Transcript Practical Electricity

Practical Electricity
Topics
•
•
•
•
Electrical Energy & Power
Electricity Generation
Electrical Safety
3 Pin Plug
ELECTRIC POWER & ENERGY
Recall
• Power = Energy / time
• Units of Power is Watts
• Units of Energy is Joules
Electrical Energy & Power
•
•
•
•
•
Power of an electric component:
P = IV
Since Energy = Power x time
Energy of an electric component
E = IVt
Electrical Energy & Power
• You may come across variants of P = IV by
substituting R = V/I (definition of resistance)
• P = I2R
• P = V2/R
Worked Example 1
• A light bulb has a current of 0.1 A and a p.d. of
1.5 V.
• (i) Determine the Power of the light bulb.
• (ii) Determine the energy consumed by the
bulb if it was left on for one minute.
Power Rating
• On some electrical appliances there is a power
rating: e.g. 230 V, 950 W [take note of the
units]
• What does this power rating mean?
• In different countries around the world, there
is different mains voltage
• Singapore’s main voltage is 230 V.
• USA 120 V, China 220 V
Power Rating
• An appliance with a power rating 230 V 950 W
means that IF it is plugged into a mains of 230
V, THEN it will generate a power of 950 W
• You should expect the power to change if it is
plugged into the mains of a different voltage
• Q: what happens when you take an appliance
designed for USA and plug it in Singapore?
• Q: what happens when you take an appliance
designed for Singapore and plug it in USA?
Power Rating
• You encounter power rating questions where
the appliance is plugged into a mains voltage
which is different from the rating’s
• Remember that the resistance of the
appliance is always constant (but the p.d. or
the power can change)
Worked Example 2
• A lamp has a rating of 60 W, 240 V, and is
connected to a 240 V power supply.
• (i) Determine current flowing through the
lamp
• (ii) Determine resistance of the lamp
Worked Example 3
• The same lamp (rating of 60 W, 240 V), and is
now connected to a 100 V power supply.
• (i) Determine the power generated by the
lamp
• (ii) Determine the current flowing through the
lamp.
Heating Appliances
• Some electrical appliances intentionally generate
heat (kettle, iron, electric hotplate, electric
radiator, etc.)
• These appliances work by using a heating
element
• A heating element is a usually a length of high
resistance wire/conductor
• One way resistance is increased is to make a coil
of long and small filament wire (e.g. light bulb)
Heating Element
• No other work is done at the heating element
other than to generate heat
• Hence, the energy conversion is
• Electrical Energy → Thermal Energy
• Since Electrical Energy = IVt, and thermal
energy is = Q
• IVt = Q
• And you may be subsequently asked thermal
questions (e.g. Q = mcθ, Q = ml)
Worked Example 4
• A kettle has power 2000 W.
• (i) Determine how long it takes for it to heat 1
kg of water from 25 °C to 100 °C. (specific heat
capacity of water 4200 JKg-1K-1)
• (ii) State an assumption you made in your
calculations
Worked Example 5
• A heating element of power 1000 W is
inserted into a beaker containing 200g of
water at 100 °C. Determine how long it takes
to completely convert all the water to steam
(specific latent heat of vaporization of water =
2 200 000 Jkg-1)
Electricity Consumption
• In real life electricity bills, we get charged for
the amount of electricity we used in our
households
• However, the units of energy used is not S.I.
Units (Joules) but rather kilo-watt hour (kWh)
• 1 kWh = 1000 x (60 x 60) J
• Another question you may be asked is to
determine electricity costs
Worked Example 6
• The price of electricity is 27 cents per kWh.
Determine how much it costs in total to use a
3kW kettle for 20 minutes and a 100 W bulb
for 5 hours.
ELECTRICITY GENERATION
Forms of Electricity Generation
•
•
•
•
•
Fossil Fuels
Nuclear Power
Solar Energy
Wind Energy
Hydroelectric Power
Forms of Electricity Generation (pg
371)
Source of Electricity
Pros
Cons
Fossil Fuels
Cheap
Not Efficient
Bad for Environment
Nuclear
Efficient
Expensive
Bad for Environment
Solar
Clean
Not Efficient
Expensive
Wind
Clean
Not Efficient
Quite Expensive
Hydroelectric
Most Efficient
Clean
Quite Expensive
ELECTRIC SAFETY
Electrical Mains
• Recall: in order for current to flow through a
component, you need TWO connections
• A light bulb will not work if only one side is
connected to a battery – that’s still an open
circuit
• Your electrical mains has 3 connections, the
live, neutral & Earth
Electrical Mains
• 1) Live – this wire is at high potential (“high
voltage”). The wire is brown in colour. The
Fuse is also attached to the Live Wire
• 2) Neutral – this wire is maintained zero
potential. The wire is blue in colour.
• 3) Earth – this wire is connected to the Earth.
It is yellow/green in colour.
Electrical Mains
• Should you touch the live wire and your feet
are not insulated, current will flow through
you to/from the Earth (through your feet), this
may result in electric shocks/electrocution
• It is safe to touch the Neutral or Earth wires,
no current will flow
• This is why the fuse is attached to the live
wire, should a short circuit happen a large
current will flow, and the fuse will blow,
disconnecting the live wire.
Electrical Mains
• This also explains why the switch is attached
to the live wire – so that the live wire is
disconnected when the appliance is not in
use.
• Every household also has a circuit breaker,
which is designed to cut the circuit when a
large current flows (works using
electromagnetic means
Electrical Mains
• When an appliance is connected to the mains,
it is connected to the live and neutral
connections.
• If the appliance is has a metal exterior, the
metal exterior is connected to the Earth.
Electrical Mains
• What happens when the live wire accidentally
makes contact with the metal surface of an
appliance?
• If it there is no fuse & no Earth wire, a human
touching the appliance may get electrocuted
(current flows through the human to/from the
ground)
• In reality, a large current will momentarily flow
from live wire to Earth, blowing the fuse in the
process
Electrical Mains
• Some appliances have non-metallic exteriors
(e.g. made of plastic). This is called double
insulation. These appliances do not need an
Earth wire, and they may use only 2 pin plugs.
Summary of Safety Features
Safety Feature
How it Works
Switch on Live Wire
Live wire is disconnected when appliance
is not in use
Fuse on Live Wire
Blows if current exceeds fuse rating,
preventing large current from flowing
Earth Wire connected to outer
metal surface of appliance
Prevents humans from being exposed to
high voltage should live wire touch casing
by accident
Double Insulation
Humans not exposed to high voltage,
even if live wire touches outer casing
Circuit Breaker Box
Cuts current off should current flow be
too large
3 Pin Plug
• Each pin of the three pin plug is connected to
one of the following:
• Brown Wire – to live
• Blue Wire – to neutral
• Yellow/Green Wire – to Earth
• The brown wire is also connected to a fuse.
This fuse is meant to protect the appliance
(not humans) should current flow be too
large.
3 Pin Plug
3 Pin Plug
• You are required to memorize which the
colour of each wire, and which wire goes
where.
Electrical Hazards
• 1) Damaged Insulation
• 2) Overheating of cables
• 3) Damp Conditions
Damaged Insulation
Damaged Insulation
• Person may accidentally touch an exposed live
wire, giving him electric shock
• Live wire may touch the neutral wire, causing
a short circuit (which in turn may cause fires)
Overheating of Cables
• Can occur when (i)power cables are too thin
or (ii)sockets are overloaded
Overheating of Cables
• Each appliance draws current
• when a socket is overloaded, a large total
current is drawn from one socket
• Which may overheat the main cable and thus
cause a fire
Damp Conditions
• Don’t use electrical appliances with
wet hands or in damp conditions
• Electrical resistance of human skin is
high when dry (around 100 000 Ω)
• When wet, it drops to just a few
hundred ohms
• Should an electric shock occur (due to
faulty insulation etc), the current is
increased if conditions are damp
• Water is also a conductor of electricity
(but not pure distilled water)
Summary
•
•
•
•
Electrical Power & Energy
Sources of Electricity Generation
Electrical Safety & 3 Pin Plug
3 Electrical Hazards