As Unit1 Electricity 01 Electric Current

8/11/2019 As Unit1 Electricity 01 Electric Current

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Electric Current

Electricity Lesson 1
http://en.wikipedia.org/wiki/File:N.Tesla.JPG


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Learning Objectives

To establish what you already understand aboutelectricity.

To know what is meant by an electric current.

To know how to calculate the charge flow in acircuit.

To be able to define the coulomb.


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The Plan...

To check what you remember from GCSE.

Build some circuits to check/change your ideas.

Discuss what is meant by electric current.

Practice some calculations.


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Electricity Random Fact

Electrons only move through a wire at a speedof about 1mm/sec.


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Practical Work!

Draw circuit diagram and then build a circuit totest your ideasrisk assessment.

Work individuallyyou have to in the exam!

Tips:- Perfect the art of fault findingreplace each

component one at a time.

Start with the simplest circuit and build on that.


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Electricity

What is electricity?


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Electric Current

The electric current is the rate of flow of chargein a wire or component.

unit is the ampere (A)

Due to the passage of charge particles referred

to as charge carriers.

In metals the charge carriers are electrons.

In liquids & gases the charge carriers are ions.


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The Coulomb

The unit of charge is the coulomb (C),

which is defined as the charge flow in one

second when the current is one ampere.

The symbol for charge is Q.

The symbol for the unit, coulomb is C.

The charge on an electron is e=1.6 10-19C


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Current

For a current I, the charge flow Q in a time tis given by:-

t

QI


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Question

If the charge on one electron is e=1.6 10-19C,how many electrons are needed to make up 1 Cof charge?


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Answer

If the charge on one electron is e=1.6 10-19C,how many electrons are needed to make up 1 Cof charge?

18

19-1025.6

101.6

C1electrons.ofno

C


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Possible Trap

There are some important multipliers forcurrent:

1 microamp (1 A) = 1 10-6A

1 milliamp (1 mA) = 1 10-3A

You must use current in amps, charge incoulombs and time in seconds for calculations.

Watch out for this!


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Worked Example

What is the charge passing a point if a current of10 pA flows for 1 year?


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Learning Objectives

To establish what you already understand aboutelectricity.

To know what is meant by an electric current.

To know how to calculate the charge flow in acircuit.

To be able to define the coulomb.


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End


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Todays Objectives

Describe the relationship between current andcharge

Know what happens to the current at anyjunction in a circuit

Describe the relationship between the currententering and leaving a component

Describe the relationship between the currentpassing through 2 or more components in series


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Spooning charge

Electric charge can be picked up and carried by aspoon, just as if it were sugar or milk. An insulatedmetal spoon can carry charge from the terminal of a

high voltage supply across to a charge-measuringinstrumente.g. Coulomb meter.

Alternatively use a charged polythene rod to spoon

charge onto the Coulomb meter.


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Spooning Charge

link to

earthsocket

5 kV supplyinternal 50MWresistor

bare 4mmplug

044

coulomb meter

insulatinghandle

metal disk on4mm plug


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Calculating the number of electrons

Knowing that the charge on an electron is1.6 1019 C, you can calculate the number ofelectrons in a 'spoonful' of charge. A typical

spoonful of negative charge is2 nC. So thenumber of electrons is:

electrons102.1C101.6

nC2

electrononcharge

spoononcharge 1019-


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Outcomes

1. Charges, which you cannot see, are realenough and can be ladled around and measured,just like other more tangible physical quantities.

2. Charges are either positive or negative.

3. The electron has a very small charge. A'charged object' is one with a slight excess ordeficit of electrons.


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Shuttling ball experiment


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Discussion: Defining current, the coulomb

Current is defined as rate of change of charge. This can be done graphically.Current is the gradient of a graph of charge transferred against time.

I = dQ/dt.

The idea of the gradient can be introduced by asking how the chargetransferred by the shuttling ball increases with time - it will go up in a series ofsteps but, given a large number of transfers, these will approximate to aconstant slope. The average current is equal to its gradient. The equation I =Q/t(familiar from pre-16 science lessons) is useful but stress that this refersto an average current Iand care must be taken when Iis changing.

A current of one amp is equivalent to a flow of one coulomb per second.

The coulomb defined as the charge passed by a current of 1 A in 1 s, i.e. 1 C= 1 A s.


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Introductory questions on charge and current

Convert 25 mA to A

2. Convert 0.50 A to mA

3. A torch bulb passes a current of 120 mA.

(a) How many coulombs of charge flow through the lamp in 1 minute?

(b) How many coulombs of charge flow through the lamp in 1 hour? (c) How many electrons leave the negative terminal of the cell each second?

4. A car battery is rated as 36 A h. In principle this means it could pass acurrent of 1 A for 36 h before it runs down. How much charge passesthrough the battery if it is completely run down?

5. An electron beam in a beam tube carries a current of 125 A.

(a) What charge is delivered to the screen of the tube every second?

(b) How many electrons hit the screen each second?


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Circuit rules Current rules

Current rules

At any junction in a circuit the total current leaving thejunction is equal to the total current entering the

junction (Kirchhoffs current Law) The current entering a component is the same as the

current leaving the component (from KS 3 and 4)

The current passing through 2 or more componentsin

seriesis the same through each component. (from KS3 and 4)


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Kirchhoffs current law

The current entering any junction is equal

to the current leaving that junction. i1 + i4

= i2 + i3
http://en.wikipedia.org/wiki/File:KCL.png


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Conclusions

The current is the charge per second : I = dQ/dt.

At any junction in a circuit the total current leaving thejunction is equal to the total current entering the

junction (Kirchhoffs current Law) The current entering a component is the same as the

current leaving the component

The current passing through 2 or more componentsin

seriesis the same through each component.

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As Unit1 Electricity 01 Electric Current