Multiple Choice Questions
Question. Which of the following correctly describes the magnetic field near a long straight current carrying wire?
(A) The field consists of straight lines perpendicular to the wire.
(B) The field consists of straight lines parallel to the wire.
(C) The field consists of radial lines originating from the wire.
(D) The field consists of concentric circles centered on the wire.
Question. What does the crowding of iron filings at the end of the magnet indicate?
(A) Magnetic field is strongest near the poles of the magnet.
(B) Magnetic field is weakest near the poles of the magnet.
(C) There is no significant magnetic field at the poles of the magnet.
(D) The significance of polarity
Question. No force acts on a current carrying conductor when it is placed
(A) perpendicular to the magnetic field
(B) parallel to the magnetic field
(C) far away from the magnetic field
(D) inside a magnetic field
Question. What should be the core of an electromagnet?
(A) soft iron
(B) hard iron
(C) rusted iron
(D) none of above
Question. Two magnetic field lines:
(A) Intersect at neutral point
(B) Never intersect each other
(C) Intersect near north-pole or south pole
(D) Intersect at the midpoint of the magnet
Question. A plotting compass is placed near the south pole of a bar magnet. The pointer of plotting compass will:
(A) point away from the south pole
(B) point parallel to the south pole
(C) point towards the south pole
(D) point at right angles to the south pole
Question. Electric motor is a device which converts
(A) mechanical energy to electrical energy
(B) electrical energy to mechanical energy
(C) chemical energy to mechanical energy
(D) mechanical energy to light energy.
Question. A student learns that magnetic field strength around a bar magnet is different at every point. Which diagram shows the correct magnetic field lines around a bar magnet?
Question. A student inserts a bar magnet in the coil. The student observes deflection in the galvanometer connected to the coil. What will happen if the magnet is continuously getting in and out of the coil?
(A) the current induced in the coil will increase
(B) the current will change its direction continuously
(C) the magnetic field will create a motion in the coil
(D) the magnetic field of the bar magnet would keep decreasing
Question. Which option explains Fleming’s left-hand rule to understand the working of a motor?
(A) When a current carrying conductor is moved with a force, it creates the magnetic field.
(B) When a conductor is moved inside a magnetic field, current is produced in the conductor.
(C) When the magnetic field is moved relative to the conductor, current is produced in the conductor.
(D) When a current carrying conductor is placed in a magnetic field, it experiences a force by magnetic field.
Assertion –Reason Type Questions
Each of these questions contain two statements, Assertion (A) and Reason (R). Each of these questions also has four alternative choices, only one of which is the correct answer. You have to select one of the codes (a), (b), (c) and (d) given below.
(a) Assertion is correct, reason is correct; reason is a correct explanation for assertion.
(b) Assertion is correct, reason is correct; reason is not a correct explanation for assertion
(c) Assertion is correct, reason is incorrect
(d) Assertion is incorrect, reason is correct.
Question. Assertion: In Fleming’s left hand rule, the direction of magnetic field, magnetic force and current are mutually perpendicular.
Reason: Fleming’s left hand rule is applied to measure the induced current.
Question. Assertion: A current carrying conductor experience a force in the magnetic field
Reason: The force acting on a current carrying conductor in a magnetic field is due to interaction between magnetic fields produced by the conductor and external magnetic field.
Question. Assertion: Magnetic Field due to a current carrying coil at its centre becomes double if current in the coil is doubled.
Reason: Magnetic Field due to a current carrying coil at its centre is directly proportional to the current.
Case Based Questions
A solenoid is a long helical coil of wire through which a current is run in order to create a magnetic field. The magnetic field of the solenoid is the superposition of the fields due to the current through each coil. It is nearly uniform inside the solenoid and close to zero outside and is similar to the field of a bar magnet having a north pole at one end and a south pole at the other depending upon the direction of current flow. The magnetic field produced in the solenoid is dependent on a few factors such as, the current in the coil, number of turns per unit length etc. The following graph is obtained by a researcher while doing an experiment to see the variation of the magnetic field with respect to the current in the solenoid. The unit of magnetic field as given in the graph attached is in milli-Tesla (mT) and the current is given in Ampere.
Question. What will happen if a soft iron bar is placed inside the solenoid?
(A) The bar will be electrocuted resulting in short-circuit.
(B) The bar will be magnetised as long as there is current in the circuit.
(C) The bar will be magnetised permanently.
(D) The bar will not be affected by any means.
Question. What type of energy conversion is observed in a linear solenoid?
(A) Mechanical to Magnetic
(B) Electrical to Magnetic
(C) Electrical to Mechanical
(D) Magnetic to Mechanical
Question. After analysing the graph a student writes the following statements.
I. The magnetic field produced by the solenoid is inversely proportional to the current.
II. The magnetic field produced by the solenoid is directly proportional to the current.
III. The magnetic field produced by the solenoid is directly proportional to square of the current.
IV. The magnetic field produced by the solenoid is independent of the current.
Choose from the following which of the following would be the correct statement(s).
(A) Only IV
(B) I and III and IV
(C) I and II
(D) Only II
Question. The magnetic field lines produced inside the solenoid are similar to that of …
(A) a bar magnet
(B) a straight current carrying conductor
(C) a circular current carrying loop
(D) electromagnet of any shape
Question. From the graph deduce which of the following statements is correct.
(A) For a current of 0.8A the magnetic field is 13 mT
(B) For larger currents, the magnetic field increases non-linearly.
(C) For a current of 0.8A the magnetic field is 1.3 mT
(D) There is not enough information to find the magnetic field corresponding to 0.8A current.
Short Answer Type Questions
Question. Why does a compass needle get deflected when brought near a bar magnet ?
Answer. The magnetic field of the magnet exerts force on both the poles of the compass needle. The forces experienced by the two poles are equal and opposite. These two forces form a couple which deflects the compass needle.
Question. What do you understand by magnetic effect of current?
Answer. When electric current flows through a conductor it produces a magnetic field around it and behaves like a magnet. This is called the magnetic effect of electric current.
Question. What is the principle of an electric motor?
Answer. A motor works on the principle of magnetic effect of current. When a rectangular coil is placed in a magnetic field and current is passed through it, a force acts on the coil which rotates it continuously. When the coil rotates, the shaft attached to it also rotates. In this way the electrical energy supplied to the motor is converted into the mechanical energy of rotation.
Question. State the principle of an electric generator.
Answer. The electric generator works on the principle that when a straight conductor is moved in a magnetic field, then current is induced in the conductor. In an electric generator, a rectangular coil is made to rotate rapidly in the magnetic field between the poles of a horse-shoe type magnet. When the coil rotates, it cuts the magnetic field lines due to which a current is produced in the coil.
Question. Explain electromagnetic induction.
Answer. A changing magnetic field in a conductor induces a current in another conductor, this is called electromagnetic induction.
• We can induce current in a coil either by moving it in a magnetic field or by changing the magnetic field around it.
• The induced current is found to be the highest when the direction of motion of the coil is at right angles to the magnetic field.
Question. Which sources produce alternating current?
Answer. Alternating current is produced by AC generators of nuclear power plants, thermal power plants, hydroelectric power stations, etc.
Question. Name some sources of direct current.
Answer. Some of the sources of direct current are dry cells, button cells, lead accumulators.
Question. An electric oven of 2 kW power rating is operated in a domestic electric circuit (220 V) that has a current rating of 5 A. What result do you expect ? Explain.
Answer. The electric oven draws a current given by I = P/V 2000/220 = 9.09A The electric oven draws current much more than the current rating of 5 A. That is the circuit is overloaded. Due to excessive
current, the fuse wire will blow and the circuit will break.
Question. Name some devices in which electric motors are used.
Answer: Electric motors are used in the appliances like electric fans, washing machine, mixers, grinders, blenders, computers, MP3 players, etc.
Question. Magnetic field lines of two magnets are shown in fig. A and fig. B.
Select the figure that represents the correct pattern of field lines. Give reasons for your answer. Also name the poles of the magnets facing each other.
Answer. Figure B represents the correct pattern of field lines. In figure A, field lines cross each other which is not possible because if they cross each other, at the point of intersection, there would be two directions of field lines.
In figure B, field lines are emerging in nature, so poles of magnet facing each other are north poles while opposite faces will have south polarity
Long Answer Type Questions
Question. Imagine that you are sitting in a chamber with your back to one wall. An electron beam, moving horizontally from back wall towards the front wall, is deflected by a strong magnetic field to your right side. What is the direction of magnetic field ?
Answer. Here the electron beam is moving from our back wall to the front wall, so the direction of current will be in the opposite direction, from front wall towards back wall or towards us. The direction of deflection (or force) is towards our right side. We now know two things :direction of current is from front towards us, and direction of force is towards our right side.
Let us now hold the forefinger, middle finger and thumb of our left hand at right angles to one another. We now adjust the hand in such a way that our center finger points towards us (in the direction of current) and our thumb points towards the right side (in the direction of force). Now, if we look at our forefinger, it will be pointing vertically downwards. Since the direction of the forefinger gives the direction of the magnetic field, therefore, the magnetic field is in the vertically downward direction.
Question. Draw a labeled diagram of an electric motor. Explain its principle and working. What is the function of a split ring in an electric motor?
Answer. Electric Motor : The device used to convert electrical energy to mechanical energy is called Electric Motor. It is used in fans, machines, etc.
Principle : Electric motor works on the principle of force experienced by a current carrying conductor in a magnetic field. The two forces in the opposite sides are equal and opposite. The direction of motion is given by Flemmings left hand rule. Motion.
Working of an electric motor : When current starts to flow, the coil ABCD is in horizontal position. The direction of current through the armature coil has the direction from A to B in the arm AB and from C to D in the arm CD. The direction of force exerted on the coil can be found through Fleming’s left hand law. According to this law, it is found that the force exerted on the part AB, pushes the coil downwards. While the force exerted on the part CD pushes it upwards. In this way, these two forces being equal and opposite form a couple that rotates the coil in anticlockwise direction.
When the coil is in vertical position, the brushes X and Y would touch the centre of the commutator and the current in the coil is stopped. Though current is is
stopped but the coil comes back in horizontal state due to momentum. After half rotation, the polarity of the commutator also changes, because now Q makes contact with brush X and P with brush Y. Therefore, now the force exerts downwards on the arm AB and upwards on the arm CD and thus again a couple of forces is formed that rotates the coil in clockwise direction. This process is repeated again and again and the coil rotates til ! the current flows across it.
Function of split ring : Split ring in a motor acts as a commutator, i.e., it reverses the flow of current in the circuit due to which the direction of the forces acting on the arms also reverses.
Question. What is a Solenoid? Draw magnetic field lines for a current carrying solenoid.
Answer. A coil of many circular turns of insulated copper wire wrapped closely in the shape of a cylinder is called a solenoid. The magnetic field lines around a current-carrying solenoid is similar to a bar magnet. One end of the solenoid acts as North Pole, while the other behaves as the South Pole. The field lines inside the solenoid are parallel straight lines, that is, the field is uniform inside the solenoid.