current electricity

Current ElectricityCurrent Electricity

Shatin Tsung Tsin Secondary SchoolShatin Tsung Tsin Secondary School

By Mr. C.K. YuBy Mr. C.K. Yu

E ’lec tric -adj., E ,lec ’tri ci ty -n., E ’lec tri cal -adj.

Electric Circuit (Electric Circuit ( 電路電路 ))

Electric Circuit (Electric Circuit ( 電路電路 ))

(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 (Electric Circuit ( 電路電路 ))(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 (Electric Circuit ( 電路電路 ))

(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

Heater Radio Television

Modem Torch Loudspeaker

Computer Candle

Microwave oven

Activities : electric devices

? ?

Heater, Torch are electric devices now andElectric 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

Currentthe amount of electric charge

time=

Q

tI

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? ______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

• 3) The reading of a device is 10A. The name of the device is ___________.

0.001

50

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 =EnergyCharge

The unit is : J/C or V (volt)

Q

E

Simple questions:

• What is the e.m.f of a AAA battery ? _________• 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

= >1.5 V 1.5 V 1.2 V

HOT questions:

What are the similarity and difference between an alkalinealkaline battery and a general purposegeneral purpose battery of size AA?

Similarity

________________________________

Difference

________________________________

________________________________

They have the same e.m.f. of 1.5V

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 VThe 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?

Battery A: 4400 mAh 1.2V : __________________

________________________________________

Battery B: 1200 mAh 1.2V : __________________

________________________________________

E = e.m.f x QE= 1.2 x 4400x 10-3 x 60 x 60=19008J

E = e.m.f x QE= 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 differencedifference 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? 1.5V1.5V

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 Differencemore 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?_______

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? ______

What is the potential difference between B and B?_______________

1 V

3 V

3 V – 1V = 2 V

Potential Differencemore examples :

Across a device in a circuit, 1 C of charge gives 3 J of energy to the device, the potential 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 ______ .

3 V

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)VoltmeterVoltmeter(伏特計 ) is an apparatus. It is used to measure potential difference or voltage.

p.d. and Voltage

The equation is :

p.d =Electrical energy

chargeV

E

Q

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 coulomb of charge ? _______1.5 J

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, PP, and it is measured in Watt (瓦特 ) ,W

W = J / s OR J / C x C /s

Electric Power

Power =Electric Energy

time

E

t

or

P

Power = Current x voltage

P = I x V

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?

• 25 J of electric energy were converted into light per second. What is the power of the light bulb?

1 W

25 W

Class Example (Power)• A 10 W electric lamp was switched on for

1 minute, how much electric energy had been consumed?

• A microwave oven of 700 W is used for 2 minutes, how much electric energy is used?

600 J

84 000 J

Class Example (Power)• A 2000 W air-conditioner is used for 8

hours, how much electric energy is consumed?

• 2554000 J of energy is required to boil 1 kg of water in a kettle of 3000 W. How long does it take to boil the water?

57 600 000 J

851s

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 Reasons : 20 W energy saving lamp used 20 J of energy per second only.used 20 J of energy per second only.

Work Example5 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 ?

(ii) What is the voltage of the charge ?I = Q/t = 5 C / 2 s = 2.5 C/s = 2.5 A

V = E/Q = 8 J / 5 C = 1.6 J/C =1.6 V

Work ExampleAn 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 ExampleElectric 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, 600,000JThen, 1kWhr means 3 600 000 J of energy.

The End

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