Magnetic Effects of Electric Current
NCERT In Text Questions
Question 10: A positively-charged particle (alpha-particle) projected towards west is deflected towards north by a magnetic field. The direction of magnetic field is
- Towards south
- Towards east
Answer: This question can be solved by using Fleming’s Left Hand Rule. We know that the direction of current is opposite to the direction of electron’s movement and hence it would be same as the direction of proton’s movement. So, the direction of current is towards west. As per Fleming’s Left Hand Rule; the middle finger shows the direction of current, the forefinger shows the direction of magnetic field and the thumb shows the direction of motion. Here, the deflection is towards north, i.e. in north westerly direction and hence, the direction of magnetic field would be towards north, i.e. upward.
Question 11: State Fleming's Left Hand Rule.
Answer: Fleming's Left Hand Rule states that if the left hand is stretched in a way that the index finger, the middle finger and the thumb are in mutually perpendicular directions; then the index finger and middle finger of a stretched left hand show the direction of magnetic field and direction of electric current respectively and the thumb shows the direction of motion or force acting on the conductor. The directions of electric current, magnetic field and force are similar to three mutually perpendicular axes, i.e. x, y and z axes.
Question 12: What is the principle of an electric motor?
Answer: The electric motor works on the principle of Fleming’s Left Hand Rule. When a rectangular coil is placed within a magnetic field and current is passed through the coil, there is deflection in the coil. The deflection changes into rotation of coil because of split ring commutator in the motor.
Question 13: What is the role of a split ring in an electric motor?
Answer: In an electric motor, after every half rotation the direction of coil gets reversed due to change in orientation of the magnetic field. To ensure a continuous rotation; a split ring is attached to the coil so that the polarity of the coil changes after every half rotation. This changes the direction of current and thus the armature keeps on rotating continuously.
Question 14: Explain different ways to induce current in a coil.
Answer: For electromagnetic induction; the coil and the magnet should be in relative motion. This can be ensured by any of the following two ways:
- The coil should be moved within a magnetic field.
- The magnet should be moved and coil can be kept static.
Question 15: State the principle of an electric generator.
Answer: Electric Generator works on the principle of electromagnetic induction which obeys Fleming’s Right Hand Rule. When coil is moved inside a magnetic field a current is induced in the coil. The electric current is thus ‘generated’ by electric generator.
Question 16: Name some sources of direct current.
Answer: Electrochemical cell, DC generator, photovoltaic cell, etc.
Question 17: Which sources produce alternating current?
Answer: AC generator
Question 18: A rectangular coil of copper wires is rotated in a magnetic field. The direction of the induced current changes once in each
- two revolutions
- one revolution
- half revolution
- one-fourth revolution
Answer: (c) Half revolution
Question 19: Name two safety measures commonly used in electric circuits and appliances.
Answer: Earth wire and electric fuse
Question 20: An electric oven of 2 kW is operated in a domestic electric circuit (220 V) that has a current rating of 5 A. What results do you expect? Explain.
Answer: The current drawn by the electric oven can be calculated using following formula: P = VI
Given, P = 2 kW = 2000 W and V = 220 V
Since, P = VI hence, I = P/V
Or, I = 2000 W ÷ 220 V = 9.09 A
Here, the oven is drawing 9.09 ampere of current from a 5 ampere source.
This means there would be an overload on the circuit. This can result in accidental fire in the circuit.
Question 21: What precaution should be taken to avoid the overloading of domestic electric circuits?
Answer: Precautions to avoid overloading of domestic electric circuit:
- Avoid using too many appliances in a single line, i.e. from a single output.
- Appliances should always be checked for potential faults and should be repaired in time.
- Fuse or MCB of proper rating should be used.