Electric & Magnetic Fields (DP IB Physics: SL): Exam Questions

Two separated, identical conducting spheres X and Y of charge –8 μC and +12 μC respectively, are brought into brief contact and then separated. What is the final charge distribution on X and Y?

A point charge q is placed near a large spherical charge Q = 10q. What is the magnitude of the force experienced by q and Q and the magnitude of the electric field E created by Q at the position of q?

Two identical point charges q create a resultant electric field at X.

The line XY is a perpendicular bisector of the line joining both point charges. 

Which vector most accurately depicts the direction of the resultant electric field at X?

Two charges, Q₁ = q and Q₂ = 4q are separated by a distance r and exert a force of magnitude F on each other. By what factor does the magnitude of the force change if the charge on Q₁ doubles and the separation distance trebles?

A plotting compass is placed next to a vertical wire AB. When there is no current in the wire, the compass points North due to an external magnetic field.

Which diagram shows a possible direction for the compass to point when a current passes from A to B?

A potential difference is applied between two metal plates that are not parallel.

Which diagram shows the electric field between the plates?

Two point charges are at rest as shown. Four positions, each of distance r from the nearest point charge, are marked in the image. 

At which position is the electric field strength greatest?

Four point charges, W, X, Y and Z, are fixed to the edges of a square with midpoint O.

W, X and Z are negatively charged, and Y is positively charged. What is the direction of the resultant electric field at O? 

Two positively charged particles, q₁ and q₂, are released from rest half-way between two oppositely charged parallel plates in a vacuum. The particles strike the negatively charged plate at the same time. 

Neglecting gravitational effects, which of the following statements is correct? 

An ion of mass m kg carrying a charge of –Q pC travels horizontally with a kinetic energy of E_k = 10 eV. 

It enters a uniform vertical electric field of strength E kV m^–1, which is directed downwards and acts over a horizontal distance of 100 cm.   

Which row in the table below correctly shows the time for the ion to pass through the field, and the magnitude of the force on the ion?

Point charges, each of magnitude Q are placed at three corners of a square as shown in the diagram.

What is the direction of the resultant electric field at the corner with no charge?

Which one of the following cannot be used as a unit for electric field strength?

What is the electric field pattern between a conducting sphere and an earthed metal plate?

A +1.0 nC and −1.5 nC point charge are placed 6 cm apart at points P and Q. A point R is equidistant to both charges.

If an additional charge of +4.0 nC is supplied to both charges, what is the magnitude of the electric field at R and the direction of this field?

A student queries their answer after solving some physics problems. The teacher explains that the problem lies in the text book solutions, some of which are incorrect.

An extract from the mark scheme is shown below:

Without calculation, which answer does the teacher know to be an error?

Electric field strength is defined as 'the electric force per unit charge experienced by a small, positive point charge q'. 

Why is the test charge described as 'small'?

The electric potential at different points between two point charges is shown.

The electric field strength E varies along the line which connects P and Q.

Which graph correctly shows this variation, where x is the distance along the line PQ?

Point charges of +2.0 nC and −3.0 nC are arranged at the corners X and Y of a triangle with sides of 50 mm and 30 mm as shown. 

Which is the best estimate for the value of the ratio of the electric field at Z due to X and the electric field due to Y, ?

The diagram shows points X, Y and Z marked on a pattern of electric field lines. The spacing between X and Y and between Y and Z are the same.

The potential at X = 0 V and at Y is − 500 V.

What is a likely value for the potential at Z?

The magnetic field produced by a straight current carrying wire is in

A positive charge is placed at S and a negative charge is placed at T. The electric potential at different points between the charges is shown below.

Which graph correctly shows the variation with x along the line ST of the electric field strength, E?

A weighted, positively charged sphere is released from rest, in a vacuum, between two parallel, vertical metal plates one at +100 V and the other grounded. The sphere is initially 4.0 cm from the edge of the grounded plate and 3.0 cm from the bottom of the plates. The sphere takes one of the paths P, Q, R or S and reaches the end of its trajectory in 60 ms.

By choosing the correct path, what is the speed of the sphere at the end of its trajectory?