Chapter 5 electromagnetism
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Transcript of Chapter 5 electromagnetism
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Chapter 5ELECTRIC FIELD
MAGNETIC FIELDELECTROMAGNETISM
TRANSFORMER
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Electric Field
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Figure 6.1 represents two parallel metal plates, A and B, charged to different potentials.
Any region in the lines of electric force between the plates in Fig. 6.1, is called an electrostatic field.
Figure 6.2(a) shows a typical field pattern for an isolated point charge, and Fig. 6.2(b) shows the field pattern for adjacent charges of opposite polarity.
Electrostatic field
Electric lines of force (often called electric flux lines) are continuous. It start and finish on point charges; also, the lines cannot cross each other.
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V = Supply Voltage (Volt, V)d = Distance between plates (meter, m)A = Area (meter2, m2)
*Electric field strength is also called potential gradient.
Electric Field Strength, E
Electric flux density D is the amount of flux passing through a defined area A that is perpendicular to the direction of the flux
A
*Electric flux density is also called charge density, σ.
Q = Charges (coulombs, C)A = Area (meter2, m2)
Electric Flux Density, D
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Example
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Exercise
TUTORIAL 1
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A
Capacitance, CQ = Charges (coulombs, C)V = Volt Applied (Volt, V)
How much charge corresponds to a given p.d. between the plates is the capacitance:
Permittivity, 0 = permittivity of vacuum = 8.85×10−12 F/m.r = relative permittivity (no unit)D = Electric Flux Density (C/m2)E = Electric Field Strenth (v/m)
CONST 32 =Typical values of εr include air, 1.00; polythene,2.3; mica, 3–7; glass, 5–10; water, 80; ceramics, 6–1000.
I = Current (Ampere, A)t = time (seconds, s)
Dielectric
Electrostatic Field established in particular materials
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Example
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TUTORIAL 1
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The parallel plate capacitor, C
0 = permittivity of vacuum = 8.85×10−12 F/m.r = relative permittivity (no unit)n = number of plates (no unit)d = distance between plates (meter/m)A = Area (meter2, m2)
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Dielectric Strength, Em
Vm = Supply Voltage (Volt, V)d = Distance between plates (meter, m)
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Example Exercise1
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TUTORIAL
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Energy stored in capacitors, WV = Voltage Applied(Volt, V)C = Capacitance (Farad, F)
Example
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Exercise
TUTORIAL 1
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Magnetic Field
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Magnetic Field
The north-seeking end of the magnet is called the north pole, N, and the south-seeking end the south pole, S. The area around a magnet is called the magnetic field and this field consist of lines of magnetic flux.
In Fig. 7.2(a), with unlike poles adjacent, attraction takes place. But in Fig. 7.2(b), with similar poles adjacent (i.e. two north poles), repulsion occurs.
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Magnetic FluxMagnetic flux is the amount of magnetic field (or the number of lines of force) produced by a magnetic source. The symbol for magnetic flux is φ (Greek letter ‘phi’).
The unit of magnetic flux is the weber, Wb. Magnetic flux density is the amount of flux passing through a defined area that is perpendicular to the direction of the flux
Flux Density, B
φ = Magnetic Flux (weber, Wb)A = Area (meter2, m2)
The symbol for magnetic flux density is B. The unit of magnetic flux density is the tesla, T, where 1 T = 1Wb/m2.
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Example
Exercise
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Magnetomotive force, mmfMagnetomotive force (m.m.f.) is the cause of the existence of a magnetic flux in a magnetic circuit
Magnetic Field StrengthN = Number of turns (No Unit)I = Current (Ampere, A)l = Magnetic Path (Meter, m)
N = Number of turns (No Unit)I = Current (Ampere, A)
Example
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Flux, Flux Density & MmfTUTORIAL 1
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Electromagnetism
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Magnetic field due to an electric current
If a current is now passed through the wire, then the iron filings will forma definite circular field pattern with the wire at the centre. If the current direction is reversed, the direction of the lines of flux is also reversed.
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Screw Rule
The direction of the magnetic lines of flux is best remembered by the screw rule which states that:If a normal right-hand thread screw is screwed along the conductor in the direction of the current, the direction of rotation of the screw is in the direction of the magnetic field.
*screw rule = right hand grip rule
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Solenoid
A magnetic field set up by a long coil, or solenoid, is shown in Fig. 8.4(a) and is seen to be similar to that of a bar magnet. If the solenoid is wound on an iron bar, asshown in Fig. 8.4(b), an even stronger magnetic field is produced, the iron becoming magnetised and behaving like a permanent magnet.
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Fleming’s left-hand rule
If the current-carrying conductor shown in Fig. 8.3(a) is placed in the magnetic field shown in Fig. 8.13(a), then the two fields interact and cause a force to be exertedon the conductor as shown in Fig. 8.13(b). The field is strengthened above the conductor and weakened below, thus tending to move the conductor downwards. This is the basic principle of operation of the electric motor
NS
+_
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Fleming’s left-hand rule
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MotorWhen current flows in the coil a magnetic field is set up around the coil which interacts with the magnetic field produced by the magnets. This causes a force F to be exerted on the current-carrying conductor which, by Fleming’s left-hand rule, is downwards between points A and B and upward between C and D for the current direction shown. This causes torque and the coil rotates anticlockwise. When the coil has turned through 90◦ from the position shown in Fig. 8.17 the brushes connected to the positive and negative terminals of the supply make contact with different halves of the commutator ring, thus reversing the direction of the current flow in the conductor.
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Electromagnetic Induction(a) When the magnet is moved at constant speed towards the coil (Fig. 9.1(a)), a deflection is noted on the galvanometer showing that a current has been produced in the coil.(b) When the magnet is moved at the same speed as in (a) but away from the coil the same deflectionis noted but is in the opposite direction (seeFig. 9.1(b)).(c) When the magnet is held stationary, even within the coil, no deflection is recorded.(d) When the coil is moved at the same speed asin (a) and the magnet held stationary the samegalvanometer deflection is noted.(e) When the relative speed is, say, doubled, thegalvanometer deflection is doubled.(f ) When a stronger magnet is used, a greater galvanometer deflection is noted.(g) When the number of turns of wire of the coilis increased, a greater galvanometer deflection isnoted.
As the magnet is moved towards thecoil, the magnetic flux of the magnet moves across, or cuts, the coil. It is the relative movement of the magnetic flux and the coil that causes an e.m.f. and thus current, to be induced in the coil. This effect is known as electromagnetic induction.
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Faraday’s laws• Faraday’s law state that the voltage induced
across a coil of wire equals the number of turns in the coil, multiply the rate of change of the magnetic flux.
Lenz’s laws• Lenz’s law state that when the current
through a coil changes, the polarity of the induced voltage is such it always oppose the change in current.
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Fleming’s Right-hand rule
NS
+_
B = Flux Density (Tesla, T)l = Length of conductor in magnetic field (meter, m)v = Conductor movement velocity (ms-1)θ = conductor movement direction towards magnetic field
In a generator, conductors forming an electric circuit are made to move through a magnetic field. By Faraday’s law an e.m.f. is induced in the conductors and thus a source of e.m.f. is created. A generator converts mechanicalenergy into electrical energy.
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Induced e.m.f.Example Exercise
TUTORIAL12
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Rotation of a loop in a magnetic field
N = Number of turnsB = Flux Density (Tesla, T)l = Length of conductor in magnetic field (meter, m)v = Conductor movement velocity (ms-1)θ = conductor movement direction towards magnetic field
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Rotation of a loop in a magnetic fieldExample
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Energy stored in inductor, W
I = Current flows (Ampere, I)L = Inductance (Henry, H)
Example TUTORIAL123
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Ideal Transformer
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Transformer
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II
PP
NN
VV
V1 = Primary VoltageV2 = Secondary Voltage
N1 = Primary TurnsN2 = Secondary Turns
P1 = Primary PowerP2 = Secondary Power
I1 = Primary CurrentI2 = Secondary Current
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Transformer 1
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II
PP
NN
VV
Example Exercise
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Transformer
TUTORIAL123
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Thank You