Unit 7J: Electrical Circuits

Transcription

1 Unit 7J: Electrical Circuits How do electrical circuits work? 7J1 Conductors and insulators 7J2 Electrolytes and non-electrolytes 7J3 Humans conduct What happens in a circuit? 7J4 Make a coin battery 7J5 Make a lemon battery 7J6 Different types of batteries 7J7 Find the fault 7J8 Connecting batteries (cells) in series 7J9 How does a light bulb work? 7J10 Connecting light bulbs in series 7J11 Connecting light bulbs in parallel How can we explain what happens in electrical circuits? 7J12 An analogy: the water circuit 7J13 Electrical potential - water circuit analogy 7J14 Electrical potential difference I - water circuit analogy 7J15 Electrical potential difference II - water circuit analogy 7J16 Electrical potential difference - thunder clouds I 7J17 Electrical potential difference - thunder clouds II 7J18 Electrical resistance I - water circuit analogy 7J19 Electrical resistance II - water circuit analogy 7J20 Build your own potentiometer 7J21 Using your potentiometer What kinds of circuits are useful and what are the hazards? 7J22 Make a quiz board 7J23 Morse code generator - the switch 7J24 Morse code generator - the circuit 7J25 Model ring main lighting circuit 7J26 Hall light switch 7J27 Blow the fuse 7J28 DPDT switch 7J29 The L.E.D 7J30 Current discriminator 7J31 Current discriminator and DPDT switch 7J32 Humidity detector I 7J33 Humidity detector II

2 7J Equipment list Specialised Equipment Household/stationary/market Cables + clips Aluminium foil Conducting strips (holes) + brackets Card Constantan cable 0.2 mm diam. Copper and Iron/Nickel coins Copper tape Drawing pins Plastic tubing Earthed tap Wood base Hole punch 1.5 litre plastic bottle x 2 Iron and Copper nails 1.5V battery x 3 Permanent marker ? resistor Plastic bottles 2 lemons Ruler 3 identical torch light bulbs + mounts Stapler 4.5V battery Sticky tape A selection of batteries Vinegar or lemon juice Ammeter Water containers Beaker 250 ml Buzzer Cables + clips Carbon probes Copper tape Humidity detector expt... Insulation tape LEDs Low voltage P/S Magnifying glass Motor Plastic bucket Plastic tubing Ruler Sticky tape Stopwatch Terminal block Torch light bulb + mount Van de Graf Voltmeter Water containers Wood base Wood strip 1 cm x 1 cm

3 7J1 Conductors and insulators 4.5V battery Torch light bulb + mount Cables + clips Various test samples ( see below) Test each sample shown in the table below and indicate which are insulators and which conductors. Sample Aluminium Foil 0.2mm copper wire 0.2mm steel wire O.2mm constantan Carbon rod Graphite Pencil Wood Rubber Plastic Light bulb Y or N Conductor (C) or Insulator (I) In order for a solid substance to conduct, electrons must be able to move within its structure. Both Copper and Rubber contain billions of billions of electrons however one is a conductor and the other an insulator. Try to find out why?

4 7J2 Electrolytes and non-electrolytes 4.5V battery Torch light bulb + mount Cables + clips Various test samples ( see below) Cables + clips Carbon probes 1 2 Connect a 4.5 V battery in series with a light bulb and a 0-20mA ammeter as shown below. Investigate various samples to see if they are good electrolytes or not using carbon rods as probes ma ammeter. Torch light bulb. Alcohol Carbon electrodes Inflammable! Keep away from ignition sources Sample Sodium Chloride Sand Test each sample shown in the table below and indicate which are electrolytes which are not: Sample Light bulb Y or N Ammeter reading mamps Strong electrolyte or weak electrolyte Water Water and salt Water and sand Alcohol Vinegar Is water an electrolyte or a non electrolyte? What happens when salt is added to water? Why does this happen?

5 7J3 Humans conduct Humidity detector expt 7J V battery 1 Use the humidity detector described in expt 7J Ω + - c e 2 Get children to form a circle holding on to the probes of the detector circuit as indicated above. 1000Ω b How does this experiment demonstrate that humans conduct electricity? Try to find out what substances in our body help to carry the electricity:?

6 7J4 Make a coin battery Beaker 250 ml Voltmeter Cables + clips Copper and Iron/Nickel based coins Vinegar or lemon juice Dip some small sections of cloth in vinegar solution and use them 1 2 to build a dry cell battery as shown below. Measure the electrical potential difference (E.P.D) produced by the combination Nickel coin (e.g. 10p piece) Aluminium foil base Damp cloth Electrolyte. Copper coin (e.g. 2p piece) 3 Add more cells to the stack and note the E.P.D produced. In the battery you have made, which coin represents the positive and which the negative electrodes? Positive: Negative: Why is it necessary to soak the cloth in an acid? What happens if the coins you use are the same? Measure the voltmeter reading produced by the combinations below: V V What happens to the voltage generated as you increase the number of cells in the pile?

7 2 lemons Iron and Copper nails Voltmeter Cable + clips 7J5 Make a lemon battery 1 Add a copper plated nail and a steel nail to a lemon as indicated below. 2 Connect two, then three, lemons in series and observe what happens to the electrical potential difference. Fill in the values of the electrical potential difference generated by the combinations below: V V What happens to the voltage generated as you increase the number of lemons? Can you think of any electrical devices that could be powered by your battery?

8 7J6 Different types of batteries A selection of batteries (See below) Voltmeter Cables + clips 1 Your teacher has provided you with a collection of various types of batteries or cells. Measure the electrical potential difference generated by each. 2 Study the information available on the battery and note what materials it is made of. [Not always available] Using the voltmeter measure the electrical potential difference generated by each battery and fill in the table below: Type PP3 Coin cell (Calculator) Voltage Mark the positive and negative terminals in each case: AA (Standard) Flat pack AA Rechargeable PP3 AA Coin cell Flat Pack Rechargeable AA The word battery really means a collection of voltaic cells joined together in series. Try to find out which of the above are batteries and which are single voltaic cells? A PP3 battery (9V) and a car battery (12V) have a similar voltage. Although you can power a radio with both you cannot start a car with both. Why not?

9 7J7 Find the fault 4.5V battery Torch light bulb + mount x2 Cables + clips One deliberately faulted item 1 Select come crocodile clips. Make sure one of the clips is faulty by cutting the lead so that there are no stray wires visible. Clamp the end back into the clip and return the plastic cover. 2 Supply students with three crocodile clips, including the faulty one. It is a good idea to use colour or length to help you identify the faulty lead. 3 Tell them that they have to connect two light bulbs in series but that there is a small problem: one of the components is faulty and they have to find out which.

10 7J8 Connecting batteries (cells) in series A selection of batteries Voltmeter Cables + clips Connect the three batteries in series as indicated in the circuit diagrams shown below. In each case measure the electrical potential difference. Measure the electrical potential difference produced by the following arrangements of batteries: V V V + - V + - V A Voltmeter is an apparatus that measures the E P D between two points in a circuit.

11 7J9 How does a light bulb work? 1.5V battery x3 Torch light bulb + mount Cables + clips Magnifying glass Connect the light bulb to one, then two, then three, 1.5V batteries. Each time examine the filament with a magnifying glass. Use torch light bulbs (6v,60mA). Draw the filament in the diagram of the light bulb Shown opposite: What happens to the filament as you increase the Voltage applied? A light bulb is nothing more than a resistant wire (the filament) that gets hot when connected to a battery. Try to find out: a) What metal the filament is made of: b) What temperature the filament reaches: c) Why the filament does not melt:

12 7J10 Connecting light bulbs in series 4.5V battery 3 identical torch light bulbs + mounts Cables + clips Voltmeter Ammeter 1 2 Connect first one, then two, and finally three light bulbs in series with a battery and ammeter. Observe what happens to: a) the current. b) the brightness of the light bulb. Each time you connect a new light bulb, measure the electrical potential difference across each bulb and fill in the table below. Use identical torch light bulbs (e.g. 6v,60mA). Observe on the side of each light bulb the voltage and current rating : Voltage Current What is the maximum power rating of the light bulb? What happens if the power consumed by the light bulb exceeds this value? Now connect the series circuit below, first with one, then two, then three similar light bulbs in series: A B C A V Fill in the table below for the current and voltage readings: Number of light bulbs in series Current in the circuit (amps) Voltage across each light bulb (volts) Power Consumed (watts) Why shouldn t the lights in a house be connected in series?

13 7J11 Connecting light bulbs in parallel 4.5V battery 3 identical torch light bulbs + mounts Cables + clips Voltmeter Ammeter Connect one, then two and finally three light bulbs in parallel with the battery and ammeter as indicated in the circuit diagram below. Observe what happens to: a) The brightness of each light bulb. 1 b) The current drawn from the battery. 2 Each time you connect a new light bulb in part 1 measure the electrical potential difference across each and write down in the table below. Observe on each light bulb the voltage and current rating : Voltage: Current: What is the maximum power rating of the light bulb? Now connect the light bulbs in parallel. First with one, then two, then three similar light bulbs in parallel: A A B C V Fill in the table opposite for the current and voltage readings: Number of light bulbs in series Current from battery (amps) Current in each light bulb(amps) Voltage across each light bulb (v) Power Consumed by each light bulb(w) What happens to the brightness of each light bulb as you increase the number of light bulbs in parallel? Should Christmas tree lights be connected in series or parallel? Give two reasons why:

14 electric circuit Unit Menu Main Menu Equipment Print and photocopy diagram below 7J12 An analogy: The water circuit analogous water circuit 1. Cable 2. Charge 3. Current 4 Electrical Potential 5.Electrical Potential Difference 6. Battery 7. The Switch M

15 7J13 Electrical potential - water circuit analogy Van de Graf Earthed tap Water containers x 2 Plastic bucket Plastic tubing Cables + clips Point out that the water has potential energy due to the fact that it is in a gravitational field. 1 2 The charges have potential energy because they are in an electric field. When the student touches the sphere he/she has the same potential. High Potential High Potential Zero Potential The gravitational potential energy of the water depends on its height and mass. Zero Potential The electrical potential of the charge depends on the amount of charge collected on the sphere. How can you increase the Gravitational Potential Energy of water in a container? How do you increase the Electrical Potential Energy of an object? In what way does the thunder cloud possess both Electrical Potential Energy and Gravitational Potential Energy?

16 7J14 Electrical potential difference I - water circuit analogy Van de Graf Earthed tap Water containers x 2 Ammeter Plastic tubing Cables + clips Point out that when you syphon the water it flows from a high state of gravitational potential energy (G.P.E) to a low one. 1 2 When you connect the conducting sphere to the tap the charges flow from a high state of electrical potential energy to a low one. High Potential High Potential Zero Potential Zero Potential Using the diagram opposite, describe a method by which the water will flow from one container into the other: Fill in the gap in the following sentence. In order for water to flow from container A to Container B in the diagram opposite, container A must be at a (HIGHER/LOWER) Gravitational Potential Energy. Using the diagram opposite, describe a method by which the charge will flow from the Van Deer Graf to the tap. Fill in the gap in the following sentence. In order for water to flow from the Van der Graf to the tap, the tap must be at a (HIGHER/LOWER) Electrical Potential Energy.

17 7J15 Electrical potential difference II - water circuit analogy 1.5 litre plastic bottle x 2 Stopwatch Permanent marker Fill two plastic bottles (1.5L) and position them at different heights.then get two students to syphon water from each so that 1 the water starts to flow at the same time. 2 Record the time taken for each bottle to empty and calculate the rate of flow of water. High Potential Zero Potential Elicit the formula for the rate of flow of water: Rate of flow = Volume of water Time taken Get students to calculate the rate of flow for the two bottles in the experiments above: Bottle Volume (cm 3 ) Time (sec) Rate of flow (cm 3 /sec) High Low Get them to see that the rate of flow depends on the difference in Gravitational Potential between the two bottles. Introduce the idea that current in the electrical circuit is analogous to the rate of flow of water and that it depends on the difference in Electrical Potential Energy.

18 7J16 Electrical potential difference: thunder clouds I IWB Print and photocopy worksheet Consider the energy changes that take place when lightening strikes. What type of energy is stored in a thunder cloud? What energy changes take place when lightening strikes?.. Electricity requires a medium. What does the lightening travel through as it passes to Earth? Air is a poor conductor. In order to carry electricity the air molecules (which are neutral) must be ripped apart to produce ions. When you tear something apart, sound is produced. How can you demonstrate this with a piece of paper? The sound produced when lightening strikes is called:. Explain how this sound is produced: Sound travels through air at speed of 340m/s yet light travels almost instantaneously. Explain why you always hear the thunder clap sometime after the lightening strike:

19 7J17 Electrical potential difference: thunder clouds II IWB Print and photocopy worksheet 1 2 Electric charges want to flow from a high electrical potential to a low electrical potential. The Earth is at an electrical potential of zero volts. 100m 3 Follow the steps below to calculate electrical potential of the thunder cloud in the diagram. What does the symbol, used in the diagram above, mean? What is the Electrical Potential of the Church? In order for a spark to travel through air, the molecules must be torn apart to form ions. This requires 30,000V for every 1 cm of air. Using this information, calculate the electrical potential energy of the thunder cloud in the diagram above.

20 7J18 Electrical resistance I - water circuit analogy Water containers Tubing of same length but differing diameters 1 2 Investigate how the rate of flow of the water in a tube depends on its cross sectional area. To do this compare the rate of flow for two different diameter tubes keeping the tube length and height difference, h, constant. h Elicit the formula for the rate of flow of water: Rate of flow = Volume of water Time taken Get students to calculate the rate of flow for the two bottles in the experiments above. Tube Diameter Volume (cm 3 ) Time (sec) Rate of flow (cm 3 /sec) Large Small Get them to see that the rate of flow of water increases with the cross sectional area of the tube. Introduce the fact that the current in an electric cable is inversely proportional to its resistance which in turn decreases as the cross sectional area of the cable increases in a similar way to the phenomenon described in this experiment.

21 7J19 Electrical resistance II - water circuit analogy Plastic bottles x 4 Tubing of same diameter but different length 1 Fill two plastic bottles with water and place them at the same height. 2 Connect each with tubes of the same diameter but different lengths. 3 Syphon water from each to a bottle positioned on the floor and record the time taken to empty. Elicit the formula for the rate of flow of water: Rate of flow = Volume of water Time taken Get students to calculate the rate of flow for the two bottles in the experiments above. Tube Length Volume (cm 3 ) Time (sec) Rate of flow (cm 3 /sec) Long Short Get them to see that the rate of flow of water decreases with the length of the tube. Introduce the fact that the current in an electric cable decreases as you increase its length in a similar way to the phenomenon described in this experiment. The current decreases as you increase the length of the cable because its resistance increases.

22 7J20 Build your own potentiometer 4.5V battery Voltmeter Cables + clips Ruler Constantan cable 0.2mm diam. Connect a 4.5V battery to about 50 cm of 0.2mm diameter constantan cable mounted as shown in the diagram. 1 2 of 5cm. Now measure the potential difference across the wire as you slide the cable from one end to the other in intervals! Wire can get hot do not touch. 4.5 Volts 50cm Use high resistance wire! e.g. 0.2 mm diameter constantan. Vary the contact position and note of the electrical potential difference (E.P.D) across the resistance in the table below: 4.5v 0cm High resistance wire 50cm V Contact Position E. P. D (Volts) Plot a graph of the variation of E.P.D with contact position. What do you notice about the relationship between E.P.D and contact position?

23 7J20 Build your own potentiometer E.P.D [volts] Position along cable [cm]

24 7J21 Using your potentiometer 4.5V battery Torch light bulb + mount Cables + clips Ruler Constantan cable 0.2 mm diam. Connect a 4.5V battery to about 30 cm of 0.2mm diameter constantan cable as shown in the diagram. 1 Instead of connecting the potentiometer 2 to a voltmeter, connect it to various electrical components such as a torch light bulb or a small 6v motor.! Wire can get hot. Do not touch. 4.5 Volts USE high resistance wire! e.g 0.2 mm diameter constantan. Label the circuit diagram for the apparatus shown above: A B What happens to the intensity of the light bulb as you slide the contact from B to A? Why is this? What is the electronic circuit symbol for the following: a) Motor: b) Voltmeter: c) Ammeter:

25 7J22 Make a quiz board 4.5V battery Torch light bulb + mount Cables + clips Coloured card Hole punch Aluminium foil Make a quiz board as shown in the diagram below. Make holes with a hole punch. Aluminium Foil. Use sellotape as an insulator. Which floats in water? Which is a non metal? Which is a liquid? Which is magnetic? Which has the highest density? Au Fe C Hg Na FRONT BACK

26 7J23 Morse code generator: the switch Card Stapler Aluminium foil Sticky tape 1 aluminium Prepare contacts by cutting some card 1cm by 6cm and wrapping in foil. Aluminium foil. 2 sticky Hold the foil in position using some tape. 3 in Staple the contacts to a piece of card making sure that they overlap the middle as shown above. 4 so Bend one of the contacts upwards that they are permanently separated.

27 7J24 Morse code generator: the circuit 4.5V battery Torch light bulb + mount Cables + clips Buzzer Switch expt 7J23 Connect the switch that you made in expt 7J23 in the circuit as shown below. Buzzer Now use the code below to send messages to your partner: A.- B -... C -.-. D -.. E. F..-. G --. H... I.. J.--- K -.- L.-.. M -- N -. O --- P.--. Q --.- R.-. S... T - U..- V...- W.-- X -..- Y -.-- Z Full stop Comma Query..--..

28 7J25 Model ring main lighting circuit Low voltage P/S Torch light bulb + mount x6 Cables + clips Drawing pins Aluminium foil Copper tape Insulation tape 1 Build a modal ring main lighting circuit, using sticky backed copper tape, on an appropriate section of MDF or plywood. ac dc 2 Connect the rails to a power supply. 3 Add light bulbs at various positions in the circuit. Fuse box Insulation tape

29 7J26 Hall light switch 4.5V battery Torch light bulb Cables + clips Drawing pins Aluminium foil Copper tape Sticky tape Card Wood base Prepare six contacts by cutting some card of approximately 1cm by 6cm and 1 wrapping in aluminium foil. Aluminium foil. 2 sticky Hold the foil in position using some tape. 3 Assemble the contacts on a wooden block using drawing pins as indicated below. Bottom landing Stairs Top landing Hall Light Top of stairs Top of stairs Bottom of stairs Stair light Bottom of stairs Stair light

30 7J27 Blow the fuse Low voltage P/S Torch light bulb + mount Cables + clips Drawing pins Aluminium foil Copper tape Motor Wood base 1 Form parallel rails by sticking copper tape to a base as indicated below Connect the rails to a power supply. ac 2 12 dc 3 Cut a thin strip of aluminium foil and secure with drawing pins to make a fuse. 4 Add components in parallel and observe what happens to the fuse. What happens to the current in the circuit each time you add a light bulb in parallel? How does the motor effect the current drawn from the power supply? What eventually happens to the aluminium strip? Explain this: In a house the lighting circuit is always separated from the appliance circuit (plug sockets). What would happen if the washing machine were connected to the lighting circuit?

31 7J28 DPDT switch 4.5V battery Cables + clips Drawing pins Conducting strips (holes) + brackets Motor Wood base Wood strip 1 cm x 1 cm Prepare the positive and negative contact rails by mounting two holed conducting strips on a wooden base 1 using brackets as indicated below. Prepare the rotating contacts by inserting two drawing pins into a 2 section of 1cm x 1cm softwood. Hammer the rotating contact in a position exactly half way between the 3 positive and negative rails. + - Connect your DPDT switch to a motor (with a fan attached) then use the switch to change the direction of spin 4 of the motor.

32 7J29 The L.E.D 4.5V battery LED Cables + clips Terminal block Ω resistor Terminal block 1 Cut a small sized terminal block into sections of four holes. 2 Use 3 Connect the resistor chart in expt 9I15 to find a resistor of between 100 and 300 ohms. the LED in series with the resistor as indicated below. Note what happens to the LED L.E.D 4 Now connect the LED correctly to the battery terminals and show that it lights up

33 7J30 Current discriminator 4.5V battery LED x 2 Cables + clips Terminal block Ω resistor x 2 Terminal block 1 Cut a small sized terminal block into sections of four holes. 2 Use two LEDs and two resistors of 100 to 300 ohms to assemble the circuit shown below: 3 Now test your current descriminator on the poles of a battery.

34 7J31 Current discriminator and DPDT switch 4.5V battery LED x 2 Cables + clips Terminal block Ω resistor x 2 Terminal block DPDT switch expt 7J Connect the current discriminator to the DPDT switch that you made in expt 7J28. 2 Observe what happens to the LEDs as you change the switch. Draw the circuit diagram for the arrangement shown opposite: What happens to the LEDs when you switch the poles of the DPDT switch?

35 7J32 Humidity detector I 4.5V battery LED x 2 Buzzer Transistor BC108C NPN Cables + clips Terminal block 120Ω resistor 1000Ω resistor 1 Use a connector block to assemble the humidity detector circuit shown below Ω c e 1000Ω b Buzzer collector base + - emitter The emitter is always the closest to the spike. c e Transistor 1000Ω b 2 Test the detector circuit by inserting the probes in a beaker of water.

36 7J33 Humidity detector II 4.5V battery LED x 2 Buzzer Transistor BC108C NPN Cables + clips Terminal block 120Ω resistor 1000Ω resistor 1 Use a connector block to assemble the humidity detector circuit shown below Ω 120Ω 120Ω c e 1000Ω + - b collector base + - emitter L.E.D Transistor The emitter is always the closest terminal to the spike Ω 1000Ω c + - b e 2 Test the detector circuit by inserting the probes in a beaker of water.