exp spa - chp 17 current of electricity
TRANSCRIPT
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Current of Electricity
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Learning Objectivesa) State that current is a rate of flow of charge and
that it is measured in amperesb) Distinguish between conventional current and
electron flowc) Recall and apply the relationship charge = current
× time to new situations or to solve related problems
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Electric CurrentWhat is Electric Current? An electric current is formed by moving charges.
Definition:An electric current is a measure of the rate of flow of electric charge through a given cross-section of a conductor.
The SI unit of electric current is the ampere (A).
where I = current; Q = charge;
t = time taken.
QI t
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Electric CurrentPractice ProblemIn one minute, 60 μC of charge passes through a point in a circuit. Calculate the current.
Solution
I = =
= 1 × 10−6 A = 1 μA
Q t60 × 10−6 C
60 s
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Electric CurrentHow do we measure current? We make use of an ammeter to measure current. The ammeter should be connected in series to the
circuit. Current must flow into positive (+)
terminal and leave negative (–) terminal
Current in Current out
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Electric CurrentConventional Current & Electron Flow Before the discovery of electrons,
scientists believed that electric current was caused by the movement of positive charges.
Although this idea was later proven wrong, the idea remains.
Conventional Current Flow Electron Flow
Movement of positive charges
From positive end of battery to negative end of battery
Movement of electrons (–)From negative end of battery topositive end of battery
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Electric CircuitMain components of a circuit A typical electric circuit consists of four main
components.
A source of electromotive force that drives electric charges around
the circuit.
A load in which moving charges can do a useful job.
Conductors that connect the components together.
A method of opening or closing the circuit.
Dry cell
Wires
Switch
Bulb
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Electric CircuitDrawing Circuit Diagrams Electric circuits can be represented by circuit diagrams. Can you identify what the symbols in the circuit diagram
below represent? ammeter
bulb
connecting wires
switch
cell
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Electric CircuitDrawing Circuit Diagrams Some common components and their symbols are
listed in the tables below.Symbol Device
switch
cell
batteryd.c. power
supplya.c. power
supply
light bulb
Symbol Device
wires joined
wires crossed
fixed resistor
Variable resistor (rheostat)
fuse
coil of wire
Symbol Device
galvanometer
ammeter
voltmetertwo-way switchearth
connector
capacitor
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Electric CircuitInterpreting Circuit Diagrams Open circuit
A circuit in which current is unable to flow through due to breaks in the circuit.
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Electric CircuitInterpreting Circuit Diagrams Short circuit
An alternative path of lower resistance is present and hence, current flows through wire X instead of the bulb.
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Learning Objectivesd) Define electromotive force (e.m.f.) as the work
done by a source in driving a unit charge around a complete circuit
e) calculate the total e.m.f. where several sources are arranged in series
f) State that the e.m.f. of a source and the potential difference (p.d.) across a circuit component is measured in volts
g) Define the p.d. across a component in a circuit as the work done to drive a unit charge through the component
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Electromotive Force & Potential Difference
What is the role of the battery in the circuit?
Why do you need a battery to make the bulb light up?
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We can compare flow of charges to flow of water.
Electromotive Force & Potential Difference
Flow of water in a pipe Flow of charge in a wireThe water in a horizontal pipe will not flow unless there is a difference in pressure between X and Y.
The free electrons in a wire will not flow unless they are forced to do so.
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Electromotive Force & Potential Difference
Flow of water in a pipe Flow of charge in a wireA pressure difference across X and Y is required such that X is at a higher pressure than Y
A source of energy is needed to drive the charge round a circuit. It may be a cell, a battery or a generator
Water pump does work to drive the water around the pipe.
Battery does work to drive electrons around the circuit.
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Electromotive Force & Potential Difference
What is Electromotive Force?Definition:The electromotive force (e.m.f) of an electrical energy source is defined as the work done by the source in driving a unit charge around a complete circuit.
ε = e.m.f. of electrical energy source;W = work done (amount of non-electrical energy converted to electrical energy);Q = amount of charge.
The SI unit of e.m.f is the volt (V).
We QQ
We
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Electromotive Force & Potential Difference
How do we measure e.m.f.? We make use of a voltmeter to measure e.m.f. The positive and negative terminals of a voltmeter
should be connected to the positive and negative terminals respectively of the electrical source.
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Electromotive Force & Potential Difference
Group ActivityObjective Measure and observe the e.m.f. of different arrangements of
dry cells.
Instructions1. In groups, set up the circuits shown below.
2. Record the voltmeter readings.
+ −
+ −
+ −
+ −
+ −
+ −
+ −
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Series Arrangement Parallel Arrangement
Electromotive Force & Potential Difference
When cells are arranged in parallel, the resultant e.m.f. is equal to that of a single cell.
When cells are arranged in series, the resultant e.m.f. is the sum of all the e.m.f.s of the cells.
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Electromotive Force & Potential Difference
What is Potential Difference?Definition:The potential difference (p.d.) across a component in an electric circuit is the work done to drive a unit charge through the component.
V = p.d. across a component;W = work done (amount of non-electrical energy converted to electrical energy);Q = amount of charge.
The SI unit of potential difference is the volt (V).
WV Q
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Electromotive Force & Potential Difference
How do we measure p.d.? We make use of a voltmeter to measure p.d. The voltmeter should be connected in parallel with the
component.
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Electromotive Force & Potential Difference
Potential Differences around a simple circuit
The cells does work by pushing the electrons through the whole circuit. They gain electrical potential energy.
As they flow through each component, the electrical potential energy is converted to other forms of energy.
Total e.m.f. in a circuit is equal to the total p.d. across each component
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Electromotive Force & Potential Difference
Potential Differences around a simple circuitFor a single charge,
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Electromotive Force & Potential Difference
Electromotive Force vs. Potential Difference
Electromotive force Potential Difference
It is the work done to drive a unit charge through two points.
It is the work done by the source in driving a unit charge around a complete circuit.
Associated with two points in an electric circuit
Associated with an electrical energy source (e.g. a dry cell)
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Electromotive Force & Potential Difference
What will the voltmeter reading show? A The e.m.f. of the electrical source (the three cells in series) B The e.m.f. of the bulbC The potential difference across the bulb
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Learning Objectivesh) State the definition that resistance = p.d. / currenti) Apply the relationship R = V / I to new situations or to
solve related problemsj) Describe an experiment to determine the resistance of
a metallic conductor using a voltmeter and an ammeter, and make the necessary calculations
k) recall and apply the formulae for the effective resistance of a number of resistors in series and in parallel to new situations or to solve related problems
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Learning Objectivesl) state Ohm’s Lawm) describe the effect of temperature increase on the
resistance of a metallic conductorn) sketch and interpret the I/V characteristic graphs for a
metallic conductor at constant temperature, for a filament lamp and for a semiconductor diode
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Resistance
QuestionPredict what will happen if a porous plate (an obstacle) is placed in the path of the water flow.
Earlier, we used the water pump analogy to help us understand e.m.f.
obstacle
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Resistance
Resistance is a measure of how difficult it is for an electric current to pass through a material.
It is the property of the material that restricts the movement of free electrons in the material.
Resistor added
Rate of flow of electric charges
reduced
resistor
Current is reduced
Ammeter reading will be reduced
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ResistanceWhat is Resistance?Definition:The resistance of a component is the ratio of the potential difference across the component to the current flowing through the component.
R = resistance of a component;V = p.d. across a component;I = current flowing through component.
The SI unit of resistance is the ohm (Ω).
VR I
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ResistanceHow do I measure resistance?
A circuit is necessary to measure resistance.
The ammeter is connected in series with the bulb.
The voltmeter is connected in parallel with the bulb.
Use
+
−
−+
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ResistanceIn the circuit shown,
All components have resistance.
Wires and ammeter assumed zero resistance current flows through without any loss of energy
Voltmeter considered infinite resistance no current flows through it
Light bulb Wires VoltmeterAmmeter
R = 0 Ω R = ∞ Ω
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ResistanceWhat are resistors?
A resistor is a conductor in a circuit that is used to control the size of the current flowing in a circuit.
There are two types of resistors — fixed resistors and variable resistors (or rheostats).
some common fixed resistors a rheostat is a type of variable resistor
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ResistancePractice Problem
A 3 V source is connected to a light bulb,as shown in the diagram.The ammeter registers a reading of 0.2 A.a) State the reading on the voltmeter.b) Determine the resistance of the light bulb.c) The circuit is switched on for five minutes.
i. Determine the amount of charge and; ii. energy dissipated in the bulb.
+
−
−+
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ResistancePractice Question(a) Voltmeter reading = 3 V
(b) R = = 15 Ω
(c) (i) Q = It = 0.2 A × 5 × 60 s = 60 C
(ii) E = QV = 60 C × 3 V = 180 J
3 V0.2 A
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Ohm’s LawOhm’s Law states that the current passing through a metallic conductor is directly proportional to the potential difference across it, provided that physical conditions remain constant.
where I = current; V = potential difference.
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Ohm’s LawOhmic Conductors are conductors that obey Ohm’s Law.
The I−V graph of an ohmic conductor is a straight line that passes through the origin.
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Ohm’s LawNon-Ohmic Conductors are conductors that do not obey Ohm’s Law.
They do not obey Ohm’s Law and their resistance R can vary.
Their I−V graphs are not straight lines, which means the ratio V/I is not a constant.
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Ohm’s LawA filament lamp is the 1st non-ohmic conductor. As the current increases, the temperature of the
filament lamp increases. It results increase in resistance increases with increasing current decreases with increasing current
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Ohm’s LawA semiconductor-diode is the 2nd non-ohmic conductor. It is a special device that allows current to flow in one
direction.
1) When potential difference is in the correct direction, current flows through it as though it is like a resistor.
2) When potential difference is in the reverse direction, almost no current flows through it.
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Learning Objectiveso) Recall and apply the relationship of the proportionality
between resistance and the length and cross-sectional area of a wire to new situations or to solve related problems
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ResistivityBesides temperature, the resistance R of a conductor depends on
1) its length l;
2) its cross-sectional area A;
3) the material it is made of (i.e. resistivity ρ).
l A
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Resistivity The formula of resistance depending on its physical
properties is:
The formula of resistivity:
The SI unit of resistivity is the ohm metre (Ω m).
where ρ = resistivity of conductor;R = resistance of conductor;A = cross-sectional area of conductor; l = length of conductor.
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Resistivity Different materials have
different resistivities.
Resistivity is a property of the material and it is independent of the dimensions of the material.
The lower the resistivity of a material, the better it is at conducting electricity.
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ResistivityComparing a wire to a tunnel
We can compare a wire to a tunnel to remember the factors affecting resistance:
Cross-sectional area The wider a tunnel,
the greater the number of people can walk through at the same time
The lower the resistance
Length The shorter a tunnel
The shorter the time taken to completely walk through it
The lower the resistance
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ResistivityTackling problems
Resistivity questions generally look like this:
1) I have a wire with resistance R1, length l1 and area A1.
2) If I use the same material but new length l2 and area A2, find its new resistance R2.
Since it is the same material, its resistivity r must be constant.
Therefore
Original wire’s
dimensions
New wire’s dimensions
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ResistivityPractice Problem 1
A conductor has a length l of 4 cm and cross-sectional area A of 2 cm2. Given that its resistance R is 5 Ω, determine the resistance of each of the following. l = 4 cm
A = 2 cm2
l = 4 cm
A = 4 cm2
l = 8 cm
A = 2 cm2(a)
(b)
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Resistivity
l = 8 cm
A = 4 cm2(c)
l = 8 cm
A = 1 cm2(d)
Read the solution for (a) and try the rest on your own first!
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Resistivity
(a)
Original wire’s
dimensions
New wire’s dimensions
Attempt (b) to (d) using the same method before checking the answers!
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Resistivity(c)
(b)
(b)
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ResistivityPractice Problem 2
A length of resistance wire 50 cm long is connected in series with an ammeter and a 3 V battery. The ammeter reading is 0.15 A.
A
50 cm
3 V
0.15 A
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ResistivityPractice Problem 2
(a) Determine the resistance of the wire.
(b) The length of the wire is doubled and its cross-sectional area halved. Determine the new resistance and hence ammeter reading.
(c) Given that the diameter of the 50 cm long wire is 5 mm, determine its resistivity.
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Resistivity(a)
(b)
(c) Remember that wires are
usually circular!