Magnetism

Transcription

Magnetism

Physics Basic Questions: Level MS
Chapter 3: Magnetism
Section 3.4.1 (Page-188)
BG 1.
What is meant by the term “magnetic field”? Is the magnetic field a vector or a scalar?
What is the direction of the magnetic field?
Solution:
* Magnetic field is the property of a region in space to exert a force on any
magnet placed in that region of space.
* Magnetic field is a vector quantity.
* Magnetic field’ s direction at a point is the direction of the force it produces
on a North pole placed at that point .
B 2.
Draw the magnetic flux pattern of a U-shaped magnet.
Solution:
Section 3.7 (Page-197)
BT 3.
I.) The diagram below shows a small compass needle placed on a bench under the
influence of the Earth’s magnetic field.
S
N
A bar magnet is now placed close to the compass in the position shown by the dotted
line. Draw a new diagram to show the new direction of the compass needle and
explain.
Solution:
Magnetic field lines around the magnet are directed from the North pole to the
South pole; the needle will be aligned with the field lines, therefore it will
point towards the south pole.
T
II.)
Solution:
BG 4.
A
Define and give the properties of magnetic field lines.
Solution:
* Magnetic field lines are imaginary, smooth lines along which the magnetic
field vector is tangent, at any point;
(The needle of a plotting compass is tangent to the field lines at any point.)
** Every field line is a smooth curve (starting at a North pole and ending at a
South pole).
**Field lines never cross. However, if extrapolated, they do meet at poles.
B 5.
Explain the following statement: “Field lines never intersect.”
Solution:
B 6.
If field lines intersected, then the point at which the lines crossed would be a
point where a plotting compass would have to point in two directions, at the
same time. This is impossible, because there can be only one resultant field,
at any point.
Describe a method used to map magnetic fields.
Solution:
Steps 1 – 5 / page 192:
1. Place a plotting compass at the point through which it is desired to plot the
line, such that the point is just at the tip of the North pole of the needle.
Draw a little arrow showing the direction in which the North pole points;
2. Move the plotting compass such that the end of the South pole is on the
little arrow just drawn, and draw another little arrow where the tip of the
North pole now is;
3. Move the compass to the new arrow drawn in point 2. and draw a new
arrow as you did before. Continue with this procedure until you reach a
point at which you wish to stop. If you approach a pole slowly, ensuring
that the needle pivots freely, you will not reverse its magnetization. It is
however dangerous to thrust a magnetic needle rapidly towards a pole;
4. Join all the little arrows with a smooth curve. This curve is approximately
the required magnetic field line. It is not exact because the small errors in
each step accumulate and add up to an appreciable error;
5. By starting at different points near the North pole of a bar magnet,
different field lines can be drawn.
B 7.
Explain how radial fields are used in the construction of current meters.
Solution:
A current carrying loop/coil is placed in a plane-symmetric radial field, with
its axis parallel to the central axis of the field. The field exerts a couple of
sideways tangential pushes on the loop, which has a rotating effect on it,
turning it around its axis. While rotating, the loop turns the pointer of the
ammeter).
The plane-symmetrical, radial shape of the field (also) ensures that the angle
turned by the loop/coil (and by the pointer of the ammeter) is directly
proportional to the current.
B 8.
Explain how radial fields are used in the construction of a loudspeaker.
Solution:
A current carrying coil placed in an axially-symmetric radial field, with its
longitudinal axis along the axis of the field, is subjected to forces of the same
direction and equal magnitude, acting all over its circumference, which
attract or repel the coil into or out of the field.
Changing direction of the current through the coil makes these forces reverse
their direction.
As the current through the coil varies (in direction) rapidly, the coil
alternatively moves in and out of the field. By this, the paper cone attached to
the coil vibrates and gives sound.
B 9.
How can a magnet lose its magnetization?
Solution:
There are several ways in which a magnet can be demagnetized, some of
which are:
-Storing it without soft iron keepers.
-Heating it to high temperatures.
-Dropping it.
-Knocking it.
-An alternating current in an electromagnet can be also used to demagnetize
magnets.
BT 10.
I.) Draw a diagram to show the magnetic field pattern around a straight wire carrying a
current into the plane of the paper. State the rule you use to determine the direction of
the field around the wire.
Solution:
×
The Right – Hand Grip Rule: If the wire is gripped with a right hand, such that the
thumb points in the direction of the current, then the fingers curl in the sense of the
magnetic field.
T
II.)
Solution:
B 11.
D
Describe a simple method to prove that the magnetic field around a wire through which an
electric current passes changes when:
(a) the current is increased.
Solution:
Use a straight horizontal wire arranged along the North-South direction,
through which a variable current can pass.
Place a plotting compass below the wire.
A
N
S
Plotting
compass
If no current passes through the wire, the needle of the compass is parallel to
the wire.
The larger the current through the wire - the greater the angle of deflection
(of the compass’ needle) from the N-S direction (maximum angle is 90o),
which means - the stronger the field created by the wire.
(b) the direction of the current is reversed.
Solution:
Use the same experimental set up as at a).
When the direction of the current in the wire is reversed, the needle of the
plotting compass deflects in the opposite direction, which means the direction
of the magnetic field is reversed.
B 12.
The diagram below represents a current-carrying solenoid. The arrows on the diagram
show the direction of the current.
(a) Determine the polarity of the solenoid.
Solution:
If the fingers of the right hand curl in the direction of the current, the thumb
points in the direction of the North-pole face.
So, in this case, the North pole is at the right side end of the solenoid:
S
N
(b) Draw a diagram to show the magnetic flux pattern (magnetic field lines) inside and
outside the solenoid.
Solution:
The field pattern is the same as the one of a bar magnet with the same polarity
as the solenoid.
S
N
I
I
BG 13.
G
(a)
What is an electromagnet? How does it work?
Solution:
* An electromagnet is a device made of a solenoid wound around a soft iron
core.
**
When a current passes through the solenoid, the device behaves like a
strong magnet, attracting magnetic materials.
[The (magnetic) domains of the iron core re-arrange themselves in the direction of the magnetic
field created by the solenoid, like tiny magnets, the result being an increased magnetic field, due
to both – the solenoid and the magnetized iron core.]
** The magnetic field of an electromagnet can be switched ON and OFF
with the electric current, since the soft iron loses its magnetization once
the “external” field, created by the solenoid, goes OFF.
[The (magnetic) domains of the soft iron re-arrange themselves once again, this time randomly.]
(b)
Give the names of some devices which use electromagnets.
Solution:
*
*
*
*
Tape recorders;
Relays;
Electric bells;
Reed switches.
B 14.
Design an experiment to demonstrate that the magnitude and direction of the
electromagnetic force depends on the relative directions of the magnetic field and the
current.
Solution:
Experiment page 215 on the text book:
Equipment needed:
1. A DC-power supply;
2. A push-button switch;
3. A high-power rheostat;
4. Connecting leads;
5. A thick copper wire,
6. Two stands;
7. A U-shaped magnet;
Method:
1) Bend the copper wire such as to resemble a garden swing;
2) Hang the bent wire (swing) from the two stands;
3) Make a series circuit of the equipment 1 - 3 above, with the
copper wire;
4) Place the U-magnet such as its magnetic field to be vertical and
the horizontal side of the copper wire to be between its poles;
5) Push the button of the switch to allow the current to pass
through the copper wire.
You will observe that the swing is deflected away from its
vertical position, under the action of the electromagnetic force.
6) Change the direction of the current in the circuit, by exchanging
the connection at the terminals of the battery and push the
button of the switch again.
You will observe that the wire swing is deflected in opposite
direction than previously.
7) Change the direction of the magnetic field by exchanging the
poles of the magnet (i.e. by turning the magnet upside down).
You will observe that the wire changes again the direction in
which it is deflected.
Remark:
At step 5) use the rheostat to adjust the current in the circuit,
such as the deflection of the swing to be within reasonable
limits.
Conclusion:
The direction of the electromagnetic force on the wire
depends on:
a) the direction of the current;
b) the direction of the magnetic field.
B 15.
Design an experiment to demonstrate that the electromagnetic force depends on the
magnitude of the magnetic field and the magnitude of the current.
Solution:
Equipment needed:
1. A DC-power supply;
2. A push-button switch;
3. A high-power rheostat;
4. Connecting leads.
5. A thick copper wire;
6. Two retort stands;
7. Two (or more) U-shaped magnets, of different
strengths;
Method:
1.) Bend the copper wire such as to resemble a garden swing;
2.) Hang the bent wire (swing) from the two stands;
3.) Make a series circuit of the equipment 1 - 3 above, with the
copper wire;
4.) Place an U-magnet such as its magnetic field to be vertical and
the horizontal side of the copper wire to be between its poles;
5.) Push the button of the switch to allow the current to pass
through the copper wire;
Notice that the wire swing is deflected away from its vertical
position, under the action of the electromagnetic force.
6.) Using the rheostat, increase/decrease the current through the
circuit.
Notice that the angle of deflection of the wire swing in the
magnetic field increases/decreases respectively, with the
current.
7.) Keep the current in the circuit constant and replace the magnet
with a stronger/weaker one.
Notice that the angle of deflection of the wire swing in the
magnetic field increases/decreases respectively, with the
strength of the magnetic field.
Conclusion:
The magnitude of electromagnetic force depends on:
a) the magnitude of the field;
b) the magnitude of the current.
Remark:
The magnitude of the electromagnetic force also depends on the
length of the wire that lies in the magnetic field.
BG 16. State the rules used to determine the direction of the force exerted by a magnetic field on a
current.
Solution:
1) The right hand rule:
If the thumb (1) of a right hand is directed along the current (I) and the
index (2) along the magnetic field (B), then the third finger (3) shows the
direction of the force (F). (“3-2-1” = “F-B-I”)
2) The Motor Force Rule:
If the fingers of a right hand are curled in the sense of the smallest rotation
from the direction of the conventional current (I), to the direction of the
magnetic field (B), the thumb points in the direction of the force (F).
(I → B => F)
3) The Right-Handed Screwdriver Rule.
If a right – handed screw is turned so that the notch in its head rotates from
I to B (in the sense of the smallest rotation), the screw advances in the
direction of F. (I → B => F)
B 17.
A wire AB is placed between the poles of a magnet when the current is turned on, the wire
AB will experience a force.
(a)
The direction of the current is reversed. What effect will this have on:
(i)
Solution:
(ii)
Solution:
(b)
The size of the force?
No effect.
The direction of the force?
The direction is reversed.
Give two ways in which the size of the force on the wire AB could be increased.
Solution:
1.
2.
Increase the current flowing in the wire.
Use a stronger magnet.
BT 18.
I.)
Describe the principle of the moving coil galvanometer.
Solution:
A coil is placed in a plane-symmetric radial magnetic field, with its axis
parallel to the axis of the field. When the coil is passed by current, a couple of
electromagnetic forces exert a torque on the coil, directly proportional to the
current.
While the coil is turning, a pointer attached to it rotates against a scale.
A spiral spring balances the torque of the motor forces, such as to a greater
current to correspond a greater angle of rotation [Without the spring, the pointer would
go every time till the end of the scale, irrespective of the magnitude of the current ( if no resistance
opposes the motion, the motion continues) and it would not come “back” when the current decreases to
zero (there will be no force on it to produce a torque of opposite direction.)]
The plane-symmetric radial shape of the field (as opposed to a uniform field),
ensures that the motor torque doesn’t change while the coil rotates. This makes
possible for the scale of the galvanometer to be linear (the angle rotated by the pointer
to be directly proportional to the current).
T
II.)
Solution:
A
BGT 19.
I.)
Describe the principle of the direct-current electric motor.
Solution:
A current carrying coil placed in a uniform magnetic field can make a half
revolution under the action of a couple of electromagnetic forces.
If the current in the coil is reversed just when the coil becomes perpendicular
to the field, the new electromagnetic forces will turn the coil (another) half
revolution (in the same direction).
A continuous motion is acquired by reversing the sense of the current after
every half revolution. (This motion can be transferred to other bodies.)
T
II.)
Solution:
(a)
When the switch is closed a current flows through the circuit and in the
wheel, the rod in between the magnetic poles of the magnet experiences
an electromagnetic or motor force and the rod turns and the process
repeats itself and the wheel turns.
(b)
In the anti-clock wise direction.
BG 20. Describe the principle of the split-ring commutator.
Solution:
•
The commutator is made of two, unconnected half–rings of copper, which
can slide, by rotation, against two, fixed, carbon brushes.
•
The ring-halves can be connected to the coil of a d.c. motor, while the
brushes will be connected to its (d.c.) power supply.
•
When the coil rotates, the two ring-halves move from one brush to the
other, alternatively, and their polarity changes according to which brush
they come in contact.
•
Consequently, the current between the two ring-halves (through the coil,
in this case) reverses its sense every half revolution.
Circuit 1
ring halves
(the coil of the d.c. motor)
brush
+
brush
Circuit 2
(circuit of the d.c.
power supply)

Magnetism

Transcription

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