IB Physics HL D.2 Electric and magnetic fields questions

Exam-style multiple choice and written questions with full markschemes. Check multiple choice answers straight away, or type, photograph or draw a written answer and get it marked.

Type

Multiple choice Written

Difficulty

Easy Medium Hard

Question 1

Multiple choiceMedium1 mark

Two fixed point charges, $+Q$ and $-Q$ are separated by $2a$ where $Q>0$ Point $P$ is midway between them. Let $k$ be Coulomb’s constant. Which row gives the electric potential at $P$ and the location chosen as the zero of electric potential?

	Electric potential at $P$	Zero-potential reference
A.	$0$	infinity
B.	$\dfrac{2kQ}{a}$	infinity
C.	$0$	$P$
D.	$\dfrac{kQ}{a}$	infinity

Show answer and explanation

Answer: A. For a point charge, $V=kq/r$ when the potential is zero at infinity. At $P$ the contributions are $+kQ/a$ and $-kQ/a$ so their scalar sum is $V(P)=0$ The reference location for zero potential is infinity.

B: adds the magnitudes of the two contributions, ignoring the negative charge’s sign.
C: confuses a point where the net potential is zero with the location chosen as the zero reference.
D: includes the contribution from $+Q$ but omits the equal contribution from $-Q$

Remember: electric potentials add as signed scalars.

Question 2

Paper 2Hard10 marks

In a simplified Millikan oil-drop experiment, negatively charged drops are held stationary between parallel plates. The polarity is chosen so that the electric force on a drop is upward. Assume buoyancy and other forces are negligible. The potential difference between the plates is $400\ \mathrm{V}$ and their separation is $4.00\ \mathrm{mm}$

Drop A has mass $4.90\times10^{-15}\ \mathrm{kg}$ Further drops are measured using the same method. The uncertainty in each charge estimate, including that for drop A, is $\pm0.08\times10^{-19}\ \mathrm{C}$ An independently calibrated candidate value is $e_0=1.60\times10^{-19}\ \mathrm{C}$ treat its uncertainty as negligible.

Drop	Measured charge magnitude / $10^{-19}\ \mathrm{C}$
B	3.18
C	4.79
D	6.39
E	8.01
F	5.55

(a)

Show that the magnitude $q$ of the charge on a stationary drop is given by

q=\frac{mgd}{V},

where $m$ is the drop mass, $g$ is gravitational field strength, $d$ is the plate separation and $V$ is the potential difference.

[2]

(b)

Calculate the magnitude of the charge on drop A. Give your answer in coulombs.

[2]

(c)

Using $e_0$ and your answer to (b), determine the nearest integer $n$ for each of drops A–E, where $q\approx ne_0$

[2]

(d)

A student claims that the results for drops A–E establish that $e_0$ is the smallest possible charge, while drop F disproves charge quantization. Evaluate this claim using the measurements and their uncertainties.

[4]

The best questions are still waiting

Unlock every question in this topic, then type, photograph or draw your answer and have it marked against the markscheme. Find out where you are dropping marks.

Unlock all questions Compare Student and Student Plus