chapter 13 science

• mains supply

  • generated in power station
  • appliances connected to main socket via 3-pin plugs
  • gives electric shock -> can kill

  electric cell -> dry cells/ batteries

  • small amounts of stored energy
  • portable electrical devices
• electric circuit - path along which current moves

  
  

  switches — tap switch, plug-key switch, rocker switch

  • used to open/ close circuit
  • disconnect circuit component from power source

  
• electric current

  : rate of flow of electric charges in a circuit

  • symbol: I
  • S.I unit: ampere (A) [1A = 1000mA]
  • ammeter — positive terminal to positive terminal (red -> positive, black -> negative)

  potential difference — voltage

  : measure of amount of energy needed to move a unit charge from one point in an electric circuit to another point

  • positive terminal of electric cells has a higher electrical potential than the negative terminal
  • S.I unit: volt (V)
  • as electric cells increase, voltage increases
  • voltmeter — connected in parallel across the bulb
• resistance

  : measure of how much an electrical component opposes the flow of electric current

  • electrical components act as an obstacle against flow of electrons
  • as resistance increases, current decreases
  • S.I unit: ohm (Ω)
  • high resistance -> insulator, low resistance -> conductors
• fixed resistors

  • resistance ranges from an ohm to thousands of ohm

  variable resistors

  • as resistance increases, current decreases
  • sliding rheostat, radio-type rheostat (light dimmer, control volume, speed train)

  _left -> minimum resistance

  -> higher current

  -> brighter bulb

  _right -> maximum resistance

  -> smaller current

  -> bulb less bright
• resistance in series

  • total resistance = sum of resistors in series
  • current = same at any point in circuit
  • voltage of battery = sum of p.d. across each resistor
  • R = R1 + R2 + R3
• resistance in parallel (combined resistance decreases)

  • total resistance = reciprocal sum of resistors
  • current (battery) = sum of current in each parallel branch
  • 1/R = 1/R1 + 1/R2 + 1/R3
  • voltage of battery = p.d. across all parallel branches (V=RI)
• 1. heating effect

  • electric current flows through wire, wire heats up
  • household fire due to overheating, dangerous
  • heating element made from high-resistance conductor (nichrome)
  • infant incubator -> provides constant temperature for pre-mature infants
  • sterilizer -> produce steam at high pressure to sterilise instruments
  • electric kettle, toaster, iron
• 2. magnetic effect

  • compass needle is deflected when electric current flows
  • electromagnet (coil of wire wound around magnetic material eg. iron) : increases strength
  • magnetic cranes -> lift iron, steel to separate metals
  • electric bells -> on and off rapidly causes hammer to vibrate and hit a metal gong
• 3. chemical effect

  • current passes through cerain chemical solutions, breaks down solution into simpler substances
  • electroplating -> prevents corrosion of metals, aesthetic reasons

  — surgical instruments (anti microbial properties) eg. gold, silver

  — coating of medical implants (reduce risk of infection, rejection by body) eg. titanium

  • electrolysis

  -> purification of metals eg. copper

  -> extracting reactive metals from their components eg. sodium, aluminium
• cause of electrical fires

  1. short circuit

  • broken/ bare wires that come in contact with each other -> results in a fire

  2. overloading

  • too many electrical appliances connected to one mains socket -> fire in wiring or adapter

  3. frayed wires

  • insulation around wire is damaged or worn out -> electric shock

  4. wet conditions

  • person touches damaged wire/ appliance with wet hands -> higher current through wet skin
• electric plug

  live pin <-> red wire

  neutral pin <-> blue wire

  earth pin <-> green wire
• fuse

  fuse rating

  — maximum amount of current that can flow through fuse before wire in it melts, breaks

  — choose the next higher rating (1.2A -> fuse rating 2A)

  — when current exceeds fuse rating, fuse melts, breaks -> opens the circuit, damage

  consumer unit

  — switchboard in homes, offices that houses a mains switch and circuit breaker

  !! circuit breaker -> special switch used as part of consumer unit eg. mains switch
• miniature circuit breaker (MCB)

  • works like a fuse but it protects a large circuit that consists of few sockets and appliances
  • when current is too large and exceeds the current rating of the breaker, the switch turns off by itself
  • advantage: easier than replacing a fuse
• residual current circuit breaker

  • a safety device that detects leakage current to ground
  • mandatory safety device in Singapore
  • reduce risks against electric shocks
• purpose of mains switch

  • switch off/ on supply of electricity to household unit with one switch
  • when there is a fire, this is switched off to cut the electricity supply to the whole unit
  • serves as main circuit breaker for whole house
• earth wire

  • safety device that protects user from electric shock
  • connects metal casing and ground via mains supply
  • when metal casing becomes live due to a fault, earth wire provides a low resistance path for current to flow from metal casing to ground

  no earth wire -> person may get an electric shock from metal casing of an appliance that is live

  with earth wire -> current flows through earth wire to ground, instead of through person (nO eLectRic sHocK!)
• power = rate of energy used or converted

  • power = energy/ time
  • S.I unit: watt (W)
  • 1kW = 1000W, 1MW = 1 000 000W

  electrical power

  : amount of electrical energy an appliance converts to other forms of energy per unit time

  • electrical power (W) = electrical energy used/ time
  • electrical power = current (I) x voltage(V)
  • electrical energy used (J) = power (W) x time (s)