9electrical measuring instruments, electric engines
9electrical measuring instruments, electric engines.docx
Teacher Guides of the Lesson
Theoretical material for the lesson,
definitions for concepts
A simple electric motor - an application of the motor effect
- uses of electric motors.
understand how a simple dc electric motor works consider the
diagram above to get the idea.
ØInstead of a
single linear wire, consider placing a d.c. current carrying loop (or
many turns of wire loops) in the magnetic field of a permanent magnet (U
shaped) or opposite poles from two permanent magnets.
ØThe wire is at 90o to
the direction of the magnetic field - lines of force in blue.
ØNow we apply
Fleming's left-hand rule because the same forces are in operation as for the
single wire demonstration.
ØI've drawn the
rule and applied it to both sides of the loop to show the directions the forces
ØThe left side of
the loop will move downwards and the right side of the loop moves upwards
giving anticlockwise rotation.
ØThis produces an
anticlockwise rotation movement - and that's quite simply, the basis of an
electric motor, but you will not get continuous rotation without
some further modifications and added 'bits' described below!
simple, but practical, working model of a simple
d.c. electric motor
how a simple dc electric motor works
as described above, the 'diagram' needs a few more bits to be a working
added 'bits' ....
an axle (spindle) about which the coil can freely rotate
between the poles of a permanent magnet, a split ring commutator that swaps the
contacts around every half-turn (swapping the +/-polarity) and keeps
the rotation in the same direction, it also enables electrical contact to
the external circuit, together with the ...
contacts (of graphite block or copper strip) which enable rotation
movement to continue but still maintain a complete electrical circuit - the
'brushes' sweep over the surface of the contacts on the axle, and of course a
frame structure to hold all the components in place!
way the forces operate was explained in the previous diagram, but I have
repeated the application of Fleming's left-hand rule to show the coil will
can reverse the direction of rotation either by either ..
swapping the polarity of the d.c. supply to change the direction of current
swapping the magnetic poles of the permanent magnet to change the direction of
the magnetic field.
there are several sources of energy loss - decreasing
the efficiency of the motor
When the electric motor starts running the current decreases a
little from its initial value.
the current flows, the thin wire coils act as a resistance, the coil heats up a
little as heat energy is lost: electrical energy ==> thermal energy store of
the motor and surroundings.
the temperature of the coils increases, its resistance increases a bit more,
leading to a greater increase in wasted energy.
Although this machine is acting as an electric motor, simultaneously it acts
as a generator!
coil rotates in the magnetic field it induces a current to flow in the opposite
can you make a simple dc (or any) electric
motor more powerful?
are three ways to do this, all involve increasing the strength of the magnetic
Increasing the number of turns of wire in the coil.
The magnetic lines of force 'cut' through more wire per unit time.
winding the coil on a soft-iron armature to increase the magnetic flux. through
The ion concentrates the lines of force, so more lines of force are 'cut'
through per unit time.
By making the field magnet as strong as possible.
The stronger the magnet, the greater the magnetic flux - the lines of force are
closer together, so
more lines of force are 'cut' per time as the armature rotates.
the p.d. across the coil to increase the current.
Increase the charge flow will intensify and strengthen the magnetic field
around the coil.
apply to any electric motor design.
factors can be used to increase the speed of rotation of the motor.
make an electric motor less powerful or slow its rotation down, (i) reduce the
current (by reducing the pd across the coils), (ii) reduce the number of turns
of wire coils and (iii) decrease the strength of the magnet to reduce the
magnetic flux density.
(i) is used to control the speed of an electric motor e.g. an electric car or
train. You can't really change any other factor in a working machine!
Principle of Electric Motor
motor is a machine which converts electrical energy into mechanical energy.
It is based on the principle that when a current-carrying conductor is placed
in a magnetic field, it experiences a mechanical force whose direction is given
Left-hand rule and whose magnitude is given by
= B I l newton
Where B is the magnetic field in weber/m2.
I is the current in amperes and
l is the length of the coil in meter.
force, current and the magnetic field are all in different directions.
If an Electric current flows through two copper wires that are between the
poles of a magnet, an upward force will move one wire up and a downward force
will move the other wire down.
Figure 1: Force in DC Motor
Figure 2 : Magnetic Field in DC Motor
Figure 3 : Torque in DC Motor
Figure 4 : Current Flow in DC Motor
The loop can be made to spin by fixing a half circle of copper which is known
as commutator, to each end of the loop. Current is passed into and out of the
loop by brushes that press onto the strips. The brushes do not go round so the wire do not get
twisted. This arrangement also makes sure that the current always
passes down on the right and back on the left so that the rotation continues.
This is how a simple Electric motor is made.
Principle of the Electrical measuring instruments
principal of working is same as that of working of a d.c motor. Whenever, a
current carrying coil is kept in the magnetic field, it experiences a force and
it tends to move. When the current to be measured is made to flow through the
coil, a deflecting torque proportional to the flux density and dimensions of
the coil is produced which makes the coil to move. as the coil moves, the
pointer shows deflection on the scale. The deflection torque is proportional to
the coils current and the magnitude of the current flowing through the coil is
indicated by a pointer which moves over a graduated scale. When the deflecting
torque is equal to the controlling torque, a balance is attained and the
pointers shows zero deflection. Here,as the deflection is directly proportional
to the current in the coil, uniform scale is obtained.
Instructions for demonstrations and safety
Warning: experiments with electricity should be performed under the
supervision of teachers or adults familiar with electricity safety procedures.
shows the effect of a magnetic field on a current loop. This is a principle of
a simple direct current motor. Show the operational principle of this motor.
several turns of wire round a cork and hang it, from a clump, between the poles
of magnet as shown. Pass a current briefly through the coil.
ü Does the coil
turn the opposite way if you reverse the current?
are two ways to find the direction of rotation in the diagram:
ØUse Fleming’s Left
Hand Rule for each side of the coil.
end of the coil becomes a N-pole. This end will be attracted to the S-pole of
guidelines for organizing a lesson
moment. Establishing emotional state. Checking for absent students.
introduces the topic and objectives of the lesson, assess criteria.
demonstrates the force on a current loop using a several
turns of wire round a cork
hang it, from a clump, between the poles of magnet. This is
a principle of a simple direct
motor. Show the operational principle of this motor.
direction of rotation can be predicted using Fleming’s Left
Hand Rule (FLHR).
sets up apparatus then learners observe to suppose what will happen answer the
are divided into three groups and study deeply the working principle of
electricity-measuring instruments, electric motor and a generator at different
three station one by one. Teacher put materials at three stations and set a
time for each station. Students were given the
opportunity to share thinking with each other throughgroup discussions and drawings.
video clip of the
working principle of an electric motor and electric
asks learners answer the filling the gap question individually.
end of the lesson students are encouraged to reflect on what they have learned
they need to improve.
Recommendations for formative assessment
Activity1. Students discuss learning objectives and assess
Activity2. Students with a teacher set up apparatus
then observe to suppose what will happen
answer the questions.
Activity3. Learners are divided into groups and study
deeply the working principle of electricity-
measuring instruments, electric motor
and a generator at different three station one by
one. Teacher put materials at three
stations and set a time for each station. Students
are given the opportunity
to share thinking with each other through group
discussions and drawings.Teacher rotated around to groups to listen, clarify thinking
and prompt further
As a result of their studying,
teacher asks from any students to explain the working
principle of electricity-measuring
Assessment criteria should be agreed
Activity4. Students individually answer the
filling the gap question.
Activity5. At the end of the lesson students are
encouraged to reflect on what they have learned
and what they need to improve.
Answers, criteria for
assignments, additional materials for the lesson
List of useful links and literature
Douglas C. Giancoli, Physics Principles with
Applications, Seventh edition 2014.
Keith Johnson, Physics for You, Updated Edition for
All GCSE Examinations 2011
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Teacher Guides of the Lesson
Explaining how a simple dc electric motor works
The ion concentrates the lines of force, so more lines of force are 'cut' through per unit time
Figure 3 : Torque in DC Motor
Wind several turns of wire round a cork and hang it, from a clump, between the poles of magnet as shown