Motion in Electromagnetic Fields (DP IB Physics: HL): Exam Questions

A proton of mass m and electric charge q enters a region of magnetic field at point P and exits at point Q. The speed of the proton at P is v. The path followed by the proton is a quarter of a circle.

State and explain whether the speed of the proton at P is the same as the speed at Q.

Outline why the path of the proton is circular.  

Show that the radius of the circular path is given by , where is the magnetic flux density.

The speed of the proton is 3.2 × 10⁶ m s^-1 at P and the magnetic flux density is 0.21 T.

Show that the radius of the path is 16 cm.

The diagram shows a charged particle entering a region of magnetic field that is directed into the page.

The path of the particle is a quarter circle. 

State and explain whether the particle is positively or negatively charged.

The proton enters the region with a speed of 5.4 × 10⁶ m s^-1. The magnetic flux density of the field is 0.35 T. 

Calculate the radius of the circular path of the proton.

Calculate the time the proton spends in the region of the magnetic field.

The diagram shows the path of a charged particle passing through a thin metallic foil.

State and explain the direction of motion of the particle.

An electron enters a uniform electric field between two charged parallel plates. The shaded region represents a uniform magnetic field which is perpendicular to the direction of motion of the electron.

The magnetic field is adjusted until the electron travels through the region undeflected.

The following data are available.

• Separation of the plates = 5.0 cm

• Potential difference between the plates = 1.3 kV

• Speed of the electron = 3.0 × 10⁵ m s^–1

(i) State the direction of the magnetic field

[1]

(ii) Determine the strength of the magnetic field.

[2]

State and explain whether a proton travelling with the same speed through the plates experiences a net force.

Another electron enters the region with a lower speed.

Explain the effect that this will have on the path of the electron.

In the national grid electricity is generated by current carrying wires within a magnetic field. 

There are two wires carrying equal currents into the page.

Determine the direction of the magnetic field at point X.    

Use a diagram to help you with your answer.

A bar magnet is placed in a uniform magnetic field as shown in the diagram.

Suggest whether there is a net force on the bar magnet, causing it to move to a different position. Explain your answer.

The bar magnet does move in a specific type of motion. Determine how it will move.

A taut electrical wire carries electricity between a generator and the step-up transformer. The current is 2000 A and the magnetic field of the Earth at the position of the wire is 4.50 × 10^–5 T and makes an angle of 45⁰ below the horizontal. 

Calculate the force experienced by a 25.0 m length of this wire.

Alpha particles travel in a vacuum at speed v and enter an area where there is a uniform magnetic field of flux density B. In this area, it begins to move in a circular trajectory. 

Show that the momentum of a single alpha particle is given by:

where e is the elementary charge and r is the orbital radius.   

An alpha particle moves across the Earth's equator towards the east. At this point, the Earth's magnetic field has a direction due north and is parallel to the surface.

Determine the direction of the force acting on the alpha particle at this instant. 

Charged particles from the Sun, carried by the 'solar wind', may become trapped in the Earth's magnetic field near its poles, causing the sky to glow. Some of these charged particles travel at a speed of 750 km s^-1 in a circle of radius 130 m in a region where the flux density is 6.0 × 10^–5 T.

Deduce whether the charged particles are protons or electrons.

Two electrons, X and Y, enter a uniform magnetic field with the same speed.

X enters the field at an angle of 30° and experiences a magnetic force . Y travels perpendicular to the field and experiences a magnetic force .

Determine the ratio .

Using a suitable calculation, compare and contrast the motion of X and Y as they travel in the field.

A cylindrical aluminium bar XY of mass 5.0 g rests on two horizontal aluminium rails, separated by 5.0 cm. 

The rails can be connected to a battery to drive a current of 4.5 A through XY. A magnetic field of flux density 0.15 T acts into the screen. 

Calculate the angle to the horizontal to which the rails must be tilted in order to keep XY stationary. 

A similar metal rod is suspended in a magnetic field by two identical, vertical springs. The cell and the rod have negligible internal resistance. 

When the switch S is closed, the metal rod is displaced a distance x from its starting point. 

Show that, when both the spring constant and electrical resistance of each spring is doubled, closing S would cause the rod to be displaced by . 

A beam of electrons, each travelling at various speeds, passes through a hole in plate P₁. P₂ is parallel to P₁, also with a hole in it. The region between the plates contains a uniform electric field and a uniform magnetic field. Both the electric field strength E and the magnetic flux density B are adjustable. 

Electrons that are undeviated travel with a particular speed v along the straight line joining the holes in P₁ and P₂. 

Deduce the direction of the electric field between the plates.

Mark with an X a position on P₂ that would indicate where electrons with a speed greater than v may strike P₂. 

The equipment is adjusted such that a single electron is shot with kinetic energy K through the hole in P₁. The distance between the plates d is fixed, and electric field is switched off, such that the electron is incident in a region of uniform magnetic flux density B only. 

Show that the maximum magnetic flux density B_max that ensures the electron reaches P₂ is given by:

where m_e is the rest mass of the electron and e is its charge. 

The image shows the main features of a loudspeaker L. A current-carrying coil is positioned within the magnetic field provided by a permanent magnet, and the current directions in the coil at a particular instant is shown. 

The dust cap D prevents dust from blocking the gap between the cardboard tube and the south pole of the magnet. 

Identify, on the diagram, the direction of the force on the coil at this particular instant with the current directions shown. 

Describe how the magnitude and direction of the force on the coil varies over a complete cycle. 

The coil consists of 200 turns, each of average diameter 2.0 cm. The magnetic flux density created by the permanent magnet is 0.40 mT. The peak current in the coil is 0.48 mA.

Calculate the maximum magnetic force on the coil.