Magnetic Effect of Electric Current

Ans. The properties of magnetic field lines are

(i) They originate from North pole of a magnet and end at its South pole

(ii) They form closed and continuous curves

(iii) They never intersect each other.

(iv) They are uniform inside the magnet.

If two magnetic field lines intersect each other, then at the point of intersection, there will be two directions of magnetic field lines, which is not possible. Hence, the magnetic field lines never intersect each other.

3. How will the strength of the magnetic field change when the point where magnetic field is to be determined is moved away from the straight wire carrying constant current? Justify your answer.

The strength of the magnetic field is inversely proportional to the distance i.e., B ∝ 1/r

As, the point where magnetic field is to be determined is moved away from the straight wire carrying constant current, the magnetic field strength decreases.

Given r₁ > r₂, where will the strength of the magnetic field be larger?

According to the right hand thumb rule, magnetic field at P is directed into the plane of paper and at Q, it is out of the plane of paper. The strength of the magnetic field at Q will be larger as strength of the field ∝ 1/r (distance)

Here, r₁ > r₂

⸫B₁ < B₂ i.e., B₂ has larger field.

Ans. According to right hand thumb rule,

(i) The direction of magnetic field at a point above the power line is from South to North.

(ii) The direction of magnetic field at a point below the power line is from North to South.

The magnetic field lines are concentric circles at every point on a current carrying circular loop.

The direction of magnetic field is determined by right hand thumb rule. At the centre of the circular loop, the magnetic field lines are straight and points towards North.

The direction of magnetic field lines at the point outside the surface of the loop is shown below.

Ans. When the coil is kept in the North-South plane and the current is flowing in the anti-clockwise through the loop, then the magnetic field is in the East to West direction.

Ans. When the coil is in vertically East-West plane and current through the coil is in anti-clockwise direction, then the magnetic field is in the South to North direction.

When a circular coil carrying current is placed horizontally and the direction of the current is clockwise, then the direction of the field for the observer positioned below the coil is in the downward direction.

Ans. A solenoid behaves like a magnet when electric current passes through it.

One end of a solenoid behaves as a North pole and the other end behaves as a South pole. We can use a bar magnet to determined the North and South poles of a current carrying solenoid by using the property, i.e., like poles repel and unlike poles attract each other.

The end of solenoid which attracts North pole of a bar magnet is magnetic South pole of the solenoid. The end of solenoid which repels the North pole of a bar magnet is the magnetic North pole of the solenoid.

Ans. The magnetic field lines are crowded (convergent) near the poles of solenoid. Hence, the magnetic field is strong and divergent, where the magnetic field is weak.

Ans. A freely suspended current carrying solenoid always points in the North-South direction even in the absence of any other magnet. Because the earth itself behaves as a magnet of solenoid to point always in a particular direction.

The field lines around a current carrying solenoid are similar to field lines of a bar magnet. So, inside the solenoid field lines are parallel to each other and the strength of magnetic field is same i.e., uniform at all points inside a solenoid.

The force experienced by a current carrying conductor placed in a magnetic field is the largest, when conductor is kept perpendicular to the direction of the magnetic field.

Fleming’s left hand rule determines the direction of force on a current carrying conductor. It states that, if the forefinger (magnetic field), middle finger (current and thumb are stretched mutually, then the direction of force acting on conductor is given by thumb.

If a charged particle moves parallel or anti-parallel to the magnetic field, no magnetic force will act on it and remains undeflected. So, in the given condition either the charged particle enters East to West or West to East as shown below.

Here, the force acting on the α-particle is directed perpendicular to the plane of paper in inward direction by Fleming’s left hand rule. If the direction of magnetic field gets reversed then the force will also act in opposite direction of α-particle i.e., the force experienced by α-particle is now in outward direction to the plane of paper.

Ans. No, it will not experience any force. As magnetic field exerts force on a moving charged particle only.

Ans. No, it will not experience any force because magnetic field exerts a force in perpendicular direction to motion of the particle.

Ans. Yes, it will experience a force in a direction perpendicular to the direction of its own motion and the direction of magnetic field can be determined by Fleming’s left hand rule.

Force on electron is maximum in case (a), because here the direction of motion of electron is perpendicular to the direction of magnetic field B.

Similarly, the force on electron is minimum in case (c) because, in this case the direction of motion of electron is along the direction of magnetic field B, as electron is moving along B.

Hence, the direction of maximum force acting on electron is perpendicular to the plane of paper and directed into it.

When the gaps of the split ring commutator are aligned with the carbon brushes, then contacts are broken and the current is temporarily cut-off.

However, the coil keeps on rotating in the same direction due to its inertia until the split ring commutator and the carbon brushes are in contact again.

Split rings acts as commutator and its function is to reverse the direction of current flowing though the coil.

Ans. The speed of rotation of the motor can be increased by

• Increasing the strength of the current in the coil
• Increasing the number of turns in the coil.
• Increasing the area of the coil
• Increasing the strength of magnetic field.

Ans. Electric motor is used in electric fans, refrigerators, mixers, washing machine, computers, MP3 player, etc.

Ans. No, induced current will be produced in the loop as the constant current flowing in the straight wire produces a constant magnetic field. Hence, no induced current is produced in the loop.

Ans. Since, current in the straight wire is changing, hence induced current will be produced in it. According to Fleming’s right hand rule, the current flowing in the loop will be in clockwise direction.

Ans. The observation that can be noted from the galvanometer reading are

(i) There are momentary galvanometer deflections that die out shortly.

(ii) The deflections are in opposite directions.

Galvanometer gets deflection in one direction

Galvanometer gets deflection in opposite direction of the first one.

Galvanometer shown no deflection.

Phenomenon involved is electromagnetic induction.

Ans. When a magnetic compass needle pointing North and South in the absence of a nearly magnet or a current loop, it is acting upon by the earth’s magnetic field only. But in the presence of a magnet or a current loop (which also has a magnetic field) the earth’s magnetic field near the compass is modified and the needle is deflected from North and South directions.

The salient features of magnetic field lines are

(i) A magnetic field line is directed from North-pole to South-pole outside the magnet.

(ii) A magnetic field line is a closed and continuous curve.

(iii) The magnetic field lines never intersected each other, because if two lines meet, it means that force is acting in two directions at that point which is not possible.

(iv) Closer the field lines, stronger is the magnetic field and vice-versa is also true.

(v) Magnetic field lines are parallel and equidistant, those represent a uniform magnetic field.

Ans. According to right hand thumb rule, the magnetic field produced by PQ at point R is into the plane of the paper and at point S is out of the plane of paper.

Ans. Here, r₁ > r₂

The magnetic field will be larger at point S as compared to that at point R.

This is because the magnetic field produced by a straight current-carrying conductor is inversely proportional to the distance from the wire. So, the magnetic field will be larger at the point which is nearer to the conductor.

As, point S is nearer to the conductor as compared to point R. So, field at S > field at R.

Ans. If the polarity of the battery is reversed, the current will be going from top to bottom in the wire and the magnetic field lines will now be in the clockwise direction on the plane, which is perpendicular to the wire carrying current.

Maxwell’s right hand thumb rule is used to find the direction of the magnetic field for a straight current carrying conductor.

This law states that, if you hold the current carrying straight wire in the grip of your right hand in such a way that the stretched thumb points in the direction of current, then the direction of the curl of the fingers will give the direction of the magnetic field.

A solenoid is defined as a coil consisting of a large number of circular turns of insulated copper wire. These turns are wrapped closely to form a cylinder.

Distinguishing features are as follow:

• Magnetic field outside the solenoid is negligible as compared to the bar magnet.
• Magnetic field of solenoid can be varied as per our requirement just by changing current or core of solenoid but in bar magnet it is fixed.

Fleming’s left hand rule states that, if the forefinger, thumb and middle finger of left hand are stretched mutually perpendicular to each other, such that the forefinger points along the direction of external magnetic field, middle finger indicates the direction of current, then the thumb points towards the direction of force acting on the conductor.

Electric motor is based on the principle that, when a rectangular coil is placed in a magnetic field and current is passed through it, two equal and opposite forces act on the coil which rotate it continuously.

It is a coil would over a soft iron core. It rotates in magnetic field, when a current flows through it.

They provide a sliding contact and facilitate current through armature while it rotates in field.

It ensures unidirectional current through armature as it rotates in field.

An electric current produced in a closed circuit by a changing magnetic field is called an induced current. This phenomenon is called electromagnetic induction. An electric generator works on the basis of electromagnetic induction

Three different ways to produce induced current in a coil of wire are as following

(a) If a magnetic field is changed around a coil, then an induced current is set up in it. It can be done by taking a bar magnet and bringing it closer to the coil or taking it away from the coil.

(b) If a coil is moved in a magnetic field, then an induced current is set up in the coil.

(c) By changing the magnitude of current flowing through another coil kept close to the coil.