# IBDP Physics 5.1 – Electric fields : IB style Question Bank – SL Paper 1

### Question

Two charges and , each equal to 2 nC, are separated by a distance 3 m in a vacuum.

What is the electric force on  and the electric field due to at the position of ?

Ans: A

$$F_{Q_2}=k\frac{Q_1Q_2}{r^2}$$
$$=\frac{9\times 10^9 \times 2\times 10^{-9}\times 2\times 10^{-9}}{3^2}=4\times 10^{-9}$$
$$E_{Q_{21}}=\frac{F}{Q_1}=\frac{4\times 10^{-9}}{2\times 10^{-9}}=2\; NC^{-1}$$

### Question

A metal wire has n free charge carriers per unit volume. The charge on the carrier is q. What additional quantity is needed to determine the current per unit area in the wire?

A Cross-sectional area of the wire

B Drift speed of charge carriers

C Potential difference across the wire

D Resistivity of the metal

Ans: B

The uniform current I, flowing through the conductor is given by
I = n e A vd
where n = number of free electrons per unit volume,
A = area of cross-section,  vd = drift velocity
In vector form, $$\Rightarrow \vec{j} =-ne \vec{v_d}$$
The negative sign is because the direction of drift velocity of electron is opposite to $$\vec{j}$$

### Question

An electron is accelerated through a potential difference of 2.5 MV. What is the change in kinetic energy of the electron?

A.  0.4μJ

B.  0.4 nJ

C.  0.4 pJ

D.  0.4 fJ

### Markscheme

C

work done = change in kinetic energy

$$=qV = e V= 1.6\times 10^{-19}\times 2.5 \times 10^6 = 4 \times 10^{-13} =0.4 \times 10^{-12} =0.4 \; pJ$$

Question

Which of the following diagrams illustrates the electric field pattern of a negatively charged sphere?