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Solving exercises in Physics Grade 9 Chapter 39: Summary of Chapter II: Electromagnetism
Exercise 1 (page 105 Physics Grade 9):
Complete the following sentence:
To determine the presence of a magnetic field at a point A in space, perform the following: Place a compass needle at A, if you observe a force acting on the needle, then there is a magnetic field at A.
* Solution:
To determine the presence of a magnetic field at a point A in space, follow these steps: Place a compass needle at A, if you observe a force acting on the needle, then there is a magnetic field at A.
Exercise 2 (page 105 Physics Grade 9):
How can you turn a steel bar into a permanent magnet?
A. Strike the steel bar forcefully with a hammer.
B. Heat the steel bar over an open flame.
C. Place the steel bar inside a tube with a one-way current running through it.
D. Place the steel bar inside a tube with a rotating current running through it.
* Solution:
Choose option C. Place the steel bar inside a tube with a one-way current running through it.
Exercise 3 (page 105 Physics Grade 9):
The rule for determining the direction of the force on a current states as follows: Place your hand ....so that the ....penetrate into the palm, the direction from the wrist to the .... indicates the direction of the current, then the ....indicates the direction of the force from the electric current.
* Solution:
Place your left hand so that the lines of force penetrate into the palm, the direction from the wrist to the middle finger indicates the direction of the current, then the thumb indicates the direction of the force from the electric current.
Exercise 4 (page 105 Physics Grade 9):
What is the condition for the appearance of induced current in a closed coil?
a. Place a magnet near the coil.
b. Place a magnet inside the coil.
c. When the number of magnetic field lines passing through the cross-sectional area S of the coil is large.
d. When the number of magnetic field lines passing through the cross-sectional area S of the coil varies.
* Solution:
Choose option d.
Exercise 5 (page 105 Physics Grade 9):
Complete the following sentence:
When a closed conducting loop rotates in the magnetic field of a permanent magnet, an induced current appears in the loop because...
* Solution:
When a closed conducting loop rotates in the magnetic field of a permanent magnet, an induced current appears in the loop because the number of magnetic field lines passing through the cross-sectional area S of the loop varies.
Exercise 6 (page 105 Physics Grade 9):
For a straight magnet with the markings indicating the pole names erased, how can you determine the North pole of that magnet?
* Solution:
Hang the magnet using a soft thread at the midpoint so that the magnet lies horizontally. The end pointing towards the geographic North is the North pole of the magnet.
Exercise 7 (page 105 Physics Grade 9):
a) State the rule for determining the direction of the magnetic field lines representing the magnetic field of a wire with a direct current running through it.
* Solution:
a) Grasp the wire with the right hand so that the four fingers curl in the direction of the current passing through the wire, then the thumb points in the direction of the magnetic field lines inside the wire.
b) Figure 39.1
Exercise 8 (page 106 Physics Grade 9):
Highlight the similarities in the structure of the two types of dynamo generators and explain the differences in their operations.
* Solution:
Similarities: Both have two main components, namely the magnet and the conducting coil.
Differences: One type has the armature as the coil, while the other type has the armature as the magnet.
Exercise 9 (page 106 Physics Grade 9):
Name two main parts of a direct current motor and explain why, when current flows through, the motor rotates.
* Solution:
- The two main parts are the magnet and the conducting frame.
- The frame rotates because when we pass a direct current through the conducting frame, the magnetic field of the magnet exerts forces on the frame, causing it to rotate.
Exercise 10 (page 106 Physics Grade 9):
Place a perpendicular magnetic field to a straight conductor with a constant current flowing through as shown in figure 39.2 Physics Grade 9. Determine the directions of the electric force acting on point N of the conductor.
* Solution:
The magnetic field lines produced by the coil of the magnet point from left to right at point N. Applying the left-hand rule, the force is directed from outside to inside and is perpendicular to the plane of the drawing.
Exercise 11 (page 106 Physics Grade 9): Transformer.
a. Why is it necessary to use transformers for long-distance electrical power transmission?
b. On the same power line, if a transformer is used to increase the voltage at both ends of the conductors by 100 times, how many times will the power loss due to heat dissipation on the power line decrease?
c. The primary coil of a transformer has 4400 turns, and the secondary coil has 120 turns. If an AC voltage of 220 is applied across the primary coil, find the voltage across the secondary coil.
* Solution:
Exercise 12 (page 106 Physics Grade 9):
Explain why direct current cannot be used to operate a transformer.
* Solution:
A constant current does not generate a changing magnetic field, and the number of magnetic field lines passing through the cross-sectional area S of the secondary coil remains unchanged. Therefore, no induced current appears in this coil.
Exercise 13 (page 106 Physics Grade 9):
In figure 39.3 Physics Grade 9, draw a conducting frame placed in a magnetic field. In which case below does the conducting frame not produce an alternating current? Explain why?
a. The conducting frame rotates around the horizontal axis PQ.
b. The conducting frame rotates around the vertical axis AB.
Solution:
Case a. When the conducting frame rotates around the horizontal axis PQ, the magnetic field lines passing through the cross-sectional area S of the frame always remain constant, always equal to 0. Therefore, no induced current appears in the frame.
