components and wire (although low) have a resistance. A copper wire which carries charge form a battery or a component like a bulb has a fairly low resistance. The filament wire inside the bulb has a very high resistance. Inside the filament wire, the electrons collide with the atoms making them vibrate and heat up. In fact so extreme is the heating effect that the filament glows white hot therefore produce light. The different components in a circuit provide a resistance to the flow of charge. The bigger the resistance the smaller the current flow. Know this: a: Know that energy from a battery is moved by a current around a circuit. b: Know what resistance is and when it is both useful and unwanted c: Know that the symbol for voltage is V, the symbol for current is I and the symbol for resistance is R
Key concepts P5.5 a Look at the photograph and information and answer all the questions: Work out the voltage in the three circuits (a, b & c) if battery a supplies 1.5. volts battery b supplies 9 volts and battery c supplies 3 volts ? If a single 1.5 battery lasts 3 hour powering a torch how long would 3 batteries last in parallel as shown by the diagram above right ? The battery in a simple circuit gives the ‘push’ to the charge/electrons so they can flow around the circuit. Batteries in series: Their voltages sum. So if you have three batteries each with a voltage of 1.5 volts in series the total circuit voltage would be 4.5. volts. Batteries in parallel: The voltage does not change, but if you place two batteries in parallel they last twice as long. Batteries in series and parallel 1.5 volt battery cells in series cells in parallel + + + + + + a b c - - - - - -
Key concepts P5.5 b Look at the photograph and information and answer all the questions: Although gold is a better conductor when compared to copper why is it not used in domestic plugs and electrical cables ? Give three device found in the home that use high resistance wire and the heating effect caused when electrons collide in to the atoms of poor conducting metals ? Resistance, measured in ohms describes the impedance to a flow of electrons or current through a conducting material. Generally metals have a low resistance, they are good conductors. Gold and copper have the lowest resistance of all the metals. Interestingly, metals like nickel and chromium are poor conductors, having quite a high resistance. Insulators like the plastic coating of electrical cables have a very high resistance. Materials like plastics do not allow any current to flow through them. What is resistance conductor (metal) electrons are free to flow as a moving charge Insulator (plastics) electrons are not free to flow in a moving charge
Key concepts P5.5 c Look at the photograph and information and answer all the questions: Give three devices found in the home that use variable resistor to control the current that they use ? As you decrease the volume of a radio, the volume dial (variable resistor) becomes hotter explain why this happens ? Using a variable resistor, we can change the resistance to a current flow through a circuit. A variable resistor allows us to change the amount of voltage and current that flows to a device. There are many examples of devices that use variable resistors. When you turn down the volume on a radio, you are using a variable resistor. Other examples include; light dimmer switches, dimmer switches for computer screens and temperature control dials found on a cooker Controlling resistance using a variable resistor A variable resistor controls the amount of current that a component like a bulb receives. variable resistor used to control volume for a radio + -
P5.5 Plenary Lesson summary: chemical larger resistance current Friday 21 October 2011 The moving charges in most metals are free electrons. As these electrons move they collide with the fixed array of atoms in the wire. These collisions make the atoms vibrate a little more – so the temperature rises. So when a current flows the temperature of a wire will rise – We make use of this effect in an electric toaster or electric kettle How Science Works: Research into how resistors are used in circuit to control current to components and what is potential difference and how is it measure across a component and in a circuit. Preparing for the next lesson: A size AAA battery will not hold as much ________ as an AA battery and so will not be able to produce as much electrical energy as the AA battery. If a battery has a high voltage it will push a _______ current round a circuit. If however the ________ of the circuit is made bigger then the _________ flow will be smaller. Decide whether the following statements are true or false : False True 3: Resistance, voltage and current are linked by Ohm’s law ? False True 2: The bigger the resistance the bigger the current flow ? False True 1: The bigger the voltage the bigger the push ?
P5.6 Measuring Resistance Decide whether the following statements are true or false:
Understand how Ohm’s law states that the current through a conductor is proportional to the voltage across it, provided the temperature remains constant.
Understand that only some metals and their alloys obey Ohm’s law and that resistance is measured in ohms
Friday 21 October 2011 First activity: On your mini white board or in your exercise book - draw a series circuit with 2 batteries (cells) facing the same direction, one switch on the positive side of the battery and then an ammeter, a resistor and a variable resistance. Finally draw in a voltmeter in parallel with the resistor. Literacy: Resistance, ohms, current, voltage, Ohm’s Law, variable resistance, light dependent resistor (LDR), thermistor (temperature dependent resistor), voltage, potential difference and voltmeter. Numeracy: The resistance of a light dependent resistor can be as high as 2000 in the dark and as low as 500 when a light is shone on it. PLTS Independent enquirers Creative thinkers Reflective learners We will focus on Team workers Effective participators Self managers
P5.6 Measuring Resistance Extension questions: 1: Give 3 devices that use high resistance wire to produce a heating effect ? 2: As a wire heats up, the resistance increase because the metal atoms vibrate more making it more difficult for the electrons to move through the wire. Explain why resistance also increase when a) the length of the wire increases and b) the diameter of the wire reduces ? 3: The resistance of a resistor is 12W and the voltage across the resistor is 6V. What is the current flowing through the resistor ? 4: Give three devices that use a variable resistor to alter the resistance (or current flow) in a circuit ? Know this: a: Know that Ohm's law relates voltage and current and that resistance is measured in Ohms. b: Know that materials such as metals and alloys that follow Ohm's Law are often called ohmic conductors. Friday 21 October 2011 Introduction: If you measure the current flowing through a conductor you will find that is proportional to the potential difference across it. This is effect is known as Ohm’s Law and is true for most metals and alloys provided that the physical conditions remain the same. If you then plot a graph of the voltage against the current it will be a straight line through the origin, if the material obeys Ohm’s law. In that graph the gradient is equal to R the resistance. Interestingly, carbon in the form of graphite is a non-metal that also follows Ohm's Law. A variable resistance or (rheostat) can be used to change the resistance of a circuit and so alter the current flowing through the circuit.
Key concepts P5.6 a Look at the photograph and information and answer all the questions: What would the resistance (R) and Current (I) be in the above copper wire if the voltage was set at 3 Volts ? Why is it important the temperature of the copper wire during this experiment remains constant ? Determining resistance in a wire requires a calculation. Ohm’s law tells us that the resistance in a wire is simply the voltage divided by the current. Resistance has the units ohms. The data above shows that the resistance in a copper wire of a fixed diameter and length does not change despite changing the voltage or current, providing the temperature of the wire also remains constant. The graph’s gradient gives us the resistance for any voltage or current. Voltage, current and resistance in an Ohmic conductor Voltage (V) Current (I) Resistance (R) 0.0 0.0 0.0 2.0 1.5 1.33 4.0 3.0 1.33 6.0 4.5 1.33 8.0 6.0 1.33 10.0 7.5 1.33 Current (Amps) Voltage (Volts) Resistance = M (Gradient) 0 2 4 6 8 10 7.5 6.0 4.5 3.0 1.5 0
Key concepts P5.6 b Look at the photograph and information and answer all the questions: Explain in terms for moving charge and metallic atoms why) the resistance decrease as you increase the diameter ? Explain use the graph showing temperature verses resistance why device that use complex circuits board like desktops and laptops are cooled using a fan ? Resistance in circuits can be useful and useless. Unwanted resistance to a current flow will waste energy, for example, when we use a laptop we want the processor to perform calculations, the screen to create and image and the speaker to create sound. The heat cause by resistance in the circuit and its components is wasteful and heat build up has to be prevented by a min fan which will prevent the circuits from over heating. The resistance in a wire can be affected by three factors Factors affecting resistance in an Ohmic conductor
Key concepts P5.6 c Look at the photograph and information and answer all the questions: Give two other uses for light dependant resistors ? How could you show that the resistance changes as the light level changes ? light dependant resistor (LDR) is a special type of resistor which can be used to respond to changing light levels. It is made from a special material which changes its resistance to a current flow according to light levels. When light levels are low, resistance through the LDR is high. This allows through less current. When light levels increase, the resistance to a current flow reduces, allowing a larger current to flow. Light dependant resistors Using lighting at the right time, whether it is day or night is very important. Street lighting needs to come on automatically and at the right time...when it begins to become dark. It also needs to be switched off when daylight starts. LDRs can be used to control these lights.
Key concepts P5.6 d Look at the photograph and information and answer all the questions: A temperature dependant resistor (thermistor) is a special type of resistor which can be used to control current in response to changing temperatures. It is made from material which change its resistance according to temperature. When the temperature is low, resistance through the thermistor is high. This allows through less current. When the external temperature is high, the resistance to a current flow reduces, allowing a larger current to flow. Thermistors Removing unwanted heat produced by, for example, the combustion engine is essential. Without cooling, engines would warp and melt. Cooling systems in cars are switched on when the temperature (sensed by a thermistor) begins to rise. The cooling action of the fan cools the engine. Give two other uses for temperature dependant resistors ? How could you show that the resistance changes as the temperature level changes ?
P5.6 Plenary Lesson summary: ohmic current resistance ratio Friday 21 October 2011 Resistance to current flow even occurs in thick copper cables designed to transport electricity from power station to consumer. During its transport resistance in the overhead cables wastes electrical energy. Resistance in these cable means that we can only transport electricity over a maximum distance of 450 to 500 km How Science Works: Research into how current is controlled in complex circuits using resistors and also look into the relationship/equation that describes how you can find the total resistance of several resistances in series. Preparing for the next lesson: If a particular conducting material behaves like an _______ conductor, then the _______ of the voltage across the material to the ________ flowing through the material is a constant called the ________ Most metals and alloys are Ohmic conductors Decide whether the following statements are true or false : False True 3: A metal with high internal resistance will produce a heating effect ? False True 2: Three factors affect resistance in a metal colour, length and diameter ? False True 1: The resistance of a thermistor increases as the temperature increases?