CHAPTER # 15
ELECTROMAGNETISM
Multiple Choice Questions
i.
A single magnetic pole does not exist
ii.
From south to north
iii.
Magnetic compass
iv.
Increases
v.
Electrical energy into mechanical energy
vi.
The commutator
vii.
Energy
viii.
Increases the input voltage
ix.
Ns = 10 Np
REVIEW QUESTIONS
15.1 Demonstrate by an experiment that a magnetic field is produced around a straight current-carrying conductor.
To demonstrate this, we will pass a straight conductor wire vertically through a piece of cardboard. Connect the two ends of the conductor wire to the terminals of the battery so that current flows clockwise through the circuit. The magnetic field lines of force generated around the wire would be concentric circles. If we place a compass needle at different points in the magnetic field region, it will align along the magnetic field direction. In addition, if we sprinkle some iron filings on the cardboard around the wire, they will align in concentric circles clockwise.
15.2 State and explain the rule by which the direction of the lines of force of the magnetic field around a current-carrying conductor can be determined.
For the determination of this, we use right hand grip rule. It states that “Grasp a wire with your right hand, thumb facing the conventional (positive) current. Curling your fingers will then point in the direction of the magnetic field”.
15.3 You are given an unmarked magnetized steel bar and bar magnet; its north and south ends are marked N and S respectively. State how would you determine the polarity at each end of the unmarked bar?
As we are given an unmarked magnetized bar magnet and a bar magnet whose north and south poles are defined, we will first bring north to the magnetized bar magnet so that if it repels, it will be the north pole and if it attracts, it will be the south pole. As we know, like repels like and unlike attracts each other, so we can use this to determine the polarity of an unknown magnetized bar magnet.
15.4 When a straight current-carrying conductor is placed in a magnetic field, it experiences a force. State the rule by which the direction of this force can be found out.
For this we use left hand Fleming’s Rule. It states that “Stretch the left hand's thumb, forefinger, and middle finger perpendicular to each other. If the forefinger points in the direction of the magnetic field, the middle finger in the direction of the current, and the thumb in the direction of the force acting on the conductor”.
15.5 State that a current-carrying coil in a magnetic field experiences a torque.
If we place a current-carrying loop inside
the magnetic field instead of a straight conductor, the loop will rotate due to
the torque acting on the coil. This is also the operation of electric motors.
For long Question Refer to PTB (Punjab Text Book) page # 123, topic 15.3.
15.6 What is an electric motor? Explain the working principle of D.C motor.
The device which is used to convert electrical energy into rotational
kinetic energy is known as an electric motor.
For long Question Refer to PTB (Punjab Text Book) page # 124, topic 15.4.
15.7 Describe a simple experiment to demonstrate that a changing magnetic field can induce e.m.f. in a circuit.
Connect the two ends of a rectangular loop of wire with a galvanometer. Hold the wire still or move it parallel to the magnetic field of a powerful U-shaped magnet. There is no current because the galvanometer shows no deflection. Moving the wire downward through the field induces current in one direction, as indicated by the deflection of the galvanometer. Moving the wire upward through the field induces current in the opposite direction.
15.8 What are the factors which affect the magnitude of the e.m.f. induced in a circuit by a changing magnetic field?
- Speed of relative motion of both coil and magnet.
- Number of turns of the coil.
15.9 Describe the direction of an induced e.m.f. in a circuit? How does this phenomenon relate to conservation of energy?
In a circuit, the direction of an induced current is always such that
it opposes the cause of the current.
Applying the law of conservation of energy to electromagnetic induction reveals that the electrical energy induced in a conductor is derived from the kinetic energy of the moving magnet. We work on the magnet to get it closer to the solenoid. As a result, this work manifests as electrical energy in the conductor. So, the mechanical energy we use to push the magnet towards or away from the coil is converted into electrical energy. As a result, Lenz's law is an indication of the law of energy conservation.
15.10 Draw a labelled diagram to illustrate the structure and working of A.C generator.
15.11 What do you understand by the term mutual induction?
Mutual induction occurs when a change in current in one circuit causes a current to flow in another. For instance; Transformer is working on the principle of mutual induction.
15.12 What is a transformer? Explain the working of a transformer in connection with mutual induction.
The device which is used to increase or decrease the alternating
current is known as transformer.
For long Question Refer to PTB (Punjab Text Book) page # 132, topic 15.9.
15.13 State two reasons why electrical power is transmitted at high voltage?
- To avoid energy loss.
- By reducing current, energy loss from heat also reduces.
15.14 Why is the voltage used for the domestic supply much lower than the voltage at which the power is transmitted?
The voltage used for the domestic supply is much lower because the high
voltage can damage the domestic appliance and also have hazardous impact.
CONCEPTUAL QUESTION
15.1 Suppose someone handed you three similar iron bars and told you one was not magnet, but the other two were. How would you find the iron bar that was not magnet?
Bring a magnet close to both metal bars one at a time. The bar that attracts or repels the magnet is magnetic, whereas the other is not.
15.2 Suppose you have a coil of wire and a bar magnet. Describe how you could use them to generate an electric current?
We use coil of wire and a bar magnet to generate electric current by their relative motion or by moving one towards or away from other due to the process of induction.
15.3 Which device is used for converting electrical energy into mechanical energy?
A devise used to convert electrical energy into mechanical energy is known as motor.
15.4 Suppose we hang a loop of wire so that it can swing easily. If we now put a magnet into the coil, the coil will start swinging. Which way will it swing relative to the magnet, and why?
According to Len's law, the coil will swing in the opposite direction of the magnet's motion. It is because current in the coil is always induced in such a way that it cancels the cause that induces it.
15.5 A conductor wire generates a voltage while moving through a magnetic field. In what direction should the wire be moved, relative to the field to generate the maximum voltage?
To generate maximum voltage through the conductor, it must be moving perpendicular to the direction of the magnetic field; in this case, the conductor will be subjected to the greatest magnetic force.
15.6 What is the difference between generator and a motor?
Motor converts electric energy into mechanical energy whereas generator converts mechanical energy to electrical energy.
15.7 What reverses the direction of electric current in the armature coil of D.C motor?
A splitting commutator consists of brushes and a split ring that reverse the direction of electric current in the armature coil of D.C motor.
15.8 A wire
lying perpendicular to an external magnetic field carries of a current in the direction
shown in the diagram below. In what direction will the wire move due to the
resulting magnetic force?
It will move in downward direction as per left hand Fleming’s rule.
15.9 Can a transformer operate on direct current?
No, a transformer cannot be operated on direct current because flux
does not change in secondary coil due to direct current.
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