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.

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.

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.

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.

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 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.

(a) Calculate the electric field strength between the plates.

(b) Calculate the electric force on a charge of 1.6 × 10⁻¹⁹ C placed in this field.

A wire of length 0.080 m carries a current of 2.5 A at right angles to a magnetic field of flux density 0.60 T.

(a) Calculate the force on the wire.

(b) The current is then tripled while everything else is kept the same. State the new force, justifying your answer.

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.

A straight metal rod rests across two horizontal parallel rails separated by 0.25 m.

A uniform magnetic field of magnitude 0.40 T is directed out of the page in the region of the rod.

A 3.0 A current is supplied to the rails, and the rod is observed to begin moving to the right (in the plane of the page).

What is the direction of the current in the rod, and the magnitude of the initial force on it?

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) State the direction of the electron's acceleration relative to the field.

(b) Calculate the electric force on the electron.

(c) Show that its acceleration is of the order 10¹⁵ m s⁻².

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.

A charged particle moving horizontally enters the gap between two charged parallel plates and is deflected so that it follows a curved path.

Outline why this path is a parabola, referring to the motion along the plates and the motion across the field.

Explain why a straight current-carrying wire experiences no force when it is placed so that the current flows parallel to the magnetic field.

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 electron and a singly-charged ion of much greater mass both enter the same velocity selector at the same speed, equal to the selected speed v = E/B.

Sketch and describe the path of each particle through the selector, and explain why.