1. FUNDAMENTAL Physics – Mains and Magnetism: 6.3 ACDC and electrical energy
AC-DC and Electrical Energy
Learning outcomes
• I know that AC-DC is a rubbish band
• I can describe AC and DC
• I know devices that can deliver AC and DC
• I can calculate the electrical energy used by a given appliance
• I know how household electricity bills are calculated
AC
AC stands for …Alternating…. Current. This means that an AC current or an AC
voltage goes ……forwards… and ……backwards……… .
V / V
+240
0
time
-240
AC is sinusoidal
Mains supply is AC. Mains is 240V ac. (it is actually 240V rms)
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2. FUNDAMENTAL Physics – Mains and Magnetism: 6.3 ACDC and electrical energy
DC
DC stands for ………Direct… Current. This means that a DC current or a DC voltage
goes only ……one way….. or only ………forwards………. . It only goes in ONE
DIRECTION.
V / V
+10
0 time
-10
DC is supplied by cells and batteries.
Is this AC or DC?
V / V
+10
0 time
-10
This is still …DC.. because the supply is only going ……foward…..
(positive) and never going …….backwards….. (negative).
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3. FUNDAMENTAL Physics – Mains and Magnetism: 6.3 ACDC and electrical energy
Rectification
All electronic devices use DC and almost all are powered by a battery.
Can AC from the mains charge a battery? Why?
Yes, this setup allows current to ONLY go one way
AC must be changed into DC in order to charge batteries and power electronic
devices. This process is called RECTIFICATION.
What device can change AC into DC?
A …diode… is, electrically, a one-way street.
+10
Input 0
AC Output -10
V
time
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4. FUNDAMENTAL Physics – Mains and Magnetism: 6.3 ACDC and electrical energy
Electrical Energy and Electricity Bills
When we pay electricity bills we pay for the energy we have consumed.
Energy = Power x time
Hence:
Electrical Energy = Current x Voltage x time
E = I V t
Eg. A home uses a 100W light bulb for 5 hours a day for one week.
What energy has been transferred by the light bulb?
240x5x60x60=240x18000=1800000 J
Obviously, the Joule is the correct SI unit of energy, but it is too small a
unit for our domestic energy bills. The Mega-Joule MJ would be a
better choice but instead we use the kW.Hr. The kW.Hr is called a
UNIT. In HK a UNIT costs approx $10.
How much did it cost the family to run the 100W light bulb for 5 hours
a day for one week?
0.1x5=0.5 kW.Hr
So 0.5x10 = $5x7 = $35
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