Ethiopian G-12 physics chapter four note

1. Unit 4
Electromagnetism
 Electromagnetism is a fundamental force in nature consisting of the elements
 electricity and magnetism.
 It is also referred to as electromagnetic force. The interaction of electrically charged
particles and uncharged magnetic force fields with electrical conductors creates the
electromagnetic fields.
 Devices that produce an electromagnetic field when electricity is applied are called
electromagnets.
 The relationship between magnetism and electricity was discovered in 1819 when,
during a lecture demonstration, Hans Christian Orested found that an electric current in
a wire deflected a nearby compass needle.
 In the 1820s, further connections between electricity and magnetism were
demonstrated independently by Faraday and Joseph Henry (1797-1878).

2.  They showed that an electric current can be produced in a circuit either
by moving a magnet near the circuit or by changing the current in a
nearby circuit. These observations demonstrate that a changing
magnetic field creates an electric field.
 Years later, theoretical work by Maxwell showed that the reverse is also
true: a changing electric field creates a magnetic field.
 Without an understanding of electromagnetism, devices such radios,
televisions, computers, tape recorders, VCR, CD players, lasers, electric
motors, and generators, could not have been invented.
 In this chapter, we discuss forces that act on moving charges as well as
forces that act on current-carrying conductors in the presence of a
magnetic field.

3. 4.1 Magnets and Magnetic field
 A magnet is any object that produces its own magnetic field that interacts with
other magnetic fields.
 Magnets have two poles, a north pole and a south pole.
 The magnetic field is represented by field lines that start at a magnet’s north
pole and end at the south pole.
 Magnet is a body having the property of attracting magnetic
materials and producing a magnetic field external to itself.
Examples of magnetic substances include:
 iron
 nickel
 cobalt
 stainless steel, and
 many rare earth metals.

4. Magnetic Fields
 All magnets are surrounded by a magnetic field
 A magnetic field is defined as:
The region around a magnet where a force acts on another magnet or on a magnetic
material (such as iron, steel, cobalt and nickel)
Magnetic Field Lines
 Magnetic field lines are used to represent the strength and direction of a magnetic field
 The direction of the magnetic field is shown using arrows
 The strength of the magnetic field is shown by the spacing of the magnetic field lines
 If the magnetic field lines are close together then the magnetic field will be strong
 If the magnetic field lines are far apart then the magnetic field will be weak
 There are some rules which must be followed when drawing magnetic field lines.
 The direction of the field lines should always be from:
 Always go from north to south (indicated by an arrow midway along the line)
 They must never touch or cross other field lines


5. Magnetic Field around a Bar Magnet
 The magnetic field is strongest at the poles
 This is where the magnetic field lines are closest together
 The magnetic field becomes weaker as the distance from the
magnet increases
 This is because the magnetic field lines are getting further
apart

6. Uniform Magnetic Field
•A uniform magnetic field will be produced in the gaps between opposite poles
• Note: Outside that gap the field will not be uniform
A uniform field is created when two opposite poles are held close together. A magnetic field is always
directed from North to South

7.  A uniform magnetic field is one that has the same strength and direction at all
points
 To show that the magnetic field has the same strength at all points there
must be equal spacing between all magnetic field lines
 To show that the magnetic field is acting in the same direction at all points
there must be an arrow on each magnetic field line going from the north pole
to the south pole
 The magnetic field lines are the same distance apart between the gaps of the
poles to indicate that the field strength is the same at every point between the
poles
Investigating Field Lines
 The shape and direction of a magnetic field may be investigated using plotting
compasses
 A plotting compass is like a small bar magnet, with a north and south pole
 The arrow of the plotting compass represents the north pole.

8. Compasses around a bar magnet show the direction of the magnetic field from
north to south

9. Differences Between Electric Field and Magnetic Field

11. 4.2 Magnetic field lines
 Magnetic field lines are imaginary lines or a visual tool used to
represent magnetic fields.
 The density of the lines indicates the magnitude of the field
 As with electric fields, the pictorial representation of magnetic
field lines is very useful
 When we sprinkle iron filings around the magnet, the iron filings
will orient themselves along the magnetic
 field lines, forming magnetic field pattern around the magnet.
 for visualizing the strength and direction of the magnetic field.
Magnetic field lines can never cross, meaning that the field is unique
at any point in space.
 Magnetic field lines are continuous, forming closed loops without
beginning or end.
 They go from the north pole to the south pole.

13. PROPERTIES OF MAGNETIC FIELD LINES:
 A magnetic field line is a closed and continuous curve.
 A magnetic field line is directed from North Pole to South Pole
outside the magnet. Whereas inside the magnet, the field lines are
directed from south pole to north pole.
 The magnetic field lines are crowded near the pole where the
magnetic field is strong and are far apart near the middle of the
magnet and far from the magnet where the magnetic field is weak.
 The magnetic field lines never intersect each other because if they
do so, these would be two directions of magnetic field at that point,
which is not possible.
 In case the field lines are parallel and equidistant, these represent
a uniform magnetic field.
 The Earth's magnetic field is uniform in a limited space.

14. 4.3 Current and Magnetism
Electricity and magnetism are essentially two aspects of the same
thing, because:
 A changing electric field creates a magnetic field, and
 A changing magnetic field creates an electric field.
 (This is why physicists usually refer to "electromagnetism" or
"electromagnetic" forces together, rather than separately.)
 Electricity is basically the presence and motion of charged
particles.
 Magnetism refers to the force which the magnets exert when they
attract or repel each other.
 Both the electric and magnetic fields exist without each other.

15. Ampere’s law
Ampere’s law states that the magnetic field around an electric current is
proportional to the current;
 each segment of current produces a magnetic field, and
 the total field of any shape current is the vector sum of the fields
due to each segment.
Applications of Ampere's Law
Ampere's Law is used to :
 Determine the magnetic induction due to a long current-carrying wire.
 Determine the magnetic field inside a toroid.
 Determine the magnetic field created by a long current-carrying conducting
cylinder.

16. Magnetic Field Created by a Long Straight Current-Carrying Wire
The kind of magnetic field is produced by a straight current-carrying
conductor:
Magnetic field lines are concentric circles lying in a plane perpendicular to
the straight conductor.
The centers of circular magnetic lines of force lie on the conductor.

18. The characteristics of the magnetic field produced by a current
flowing in a straight wire have the following properties.
 The magnetic field lines form a circular pattern.
 The magnetic field strength increases when current increases.
 The magnetic field strength is stronger near the wire and weaker
further away.
 When the direction of the current is reversed, the direction of the
magnetic field is reversed too.
 The direction of this magnetic field is given by the right-hand thumb rule.
 the thumb of the right-hand points in the direction of the current.
 The curled fingers give the direction of the magnetic field around the
wire.

21. Ampere’s Circuital Law and Magnetic Field: Applications
There is an enormous application of the Ampere's Law, but a few of the
important applications are:
 To calculate the Magnetic field at the inner side of a long solenoid.
 To calculate the magnetic field inside of a conductor.
 To calculate the Magnetic field inside of a toroidal coil.

22. Electromagnets at Home or School
 Some electromagnet uses in the home include:
 an electric fan
 electric doorbell
 induction cooker
 magnetic locks, etc.
 In an electric fan, the electromagnetic induction keeps the motor
rotating on and on making the blade of the fan to rotate.
 Electromagnets can be created by wrapping a wire around an iron nail
and running current through the wire.
 The electric field in the wire coil creates a magnetic field around the
nail.
 In some cases, the nail will remain magnetised even when removed
from within the wire coil.

23. Electromagnets are used for various purposes on a day-to-day basis.
For example:
 in generators
 motors
 transformers
 electric bells
 headphones
 loudspeakers
 relays
 MRI machines and others.
Most of the electric appliances used in the home use electromagnetism as
the basic working principle.

24. Magnetic Relays
Electromagnetic Relay Applications
Automobiles.
 Fuel pump, horns, starter motors, windshield vipers.
Building automation.
 Access control systems, elevators, control panels.
Industrial automation.
 Motor controllers, light controllers, power supply distribution and
switching.
Domestic electric appliances.
 Air conditioners, dishwashers, clothes dryers, drying cabinets, freezers,
refrigerators, kitchen stoves, water heaters, washing machines, trash compactors,
microwave ovens, and induction cookers are just a few examples of major
appliances.

26. Electric bell
 An electric bell is a mechanical or electronic bell that functions by
means of an electromagnet.
 When an electric current is applied, it produces a repetitive buzzing,
clanging or ringing sound.
 Electromechanical bells have been widely used at railroad crossings, in
telephones, fire and burglar alarms, as school bells, doorbells, and
alarms in industrial plants, since the late 1800s, but they are now being
widely replaced with electronic sounders.
 An electric bell consists of one or more electromagnets, made of a coil
of insulated wire around an iron bar, which attract an iron strip
armature with a clapper.

27. DC Electric Motor
 A DC motor is an electrical motor that uses direct current (DC) to
produce mechanical force.
 The most common types rely on magnetic forces produced by currents
in the coils.
 Nearly all types of DC motors have some internal mechanism, either
electromechanical or electronic, to periodically change the direction of
current in part of the motor.
 Small DC motors are used in tools, toys, and appliances.
 The universal motor, a lightweight brushed motor used for portable
power tools and appliances can operate on direct current and
alternating current.
 Larger DC motors are currently used in propulsion of electric vehicles,
elevator and hoists, and in drives for steel rolling mills.

29.  A DC power source supplies power motor.
 The commutator is the rotating interface of the rotating
loop (or coil) with a stationary circuit.
 The permanent magnetic field helps to produce a
torque on the rotating coil.
 The brushes is a device that conducts current between
stationary wires and moving parts.

30. 4.4 Electromagnetic Induction
What Is Electromagnetic Induction?
 Electromagnetic Induction was discovered by Michael Faraday in 1831, and James
Clerk Maxwell mathematically described it as Faraday’s law of induction.
 Electromagnetic Induction is a current produced because of voltage production
(electromotive force) due to a changing magnetic field.
 This either happens when a conductor is placed in a moving magnetic field (when
using an AC power source) or when a conductor is constantly moving in a stationary
magnetic field.
 Michael Faraday arranged a conducting wire attached to a device to measure the
voltage across the circuit.
 When a bar magnet is moved through the coiling, the voltage detector measures the
voltage in the circuit.

32. Magnetic flux
 Magnetic flux is a measurement of the total magnetic lines of force which passes
through a given area A.
 For a plane of surface area A placed in a uniform magnetic field B, magnetic flux is
mathematically written as:
'B = B.A =B Acosθ
where θ is angle between B and A.

33. Applications of Electromagnetic Induction
According to Faraday’s law:
the amount of voltage induced in a coil is proportional to the number of turns of the
coil and the rate of changing magnetic field.
 AC generators work on the principle of electromagnetic induction.
 The working of electrical transformers are based on electromagnetic induction.
 The magnetic flow meter is based on electromagnetic induction.
Electromagnetic Induction Formula
Mathematically, the induced voltage can be given by the following relation:
𝜀 =
_𝑁∆𝜑𝐵
∆𝑡
=
The negative sign indicates the direction of emf and hence the direction of current in a closed loop
 e is the induced voltage (in volts)
 N is the number of turns in the coil
 Φ is the magnetic flux – the amount of magnetic field at a surface (in Webbers)
 t is the time (in seconds)

34. Who discovered Electromagnetic Induction?
 Electromagnetic Induction was discovered by Michael Faraday in 1831.
What is the significance of Electromagnetic Induction?
 The significance of this discovery is a way of producing electrical
energy in a circuit by using magnetic fields and not just batteries.
Which machines work on the principle of Electromagnetic Induction?
 Everyday machines like motors, generators and transformers work on
the principle of electromagnetic induction.
What are the applications of Electromagnetic Induction?
 AC generators work on the principle of electromagnetic induction.
 The working of electrical transformers are based on electromagnetic
induction.
 The magnetic flow meter is based on electromagnetic induction.

35. There are two laws of electromagnetic induction:
• Faraday’s law
• Lenz’s Law
Lenz's law states that:
The induced electromotive force with different polarities induces a current whose
magnetic field opposes the change in magnetic flux through the loop in order to ensure
that the original flux is maintained through the loop when current flows in it.
Faraday's Law and Lenz's Law of
Electromagnetic Induction

36. Transformers
 A transformer is a device that transfers electric energy from one
alternating-current circuit to one or more other circuits, either
increasing (stepping up) or reducing (stepping down) the
voltage.
 A Step-up Transformer converts the low primary voltage to a
high secondary voltage and steps up the input voltage.
 A step-down transformer steps down the input voltage.
 A transformer is simply a pair of coils wound on the same core.
 The construction of a transformer allows the magnetic flux
generated by a current changing in one coil to induce a current
in the neighboring coil.

39. Applications of Transformer
 The transformer transmits electrical energy through wires
over long distances.
 Transformers with multiple secondaries are used in radio and
TV receivers, which require several different voltages.
 Transformers are used as voltage regulators.
 Transformers are used in various fields like:
 power generation grid
 distribution sector
 transmission and
 electric energy consumption.
 The primary and secondary windings are electrically isolated from each
other but are magnetically linked through the common core allowing
electrical power to be transferred from one coil to the other.

40. The transformer formula is given by:
𝑉𝑃
𝑉𝑆
=
𝑁𝑃
𝑁𝑆
=TURNS RATIO
Where:
 𝑉𝑃 =represents the main voltage
 𝑉𝑆= represents the secondary voltage
 𝑁𝑃 =represents the number of turns in the primary coil
 NS= represents the number of turns in the secondary coil
Transformer Efficiency
 For a transformer operating at a constant AC voltage
and frequency its efficiency can be as high as 98%.

41. EXAMPLES
1. A transformer has a primary and a secondary coil with the number of loops of 500
and 5000 respectively. If the input voltage is 220 V. What is the output voltage?
2. A transformer has primary coil with 1200 loops and secondary coil with
1000 loops. If the current in the primary coil is 4 Ampere, then what is the
current in the secondary coil.
3. A 95% efficient transformer is used to operate a lamp rated 60 W, 220 V from a 4400 V
a.c. supply. Calculate the (i) ratio of the number of turns in the primary coil to the number
of turns in the secondary coil of the transformer; (ii) current taken from the mains circuit?
4. A transformer is required to give 12V from the 240V Mains supply. If the primary coil has
30000 turns, how many has the secondary coil?
5. A transformer connected to a 120V power supply produces a voltage of 9.2V and a
current of 0.02A to power a toy car. What is the ratio of primary windings to secondary
windings in the transformer?

42. Working principle of transformer in house appliances
 An alternating current (AC) changes its direction periodically and
typically supplies power to run household appliances and industrial
equipment.
 We use some of the equipment and systems in our everyday lives.
 We use transformers around devices in the home that’s operating
voltage does not match the input voltage supplied from a power outlet.
Transformers are used in:
 Laptops and desktop computers
 Mobile phone chargers
 Games consoles
 Air conditioning unit
 Microwaves
 Battery charger
 Shaver socket
 Sound systems

44. Transformers can be found in the following industrial applications:
 Machinery control circuits
 Compressor panels
 Electrical distribution
 Electrical isolation

45. Electric, Magnetic, and Electromagnetic Field Safety
Electric, Magnetic, and Electromagnetic Fields (EMF) include:
 static fields
 low frequency waves, and are categorized as non-ionizing radiation.
The guidelines and recommendations for limiting EMF exposure differ from other types of
radiation such as gamma/X-rays or UV/IR lasers.
EMF radiation is classified by frequency or by corresponding wavelength.
Electromagnets help us to lift metal plates and transport them comfortably and
Quickly.
The best safety tips for electromagnets' uses
 Define the type of plates you want to lift.
 Check potential hazards in the environment.
 Protect your business and your lifting system from power failures.
 Check the electromagnets before uses.
 Choose the permanent electromagnet that best suits your project.