# Magnetic effects of current: Short Questions Answers Physics 12

## Magnetic effect of electric current

#### 1. An alpha particle moving in a straight line enters a strong magnetic field in a direction parallel to the field. What will be the change in speed and the direction of motion on entering the magnetic field?

If a charged particle of charge q is moving with a velocity v inside the magnetic field of strength B making an angle 0 with the direction:f magnetic field, it experiences a force given by F = B qv sinθ. For an alpha particle q = +2e, therefore, F = 2Bev sinθ.

When the alpha particle enters parallel to B,  θ = 0°. Hence F = 0. As no force is experienced by the particle, there is no change in the speed and the direction of motion of the alpha particle.

#### 2. Equal currents I am flowing through two infinitely long parallel wires. Will there be a magnetic field at exactly halfway between the wires when the current is (a) in the same direction? (b) in the opposite direction?

(a) The magnetic fields at the point P halfway between the wires due to long conductors are given by B1 = B2 = μoI/ 2πr But B1 is inward the plane of the page and B2 is outward the plane of the page according to right-hand thumb rate, Hence net magnetic field at P is zero.

(b) When the current is in opposite direction then the same magnitude of the magnetic field is produced by the wires in the same direction (i.e. inward the plane of the page). Hence, the magnetic field at the point exactly halfway between the wires will be double that due to one wire. i.e.
B = 2μoI/2πr

#### 3. A uniform magnetic field and a uniform electric field are produced in the same direction. An electron is projected with its velocity in the same direction, how will the velocity affect?

An electron of charge e moving with a velocity v in the uniform electric field of intensity E experiences a force of Fe = eE. Similarly, the electron in the uniform magnetic field experiences a force of Fm = Bev sinθ. where θ is the angle between the direction of motion of the electron and the applied magnetic field.

If the electron is moving parallel to the direction of the magnetic field, θ = 0 and hence Fm = 0. But the electron experiences electric force Fe in the direction opposite to that of the direction of the electric field. Then the electron will be retarded as the electric force acts in the opposite direction of its motion.

#### 4. State right-hand thumb rule

The direction of the magnetic field produced by a straight current-carrying conductor is found by the right-hand thumb rule. According to this rule, if a current-carrying straight conductor is held in the right hand so that the thumb points in the direction of flow of current, the tips of the curled fingers encircling the conductor will give the direction of the magnetic lines of force.

#### 5. Why does a current-carrying conductor inside a magnetic field experience a force?

A conductor consists of a large number of free electrons which drift along the conductor when current passes through it. When a moving charge is placed inside a magnetic field, it experiences a force. So, if such a current-carrying conductor is placed in a magnetic field, each of the free electrons experiences a Lorentz’s force. Since the electrons are bound in the conductor, the conductor itself experiences a force. Hence a current-carrying conductor when placed in a magnetic field experiences a force.

#### 6. How do you find the direction of Lorentz’s force on a current-carrying conductor moving in a uniform magnetic field? OR State Fleming’s left-hand rule.

To find the direction of Lorentz’s force, Fleming’s left-hand rule is applied. It states that if the forefinger, central finger, and the thumb of the left hand are stretched mutually perpendicular to each other. Such that the forefinger points in the direction of the magnetic field (B), the central finger in the direction of current (I) then the thumb points is the direction of the force (F) experienced by the conductor.

#### 7. Explain what do you mean by the sensitivity of the moving coil galvanometer?

The sensitivity of a galvanometer is the deflection produced in it per unit current or voltage. The deflection per unit current passed through the galvanometer is called its Current Sensitivity. If θ is the deflection produced when a current I is passed through the galvanometer,

the current sensitivity = θ/I= BAN/C,

Where C is the torsion constant, N is the number of turns and A is the area of the coil. Similarly, voltage sensitivity is the deflection per unit difference in potential across the galvanometer. If θ is the deflection in the voltmeter when the p.d. of V volt is applied, then the

voltage sensitivity = θ/V = θ/IR = NAB/CR

Where R is the coil resistance of the galvanometer.

#### 8. What do you mean by a radial magnetic field?

The magnetic field in which the plane of the rectangular coil of the galvanometer always lies parallel to the magnetic lines of force is called Radial Magnetic Field. In practice, concave pole pieces of a magnet are used to produce a radial magnetic field. The magnetic lines of force are along the radii of the concave poles of the magnet and are normal to the surfaces.

#### 9. How do you define the galvanometer constant?

Galvanometer constant is defined as the couple produced on the coil when unit current flows in the coil. For a given galvanometer with a coil of area A, a number of turns N, and torsion constant C in a magnetic field B, the quantity C/BAN is constant and is called the Galvanometer Constant.

#### 10. What do you mean by the current reduction factor of a galvanometer?

The current reduction factor of a galvanometer is the value of current required to produce one-millimeter deflection on the scale when placed at a distance of one meter from the mirror. It is the reciprocal of the current sensitivity.

#### 11. What is the figure of merit of a galvanometer?

The figure of merit is defined as the current required to produce a deflection of 1 mm on a scale placed one meter away from the mirror of the galvanometer. OR
The figure of merit is defined as the current which causes a deflection of one scale division. If a current I produces a deflection of θ divisions, then figure of merit = I/θ

#### 12. What is the function of the spring connected to the lower end of the coil of the galvanometer?

The lower end of the coil is connected to the spring which is a coil of a very fine strip having a large number of turns. This is used to provide the restoring couple on the coil when it is deflected from its mean position of rest. By taking in the form of strips and having a large number of turns, the controlling couple is made small.

#### 13. Why the suspension fiber of phosphor bronze is taken in the form of a strip and not a wire?

Phosphor bronze is an alloy of copper (92.5%), tin (7%), and phosphorous (5%). The couple for a unit twist in the case of a strip of a rectangular cross-section is very much smaller than in a wire of circular cross-section. Hence for a given couple, the deflection obtained by a strip will be greater. It also provides a larger surface area for getting cooled if heated by the current.

#### 14. A non-uniform magnetic field that varies in magnitude from point to point but has a constant direction is set up in a region of space. A charged particle enters that region and travels undeflected in a straight line with a constant speed. What can you say about the direction of the initial velocity of the particle?

Since the direction of the velocity of the particle remains unchanged, no magnetic force acts on the particle. The force experienced by a particle of charge q moving with a velocity v in a magnetic field B is F= q (v x B) Since F = 0, v x B = 0. i.e. v and B are parallel to each other. Thus, the initial velocity of the particle is either along the direction of the field or opposite to it.

Note: F, v and B are vectors

#### 15. we define electric field E as the force acting on a test charge as F= q0E. Why is B not defined in this manner?

We can have an isolated electric charge for which the electric field can be defined as the force acting on a test charge as F = qo E. But we cannot have an isolated magnetic pole. For this reason, we cannot define B in the manner we define E.

Note: E, F & B are vectors