Transcript Today`s Objectives - RanelaghALevelPhysics

Electric Current
Electricity Lesson 1
Learning Objectives
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To establish what you already understand about
electricity.
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To know what is meant by an electric current.
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To know how to calculate the charge flow in a
circuit.
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To be able to define the coulomb.
The Plan...
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To check what you remember from GCSE.
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Build some circuits to check/change your ideas.
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Discuss what is meant by electric current.
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Practice some calculations.
Electricity Random Fact
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Electrons only move through a wire at a speed
of about 1mm/sec.
Practical Work!
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Draw circuit diagram and then build a circuit to
test your ideas – risk assessment.
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Work individually – you have to in the exam!
Tips: Perfect the art of fault finding – replace each
component one at a time.
 Start with the simplest circuit and build on that.
Electricity
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What is electricity?
Electric Current
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The electric current is the rate of flow of charge
in a wire or component.
 unit is the ampere (A)
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Due to the passage of charge particles referred
to as charge carriers.
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In metals  the charge carriers are electrons.
In liquids & gases  the charge carriers are ions.
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The Coulomb
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The unit of charge is the coulomb (C),
which is defined as the charge flow in one
second when the current is one ampere.
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The symbol for charge is Q.
The symbol for the unit, coulomb is C.
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The charge on an electron is e=1.6 × 10-19 C
Charge Flow
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For a current I, the charge flow ΔQ in a time Δt
is given by:-
Q  It
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The symbol Δ is delta, a Greek capital letter Δ,
meaning “change in”.
charge flow  current  time interval
Current
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For a current I, the charge flow ΔQ in a time Δt
is given by:-
Q
I
t
Question
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If the charge on one electron is e=1.6 × 10-19 C,
how many electrons are needed to make up 1 C
of charge?
Answer
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If the charge on one electron is e=1.6 × 10-19 C,
how many electrons are needed to make up 1 C
of charge?
1C
18
no .of electrons 
 6.25 10
-19
1.6 10 C
Possible Trap
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There are some important multipliers for
current:
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1 microamp (1 μA) = 1 × 10-6 A
1 milliamp (1 mA) = 1 × 10-3 A
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You must use current in amps, charge in
coulombs and time in seconds for calculations.
Watch out for this!
Worked Example
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What is the charge passing a point if a current of
10 pA flows for 1 year?
Learning Objectives

To establish what you already understand about
electricity.

To know what is meant by an electric current.

To know how to calculate the charge flow in a
circuit.

To be able to define the coulomb.
End
Today’s Objectives
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Describe the relationship between current and
charge
Know what happens to the current at any
junction in a circuit
Describe the relationship between the current
entering and leaving a component
Describe the relationship between the current
passing through 2 or more components in series
Spooning charge
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Electric charge can be picked up and carried by a
spoon, just as if it were sugar or milk. An insulated
metal spoon can carry charge from the terminal of a
high voltage supply across to a charge-measuring
instrument – e.g. Coulomb meter.
Alternatively use a charged polythene rod to ‘spoon’
charge onto the Coulomb meter.
‘Spooning’ Charge
internal 50MW resistor
5 kV supply
link to
earth
socket
insulating
handle
bare 4mm
plug
metal disk on
4mm plug
044
coulomb meter
Calculating the number of electrons
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Knowing that the charge on an electron is –1.6 ´
10–19 C, you can calculate the number of
electrons in a 'spoonful' of charge. A typical
spoonful of negative charge is –2 nC. So the
number of electrons is:
charge on spoon
2 nC

 1.2 10 electrons
charge on electron 1.6 10 C
10
-19
Outcomes
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1. Charges, which you cannot see, are real
enough 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 or
deficit of electrons.
Shuttling ball experiment
Discussion: Defining current, the coulomb
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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 charge
transferred by the shuttling ball increases with time - it will go up in a series of
steps but, given a large number of transfers, these will approximate to a
constant 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 refers
to an average current I and care must be taken when I is 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.
Introductory questions on charge and current
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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 a
current of 1 A for 36 h before it runs down. How much charge passes
through 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?
Circuit rules Current rules
Current rules
 At any junction in a circuit the total current leaving the
junction is equal to the total current entering the
junction (Kirchhoff’s 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 components in
series is the same through each component. (from KS
3 and 4)
Kirchhoff’s current law
• The current entering any junction is equal
to the current leaving that junction. i1 + i4
= i2 + i3
Conclusions
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The current is the charge per second : I = dQ/dt.
At any junction in a circuit the total current leaving the
junction is equal to the total current entering the
junction (Kirchhoff’s current Law)
The current entering a component is the same as the
current leaving the component
The current passing through 2 or more components in
series is the same through each component.