A Level Physics 18.1 Electric Fields and Field Lines MCQs – 30 Practice Questions with Answers (Cambridge 9702)
This page covers Section 18.1 of Cambridge AS and A Level Physics (9702), an A Level topic introducing electric fields and the field line patterns used to represent them. Work through 30 original practice questions on the definition of electric field strength, field patterns for charges and plates, and simple calculations with E = F/Q, each with a worked answer.
Formula recap: Electric field strength E = F/Q, the force per unit positive charge, measured in N/C (equivalently V/m). The force on a charge in a field is F = EQ, in the direction of E for a positive charge and opposite to E for a negative charge. See the full A Level Physics Formula Sheet for all the equations for this topic.
Electric Field Concepts
Q1. An electric field is best described as a region of space in which
- (A) only a magnet can exert a force
- (B) an electric charge experiences a force
- (C) there is no matter present at all
- (D) a mass experiences a gravitational force
Show Answer
Answer: B – An electric field exists at any point where an electric charge placed there would experience an electric force. A mass experiencing a force belongs to the idea of a gravitational field.
Q2. Electric field strength at a point is defined as
- (A) the product of force and charge
- (B) the charge divided by the force
- (C) the force per unit positive charge placed at the point
- (D) the work done per unit distance moved
Show Answer
Answer: C – By definition E = F/Q: the force that would act on a small positive charge divided by the size of that charge.
Q3. The unit of electric field strength is
- (A) N/C
- (B) C/N
- (C) J/C
- (D) N m
Show Answer
Answer: A – From E = F/Q the unit is newtons per coulomb, written N C−1. J/C is the volt, the unit of potential, not of field strength.
Q4. The direction of the electric field at a point is defined as the direction of
- (A) the force on an electron placed at the point
- (B) the force on a negative charge placed at the point
- (C) a line drawn from a negative charge to a positive charge
- (D) the force on a positive charge placed at the point
Show Answer
Answer: D – E is a vector pointing the way a positive charge would be pushed. An electron, being negative, would be pushed in exactly the opposite direction.
Q5. A negative charge is placed at a point in an electric field. The force on the charge acts
- (A) in the same direction as the field
- (B) in the opposite direction to the field
- (C) at right angles to the field
- (D) with zero size, because negative charges are unaffected by fields
Show Answer
Answer: B – In F = EQ, a negative Q reverses the direction of the force, so a negative charge is forced against the field direction.
Q6. Electric field strength can also be measured in the equivalent unit
- (A) J/m
- (B) W/C
- (C) V/m
- (D) A/m
Show Answer
Answer: C – The volt per metre (V m−1) is equivalent to the newton per coulomb; the connection is shown for uniform fields in Section 18.2.
Q7. Which comparison of gravitational field strength g and electric field strength E is correct?
- (A) g is force per unit mass; E is force per unit positive charge
- (B) g and E are both force per unit charge
- (C) g is force per unit charge; E is force per unit mass
- (D) g and E are both scalars
Show Answer
Answer: A – Each field strength is the force per unit of the quantity the field acts on: mass for gravity (g = F/m, in N/kg) and positive charge for electricity (E = F/Q, in N/C). Both are vectors.
Q8. A key difference between electric and gravitational fields is that
- (A) electric fields can only exist near the Earth
- (B) gravitational fields cannot act through a vacuum
- (C) electric field strength is measured in N/kg
- (D) an electric force on a charge can be attractive or repulsive, while the gravitational force between masses is always attractive
Show Answer
Answer: D – Charges come in two signs, so electric forces can attract or repel. Mass has only one sign, so gravity between masses always attracts.
Q9. A charge Q placed in an electric field E experiences a force F. A charge of 2Q is now placed at the same point. The force it experiences, and the field strength at the point, are
- (A) F/2, and E is halved
- (B) 2F, and E is unchanged
- (C) 2F, and E is doubled
- (D) F, because the field is used up by the first charge
Show Answer
Answer: B – F = EQ, so doubling the charge doubles the force to 2F. E is fixed by the charges creating the field and does not depend on the test charge placed in it.
Q10. The electric field strength at a point in space depends on
- (A) the size of the test charge used to measure it
- (B) whether a test charge is present at the point
- (C) the charges creating the field and the position of the point
- (D) the mass of the test charge used to measure it
Show Answer
Answer: C – The field is a property of the source charges and of position. A test charge only reveals the field; the field exists at the point whether or not a charge is placed there.
Field Lines and Patterns
Q11. The arrows on electric field lines show the direction in which
- (A) a free positive charge would move
- (B) a free negative charge would move
- (C) a compass needle would point
- (D) energy flows out of a battery
Show Answer
Answer: A – Field lines are drawn from positive charge towards negative charge, in the direction of the force on a positive charge.
Q12. The field pattern around an isolated positive point charge consists of lines that are
- (A) radially inwards
- (B) concentric circles around the charge
- (C) straight and parallel
- (D) radially outwards
Show Answer
Answer: D – A positive charge pushes a positive test charge away in every direction, so the lines spread radially outwards and get further apart (weaker field) with distance.
Q13. The field pattern around an isolated negative point charge consists of lines that are
- (A) radially outwards
- (B) radially inwards
- (C) concentric circles around the charge
- (D) absent, because a negative charge has no field
Show Answer
Answer: B – A positive test charge is attracted towards a negative charge from every direction, so the arrows point radially inwards towards the charge.
Q14. Two equal positive charges are held a short distance apart. At the point midway between them,
- (A) the field lines cross each other
- (B) the field is strongest, because the fields add
- (C) the resultant field is zero, because the two fields are equal and opposite there
- (D) the charges attract each other
Show Answer
Answer: C – At the midpoint the field from each charge has the same size but points in opposite directions, so they cancel. This neutral point is a feature of the pattern for two like charges; the charges themselves repel.
Q15. For a positive charge and a negative charge of equal size placed near each other (a dipole), the field lines
- (A) leave the positive charge and end on the negative charge
- (B) form closed circles around both charges
- (C) never connect the two charges
- (D) are absent between the charges, where the field is zero
Show Answer
Answer: A – Lines begin on the positive charge and end on the negative charge. Between the charges the fields of both point the same way, so the field there is strong, not zero.
Q16. Between two oppositely charged parallel plates, the electric field is
- (A) radial, spreading out from the centre
- (B) zero everywhere, because the charges cancel
- (C) strongest in the middle and zero at the edges
- (D) uniform in the central region, shown by parallel equally spaced lines, with fringing (bulging) near the edges
Show Answer
Answer: D – Away from the edges the lines are straight, parallel and evenly spaced, meaning the same field strength everywhere: a uniform field. Near the edges the lines bulge outwards.
Q17. Electric field lines never cross each other because
- (A) the lines repel one another
- (B) at any point the field has only one direction, while crossing lines would mean two directions at that point
- (C) the lines are made of a material that cannot bend back
- (D) charges at the crossing would cancel out
Show Answer
Answer: B – A field line shows the direction of the field. If two lines crossed, the field at that point would point two ways at once, which is impossible.
Q18. On a field line diagram, the field is strongest where the lines are
- (A) furthest apart
- (B) missing altogether
- (C) closest together
- (D) drawn the longest
Show Answer
Answer: C – Line spacing represents strength: closely spaced lines mean a strong field and widely spaced lines mean a weak field.
Q19. Electric field lines always
- (A) start on positive charges and end on negative charges
- (B) start on negative charges and end on positive charges
- (C) form closed loops with no beginning or end
- (D) exist only inside solid conductors
Show Answer
Answer: A – Lines begin on positive charge and end on negative charge; if there is no negative charge nearby they continue out to infinity. Unlike magnetic field lines, they do not form closed loops.
Q20. The tangent to a field line at a point gives
- (A) the velocity of any charge at that point
- (B) the direction of the force on a negative charge at that point
- (C) the value of the field strength at that point
- (D) the direction of the force on a positive charge at that point
Show Answer
Answer: D – The tangent shows the direction of E, and so of the force on a positive charge. The path a moving charge actually follows also depends on its velocity, so it does not simply run along the line.
Using Fields and Field Strength
Q21. A charge of 2.0 × 10−6 C is placed in an electric field of strength 400 N/C. The force on the charge is
- (A) 2.0 × 10−4 N
- (B) 8.0 × 10−4 N
- (C) 2.0 × 108 N
- (D) 5.0 × 10−9 N
Show Answer
Answer: B – F = EQ = 400 × 2.0 × 10−6 = 8.0 × 10−4 N.
Q22. A small charge placed in an electric field of strength 1.5 × 103 N/C experiences a force of 3.0 × 10−3 N. The size of the charge is
- (A) 4.5 C
- (B) 5.0 × 10−7 C
- (C) 2.0 × 10−6 C
- (D) 2.0 × 106 C
Show Answer
Answer: C – Q = F/E = (3.0 × 10−3) / (1.5 × 103) = 2.0 × 10−6 C.
Q23. A charge of 3.0 × 10−9 C experiences a force of 6.0 × 10−5 N at a point in an electric field. The field strength at that point is
- (A) 2.0 × 104 N/C
- (B) 1.8 × 10−13 N/C
- (C) 5.0 × 10−5 N/C
- (D) 2.0 × 10−4 N/C
Show Answer
Answer: A – E = F/Q = (6.0 × 10−5) / (3.0 × 10−9) = 2.0 × 104 N/C.
Q24. An electric field at a point points horizontally to the right. An electron released at that point will begin to move
- (A) to the right, along the field
- (B) vertically downwards, under gravity only
- (C) at right angles to the field
- (D) to the left, opposite to the field
Show Answer
Answer: D – The electron carries a negative charge, so the force on it is opposite to E: it accelerates to the left, against the field direction.
Q25. A proton is placed at a point in an electric field. The force on the proton acts
- (A) opposite to the direction of the field
- (B) in the direction of the field
- (C) in a circle around the field lines
- (D) with zero size, because protons are inside nuclei
Show Answer
Answer: B – The proton is a positive charge, and by definition a positive charge is pushed in the direction of the field.
Q26. A proton (charge +e) and an alpha particle (charge +2e) are placed separately at the same point in an electric field. Compared with the force on the proton, the force on the alpha particle is
- (A) the same
- (B) half as large
- (C) twice as large
- (D) four times as large
Show Answer
Answer: C – F = EQ at the same point means the same E, so the force is in proportion to the charge: +2e gives twice the force of +e.
Q27. Two regions of an electric field are drawn. In region X the field lines are closely spaced; in region Y the field lines are widely spaced. The field is stronger in
- (A) region Y, because more space between lines means a stronger field
- (B) neither region, because straight lines mean equal strength
- (C) region Y, because wide spacing shows the field reaching further
- (D) region X, because closer line spacing shows a stronger field
Show Answer
Answer: D – Line density represents strength. The closely spaced lines in region X show the stronger field; the spread-out lines in Y show a weaker field.
Q28. In fair weather there is an electric field of about 100 N/C pointing vertically downwards near the surface of the Earth, caused by charge separation between the ground and the upper atmosphere. A small positive charge held near the surface experiences a force that is
- (A) vertically downwards
- (B) vertically upwards
- (C) horizontal
- (D) zero, because the field is a natural one
Show Answer
Answer: A – A positive charge is forced in the direction of the field, so downwards. (An electron at the same place would be forced upwards.)
Q29. A metal conductor is charged and left until everything is steady (electrostatic equilibrium). The electric field inside the material of the conductor is
- (A) largest at the centre of the conductor
- (B) the same as the field just outside the surface
- (C) radial, pointing outwards from the centre
- (D) zero
Show Answer
Answer: D – If any field remained inside, the free charges in the metal would keep moving. They settle on the surface in positions that cancel the internal field exactly, so in equilibrium E = 0 inside the conductor.
Q30. A small positive charge is released from rest in a uniform electric field. The charge will
- (A) move with a constant velocity along the field
- (B) accelerate along the direction of the field
- (C) accelerate against the direction of the field
- (D) stay at rest, because it starts with no velocity
Show Answer
Answer: B – The field exerts a constant force F = EQ, so the charge has a constant acceleration a = EQ/m in the field direction, and its speed grows as it moves.
Keep practising: revise the rest of A Level Physics and work through more topic questions on the Physics MCQs hub, and keep the A Level Physics Formula Sheet open while you study this topic.
These are original practice questions written for revision purposes only; they are not Cambridge past-paper questions. QuaidianPoint is not affiliated with or endorsed by Cambridge Assessment International Education.