Electromagnetism
Electromagnetism
Unit
21
Unit
21
Electric motors are machines that use
magnetism and electricity to make things move. We use d.c. motors to drive electric trains. The
electric current is supplied to the train from overhead wires or from the rails below.
Electric motors are machines that use
magnetism and electricity to make things move. We use d.c. motors to drive electric trains. The
electric current is supplied to the train from overhead wires or from the rails below.
contents
contents
Force on a Current-carrying Conductor
D.C. Motors
http://www.youtube.com/watch?v=HQdLFEiVeCA
Force on a Current-carrying Conductor
D.C. Motors
Unit 21.1: Magnetic Effect of a Current
• When the circuit is closed, a compass A placed above the wire XY would point to the East. Another compass B is placed below the wire would point to the West.
• A current-carrying conductor produces a magnetic field around it.
Demonstrating the magnetic effect of a
current—Oersted’s Experiment
Fig. 21.4 Oersted’s
A Straight Wire
1. A straight wire carrying a current produces
circular
lines of force.
What happens when the direction of current is
reversed?
The direction of the magnetic field will also be
reversed!
A Straight Wire
The strength of the magnetic field in a
straight wire is stronger when
•
a larger current flows through it.
•
(the circular lines of force are closer) i.e.
Direction of arrow
Direction of current or
magnetic field
Current-carrying wire is perpendicular
to the
plane of board.
current is directed
out
of paper
(point of arrow)
current is directed
into
paper
(tail of arrow)
A Flat Circular Coil
A flat coil carrying a
current produces
circular lines of force
around the wires
and
almost parallel lines of
Unit 21.1: Magnetic Effect of a Current
Test Yourself 21.1
1. A current flows in a long straight wire in the direction shown in Figure 21.17. Draw, in the diagram, the pattern and direction of the magnetic field produced
.
Unit 21.1: Magnetic Effect of a Current
Test Yourself 21.1
2. (a) Draw the magnetic field lines around a current-carrying solenoid.
(b) Name three ways to increase the magnetic field strength of a solenoid.
Answer:
(a)
(b) 3 ways to increase magnetic field of solenoid:
•
Increase the no. of turns per unit length of the solenoid,
•
Increase the magnitude of the current
•
Place a soft iron core in the solenoid.
force on a current-carrying
conductor
2. Current-carrying conductor
2. Current-carrying conductor
The setup investigates the interaction between a current and a magnetic field.
The setup investigates the interaction between a current and a magnetic field.
soft iron
c-core thick bare wire
powerful magnadur magnet 2V power pack or lead-acid accumulator
The direction of the force can be deduced by using this rule.
The direction of the force can be deduced by using this rule.
3. Fleming’s left-hand rule
3. Fleming’s left-hand rule
force on a current-carrying
conductor
Motion
(thumb)
Motion
(thumb) (first finger)(first finger)FieldField
the fingers are at right angles to one another
Current
(second finger)
Current
To explain the force exerted on the wire, consider the
combined magnetic fields due to the current flowing through the straight wire and the magnets.
To explain the force exerted on the wire, consider the
combined magnetic fields due to the current flowing through the straight wire and the magnets.
Fleming’s left-hand rule
Fleming’s left-hand rule
force on a current-carrying
conductor
magnetic field between
two magnadur magnets magnetic field due to the current in the wire
N
Fleming’s left-hand rule
Fleming’s left-hand rule
force on a current-carrying
conductor
The two fields acting in the same direction combine to give a stronger field, but the two fields opposing each other
combine to give a weaker field.
The unbalanced fields on both sides exert produce a force that exerts on the wire.
The two fields acting in the same direction combine to give a stronger field, but the two fields opposing each other
combine to give a weaker field.
The unbalanced fields on both sides exert produce a force that exerts on the wire.
Further explanation Unit 21.2: Force on
Current-carrying Conductors
Worked Example 21.1
Figure 21.20(a) shows a wire placed between two magnetic poles.
(a) If the current in the wire flows from A to B, in which direction does a force act on the wire?
(b) What will happen if the current flows from B to A instead?
Unit 21.2: Force on Current-carrying Conductors
Worked Example 21.1 – Solution
(a) By using Fleming’s Left-Hand Rule, we find that the force acts vertically downward on the wire AB (Figure 21.20(b)).
(b) If the current flows from B to A, the force reverses in direction and acts vertically upward.
Unit 21.2: Force on Current-carrying Conductors
Why does a current-carrying conductor experience a force when
placed in a magnetic field?
Fig. 21.21(a) & (b) Separate magnetic fields of a current
Unit 21.2: Force on Current-carrying Conductors
Why does a current-carrying conductor experience a force when
placed in a magnetic field?
Fig. 21.21(c) Superimposed
Unit 21.2: Force on Current-carrying Conductors
Why does a current-carrying conductor experience a force when
placed in a magnetic field?
Fig. 21.21(d) Combined magnetic field when the
Unit 21.2: Force on Current-carrying Conductors
Why does a current-carrying conductor experience a force when
placed in a magnetic field?
From Fig. 21.21(d), we can
see that there is a stronger
field on one side of the wire at
A, since all the magnetic field
lines are in the same direction.
At B, the combined field is
weaker due to opposing
magnetic field lines.
A force then acts on the wire
from the stronger field to the
weaker field.
Fig 21.21(d) Combined
magnetic field when the wire is placed between the poles of the magnet.
4. Force on a beam of
charged particles
4. Force on a beam of
charged particles
Fleming’s left hand rule can be applied to all moving charges.
Fleming’s left hand rule can be applied to all moving charges.
force on a current-carrying
conductor
motion field (magnetic)
Conventional current flow
Electron current flow
The conventional current travels in an opposite
direction to that of the electron flow.
The conventional current travels in an opposite
direction to that of the electron flow.
force on a beam of
charged particles
force on a beam of
charged particles
force on a current-carrying
conductor
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magnetic field into paper path of positively charged particle (part of a circle) positively charged particle direction of positively charged particle beforeentering the magnetic field
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motion (force) current
force on a beam of
charged particles
force on a beam of
charged particles
force on a current-carrying
conductor
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magnetic field into paper path of electron or negatively charged particle(part of a circle) electron or
negatively charged particle
direction of electron or negatively charged particle
before entering the magnetic field
x
x
motion (force) current
Unit 21.2: Force on Current-carrying Conductors
Force on a moving charge in a magnetic field
Fig. 21.22(a) A positively charged particle in a
magnetic field is deflected upwards in a circular path.
• When a beam of positive charges enter the magnetic field region, it is deflected upwards in a circular path as the
moving charges experience a force perpendicular to its direction of motion.
• The direction of the force can be predicted by Fleming’s
Unit 21.2: Force on Current-carrying Conductors
Forces between two parallel current-carrying wires
•
Currents in opposing directions cause repulsion.
Fig. 21.24 Combined magnetic field due to currents in the
Unit 21.2: Force on Current-carrying Conductors
Forces between two parallel current-carrying wires
•
Currents in similar directions cause attraction.
Fig. 21.25 Combined magnetic field due to currents
d.c. motors
5. Turning effect on a current carrying coil
5. Turning effect on a current carrying coil
A current- carrying coil placed in a magnetic field of a horseshoe magnet experiences a turning effect. A current- carrying coil placed in a magnetic field of a horseshoe magnet experiences a turning effect.
d.c. motors
turning effect on a current carrying coil
turning effect on a current carrying coil
A catapult field is produced when the field produced by the coil superimposes on the field of the horseshoe magnet.
A catapult field is produced when the field produced by the coil superimposes on the field of the horseshoe magnet.
increasing the number of turns on the coil increasing the magnitude of the current
inserting a soft iron core within the coil to concentrate the magnetic lines of force
increasing the number of turns on the coil increasing the magnitude of the current
inserting a soft iron core within the coil to concentrate the
magnetic lines of force
d.c. motors
turning effect on a current carrying coil
turning effect on a current carrying coil
The turning effect can be increased by
principles of a d.c. motor
principles of a d.c. motor
make use of the turning effect of a current-carrying coil in magnetic field to convert
electrical energy to mechanical (kinetic) energy
works on direct current
are the basic components in electric fans, hair dryers and many other electrical
appliances
make use of the turning effect
of a current-carrying coil in magnetic field to convert
electrical energy to mechanical (kinetic) energy
works on direct current
are the basic components in
electric fans, hair dryers and many other electrical
appliances
d.c. motors
principles of a d.c. motor
principles of a d.c. motor
a. when the circuit is closed, current flows from the battery through P and X, through the coil and back to the battery through Y and Q
a. when the circuit is closed, current flows from the battery through P and X, through the coil and back to the battery through Y and Q
using Fleming’s left-hand rule, the left side of the coil
experiences a downward force and the right-hand side experiences an equal upward force using Fleming’s left-hand rule, the left side of the coil
experiences a downward force and the right-hand side
experiences an equal upward force
principles of a d.c. motor
principles of a d.c. motor
b. this pair of forces causes the coil to rotate
anticlockwise until it reaches a vertical position
b. this pair of forces causes the coil to rotate
anticlockwise until it reaches a vertical position
at this point, current is cut off because neither X nor Y is in contact with P or Q at this point, current is cut off because neither X nor Y is in contact with P or Q
d.c. motors
principles of a d.c. motor
principles of a d.c. motor
(c) momentum of the coil carries it slightly beyond this vertical position
(c) momentum of the coil carries it slightly beyond this vertical position half-ring Y will then touch P while X comes into contact with Q turning forces act again and coil continues to rotate in the same direction half-ring Y will then touch P while X comes into contact with Q turning forces
act again and coil continues to rotate in the same direction
If a soft iron cylinder is placed between the curved poles of the magnet in a motor:
If a soft iron cylinder is placed between the curved poles of the magnet in a motor:
d.c. motors
principles of a d.c. motor
principles of a d.c. motor
this arrangement increases the magnetic field strength and thus increases the turning effect for a givencurrent in the coil this arrangement
increases the magnetic field strength and thus increases the turning effect for a given
current in the coil
a radial field will be created
radial field keeps the pair of forces acting on the coil almost constant as it turns
a radial field will be created
radial field keeps the pair of forces acting on the coil almost
Unit 21.3: Force on a Current-carrying Rectangular
Coil in a Magnetic Field
How does a d.c motor work?
– When current flows through the coil ABCD, using Fleming’s left-hand rule, a downward force will act on side AB, and an upward force on side CD.
– The coil thus rotates anticlockwise about axis PQ until it reaches a vertical position.
– Here, the current is cut off because X and Y are both not in contact with the carbon brushes
– The turning effect of the coil, however, carries it past the vertical position.
– This reverses the current direction in the wire arm CD and now a downward force acts on it.
– Similarly, an upward force acts on wire arm AB.
Unit 21.3: Force on a Current-carrying Rectangular
Coil in a Magnetic Field
How does a d.c motor work?
•
The purpose of the split-ring commutator is to
reverse
the direction of the current in the coil every half a
revolution to ensure that the coil will always turn in one
direction.
•
To increase the turning effect of the coil, we can:
1.
Insert a soft iron core or cylinder into the coil to
concentrate the magnetic field lines.
2.
Increase the number of turns in the coil
Unit 21.3: Force on a Current-carrying Rectangular
Coil in a Magnetic Field
Key Ideas
1.
The d.c. motor works on the principle that a current-carrying coil
in a magnetic field experiences a turning effect.
2.
The function of a split-ring commutator is to reverse the direction
of current in the coil when the coil passes the vertical position so
that it continues to turn in the same direction.
3.
The turning effect on the coil can be increased by
(a) increasing the current in the coil
(b) having more turns on the coil, or
Unit 21.3: Force on a Current-carrying Rectangular
Coil in a Magnetic Field
Test Yourself 21.3
1.
In the d.c. motor, what change(s) must be made so that
the coil rotates clockwise instead of anti-clockwise?
Answer:
To change the direction of rotation to turn clockwise, we
can do one of the following:
•
reverse the poles of the magnets, or
•
reverse the direction of the current, by switching the
terminals of the battery
Unit 21.3: Force on a Current-carrying Rectangular
Coil in a Magnetic Field
Test Yourself 21.3
2.
Explain the purpose of the rheostat in the d.c. motor.
Answer:
The resistance of the rheostat is varied so that the current flowing
in the coil can be controlled.
By lowering the resistance, the current will increase and the turning
force on the coil will increase. This results in an increased speed of
rotation.
Unit 21.3: Force on a Current-carrying Rectangular
Coil in a Magnetic Field
Test Yourself 21.3
3.
State the energy conversion that takes place in the d.c. motor.
Answer:
Purpose of the split ring commutator
•
To reverse the direction of the current in the coil every half a
revolution whenever the commutator changes contact from one
brush to another. This is to ensure that the current continue to
flow in the same direction in the coil.
Turning effect on
Turning effect on
a current
a current--carrying carrying coil coil Turning effect is Turning effect is increased by increased by increasing increasing (a)
(a) number of turnsnumber of turns (b) (b) currentcurrent Force on a beam Force on a beam of charged of charged particles in a particles in a magnetic field magnetic field Force on a current Force on a current- -carrying conductor carrying conductor in a magnetic field in a magnetic field Fleming’s Fleming’s Left hand Left hand rule rule Electromagnetism Electromagnetism Electric motor Electric motor Electric motor is shown by is shown by results in results in helps to helps to determine the determine the direction of direction of is the basis of is the basis of
