Practical Electricity

Transcription

1 Practical Electricity This unit of work is concerned with examining electrical circuits, electrical components and how we use electricity in the home. If we had to draw pictures of electrical components then anyone who was not a good artist would have difficulty. To get around this problem we use simple symbols to represent the components. Instead of drawing a lamp we use the symbol You need to recognise and be able to draw the following symbols. battery lamp switch

2 In any electrical circuit some energy source must be used. In the circuits that we examine in class the battery is used as a source of electrical energy. There are many different types of battery but they all supply energy. Electric Circuits We can combine our symbols to represent electric circuits. The actual circuit is shown on the left and our simpler circuit diagram is shown on the right. - Actual Circuit Circuit Diagram The circuit shown above has only one path around it. Going from the negative terminal on the battery to the positive it is only possible to have one path. A circuit that has only one path is called a SERIES CIRCUIT.

3 - Actual Circuit X X Circuit Diagram The circuit shown above has more than one path around it. Going from the negative terminal on the battery to the positive it is possible to have more than one path.[a choice can be made at point X] A circuit that has more than one path is called a PARALLEL CIRCUIT. Measuring circuit conditions There are two quantities that can be measured in an electrical circuits, CURRENT and VOLTAGE. CURRENT is the measure of the number of charges passing a point in a circuit each second. VOLTAGE is the measure of energy given out by a battery or the energy used by a component.

4 If we want to measure the current then we must place our measuring device so that the charges can pass through it. This means that any circuit already built up must be broken and the measuring device placed into the gap. To measure current we use a device called an AMMETER. The current is measured in the unit called the AMPERE [AMP for short] Symbol A Example: current = 4.5 A A Original circuit Broken circuit Ammeter in place When we wish to measure voltage the device used is placed in the circuit differently. This device measures the difference in the energy carried by the charges either side of the component. The circuit does not have to be broken to measure voltage. To measure voltage we use a device called an VOLTMETER. The current is measured inthe unit called the VOLT Symbol V Example: voltage = 6 V V Original circuit Voltmeter in place

5 Circuit Rules There are rules that allow you to find out currents and voltages in series and parallel circuits. Series Rules: 6V The charges have only one way to go around the circuit. This means that all the charges must pass every part of the circuit. In a series circuit this means that the current is the same at all points in the circuit. 6V 6 Volts supplied 4 Volts used 2 Volts used The battery supplies energy to the charges in the circuit. The components use up this energy so that the charges have no energy left when they get back to the battery. In a series circuit the voltages across the components in the circuit add up to the supply voltage.

6 Parallel Rules: 4.5V 6A 3A 1A 2A In a parallel circuit the charges have a choice of paths to take. They can only go down one of the paths at any time. This means that the currents through the different paths must add up to the main current. 6A 4.5V 4.5 V supplied 4.5 V used 4.5 V used 4.5 V used In a parallel circuit each charge gains the same quantity of energy from the battery. Each charge can only go down one of the paths at any time. The charges give up this energy to the components in the branches. This means that the voltage across each component is the same.

7 Worked Examples. 5 V 2A 3 V? V SERIES Find the current through the 3V lamp and the voltage across the other lamp. Voltage: The voltage across both lamps must add up to 5V[the supply voltage] So 5V = 3V +?V?V = 5V 3V?V = 2V Current: The current is the same at all points in the circuit. So the current through the 3V lamp is 2A. PARALLEL Find the voltage across the resistor R 1 and the current flowing in the main circuit. Voltage: The voltage across components connected in parallel is the same. So voltage across R 1 is 10V. Current: the current in the branches adds up to the main current. So?A = 1.2A + 1.6A?A = 2.8A?A 1.2 A 1.6 A 10 V R 1 R 2

8 Resistance A resistor is acomponent which tries to stop [resist] the flow of charges through a circuit. There are two types of resistor: (i) Fixed resistors,normally just called resistors, that have a particular value. (ii) Variable resistors that can have their values altered. These resistors are shown below along with their circuit symbols. Resistor Variable resistor Resistors are used in many different ways in electrical circuits. Any time a current passes through a resistor the resistor changes electrical energy into heat energy. This property is used in many heat producing electrical devices.

9 Measuring resistance The resistance of a component can be measured directly using a meter. The device used to measure resistances is called an OHMMETER. The unit of resistance is called the OHM. Symbol Ω The meter is simply connected across the resistance with nothing else in the circuit. It is possible to find the value of a resistor if you do not have an ohmmeter. The current through the resistor and the voltage across the resistor need to be measured. The circuit below shows how this could be done. ammeter to measure current through resistor A Ω V voltmeter to measure voltage across resistor If the resistance is increased the current flowing will decrease. This is because the resistor tries to stop the charges flowing.

10 Calculating the resistance. Once the voltage and current have been measured we must use an equation to find the resistance. voltage resistance = current A 3A V 9V To find the resistance of the resistor in the circuit above: Current = 3 A Voltage = 9 V Resistance =? voltage 9 resistance = = current 3 = 3 Ω The resistance is 3 Ω.

11 Uses of variable resistors Variable resistors change the current flowing in a circuit. This allows them to change the output from electrical devices. Speed control on a fan Dimmer switch for a lamp Heat control on an iron Speed control on a toy train

12 Mains electricity For most of us our most common use of electricity is in the home, TV, hi-fi, Playstation, computer, microwave, the list is endless. It is important then to have some knowledge of how the electricity is supplied in the home and how we can keep ourselves safe. The mains supply voltage is 230 volts. All electrical appliances that use the mains voltage directly are designed to work from a voltage of 230 volts. The only way that this can happen is if the wiring connects the appliances in PARALLEL.[ see circuit rules] In order to use most electrical appliances we plug them into the mains socket using a plug. Earth green/yellow Neutral blue Fuse Cable Grip Live brown Flex Diagram of household plug

13 Safety The plug contains two parts that are involved in safety. These are the fuse and the earth wire. The Fuse The fuse is normally a thin wire inside some type of casing. If the current flowing in the circuit becomes too large the thin wire melts and breaks. We normally say that the fuse has blown. This is important because the flex connecting the appliance to the socket could overheat and even start a fire if the current flowing is too large. The earth wire The earth wire gives an easy path for the current if a fault makes the casing of an appliance live. This easy path produces a large current and the fuse blows making the appliance safe. Double insulation Some appliances have plastic insulating coverings [hairdryers, power tools, etc.] and you cannot make the casing live. These appliances do not need an earth wire. They have the symbol above on their casing, normally on the ratings plate. Circuit Breakers 5A Very often circuit breakers are used instead of fuses in the mains fuse box in the home. Circuit breakers are much easier to reset and are more difficult to tamper with than fuses.

14 How much will it cost? You probably don t think too much about the cost of energy at the moment, mainly because you don t have to pay for it!! Every time you switch on an electrical appliance it costs money. It is fairly easy to work out roughly how much it costs to run an appliance. The steps set out below show how you can do this at home. 1. Find the ratings plate on the appliance. It normally looks something like this. 230 V ~ 50 Hz 800W made in UK 2. Find out the power rating of the appliance. This is the number followed by the symbol W. The symbol stands for watts, the unit of power. On the ratings plate above the power is 800W. 3. Multiply the power by the number of hours the appliance is switched on for. 4. The higher your answer to number 3. the more money you will have to pay. Cost = power x time Extra Actual cost is calculated by dividing your answer to number 3 by 1000 and multiplying this by 6p.

15 Safety in the Home Many people in Scotland are injured or even killed by problems involving household electrical appliances. Some knowledge of the dangers can help you to keep yourself safe. Danger - humans conduct electricity Humans conduct electricity so if you touch a faulty appliance the electric current will pass through your body. This will cause serious injury or even death. If you are wet then you conduct electricity even better. This is why there are no electrical sockets and special light switches in a bathroom. Faulty Iron Danger overloading sockets Another common danger is overloading sockets. If too many high power appliances are connected to one socket the socket can overheat and could even start a fire. Appliances that produce heat are usually high powered. Danger Stupidity A cause of many accidents involving electricity in the home is thoughtlessness. Sticking a fork into the switched on toaster to free a stuck piece of bread can lead to serious injury.

16 Testing for faults You switch on an electrical appliance and it does not work. It may be that the fuse is blown. In some situations a wire may have broken. If this break is inside the insulation on a flex then you will not be able to see it. This type of fault is known as an open circuit. It would be useful to have some device that would allow us to test if the fault is simple like the two mentioned above, or whether there is a more serious problem with the appliance itself. The device we use to test for broken wires is called a continuity tester. The actual circuit and the circuit diagram are shown below. Probes The wire to be tested is connected between the probes. If the wire is broken the lamp stays out. If there is no break the lamp lights. Before the tester is used it must be checked to see if it is working properly. The probes are touched together and the lamp will light if the tester is working.

17 Electrical Circuits Draw the circuit symbols for a battery, lamp and switch. A battery is a source of electrical energy. Lamps convert electrical energy into light energy. Heaters convert electrical energy into heat energy. Motors convert electrical energy into Kinetic energy. Describe a series circuit. An ammeter is used to measure electric current. Draw the circuit symbol for an ammeter. Draw a circuit diagram showing an ammeter connected in series. Current is measured in amperes. In a series circuit the current is the same at all points. Describe a parallel circuit. Adding the currents in two parallel branches is equal to the current drawn from the supply. A voltmeter is used to measure voltage. Draw the circuit symbol for a voltmeter. Draw a circuit diagram showing a voltmeter connected in parallel. Voltage is measured in volts. Adding voltages across components in a series circuit is equal to the voltage of the supply. The voltage across two parallel branches is the same as the supply voltage. RESISTANCE Draw the circuit symbol for a resistor and a variable resistor. Resistors convert electrical energy into heat energy. An ohmmeter is used to measure resistance. Resistance is measured in ohms. An increase in resistance of a circuit leads to a decrease in the current in the circuit. Calculate the resistance using: Give two uses of a variable resistor. resistance = voltage current

18 MAINS ELECTRICITY Household wiring connects appliances in parallel so that they receive the same voltage. The value for mains voltage in the UK is 230 V. Draw the circuit symbol for a fuse. Identify live, neutral and earth wires from the colour of their insulation. Identify the live, neutral and earth terminals in a plug. Explain how a fuse acts as a safety device. Electrical energy costs for the home increase as: (a) the power of the appliance increases (b) the time of use of the appliance increases. Calculate the current using: power current = voltage to determine fuse values. A circuit breaker is an automatic switch, which can be used instead of a fuse. The human body is a conductor of electricity and that moisture increases its ability to conduct. The earth wire is a safety device. Electrical appliances, which have the double insulation symbol, do not require an earth wire. Explain why connecting too many appliances to one socket is dangerous. Explain why situations involving electricity could result in accidents. Describe how to make a simple continuity tester. Describe how a continuity tester may be used to identify an open circuit.

19 1. Draw the circuit symbols for (a) a battery, (b) a lamp, (c) a switch 2. What does a battery supply to an electrical circuit? 3. Write down the energy changes for the following devices (a) Lamp (b) Heater (c) Motor 4. Draw a series circuit including a battery, a switch and a lamp. 5. Draw the circuit symbol for an ammeter and state what it is used for. 6. What quantity is measured in amperes. 7. Redraw the circuit below to include (a) an ammeter. (b) a voltmeter to measure the voltage across the lamp 8. Draw a parallel circuit that includes a battery and two lamps.

20 9.Two identical 3V bulbs are connected to a supply as shown. What is the voltage of the supply? 3 V 3 V 10. Two resistors are connected in parallel to a 10 V battery. 0 4 A 10 V R 1 (a) What is the voltage across R 1? (b) What is the voltage across R 2? 0 6 A R 2 (c) What size of current is drawn from the battery? 11. Draw the circuit symbol for a resistor and a variable resistor. 12. What is the energy change in a resistor? 13. What device could be used to measure resistance? 14. What is the unit of resistance? 15. What happens to the value of the current in a circuit if the resistance of the circuit is increased? 16. A power drill is operated at mains voltage 230 V and has a current of 0.1 A through it. Calculate the resistance of the drill.

21 17. Give two uses of a variable resistor. 18. What is the value for the mains voltage? 19. (a) Copy and complete the table. Wire A Wire B X Wire A B C name of terminal colour of wire Wire C (b) What is the name of component X? (c) How does component X act as a safety device? 20. What two factors affect the cost of running an electrical appliance? 21. Give examples of common dangers associated with electricity in the home. (What should you not do around electrical appliances or power points.) 22.(a) Draw a simple circuit for a continuity tester. (b) Explain how it could be used to identify an open circuit.