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ELECTRICITY 1. Label the pictures with the name of the device + 1 2. Electric circuit All electric circuits include: An electrical supply, for example a battery. An electrical conductor (or conductor), that is, an electrical path like wires. One or more electrical loads like a bulb. Control elements like a switch. 3. Units A battery is a source of electrical energy. It provides the necessary “pressure” for electricity to flow. This electrical pressure is generally called voltage and is measured in volts, V. The flow of electricity is called a current and is measured in amperes or amps, A. Electrical resistance (or resistance) is the opposition to the flow of current in a circuit. This value - in ohms, Ω – is a measure of how easily current can flow through the conductors and components in a circuit. 4. Ohm´s Law Is a special relation between voltage, current and resistance. The current passing through a lamp is proportionate to the voltage. resistance voltage current or in short R V I 5. Electrical Power The text below, about electrical power, is from a home improvements magazine. The amount of current, in amps, required by an electrical appliance — such as a TV or an electric kettle — depends on the power of the appliance. This number — expressed in watts (W) — will be marked somewhere on the appliance. To calculate the required current, simply take the wattage and divide it by the voltage of the electrical supply in your home — around 230 volts in most of Europe. Therefore, for an electric kettle with a power rating of 2,000 watts (as specified by the manufacturer), the current required is: 2,000 watts / 230 volts = 8.7 amps 2 6. Exercise 1 Complete the exercise below about current and power calculations using the words in the box. amps conductor components current circuit resistance voltage wattage ohms supply volts watts In electrical calculations, voltage is expressed by the letter V, resistance by the letter R, and current by the letter I (which comes from the word `intensity'). In other words, the (1) ................................flowing through a (2) .................................measured in (3) .............................. equals the (4) ................................of the electrical (5) ................................, measured in (6) ................................. divided by the total (7) .................................measured in (8)............................... To work out the value of R, it is necessary to calculate the total resistance of all the (9) ................................and connecting lengths of (10) ................................that make up the circuit. Once both the voltage and amperage are known, it is possible to work out the power, measured in (11) ............................ that will be consumed. Power (P) can be calculated using the equation P = VI. Therefore (12)................................ equals voltage multiplied by amperage. 7. DC and AC There are two types of current: DC and AC. These acronyms mean direct current and alternating current. Batteries are examples of direct current sources; they always produce a positive charge, and are written with a positive (+). AC generators produce power from wall sockets, and are examples of alternating currents; they produce a voltage with alternating signs that changes from positive (+) to negative voltage (-). 8. Why do some electrical appliances use DC and some use AC? The difference between these appliances is basically how they work. DC is useful for appliances that run on a constant current. All electronic devices use DC. For example, your TV plugs into an AC wall outlet, but the first thing that the circuitry does is convert the current into DC, which is what the circuitry requires. AC is useful for appliances that work a current that is constantly changing. Most motors are designed to operate by ac. 9. AC generation and use Electrical mains power is generated (produced) at sites called power stations (or power plants), which use large generators. A generator converts mechanical energy to electrical energy. A generator rotates a magnet within an iron case. 3 Current from the generators leaves the power station and enters the power grid (or grid) - the network of power lines (cables) which transmit power around the country. At the point where it enters the grid, the electricity flows through transformers - specifically step-up transformers, which increase voltage up to 400,000 volts and decrease amperage or amps. This reduces the energy lost from the power lines over long distances, as high-voltage (HV) supplies flow more efficiently than low-voltage (LV) supplies. Before the supply is used by homes and other buildings, it passes through several step-down transformers, which reduce the voltage and increase the amperage. 10. DC generation and use The extraction below is from a consumer magazine. Photovoltaic cells (PVs) - or solar cells - are an effective way of generating your own electricity from sunlight. The current they produce can be used immediately, may be stored in rechargeable batteries (like the ones in cars), or can be fed into the power grid and sold to the electric company. But PVs produce direct current. This is fine for charging batteries, but is not suitable for powering household appliances, which require alternating current. For this, the DC supply from PVs and batteries needs to go through an inverter a device which converts DC to AC. 11. Exercise 2 Decide whether the sentences below are true or false, and then correct the false sentences. 1 Photovoltaic cells produce direct current. 2 Electrical supply from PVs can be used to charge rechargeable batteries. 3 Rechargeable batteries supply electricity as an alternating current. 4 Inverters convert sunlight to alternating current. 12. Types of Circuit Series circuits: In this circuit, the current passes through several parts. In the circuit below, current flows through bulb A, then to bulb B and finally to bulb C. If we add more bulbs, the lights will be less bright. If any of the bulbs fail or burn out, current cannot flow through the circuit and the other components will not work. 4 Parallel circuits: Current flows through each of the bulbs without having to flow through any others. If any of the bulbs fail the others will work because current can flow through the rest of the circuit. Series / Parallel Circuit: The circuitry below is both a series circuit and a parallel circuit. The bulbs are in a series circuit but the motors are in a parallel circuit. Questions: 1. What will happen if one of the motors fails? Will the entire circuit fail? 2. Draw another example of a series and parallel circuit. 5 13. Exercises a. Explain, in English, if the following circuits will work properly and if not correct the mistakes. b. Look at the circuit below and mark the bulbs that are lit when the following switches are pressed: C, E and F __________________________________________ B, C, D, E and F __________________________________________ F __________________________________________ c. Draw a circuit with: 2 batteries in series 3 bulbs in parallel 1 switch in series with the batteries d. Draw a circuit with: 2 batteries in parallel 2 bulbs in series 1 switch in series with the bulbs 6 e. Complete the following sentences: 1. Electrical systems contain four major components: 1. ____________________ 2. ____________________ 3. ____________________ 4. ____________________ 2. The _________ in an electrical system is usually an appliance or machine such as an electric motor. 3. An electrical __________________ is a material through which charge can easily flow. 4. The ____________________________ can be a switch that turns the current on or off. 5. An _____________________ is a closed path for current to flow created by connecting voltage sources, conductors, control elements, and loads. 6. There are two types of current: ___________________ and ______________________. 7. In direct current (DC), the electric charge flows in _________ direction. 8. In alternating current (AC), the electric charge flows ____________________________. 9. Batteries produce ____________________ and alternators produce _________________. 10. Batteries can be added together in ___________ to produce a higher voltage. 11. In a series circuit, there is only _______ path for current. 12. In a parallel circuit, there are ______________________ paths for the current to flow. 13. Shorthand symbols: 1. Battery - ____________ 2. Light bulb - __________ 3. Switch - ____________ 4. Conductors - __________ 14. Schematic of a simple circuit: 15. An AC voltage source reverses the positive and negative terminals many times per second. A majority of AC circuits cycle at a rate of ____________. The cycling rate is called the _________________. 16. Frequency is measured in cycles per second, or ___________. 7 f. Create your own wordsearch using the following words or your own ‘relevant’ words. WIRE PLIERS BATTERY RESISTANCE BUZZER BULB ELECTRICS COMPONENT POSITIVE CABLE SAFETY NEGATIVE SWITCH CIRCUIT FUSE CURRENT You need two colours for this exercise. Write the above words in one colour and add extra letters in another. E L E C T R I R C U I T 8 I C S