Question 1Multiple choice · 1 markA* challenge
Point charges of +2 μC and +1 μC are 0.4 m apart.
What is the electric potential at the midpoint between them?
- A 45 000 V
- B 67 000 V
- C 1.3 × 105 V
- D 6.7 × 105 V
Show answer
Answer: C
Free D.2 Electric & magnetic fields questions for IB Physics (SL & HL), part of D Fields. Each question has the answer and the marking points an examiner would look for.
Generate a full D.2 Electric & Magnetic Fields worksheet →All D Fields questionsPoint charges of +2 μC and +1 μC are 0.4 m apart.
What is the electric potential at the midpoint between them?
Answer: C
Which statements about electric field lines are correct?
I. They are perpendicular to equipotential surfaces.
II. A positive charge always moves along a field line.
III. They never cross.
Answer: B
Two parallel plates 4.0 cm apart have a potential difference of 2000 V between them.
Calculate the electric field strength between the plates.
An electron is released from rest at the negative plate. Calculate its speed when it reaches the positive plate.
A proton is released from rest at the positive plate. Compare the time it takes to cross with that of the electron.
The plate separation is doubled with the same potential difference. State and explain the effect on the final speed of the electron.
| Part | Answer and marking points | Marks |
|---|---|---|
| (a) | 5.0 × 104 V m−1 E = 2000/0.040 = 5.0 × 104 V m−1 ✓ | 1 |
| (b) | 2.7 × 107 m s−1 eV = ½mv2 ✓ v = 2.65 × 107 m s−1 ✓ | 2 |
| (c) | about 43 × longer the same size of force acts on both ✓ the proton’s mass is about 1840 times greater so its acceleration is smaller ✓ time ∝ √m so about 43 times longer ✓ | 3 |
| (d) | unchanged work done = eV is unchanged ✓ so the speed is unchanged ✓ | 2 |
A point charge of +2.0 nC and a point charge of −2.0 nC are 0.20 m apart. M is the midpoint between them.
Calculate the electric field strength at M.
Calculate the electric potential at M.
Calculate the work done in moving a +1.0 nC charge from infinity to M.
Outline why the electric field strength at M is not zero although a point between the charges might be expected to be one.
| Part | Answer and marking points | Marks |
|---|---|---|
| (a) | 3600 N C−1 fields from both charges point the same way at M ✓ E = kq/r2 for each with r = 0.10 m ✓ total = 3600 N C−1 ✓ | 3 |
| (b) | 0 V V = Σkq/r ✓ = 0 V ✓ | 2 |
| (c) | 0 J W = qV = 0 J ✓ | 1 |
| (d) | opposite charges, fields add between opposite charges the fields from each charge are in the same direction ✓ so they add rather than cancel ✓ | 2 |
A straight wire of length 0.25 m carries a current of 3.0 A in a uniform magnetic field of 0.40 T.
Calculate the force on the wire when it is perpendicular to the field.
Calculate the force when the wire is at 30° to the field.
Calculate the magnetic field strength 5.0 cm from a long straight wire carrying 3.0 A.
Explain why two parallel wires carrying currents in the same direction attract.
| Part | Answer and marking points | Marks |
|---|---|---|
| (a) | 0.30 N F = BIL = 0.30 N ✓ | 1 |
| (b) | 0.15 N F = BIL sin30° = 0.15 N ✓ | 1 |
| (c) | 1.2 × 10−5 T B = μ0I/2πr ✓ = 1.2 × 10−5 T ✓ | 2 |
| (d) | each in other’s field each wire is in the magnetic field of the other ✓ the force from F = BIL on each is towards the other wire ✓ | 2 |
Two point charges of +4.0 μC and −2.0 μC are 0.30 m apart.
Calculate the force between the charges.
Calculate the electric field strength at the midpoint.
Sketch the electric field pattern around the two charges.
| Part | Answer and marking points | Marks |
|---|---|---|
| (a) | 0.80 N attractive F = kq₁q₂/r2 ✓ 0.80 N (attractive) ✓ | 2 |
| (b) | 2.4 × 106 N C−1 fields add (same direction) ✓ 2.4 × 106 N C−1 ✓ | 2 |
| (c) | lines from + to −; more from + lines start on + and end on − ✓ twice as many lines from +4 μC ✓ | 2 |
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