Electricity Past Paper PDF

8 Making use of electricity 8.1 Simple circuits (Book 2A, p.90–103) 1 An electric circuit that forms a closed loop is called a closed circuit, one that does not is called an open circuit. Ç Closed Ç Open...

8 Making use of electricity 8.1 Simple circuits (Book 2A, p.90–103) 1 An electric circuit that forms a closed loop is called a closed circuit, one that does not is called an open circuit. Ç Closed Ç Open circuit circuit 2 Conditions for an electrical appliance to work:  there must be a source of electrical energy (e.g. a cell or battery).  the source of electrical energy and the appliance must form a closed circuit. 3 Materials that conduct electricity are electrical conductors. Examples: metal, pencil lead (graphite), vinegar 4 Materials that do not conduct electricity are electrical insulators. Examples: plastics, wood, distilled water 5 A switch is used to open or close an electric circuit. Junior Secondary Science Mastering Concepts and Skills © Oxford University Press 2023 1  8 Making use of electricity 6 Circuit symbols of some circuit components of a circuit: Circuit component Circuit symbol Circuit component Circuit symbol Connecting wire / Cell connector Two or more (connected) Battery or connecting wires / connectors (not connected) (open) Switch Bulb (closed) 7 Drawing circuit diagrams: Steps aDraw the circuit symbol for each component in the same position as it appears in the circuit bUse straight lines to represent connecting wires. cPut a solid dot at each wire junction. Junior Secondary Science Mastering Concepts and Skills © Oxford University Press 2023 2  8 Making use of electricity 8.2 Current (Book 2A, p.104–116) 1 Electric current is a flow of electric charges. It flows from the positive terminal of a cell to the negative terminal through the circuit. 2 We use an ammeter to measure the size of a current. The unit of current is the ampere (A). 3 The circuit symbol for an ammeter: Ç An ammeter 4 Connecting an ammeter:  The red knob of the ammeter should lead to the positive terminal of the cell and the black knob to the negative terminal. switch switch black ammeter knob ammeter black red knob knob red knob  Never connect the ammeter to the cell directly. Otherwise, a large current may flow through the ammeter and damage it. ammeter cell Junior Secondary Science Mastering Concepts and Skills © Oxford University Press 2023 3  8 Making use of electricity 5 The water flow model can be used as an analogy for an electric circuit: water flow connecting wire current water channel bulb water cell pipe turbine water pump Ç Water flow model Ç Electric circuit water pipe and Part in water flow model water pump water flow turbine water channel Part in electric circuit cell connecting wire current bulb provides a path for transfers consumes Function supplies energy water flow / current energy energy 6 When a current flows through a conductor, the temperature of the conductor increases. Some electrical energy is converted to thermal energy. This is the heating effect of current. The effect becomes greater when the current increases. 7 Many appliances work using the heating effect of current. Examples: electric kettle, iron, hairdryer 8 A current produces a magnetic effect.  When a current flows through a wire near a compass, the needle of the compass changes direction.  When a current flows through a coil of wire, the coil becomes an electromagnet. 9 Electromagnet is useful in many areas. Examples: electromagnetic door lock, electric motor, electromagnetic crane Junior Secondary Science Mastering Concepts and Skills © Oxford University Press 2023 4  8 Making use of electricity 8.3 Voltage (Book 2A, p.117–126) 1 The voltage of a cell indicates how much energy is supplied by the cell to the electric charges in a circuit. 2 We use a voltmeter to measure the size of voltage. The unit of voltage is the volt (V). 3 The circuit symbol for a voltmeter: Ç A voltmeter 4 Connecting a voltmeter:  A voltmeter is connected across a cell to measure its voltage.  The red knob of the ammeter should be connected to the positive terminal of the cell and the black knob to the negative terminal. 5 When more than one cell is connected in the same direction, the voltage across the cells is added up. The overall voltage is the sum of the individual voltages. 6 When one of the cells is connected in the opposite direction, the voltage across the cells is lower than the sum of the individual voltages. 7 The higher the voltage, the larger the current flowing through the circuit. 8.4 Resistance (Book 2A, p.127–140) 1 Resistance shows the opposition of a material to the flow of current. Its unit is the ohm (Ω). 2 A resistor is a circuit component that has a certain value of resistance. The circuit symbol for a resistor: Junior Secondary Science Mastering Concepts and Skills © Oxford University Press 2023 5  8 Making use of electricity Junior Secondary Science Mastering Concepts and Skills © Oxford University Press 2023 6  8 Making use of electricity 3 If a resistor with a higher resistance is connected to a cell, it will result in a smaller current in the circuit. We can connect a resistor to a circuit regardless of its polarity. 4 Resistors are used to control the size of current in a circuit. E 5 Factors affecting resistance: Factor How it affects the resistance of a wire Material of wire A copper wire has a lower resistance than a nichrome wire of the same length and thickness. Thickness of wire A thin wire has a higher resistance than a thick wire of the same material and length. Length of wire A long wire has a higher resistance than a short wire of the same material and thickness. E 6 A variable resister or rheostat is a resistor with an adjustable resistance. It is used to adjust the current in a circuit. E 7 The circuit symbol for a variable resistor or a rheostat: E 8 Volume controls and dimmer switches are examples of applications of rheostats. 8.5 Electrical circuits (Book 2A, p.141–150) 1 In a series circuit,  current flows along one path only;  the current is the same at all points;  the sum of the voltages across each appliance is equal to the voltage of the energy source. 2 If a series circuit is broken at any point, current will not flow in the circuit. Junior Secondary Science Mastering Concepts and Skills © Oxford University Press 2023 7  8 Making use of electricity 3 In a parallel circuit,  current can flow in more than one path;  the current in the main loop is equal to the sum of the currents in the branches;  the voltage across each branch is the same;  the current flowing in the branch with a lower resistance is larger. 4 If a parallel circuit is broken at any branch, current will flow in other branches. 8.6 Household electricity (Book 2A, p.151–170) 1 The appliances at home are supplied with mains electricity. 2 The mains voltage in Hong Kong is 220 V. 3 Electrical appliances are energy converters. They convert electrical energy to other forms of energy. 4 Many household appliances make use of the heating and magnetic effects of current.  The heating element of some appliances becomes hot when a current flows through it. Most of the electrical energy is converted to thermal energy.  The electric motors of some appliances make the moving part rotate using the magnetic effect of current. They convert electrical energy into kinetic energy. 5 Wall socket in Hong Kong: earth hole switch neutral hole live hole Junior Secondary Science Mastering Concepts and Skills © Oxford University Press 2023 8  8 Making use of electricity 6 A three-pin plug: earth pin live pin neutral pin 7 The wires of a three-pin plug and their colours: earth wire fuse (green and yellow) neutral wire (blue) live wire (brown) flex grip 8 Electricity is supplied to our homes through the live wire and the neutral wire. 9 All sockets are connected in parallel in a domestic circuit.  Switching on or off each appliance does not affect the operation of other appliances.  Each socket offers the same voltage for any appliances plugged in. Junior Secondary Science Mastering Concepts and Skills © Oxford University Press 2023 9  8 Making use of electricity 10 When several electrical appliances are connected to a single mains socket via a universal adaptor or an extension unit: If too many electrical the current flowing appliances are connected through the socket may to a single mains socket, become very large. The strong heating effect on the mains circuit may result in a fire. Overloading may occur. 11 To avoid overloading, we should connect appliances that draw a large current to different main sockets. 12 In the case of a short circuit: The wire connected in parallel to the bulb provides a branch with very low resistance for current to flow. The bulb is said to be short-circuited. Since resistance of the wire is very This may overheat the circuit or low, the current flowing through it cause a fire. becomes large. 13 Fuse rating shows the maximum current the fuse can stand. 14 The circuit symbol for a fuse: In a fuse, there is a metal wire with a low melting point. Junior Secondary Science Mastering Concepts and Skills © Oxford University Press 2023 10  8 Making use of electricity 15 If the current passing through a fuse is larger than the fuse rating, the fuse will blow and cut off the current. This prevents the circuit from overheating. 16 A suitable fuse should have a rating slightly higher than the normal operating current of the electrical appliance. 17 A circuit breaker turns to the ‘OFF’ mode and cuts off the current when the current exceeds its rating. 18 A circuit breaker can be reset by switching it back to the ‘ON’ mode. Ç Circuit breakers in consumer unit 19 The earth wire connects the metal case of an electrical appliance to the ground. It protects us from an electric shock. 20 Function of the earth wire: If the live wire becomes loose large current flows to the ground through the and touches the metal case of an earth wire and blow the fuse which electrical appliance, disconnect the appliance from mains supply. The current from the mains is cut off. This protects us from an electric shock. 21 Safety precautions in using electricity: We should… We should NOT…  use three-pin plugs with an earth wire. ✗ overload sockets.  only switch on an appliance after firmly ✗ cover appliances while they are plugging it into a socket outlet. working.  use fuses with suitable fuse ratings. ✗ let water seep into any appliances.  ask a registered electrical worker to ✗ touch sockets or switches with wet immediately replace damaged cables, hands. sockets and plugs. Junior Secondary Science Mastering Concepts and Skills © Oxford University Press 2023 11  8 Making use of electricity E 8.7 Power and efficiency (Book 2A, p.171–181) 1 The electrical power of an appliance is the amount of electrical energy transferred to the appliance in one second. energy 2 Power = time 3 The unit of power is the watt (W). 1 W = 1 J/s A larger unit of power is the kilowatt (kW). 1 kW = 1000 W 4 The power rating is the power of the appliance when it is working at the indicated voltage rating. 5 The electrical energy consumed by the appliance can be calculated by: Energy = power × time 6 The efficiency of an electrical appliance tells us what percentage of the power input is converted to useful power output. useful power output × 100% Efficiency = power input 7 An appliance with high efficiency uses less power to do the same job than one with low efficiency. 8 We can find an energy label on many appliances. Grade 1 products are the most efficient while grade 5 products are the least efficient. 9 A joulemeter can be used to measure the electrical energy consumed by appliances in joules (J). 10 A kilowatt-hour meter measures the electrical energy in a large unit called kilowatt-hour (kW h). 1 kW h = (1000 W) × (60 × 60 s) = 3 600 000 J Ç A kilowatt-hour meter Junior Secondary Science Mastering Concepts and Skills © Oxford University Press 2023 12  8 Making use of electricity 11 The amount of electrical energy consumed by an electrical appliance can be read from the kilowatt-hour (218.7 kW h in the photo below). 12 The kilowatt-hour (kW h) is a unit of electrical energy for calculating the cost of electricity. 13 Ways to save electricity at home:  Choose energy efficient appliances, i.e. appliances with Grade 1 energy label.  Do not leave the appliances in standby mode. Always switch off or plug off appliances from power source after use.  Switch lighting on only when you need it. Use daylight whenever possible.  Running air conditioner and fan together to spread the cooled air more effectively.  Remove battery charger from mains socket after charging is completed. Junior Secondary Science Mastering Concepts and Skills © Oxford University Press 2023 13