Force on a Current-carrying Conductor in a Magnetic Field Questions in English

(D) When a proton moves in a magnetic field,it experiences a magnetic force given by $F = q(v \times B)$.
This force acts perpendicular to the direction of motion,which causes the proton to follow a circular path.
Since the direction of motion changes continuously in a circular path,the velocity of the proton changes.
Because momentum is defined as $p = mv$ and velocity changes,the momentum also changes.
However,the speed (magnitude of velocity) and mass remain constant because the magnetic force does no work on the particle.

(N/A) current-carrying conductor placed in a magnetic field experiences a magnetic force. The magnitude of this force is given by $F = BIl \sin \theta$,where $B$ is the magnetic field strength,$I$ is the current,and $l$ is the length of the conductor.
$(i)$ If the current $(I)$ in rod $AB$ is increased,the magnetic force $F$ increases,leading to a greater displacement of the rod.
$(ii)$ If a stronger horse-shoe magnet is used,the magnetic field strength $(B)$ increases,which increases the magnetic force $F$,resulting in a greater displacement of the rod.
$(iii)$ If the length $(l)$ of the rod $AB$ is increased,the magnetic force $F$ increases,which also leads to a greater displacement of the rod.

(B) The direction of the magnetic field can be determined by the $Fleming's$ left-hand rule.
According to this rule,if we arrange the thumb,the middle finger,and the forefinger of the left hand at right angles to each other,then the thumb points towards the direction of the magnetic force,the middle finger gives the direction of current,and the forefinger points in the direction of the magnetic field.
Since the direction of the positively charged alpha particle is towards the west,the direction of the current will be the same,i.e.,towards the west.
The direction of the magnetic force (deflection) is towards the north.
Applying $Fleming's$ left-hand rule: point the middle finger towards the west and the thumb towards the north; the forefinger will point in the upward direction.
Therefore,the direction of the magnetic field is upward.

(N/A) Fleming's left-hand rule states that if we stretch the thumb,the forefinger,and the middle finger of the left hand such that they are mutually perpendicular to each other,then:
$1$. The forefinger points in the direction of the magnetic field.
$2$. The middle finger points in the direction of the current.
$3$. The thumb points in the direction of the force or motion experienced by the conductor.

(B) The split ring in an electric motor acts as a commutator.
It reverses the direction of the current flowing through the coil after every half rotation.
Due to this reversal of the current,the force acting on the coil remains in the same direction,allowing the coil to continue rotating in the same direction.

(B) The force $F$ on a current-carrying conductor in a magnetic field is given by the formula $F = BIl \sin(\theta)$,where $B$ is the magnetic field strength,$I$ is the current,$l$ is the length of the conductor,and $\theta$ is the angle between the direction of the current and the magnetic field.
Since the maximum value of $\sin(\theta)$ is $1$ (which occurs when $\theta = 90^{\circ}$),the force is largest when the direction of the current is perpendicular to the direction of the magnetic field.

(DOWNWARD) The direction of the magnetic field is determined using Fleming's left-hand rule.
According to this rule,if the forefinger points in the direction of the magnetic field,the middle finger in the direction of the current,then the thumb points in the direction of the force (deflection).
$1$. The electron beam moves from the back wall to the front wall. Since electrons are negatively charged,the direction of the conventional current is from the front wall to the back wall.
$2$. The force (deflection) is directed towards your right side.
$3$. Applying Fleming's left-hand rule: Point your middle finger towards the back wall (direction of current) and your thumb towards your right (direction of force). Your forefinger will then point downwards.
Therefore,the direction of the magnetic field is downward.

(N/A) An electric motor converts electrical energy into mechanical energy.
It works on the principle of the magnetic effect of current. $A$ current-carrying coil rotates in a magnetic field. The following figure shows a simple electric motor.
When a current is allowed to flow through the coil $MNST$ by closing the switch,the coil starts rotating anti-clockwise. This happens because a downward force acts on length $MN$ and at the same time,an upward force acts on length $ST$. As a result,the coil rotates anti-clockwise.
Current in the length $MN$ flows from $M$ to $N$ and the magnetic field acts from left to right,normal to length $MN$. Therefore,according to Fleming's left-hand rule,a downward force acts on the length $MN$. Similarly,current in the length $ST$ flows from $S$ to $T$ and the magnetic field acts from left to right,normal to the flow of current. Therefore,an upward force acts on the length $ST$. These two forces cause the coil to rotate anti-clockwise. After half a rotation,the position of $MN$ and $ST$ interchange. The half-ring $D$ comes in contact with brush $A$ and half-ring $C$ comes in contact with brush $B$. Hence,the direction of current in the coil $MNST$ gets reversed.
The current flows through the coil in the direction $TSNM$. The reversal of current through the coil $MNST$ repeats after each half rotation. As a result,the coil rotates unidirectionally. The split rings help to reverse the direction of current in the circuit. These are called the commutator.

(C) According to Fleming's Left-Hand Rule,the direction of the force on a moving charge in a magnetic field is determined by the orientation of the magnetic field,the direction of the current (or motion of positive charge),and the force.
For the proton (positive charge moving upwards),the magnetic field is to the right. Using the left-hand rule,the force points into the plane of the paper.
For the electron (negative charge moving downwards),the direction of the conventional current is upwards. Since the magnetic field is to the right,the force on the electron points out of the plane of the paper.
Therefore,the proton experiences a force into the plane,and the electron experiences a force out of the plane.

(C) An $AC$ generator uses slip rings to maintain a continuous connection with the rotating coil,which results in an alternating current output.
To convert this into a $DC$ generator,the slip rings must be replaced with a split-ring type commutator.
The split-ring commutator acts as a reversing switch that reverses the direction of the current in the external circuit every time the coil passes the vertical position,ensuring that the current flows in only one direction in the external circuit.

(C) In a simple electric motor,the two conducting stationary brushes (usually made of carbon) are used to maintain a sliding contact with the split rings (commutator).
These brushes are connected to the external battery circuit.
As the split rings rotate with the coil,the brushes remain stationary and allow the electric current to flow continuously from the external source into the rotating coil without tangling the wires.

(N/A) Activity: Take a small aluminum rod $AB$ suspended horizontally from a stand using two connecting wires. Place a strong horseshoe magnet in such a way that the rod lies between the two poles with the magnetic field directed upwards. Connect the rod $AB$ in series with a battery,a key,and a rheostat. When current is passed through the rod from $B$ to $A$,the rod is observed to get displaced. This displacement shows that a force is acting on the current-carrying conductor when it is placed in a magnetic field. The direction of this force is perpendicular to both the length of the conductor and the magnetic field.
Fleming's Left-Hand Rule: According to this rule,stretch the thumb,forefinger,and middle finger of your left hand such that they are mutually perpendicular. If the forefinger points in the direction of the magnetic field and the middle finger points in the direction of the current,then the thumb will point in the direction of the motion or the force acting on the conductor.

(N/A) To determine the direction of the magnetic field,we use Fleming's Left-Hand Rule.
According to Fleming's Left-Hand Rule,if we stretch the thumb,forefinger,and middle finger of our left hand such that they are mutually perpendicular to each other,then:
$1$. The forefinger points in the direction of the magnetic field.
$2$. The middle finger points in the direction of the current.
$3$. The thumb points in the direction of the force on the conductor.
In the given figure,the current is directed towards the left and the force on the conductor is directed upwards.
By applying Fleming's Left-Hand Rule,we find that the magnetic field is directed out of the page (towards the observer).

(A) The rule used to determine the direction of the force acting on a current-carrying conductor placed in a magnetic field is $Fleming's \text{ left-hand rule}$. According to this rule, if we stretch the thumb, forefinger, and middle finger of our left hand such that they are mutually perpendicular to each other, then the forefinger points in the direction of the magnetic field, the middle finger points in the direction of the current, and the thumb gives the direction of the force or motion of the conductor.

(D) The force $(F)$ acting on a current-carrying conductor placed in a magnetic field is given by the formula $F = BIl \sin \theta$.
Therefore,the factors are:
$(i)$ The current $(I)$ flowing through the conductor.
$(ii)$ The strength of the magnetic field $(B)$.
$(iii)$ The length of the conductor $(l)$.
$(iv)$ The angle $(\theta)$ between the direction of the current and the direction of the magnetic field.

(B) Fleming's right-hand rule states that if you stretch the thumb,forefinger,and middle finger of your right hand such that they are mutually perpendicular to each other,then the thumb points in the direction of the motion of the conductor relative to the magnetic field,the forefinger points in the direction of the magnetic field,and the middle finger points in the direction of the induced current.

(N/A) According to Lenz's Law,the induced current will oppose the motion of the magnet. As the North pole $(N)$ of the magnet moves towards the coil,the coil will develop a North pole on its upper face to repel it. Therefore,the direction of the induced current,as seen from the top,will be anticlockwise.
$(b)$ As the North pole $(N)$ of the magnet moves towards the coil,the face of the coil facing the magnet will act as a North pole to oppose the motion. Looking from the right-hand side (where the magnet is approaching),the current will appear to flow in a clockwise direction.

(D) According to Fleming's left-hand rule,the direction of the force on a moving charge is determined by the orientation of the magnetic field and the direction of the conventional current.
Since the electron is a negatively charged particle moving downwards,the direction of the conventional current is upwards.
The magnetic field is directed towards the right.
Applying Fleming's left-hand rule: point the index finger in the direction of the magnetic field (right),and the middle finger in the direction of the current (upwards). The thumb will point into the page.
Therefore,the force acting on the electron is directed into the page.

(C) The force $F$ experienced by a current-carrying conductor of length $l$ carrying current $I$ in a magnetic field $B$ is given by the formula $F = BIl \sin(\theta)$,where $\theta$ is the angle between the direction of the current and the magnetic field.
Since the maximum value of $\sin(\theta)$ is $1$ (which occurs when $\theta = 90^{\circ}$),the force is largest when the current flows perpendicular to the applied magnetic field.

(C) The force $F$ experienced by a current-carrying conductor of length $l$ carrying current $I$ placed in a magnetic field $B$ is given by the formula $F = BIl \sin(\theta)$,where $\theta$ is the angle between the direction of the current and the magnetic field.
Since the value of $\sin(\theta)$ is maximum $(1)$ when $\theta = 90^{\circ}$,the force experienced by the conductor is largest when the conductor is placed perpendicular to the magnetic field,i.e.,at an angle of $90^{\circ}$.

(C) Fleming's right-hand rule is used to determine the direction of the induced current in a conductor moving in a magnetic field.
According to this rule,if you stretch the thumb,forefinger,and middle finger of your right hand such that they are mutually perpendicular to each other,then:
$1$. The forefinger points in the direction of the magnetic field.
$2$. The thumb points in the direction of the motion of the conductor.
$3$. The middle finger points in the direction of the induced current.
Therefore,the thumb indicates the direction of motion of the conductor.

(N/A) According to Fleming's left-hand rule,if the forefinger points in the direction of the magnetic field and the central finger points in the direction of the current (the direction of motion of the positively charged alpha particle),then the thumb points in the direction of the force acting on the particle.
In this case,the magnetic field is directed upwards and the alpha particle is moving towards the right. Applying Fleming's left-hand rule,the force acting on the alpha particle will be directed out of the plane of the paper (perpendicular to the plane of the paper and towards the observer).

(N/A) When a magnetic needle is brought near a current-carrying conductor,it deflects because the magnetic field produced by the current-carrying conductor exerts a magnetic force on the needle.
The direction of the force experienced by a current-carrying conductor placed in a magnetic field is determined by Fleming's Left-Hand Rule. According to this rule,stretch the thumb,forefinger,and middle finger of your left hand such that they are mutually perpendicular to each other. If the forefinger points in the direction of the magnetic field and the middle finger points in the direction of the current,then the thumb will point in the direction of the force (or motion) experienced by the conductor.

(D) The force $F$ experienced by a current-carrying conductor of length $l$ carrying current $I$ placed in a uniform magnetic field $B$ is given by the formula $F = BIl \sin(\theta)$.
Based on this formula, the force depends on the following factors:
$(i)$ The strength of the magnetic field $(B)$.
$(ii)$ The strength of the electric current $(I)$.
$(iii)$ The length of the conductor $(l)$.
$(iv)$ The angle $(\theta)$ between the direction of the current and the direction of the magnetic field.

(N/A) The following are the characteristics of magnetic force:
$(i)$ Magnetic force acts only on moving charges and not on stationary charges.
$(ii)$ No magnetic force acts on a charge if it is moving parallel or anti-parallel to the direction of the magnetic field.
$(iii)$ The direction of magnetic force is perpendicular to both $(a)$ the velocity vector of the charge and $(b)$ the magnetic field vector.
$(iv)$ The magnitude of magnetic force $(F)$ depends on the charge $(q)$,velocity $(v)$,and the strength $(B)$ of the magnetic field; i.e.,$F = qvB \sin \theta$,where $\theta$ is the angle between velocity and magnetic field.
$(v)$ The magnetic force $(F)$ acting on a current-carrying conductor of length $(L)$ placed in a magnetic field $(B)$ is given by $F = BIL \sin \theta$,where $I$ is the current flowing through the conductor.

(C) $(i)$ In series: When connected in series,the current flows through the wires in opposite directions relative to each other. According to the laws of electromagnetism,parallel wires carrying current in opposite directions exert a repulsive force on each other. Therefore,they will move apart.
$(ii)$ In parallel: When connected in parallel,the current flows through both wires in the same direction. Parallel wires carrying current in the same direction exert an attractive force on each other. Therefore,they will move towards each other.

(N/A) current-carrying conductor experiences a force when placed in a magnetic field due to the interaction between the magnetic field produced by the current flowing through the conductor and the external magnetic field in which it is placed.
Statement of Fleming's left-hand rule:
Stretch the thumb,forefinger,and middle finger of your left hand such that they are mutually perpendicular to each other. If the forefinger points in the direction of the magnetic field and the middle finger points in the direction of the current,then the thumb will point in the direction of the force acting on the conductor. An example of a device based on this principle is the electric motor.

(N/A) The electron will move perpendicular to the plane of the paper and downwards.
Explanation:
$1$. The direction of the magnetic field is towards the right.
$2$. The electron is moving upwards. Since the current is defined as the flow of positive charges,the direction of conventional current is opposite to the direction of the electron's motion,i.e.,downwards.
$3$. Applying Fleming's Left-Hand Rule: Stretch the thumb,forefinger,and middle finger of the left hand such that they are mutually perpendicular. If the forefinger points in the direction of the magnetic field (right) and the middle finger points in the direction of the current (downwards),then the thumb points in the direction of the force acting on the electron.
$4$. According to this rule,the force on the electron acts perpendicular to the plane of the paper and downwards.

(A) Observation $(i)$ is correct.
The force $(F)$ experienced by a current-carrying conductor placed in a magnetic field is given by the formula $F = BIl \sin \theta$,where $B$ is the magnetic field strength,$I$ is the current,and $l$ is the length of the conductor.
According to this relationship,the force is directly proportional to the current $(I)$ flowing through the conductor. Therefore,as the current increases,the force experienced by the conductor also increases.
Observation $(ii)$ is incorrect because the force is also directly proportional to the magnetic field strength $(B)$. Thus,increasing the magnetic field strength would increase the force,not decrease it.

(N/A) The rule used to determine the direction of the force is $Fleming's \text{ left-hand rule}$.
Statement: Stretch the thumb,forefinger,and middle finger of your left hand such that they are mutually perpendicular to each other. If the forefinger points in the direction of the magnetic field and the middle finger points in the direction of the current,then the thumb will point in the direction of the force acting on the conductor.
$A$ device that uses a current-carrying conductor and a magnetic field is an $electric \text{ motor}$.

(EAST) The $\alpha$-particle is a positively charged particle. According to Fleming's Left-Hand Rule,the force $(F)$ on a moving charge is perpendicular to both the magnetic field $(B)$ and the velocity $(v)$ of the charge.
Given: Magnetic field $(B)$ is vertically upwards. Force $(F)$ is towards the south.
Using Fleming's Left-Hand Rule: If the forefinger points upwards (magnetic field) and the thumb points south (force),the middle finger will point towards the east.
Therefore,the $\alpha$-particle should be projected towards the east.
Fleming's Left-Hand Rule states: Stretch the thumb,forefinger,and middle finger of your left hand such that they are mutually perpendicular. If the forefinger points in the direction of the magnetic field and the middle finger points in the direction of the current (or motion of positive charge),then the thumb points in the direction of the force acting on the particle.

(N/A) current-carrying conductor produces its own magnetic field around it. When this field interacts with an external magnetic field,it experiences a mechanical force.
The direction of this force depends on two factors:
$1$. The direction of the electric current flowing through the conductor.
$2$. The direction of the external magnetic field.
The rule used to determine the direction of this force is $Fleming's \text{ } Left-Hand \text{ } Rule$.
Statement of $Fleming's \text{ } Left-Hand \text{ } Rule$: Stretch the thumb,forefinger,and middle finger of your left hand such that they are mutually perpendicular to each other. If the forefinger points in the direction of the magnetic field,the middle finger points in the direction of the current,then the thumb will point in the direction of the force (or motion) acting on the conductor.

(N/A) current-carrying conductor experiences maximum force when it is placed perpendicular to the direction of the magnetic field.
The magnitude of this force depends on the following factors:
$(i)$ The strength of the magnetic field $(B)$.
$(ii)$ The magnitude of the current flowing through the conductor $(I)$.
$(iii)$ The length of the conductor $(L)$ placed within the magnetic field.
The rule used to determine the direction of this force is Fleming's Left-Hand Rule.
Statement of Fleming's Left-Hand Rule: Stretch the thumb,forefinger,and middle finger of your left hand such that they are mutually perpendicular to each other. If the forefinger points in the direction of the magnetic field and the middle finger points in the direction of the current,then the thumb will point in the direction of the force acting on the conductor.

(N/A) Electric motors are commonly used in appliances such as electric fans,washing machines,and mixers.
$(b)$ An electric motor works on the principle that a current-carrying conductor placed in a magnetic field experiences a mechanical force.
Fleming's Left-Hand Rule: If we stretch the thumb,forefinger,and middle finger of our left hand such that they are mutually perpendicular to each other,then the forefinger points in the direction of the magnetic field,the middle finger points in the direction of the current,and the thumb gives the direction of the force acting on the conductor.

(N/A) thick,copper wire $AB$ is suspended vertically from a support $T$ by means of a flexible joint $J.$ The lower end $B$ of this wire is free to move between the poles of a $U$-shaped magnet $M.$ The lower end $B$ of the wire just touches the surface of mercury kept in a shallow vessel $V$ so that it can move when a force acts on it. The positive terminal of a battery is connected to end $A$ of the wire. The circuit is completed by dipping another wire from the negative terminal of the battery into the mercury in the vessel. We know that mercury is a liquid which is a good conductor of electricity,so the circuit is completed.
On pressing the switch,a current flows in the wire $AB$ in the vertically downward direction. The wire $AB$ is pushed in the forward direction (towards south) and its lower end $B$ reaches the position $B',$ so that the wire comes to the new position $AB',$ as shown by the dotted line in the figure. When the lower end $B$ of the hanging wire comes forward to $B',$ its contact with the mercury surface is broken,due to which the circuit breaks and the current stops flowing in the wire $AB.$
Since no current flows in the wire,no force acts on the wire in this position and it falls back to its original position. As soon as the wire falls back,its lower end again touches the mercury surface,current starts flowing in the wire,and it is pushed again. This reaction is repeated as long as the current is passed in wire $AB.$ It may be noted that the current-carrying wire is pushed forward because a force is exerted on it by the magnetic field of the $U$-shaped magnet. From this experiment,we conclude that when a current-carrying conductor is placed in a magnetic field,a mechanical force is exerted on the conductor which makes it move.

(N/A) Experiment: Take a small aluminum rod $AB$ suspended horizontally from a stand by two connecting wires. Place a strong horseshoe magnet in such a way that the rod lies between the two poles with the magnetic field directed vertically upwards. Connect the ends of the rod in series with a battery,a key,and a rheostat. When current is passed through the rod from $B$ to $A$,the rod gets displaced. This displacement shows that a force is exerted on the current-carrying conductor when placed in a magnetic field.
$(b)$ The force acting on the conductor is directly proportional to the current flowing through it $(F \propto I)$. Therefore,if the current in the conductor is increased,the force acting on it will also increase,resulting in a larger displacement of the rod.
$(c)$ An electric motor is a device that works on this principle.

(D) The direction of the induced current in a conductor moving in a magnetic field is determined by Fleming's right-hand rule.
According to this rule,if you stretch the thumb,forefinger,and middle finger of your right hand such that they are mutually perpendicular to each other,then:
$1$. The thumb points in the direction of the motion of the conductor.
$2$. The forefinger points in the direction of the magnetic field.
$3$. The middle finger points in the direction of the induced current (or induced potential difference).

(B) The statement is $False$.
Maxwell's corkscrew rule (also known as the Right-Hand Grip Rule) is used to determine the direction of the magnetic field produced by a current-carrying conductor.
To find the force experienced by a current-carrying conductor placed in a magnetic field,we use Fleming's Left-Hand Rule.

(C) The magnetic force $(F)$ acting on a current-carrying conductor of length $(l)$ carrying current $(I)$ placed in a magnetic field $(B)$ is given by the vector product: $F = I(l \times B)$.
According to the right-hand rule or Fleming's left-hand rule,the direction of the magnetic force is always perpendicular to the plane containing both the direction of the electric current and the direction of the magnetic field.
Therefore,the force is perpendicular to both the magnetic field and the electric current.

(A) The magnetic force $F$ acting on a current-carrying wire of length $L$ carrying current $I$ in a magnetic field $B$ is given by the formula: $F = I L B \sin(\theta)$,where $\theta$ is the angle between the direction of the current and the magnetic field.
For the magnetic force to be zero $(F = 0)$,the value of $\sin(\theta)$ must be zero.
This occurs when $\theta = 0^{\circ}$ or $\theta = 180^{\circ}$.
Therefore,when the current-carrying wire is placed parallel to the magnetic field,the magnetic field does not exert any force on it.

(B) An $Electric \text{ } motor$ is a device that converts electrical energy into mechanical energy.
It works on the principle of the magnetic effect of electric current, where a current-carrying coil placed in a magnetic field experiences a force that causes it to rotate.
In contrast, an $Electric \text{ } generator$ converts mechanical energy into electrical energy.
$Electric \text{ } iron$ and $Electric \text{ } oven$ convert electrical energy into heat energy.

(B) Fleming's Right-Hand Rule is used to determine the direction of the induced current in a conductor moving in a magnetic field.
According to this rule,if you stretch the thumb,forefinger,and middle finger of your right hand such that they are mutually perpendicular to each other,then the thumb points in the direction of the motion of the conductor,the forefinger points in the direction of the magnetic field,and the middle finger points in the direction of the induced current.

(D) The magnetic force $F$ acting on a current-carrying conductor placed in a magnetic field is given by the formula $F = I L B \sin(\theta)$.
Here,$I$ is the electric current,$L$ is the length of the conductor within the magnetic field,$B$ is the magnetic field strength,and $\theta$ is the angle between the current direction and the magnetic field.
Since the wire is placed perpendicular to the magnetic field,$\theta = 90^\circ$,and $\sin(90^\circ) = 1$.
Therefore,the force $F = I L B$.
This shows that the magnetic force is directly proportional to the electric current $(I)$,the length of the wire $(L)$,and the magnetic field strength $(B)$.
Thus,all the given options are correct.

(A) When a current-carrying conductor is placed in a magnetic field,it experiences a magnetic force.
According to Fleming's Left-Hand Rule,if we stretch the thumb,forefinger,and middle finger of our left hand such that they are mutually perpendicular to each other,then the forefinger points in the direction of the magnetic field,the middle finger points in the direction of the current,and the thumb gives the direction of the force acting on the conductor.
Therefore,Fleming's Left-Hand Rule is used to determine the direction of the magnetic force.

(D) The phenomenon that a current-carrying conductor experiences a force when placed in a magnetic field was first observed by $Andre-Marie$ $Ampere$.
He demonstrated that magnetic fields exert forces on moving charges,which results in a mechanical force on the conductor carrying the current.

(A) The force $F$ acting on a current-carrying conductor placed in a magnetic field is given by the formula $F = BIl \sin(\theta)$,where $B$ is the magnetic field strength,$I$ is the current,$l$ is the length of the conductor,and $\theta$ is the angle between the direction of the current and the magnetic field.
When the direction of the current is perpendicular to the magnetic field,the angle $\theta = 90^\circ$.
Since $\sin(90^\circ) = 1$,the force becomes $F = BIl(1) = BIl$.
This value represents the maximum force that can act on the conductor.

(A) An electric motor is a device that converts electrical energy into mechanical energy.
It works on the principle that when a rectangular coil carrying current is placed in a magnetic field,it experiences a magnetic force,which causes the coil to rotate.

(B) In an electric motor,a large number of turns of insulated copper wire wound on a soft iron core is called an $Armature$.
This $Armature$ is placed on an axis and rotates within a magnetic field when current flows through it.

(A) Fleming's Left-Hand Rule is used to determine the direction of the force acting on a current-carrying conductor placed in a magnetic field.
According to this rule,if we stretch the thumb,forefinger,and middle finger of the left hand such that they are mutually perpendicular to each other:
$1$. The forefinger points in the direction of the magnetic field.
$2$. The middle finger points in the direction of the current.
$3$. The thumb points in the direction of the force (or motion) acting on the conductor.
Therefore,the thumb indicates the direction of the magnetic force.

(A) In a $DC$ electric motor,a split-ring commutator is used to reverse the direction of the current flowing through the coil after every half rotation.
This reversal of current ensures that the force acting on the coil continues to push it in the same direction,allowing the motor to rotate continuously.
Therefore,the correct answer is half rotation.