IBDP Physics 5.1 – Electric fields : IB Style Question Bank HL Paper 1

Question

Two charges Q_1 and Q_2, each equal to 2 nC, are separated by a distance 3 m in a vacuum.

What is the electric force on Q_2 and the electric field due to Q_1 at the position of Q_2?

Answer/Explanation

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

An ion of charge +Q moves vertically upwards through a small distance s in a uniform vertical electric field. The electric field has a strength E and its direction is shown in the diagram.

What is the electric potential difference between the initial and final position of the ion?

A. \(\frac{{EQ}}{s}\)

B. EQs

C. Es

D. \(\frac{E}{s}\)

Answer/Explanation

Markscheme

C

RELATION BETWEEN ELECTRIC FIELD AND POTENTIAL

The relation between electric field (E) and potential (V) is
For 3-D we can write
, and
So electric field is equal to negative potential gradient.
In this relation negative sign indicates that in the direction of electric field, potential decreases. Consider two points A and B situated in a uniform electric field at a distance d then,
The potential difference between A and B is

Question

Two wires, X and Y, are made from the same metal. The wires are connected in series. The radius of X is twice that of Y. The carrier drift speed in X is vX and in Y it is vY.

What is the value of the ratio \(\frac{{{{\text{v}}_{\text{X}}}}}{{{{\text{v}}_{\text{Y}}}}}\)?

A. 0.25

B. 0.50

C. 2.00

D. 4.00

Answer/Explanation

Markscheme

A

\(i=neAv_d\)
Now \(x \) and \( y \) are in series , hence \( i_x=i_y\)
or  \(neA_xv_{d_x}= neA_yv_{d_y}\)
\(A_x=\pi (r_x)^2 \; ,\; A_y=\pi (r_y)^2\)
\(ne\pi (r_x)^2v_{d_x} =ne\pi (r_y)^2 v_{d_y}\)
\(\frac{v_{d_x}}{v_{d_y}}=\frac{(r_y)^2}{(r_x)^2}=\frac{(r_y)^2}{(2r_y)^2} =\frac{1}{4}=0.25\)

Question

Positive charge is uniformly distributed on a semi-circular plastic rod. What is the direction of the electric field strength at point S?

Answer/Explanation

Markscheme

B

The component along y axis ( Along A and Along C) are cancelled out and along x – axis ( along B) it is added up

Question

Electrons, each with a charge e, move with speed v along a metal wire. The electric current in the wire is I.

Plane P is perpendicular to the wire. How many electrons pass through plane P in each second?

A.  \(\frac{e}{I}\)

B.  \(\frac{{ve}}{I}\)

C.  \(\frac{I}{{ve}}\)

D.  \(\frac{I}{e}\)

Answer/Explanation

Markscheme

D

ELECTRIC CURRENT

It is the rate of flow of charge through any cross section. 
\(I=\frac{q}{t}\)
\(q= n\times e\)
\(t= 1 \) second
\(I = \frac{n\times e}{1}\)
\(\therefore n=\frac{I}{e}\)