Transcript File - Garbally Science

Electricity in the home
Chapter 52
Fuses
• A fuse is a safety device which cuts off the
current in a circuit if the current goes above
a certain level. The thin fuse wire overheats
and melts and so breaks the circuit. Fuses
are now replaced by circuit breakers.
Plugs
• Brown wire = Live wire
connected to the fuse.
• Blue wire = neutral
connected to the left hand
side of the plug.
• Yellow green=Earth wire.
Connected to the metal
casing of the electrical
device and runs to a metal
plate buried in the ground
Measuring Electricity
• The watt is the unit of electrical power.
• It is a measure of how quickly an electrical appliance
converts electrical energy to other forms of energy.
E.g. A 100 watt light bulb converts 100 Joules
of energy into light every second.
• The ESB use a unit called the kilowatt-hour(kWh).
• A kilowatt-hour is the electric energy used by a 1 kw
appliance for 1 hour.
No. of kilowatt-hrs = No. of kilowatts x No of hours
Examples
• A 6 kw cooker is on for half an hour.
a) How many units does it use?
b) If each unit costs 11.5 cents how much does the electricity
cost?
Answer
a) No of units = 6 x 0.5hrs = 3kWh(units)
b) Cost = 3 x 11.5 = 34.5 cent
Example 2
If each unit of electricity costs 12 cents, calculate the total
cost of using a 1500 W electric heater for 2 hours, a 150 W
freezer for 4 hours, a 1000W hoover for 12 minutes, a
1200W iron for 10 minutes and an 8kW electric shower for
15 minutes.
Answer:
Total No of units = (1.5 x 2) + (0.15 x 4) + (1 x 1/5)
+ (1.2 x 1/6) + (8 x 1/4)
= 3 + 0.6 + 0.2 + 0.2 + 2
= 6kWh or 6 units.
Cost of electricity
= 6 units x 12 cents
=
72 cents
Example 3
Question
The reading on an electricity bill 2 months ago was
21,185.Now the reading is 21,935. The price per unit is 11.75
cent. The standing charge is 6.52 euro and VAT @ 13.5% is
then added. What is the total bill for electricity over these 2
months?
Answer
No of units = present - previous = 21935 - 21185 = 750
Cost of electricity
= 750 x 11.75 = 88.13
Standing charge
=
6.52
Total (before VAT) =
94.65
VAT @ 13.5%
=
12.78
Total including VAT =
107.43
Electrical Power
Electrical power can be calculated using the formula
Power = Voltage x current
(watts)
(volts)
(amps)
The power rating of any electrical appliance is always marked
on it. The ESB provides a voltage of 220 V.
Knowing these two values we can calculate the current an
appliance uses.
Current(amps) = Power(watts)
Voltage(volts)
Example
Given the mains voltage is 220 V calculate the current
flowing and the fuse s needed in a) 2.4 kW kettle b) a 720
W toaster.
Answer
a) Current(I) = P / V
b) Current =
P/V
= 2400W / 220
= 720 W/ 220V
= 10.9 A
= 3.3 A
Fuse needed :13 A
Fuse needed:5 A
Alternating current and Direct current
• Direct Current (d.c.) travels in one direction only.
• Alternating current (a.c.), supplied by the ESB changes
direction many times per second.
• a.c. can be converted to d.c. using a rectifier.
Homework
Textbook Chapter51-ElectricityintheHome
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•
•
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•
A1.Fuse,melts,circuitbreakers.
A2.Earth,live.
A3.Watt,changes.
A4.Kilowatthour,kWh.
A5.Direct,d.c.,
• B1.
• (a)230V.
• (b)Whenthecurrentinthecircuitgetstoohig
h,theheatingeffectcausesthethinfusewiret
omelt,breakingthecircuit.
• (c)Itdoesnothavetobereplacedwhenittrips.
(d)Onthelivewire,itcutsoffallcurrenttothe
appliance.
• (e)(i)30A;(ii)10A.
•
•
•
•
•
B2.
(a)A=neutral,B=earth,C=live.
(b)A=blue,B=yellow/green,C=brown.
(c)Replacethe13Afusewitha3Afuse.
(d)Themetalcasetoacopperrod,buriedinth
eground.
• (e)Ifthemetalcaseofanappliancebecameliv
e,theearthwirewouldcarrythecurrentsafel
ytoearth.
•
•
•
•
•
•
•
•
B3.
(a)2kW.
(b)12units.
(c)€1.80.
(d)€1.20.
(e)€0.90.
(f)Kilowatthour.
(g)806.
• B4.
• (a)Fuse.
• (b)X=neutral, Y=live.
• (c)Thefuseisplacedonthelivewireforsafetyreasons.I
fafaultdevelopsandthefuseblows,itwillcutoffallcur
renttotheappliance.
• (d)Parallel.
• (e)Thebulbsshinebrighter;ifonefails,theotherskeep
lighting.
• (f)Anearthwire.
• (g)3x100=300W=0.3kW;5x24=120hours;0.3x120x
0.15=€5.40.(h)Atwowayswitchisfoundinthehallandlanding.