electric current lecture
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Introduction
1.Electrostatics stationary charges
What is the relationship betweenelectric charges and electricity?
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Electric Current
2. For an amount of current,
3. The current is measured in Ampere, A.
1 A = 1 C s-1
tQI !
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Potential Difference (p.d.), V
1.Current is anologous to water flowing in asystem of pipes. Water flows from a point ofhigher pressure to a point of lower pressure.T
hus, a pressure difference.2.Electric current flows from a point of higher
electric potential to a lower electric potential. Apotential difference, V is required for a currentflow.
3.P.d. between 2 points in a circuit is the energyrequired by a Coulomb of charge to move fromone point to other.
4. 1 Volt = 1 Joule per Coulomb
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Potential Difference (p.d.), V
Resistor
Higher
potential,V1
Lower
potential,V2
V = V1 V2
I
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Electric energy, E
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ElectricalPower, P
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Resistance
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Electromotive Force (e.m.f.)
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Electromotive Force (e.m.f.)
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For a current to flow continuously between two points
PQ in a circuit, a source of e.m.f. is required, such as abattery, to maintain the p.d. between the two points.
The source of e.m.f. is the energy source in an electriccircuit.
El tr tiv F r ( . .f.)
P Q
E
V
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Ohms Law
A steady current through a metallic
conductor is directly proportional to the
potential difference across the conductor
if the temperature and other physicalconditions remain constant
Current I w V if temperature is constant
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Ohmic conductor
Obeys Ohms Law:
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Non-Ohmic Conductor- are conductors not obeying Ohms Law
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Resistors in series
The same current Ipasses through all resistors.
The potential difference across each resistor is
The total potential difference,
Hence,
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Resistors in parallel
P.d. are the same for all resistors
Current Iis divided into I1, I2, I3.
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Hence,
IfR is the effective resistance,
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Example:
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Questions:
1. Two 1.5 V battery is connected toresistors 3 , 4 and 5 connected in series. Calculate (i)
the current in the circuit and (ii)the potential difference acrosseach resistor.
2. Three 1.5 V cells are connected in
parallel.This circuit is connected toa resistor of 5.0 . Calculate the
current flowing through theresistor.
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Electromagnetism
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Using electromagnets
Many objects around youcontain electromagnets.They are found in electricmotors and loudspeakers.Very large and powerfulelectromagnets are usedas lifting magnets in scrap
yards to pick up, thendrop, old cars and otherscrap iron and steel.
They are better thanmagnets because themagnetism can be turned
off and on.
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Electromagnet Corkscrew rule or
right hand rule
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Electromagnet
S N
Use right-hand rule todetermine the magnetic poles
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Flemings Left Hand Rule (motor)
Force(Thumb)
Field (First finger)
Current (seCond finger)
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Electromagnetic Induction
Faradays Law:
the induced e.m.f. is directly
proportional to the rate of change offlux linkage or rate of flux cutting
Lenzs law
the direction of the induced e.m.f.is such that it tends to oppose theflux change causing it.
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What is magnetic flux
Consists of many magnetic field linesin an area;
Also known as magnetic flux density,B;
Unit ofB is Tesla, T
A
B
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Electromagnetic induction
N N
1. Magnet moving intosolenoid;
2. Direction of current isaccording to Lenz Law.
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N S
1. Magnet moving out ofsolenoid;
2. Direction of current isaccording to Lenz Law.
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This is a rule to help you to remember their directions.(NOTNOT a physics principle!)
Direction of induced current
a Fleming's right-hand rule
For a wire cutting through a B-field...
motion or force F
magneticfield B
inducedcurrent I
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Direction of induced current
b Lenz's law
induced I opposesthe motion ofmagnet (beingattracted)
induced I opposes
the motion ofmagnet (beingrepelled)
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Direction of induced current
In both cases, magnet
moves against a force.Work is done duringthe motion & it istransferred as electrical
energy.
Induced I always flows to oppose themovement which started it.
b Lenz's law
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Example 1Current induced along a coil
A bar magnet passes through a coil:
(a) Indicate the direction of the induced I ineach case. Explain briefly.
(i) (ii) (iii)
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Example 1Current induced along a coil
(a) Indicate the direction of the induced I. Explain.
(i)
When magnets N-pole is movinginto coil,
induced I flows in such a direction asto produce a N-pole
to oppose the approaching of magnet.Lenzslaw
I
S N
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Induction
)( *!\ Ndt
d
Induced e.m.f. depends on:
1.No. of coils
2.Magnetic field strength
3.Velocity of magnetic field
4.Cross-sectional area of coil
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Self-inductance
A current-carrying coil has amagnetic flux, B;
When the current in the coilchanges, B in the coil changes;
This induces an e.m.f. in the coil;
Induced e.m.f. is due to the changein the current in the same coil.
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NsNp
Vp Vs
Ip
Is
Transformer
S
P
P
S
P
S
S
P
SSPP
S
P
S
P
N
N
I
I
II
VV
VIVI
poweroutputpowerInput
N
N
V
V
!
!
!
!
!
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Transmission of current
Powerstation
houses
Step-downtransformer
Step-uptransformer
cables
50 kV 100 kV
240 V
13 A
0.5 A
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