Chapter 16: Magnetic Fields

A magnetic field is the field of magnetic lines of force produced by current-carrying conductors or permanent magnets. Which statement about magnetic lines of force is not true?

• The magnetic lines of force always emanate from the North pole along a curved path outside the magnet to the South pole.

• The direction of magnetic lines of force inside the magnet is from North pole to South pole.

• The magnetic lines of force outside the magnet do not intersect with each other.

• The magnetic lines of force in a uniform magnetic field are parallel and equidistant.

A proton moving at a speed of 2.0 $\times10^7m\ s^{-1}$ enters a uniform magnetic field perpendicular to the field. If the magnetic flux density is 2.5 Tesla, what is the magnetic force acting on the proton?

• 3.2 $\times10^{-12}N$  

• 4.0 $\times10^{-12}N$  

• 7.5 $\times10^{-12}N$  

• 8.0 $\times10^{-12}N$  

An  $\alpha-$ particle moving with a speed of 3.4 $\times10^7$ m  $s^{-1}$ enters a magnetic field in a direction perpendicular to the field with flux density 1.2 T. The particle moves in a circle of radius 0.45 m. If the charge of the  $\alpha-$ particle is 3.2 $\times10^{-19}$ C, what is the mass of the  $\alpha-$ particle?

• 1.7 $\times10^{-27}kg$  

• 3.8 $\times10^{-27}kg$  

• 5.1 $\times10^{-27}kg$  

• 6.8 $\times10^{-27}kg$  

An electron is moving with a speed of 3.4 $\times10^7m\ s^{-1}$ perpendicular to a uniform magnetic field of flux density 2.0 T. If the mass of the electron is 9.0 $\times10^{-31}kg$ , calculate the acceleration of the electron in the magnetic field.

• 1.2 $\times10^9\ m\ s^{-2}$  

• 2.7 $\times10^9\ m\ s^{-2}$  

• 5.4 $\times10^9\ m\ s^{-2}$  

• 6.1 $\times10^9\ m\ s^{-2}$  

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An electron enters an electric field between two parallel charged plates with separation 15 mm as shown in the diagram. The velocity of the electron is 4.2 $\times10^6\ m\ s^{-1}$ . If the electron continues to move in a straight line without any deflection, what is the magnitude and direction of the magnetic field?

• 2.0 mT, perpendicular and out of the page.

• 4.8 mT, perpendicular and out of the page.

• 5.2 mT, perpendicular and into the page.

• 7.3 mT, perpendicular and into the page.

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Which of the following will not influence the magnetic force acting on a current-carrying conductor?

• The current flowing in the conductor

• Length of the conductor

• Magnetic field strength

• The speed of the conductor

A straight wire of length 12 cm carrying a current of 10 A is placed at an angle of $60\degree$ to the direction of the magnetic field. If the magnetic flux density is 5.0 $\times10^{-4}$ T, what is the force acting on the wire?  

• 1.2 $\times10^{-4\ }N$  

• 3.0 $\times10^{-4\ }N$  

• 5.2 $\times10^{-4\ }N$  

• 1.0 $\times10^{-3}N$  

A horizontal metal rod PQ of length L and mass m is suspended by two wires in a uniform magnetic field as shown in the diagram. T is the tension in each supporting wire. If a current I flows from end Q to end P, find the tension T.

• $\frac{mg}{2}$

• $\frac{BIL}{2}$

• $\frac{mg-BIL}{2}$

• $\frac{mg+BIL}{2}$

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Two long straight wires each carrying a current of 3 A are separated by a distance of 10 cm as shown in the diagram. Calculate the magnitude of the resultant magnetic flux density at point P.

• 5.0 $\times10^{-6}$ T

• 6.5 $\times10^{-6}$ T

• 2.5 $\times10^{-5}$ T

• 4.2 $\times10^{-5}$ T

A current of 20 mA flows in a circular coil of radius 40 mm as shown in the diagram. If the number of turns in the circular coil is 250, what is the magnetic flux density at the centre of the coil?

• 3.14 $\times10^{-5}$ T

• 4.28 $\times10^{-5}$ T

• 6.25 $\times10^{-5}$ T

• 7.85 $\times10^{-5}$ T