Topic 4.3: Motion in Electromagnetic Fields Questions

A small charged sphere is held at rest in a uniform electric field.

State the formula for the electric force on the sphere, and state how the direction of this force compares with the field direction if the charge is negative.

An ion carrying charge 1.6 × 10⁻¹⁹ C moves at 5.0 × 10⁶ m s⁻¹ at right angles to a magnetic field of strength 0.30 T.

Calculate the magnitude of the magnetic force on the ion.

State the equation for the magnitude of the force on a straight current-carrying conductor in a magnetic field, and define each symbol in it.

A straight wire of length 0.30 m lies at right angles to a uniform magnetic field of flux density 0.45 T and carries a current of 8.0 A.

Calculate the magnitude of the force on the wire.

A charged particle is held stationary in a uniform magnetic field.

State the size of the magnetic force acting on it, and explain your answer.

A proton (charge 1.6 × 10⁻¹⁹ C, mass 1.7 × 10⁻²⁷ kg) is placed in a uniform field of strength 5.0 × 10⁶ N C⁻¹.

Estimate the order of magnitude of the proton's acceleration in the field.

A metal rod rests on two horizontal rails and carries a current flowing from the left rail to the right rail.

A uniform magnetic field points vertically downward, into the plane of the rails.

Use Fleming's left-hand rule to determine the direction of the force on the rod, and state what happens to that direction if the current is reversed.

A 0.40 m length of wire carries a current of 6.0 A through a uniform magnetic field of strength 0.25 T.

The current makes an angle of 30° with the field direction.

Calculate the force on the wire.

Two parallel plates are separated by 0.040 m and connected to a 600 V supply, producing a uniform field between them.

In a demonstration of the motor effect, a wire experiences a maximum force of 0.40 N when it carries a current of 5.0 A at right angles to a uniform magnetic field of strength 0.20 T.

Calculate the length of wire that lies within the field.

A horizontal wire passes through the gap of a horseshoe magnet, with the magnetic field directed horizontally from the north pole to the south pole.

The current in the wire flows horizontally and at right angles to the field.

Deduce the direction of the force on the wire, and describe how the force changes if the wire is rotated until the current flows along the field direction.

A magnetic field acts alone on a charged particle moving across it.

Explain why the magnetic force changes the particle's direction but never its speed.

An electron (charge 1.6 × 10⁻¹⁹ C, mass 9.1 × 10⁻³¹ kg) is placed in the uniform field between two parallel plates, where the field strength is 4.0 × 10³ N C⁻¹.

A proton enters a velocity selector moving more slowly than the selected speed v = E/B.

Deduce whether it is deflected, and state which of the two forces is responsible.

A velocity selector has parallel plates 2.0 cm apart with a potential difference of 600 V across them, and a magnetic field of 0.15 T crossed at right angles.

Show that the speed selected by this device is about 2 × 10⁵ m s⁻¹.

A proton travelling at 3.0 × 10⁶ m s⁻¹ enters a velocity selector with crossed fields.

The electric field between the plates is E = 6.0 × 10⁵ N C⁻¹.

Calculate the magnetic field strength B needed for the proton to pass through undeflected, and explain why a second proton entering at twice this speed is deflected and which force is responsible.

An alpha particle passes undeflected through a velocity selector in which the electric field is E = 2.7 × 10⁵ N C⁻¹.

The alpha particle is travelling at 1.5 × 10⁶ m s⁻¹.

Calculate the magnetic field strength in the selector.

In the same uniform field, a proton and an electron each experience an electric force of the same magnitude.

Explain why the electron has a much larger acceleration than the proton, and estimate roughly how many times larger it is.

A straight horizontal wire of length 0.15 m lies at right angles to a uniform magnetic field.

When it carries a current of 2.0 A the force on it is 0.090 N.

When the current is increased to 6.0 A (everything else unchanged), state the new force, and calculate the magnetic field strength B.

An alpha particle (charge +2e = 3.2 × 10⁻¹⁹ C, mass 6.6 × 10⁻²⁷ kg) is in a uniform field of strength 2.5 × 10⁴ N C⁻¹.

Show that the acceleration of the alpha particle in the field is approximately 1.2 × 10¹² m s⁻².