Download Electricity - Shatin Tsung Tsin Secondary School

Current Electricity
Shatin Tsung Tsin Secondary School
By Mr. C.K. Yu
Electric Circuit (電路)
E ’lec tric -adj.,
E ,lec ’tri ci ty -n.,
E ’lec tri cal -adj.
Electric Circuit (電路)
(a) When some wires (電線) are used to
connect (連接) the supply source (電源)
and some electric devices (零件), the
complete (完整的) set up (組合) is called
an electric circuit.
Electric Circuit (電路)
(b)Supply sources, such as electric cell (電
芯) and battery (電池組), provide electrical
energy (Electricity).
(c) Electric devices, such as lamp, work
for us.
Electric Circuit (電路)
(d) If there is a switch (開關) within the
circuit, "Open Circuit" can be described as
the switch being opened (打開) and
"Closed Circuit" as the switch being
_____________.
closed
Activities : electric devices
 Heater  Radio  Television
 Modem  Torch ? Loudspeaker
 Computer ? Candle
 Microwave oven
Heater, Torch are electric devices now and
Electric candle becomes more popular.
Circuit Parameters
Items to describe the circuit
Circuit Parameters : Current
• (i) Electric current passed through a
point is the amount of electric charge
(電荷), or amount of electrons (電子)
passes through that point in one second
(s).
• (ii) The symbol of current is I
the amount ofQelectric charge
=
Current
I
time
t
Circuit Parameters : Current
• (iii) Electric current is measured in
Ampere (安培), A
• (iv)Ammeter (安培計) is an apparatus. It is
used to measure current.
• (v) The amount of charge is measured in
Coulomb (庫倫), C. Its symbol is Q.
• Therefore , A = C / s
Current :Simple questions
• 1) What is the meaning of 1 mA?
0.001
______A.
• 2) 5 Coulombs of electric charge passes
through a lamp in 100 s. What is the electric
current in mA, passing through the lamp? Ans :
_________
mA
50
• 3) The reading of a device is 10A. The name of
the device is ___________.
Ammeter
Electromotive force (e.m.f.)
• A term to describe the ability of a supply
source to supply energy to a charge when
the charge passed through the supply
source.
Electromotive force (e.m.f.)
(i) Supply sources (e.g. cells or battery)
convert (轉化) chemical energy into
electrical energy.
(ii)Electromotive force is the amount of
energy converted into electrical energy per
coulomb of charge or the energy
supplied by a cell to each coulomb of
charge.
Electromotive force (e.m.f.)
(iii) Electromotive force is measured in Volt
(伏特), V.
i.e if the e.m.f of a cell is 1.2 V, it means
that the cell can provide 1.2 J of energy
to 1C of charge passing it.
(iv) The amount of energy is measured in
Joule (焦耳), J. Its symbol is E.
Electromotive force (e.m.f.)
The mathematical equation is :
e.m.f =
E
Energy
Charge
Q
The unit is : J/C or V (volt)
Simple questions:
1.5 V
• What is the e.m.f of a AAA battery ? _________
1.5
• A battery of size AA can supply energy of ______
Joules to each coulomb of charge passing through
it.
• Arrange in descending order the amount of energy
supplied to each coulomb of charge by the
following cells.
AA battery = AAA battery >rechargeable AA battery
1.5 V
1.5 V
1.2 V
HOT questions:
What are the similarity and difference
between an alkaline battery and a
general purpose battery of size AA?
Similarity
They have the same e.m.f. of 1.5V
________________________________
Difference
Usually, the alkaline battery stores
________________________________
more energy
________________________________
Home activity
•
What is the e.m.f. of your home electric
mains ?
The e.m.f is 220 V
Question No. 6
•
The following pieces of information are printed
on two rechargeable batteries, how much
energies are stored in each battery?
E = e.m.f x Q
Battery A: 4400 mAh 1.2V : __________________
-3 x 60 x 60=19008J
E=
1.2
x
4400x
10
________________________________________
E = e.m.f x Q
Battery B: 1200 mAh 1.2V : __________________
E= 1.2 x 1200x 10-3 x 60 x 60= 5184 J
________________________________________
Potential and potential difference
• Potential is the amount of electric energy
carried by a coulomb of charge (J/C, V)
• Potential difference is the difference in
potential between two points. (J/C, V)
• The difference can be due to :
Consumption (used by a lamp, TV)
Energy supply (supplied by a battery)
• What is the potential difference before and
after a battery of e.m.f of 1.5 V? 1.5V
Potential difference (電位差)
• Potential difference is the amount of
electric energy per coulomb of charge
changed into other forms of energy
across/between two points in a circuit.
e.g. potential difference of a battery of 1.5V
= 1.5 J of energy changed from chemical
to electrical per coulomb of charge
Potential difference (電位差)
• Potential difference is the amount of
electric energy per coulomb of charge
changed into other forms of energy
across/between two points in a circuit.
e.g. potential difference of lamp of 12 V = 12
J of energy changed from electrical energy
to light and heat energy.
Potential Difference
more examples :
At a point A in a circuit, 1 C of charge is found
to carry 1J of energy. What is the potential at
this point A?_______
1V
At a point B in a circuit, 2 C of charge are
found to carry a total of 6 J of energy. What
is the potential at this point B? ______
3V
What is the potential difference between B and
3 V – 1V = 2 V
B?_______________
Potential Difference
more examples :
Across a device in a circuit, 1 C of charge gives
3 J of energy to the device, the potential
3V
difference across this device is _______
.
After going through a lamp, each coulomb of
charge is found to have 1.5J of energy less,
the potential difference across this lamp is
1.5 V .
______
Voltage (電壓)
(ii) Both e.m.f and p.d. are often called
Voltage.
(iii)Voltage has the symbol, V, and potential
difference p.d..
(iv)Voltage and potential difference are
measured in Volt (V).
(v)Voltmeter(伏特計) is an apparatus. It is
used to measure potential difference or
voltage.
p.d. and Voltage
The equation is :
p.d
V =
Electrical
E energy
Q
charge
The unit of e.m.f , p.d and voltage is
V (volt) or J/C
Simple questions
How much chemical energy will convert into
electrical energy when 1 coulomb of charge
passes through two AA batteries? Ans :
1.5 J+ 1.5 J = 3.0 J
_________________.
Simple questions
Same amount of charges pass through two
batteries, which are AA battery of 1.5V and
AAA battery of 1.5V. Will there be any
difference in the amount of energy carried
away by the same amount of charges?
No, it is because they have the same e.m.f
__________________________________
Simple questions :
2 C of charge passed through an AA battery of
1.5V.
How much energy will be carried away by each
1.5 J
coulomb of charge ? _______
Simple questions :
2 C of charge passed through an AA battery of
1.5V.
What is the total chemical energy converted to
electrical energy? _________________
1.5 J/C x 2 C = 3 J
Simple questions :
2 C of charge passed through an AA battery of
1.5V.
Will there be any difference in the amount of
energy carried away by 1 coulomb of charge?
____________________________________
No, it is because the battery gives each
coulomb of charge the same amount of
____________________________________
energy , i.e. 1.5 J
____________________________________
Electric power (電功率)
(i) Electric power is the rate (速率, how quick)
of electric energy dissipated (使用)
(ii) Power has the symbol, P, and it is
measured in Watt (瓦特) ,W
W = J / s OR J / C x C /s
Electric Power
Power
P =
ElectricEEnergy
time
t
or
Power = Current x voltage
P =IxV
Class Example (Power)
• 50 J of electric energy were consumed by
a light bulb in 50 s. What is the power of
the light bulb? 1 W
• 25 J of electric energy were converted into
light per second. What is the power of the
light bulb? 25 W
Class Example (Power)
• A 10 W electric lamp was switched on for
1 minute, how much electric energy had
been consumed? 600 J
• A microwave oven of 700 W is used for 2
minutes, how much electric energy is
used? 84 000 J
Class Example (Power)
• A 2000 W air-conditioner is used for 8
hours, how much electric energy is
consumed? 57 600 000 J
• 2554000 J of energy is required to boil 1
kg of water in a kettle of 3000 W. How long
851s
does it take to boil the water?
Simple question
• The following devices give equal
brightness. Which one will save you
money in operation?
 1) A 20 W energy saving lamp

 2) A 100 W light bulb.
Reasons : 20 W energy saving lamp
used 20 J of energy per second only.
Work Example
5 C of charge passes a point A in a
circuit in 2 s. The charge has an
electrical energy of 8 J.
(i)
What is the current at point A ?
I = Q/t = 5 C / 2 s = 2.5 C/s = 2.5 A
(ii)
What is the voltage of the charge ?
V = E/Q = 8 J / 5 C = 1.6 J/C =1.6 V
Work Example
An electric device of 1000W operates for
one hour. How much energy has been
used?
E = P x t = 1000 W x 3600 s = 3 600 000 J
Work Example
Electric bill in Hong Kong is charged at $0.9
for a unit of kW hr (kW is unit of power, hr is
the unit of time). If also means an electric
device of 1kW operates for one hour.
What does the unit kW hr represent?
The unit represents P x t,
which means energy.
E = P x t = 1000 W x 60 x 60 s = 3, 600,000J
Then, 1kWhr means 3 600 000 J of energy.
The End
Thanks you. A big clap please!!