Electric and Magnetic FieldsEdexcel International A Level Physics: Topic test

20 questions, 54 marks

Edexcel International A Level Physics

Electric and Magnetic Fields topic test

Total 54 marks

Name

Class

Date

1
The metal dome of a small Van de Graaff generator has a radius of 0.15 m and carries a charge of +1.5 × 10⁻⁷ C. The dome is isolated in air and its charge may be treated as a point charge at its centre. Take 1/(4πε₀) = 8.99 × 10⁹ N m² C⁻².
(a)
What is the electric potential at the surface of the dome?
[1 mark]
A9.0 × 10³ V
B6.0 × 10⁴ V
C1.3 × 10³ V
D4.5 × 10³ V
(b)
What is the electric field strength at the surface of the dome?
[1 mark]
A9.0 × 10³ V m⁻¹
B1.3 × 10³ V m⁻¹
C3.0 × 10⁴ V m⁻¹
D6.0 × 10⁴ V m⁻¹
(c)
Sparking occurs in air when the field strength exceeds about 3.0 × 10⁶ V m⁻¹. State, with a calculation, whether a spark would form at the surface of the dome and explain what would have to change for a spark to occur.
[2 marks]
Total for question 1: 4 marks
2
A capacitor of capacitance 680 μF is charged until the potential difference across it is 15 V.
(a)
What is the charge stored on the capacitor?
[1 mark]
A4.5 × 10⁻⁵ C
B7.7 × 10⁻² C
C1.0 × 10⁻² C
D2.2 × 10⁴ C
(b)
What is the energy stored in the capacitor?
[1 mark]
A1.5 × 10⁻¹ J
B7.7 × 10⁻² J
C5.1 × 10⁻³ J
D1.0 × 10⁻² J
(c)
The p.d. across the capacitor is now increased from 15 V to 30 V. Calculate the additional energy that must be supplied to the capacitor.
[2 marks]
Total for question 2: 4 marks
3
A 330 μF capacitor is charged to a p.d. of 9.0 V and is then discharged through a 15 kΩ resistor.
(a)
Calculate the time constant of the circuit and the p.d. across the capacitor 10.0 s after the discharge begins.
[3 marks]
(b)
A graph of ln V against t is plotted for the discharge, where V is in volts and t is in seconds. Describe the graph, giving the value of its gradient and of its intercept on the ln V axis.
[4 marks]
Total for question 3: 7 marks
4
A straight conducting rod of length 0.40 m slides at a constant speed of 2.5 m s⁻¹ along two horizontal parallel rails that are 0.40 m apart. The rails are joined at one end, and the total resistance of the circuit is 0.50 Ω. A uniform magnetic field of flux density 0.15 T acts vertically downwards through the circuit, and the rod stays perpendicular to the rails.
(a)
Calculate the e.m.f. induced in the circuit, the current in the rod, the magnetic force on the rod and the external power needed to keep the rod moving at constant speed.
[6 marks]
(b)
Explain, using Lenz's law and the principle of conservation of energy, why the magnetic force on the rod acts against its motion. Describe what would happen if the force acted in the direction of motion.
[6 marks]
Total for question 4: 12 marks
5
An electron travelling at 4.0 × 10⁷ m s⁻¹ in a vacuum enters a region of uniform magnetic field of flux density 2.5 mT, with its velocity perpendicular to the field. The charge of an electron is 1.60 × 10⁻¹⁹ C.
(a)
What is the magnitude of the magnetic force on the electron?
[1 mark]
A4.0 × 10⁻²² N
B1.6 × 10⁻¹⁴ N
C1.6 × 10⁻¹¹ N
D1.6 × 10⁻¹⁷ N
(b)
The electron instead enters the field with its velocity at 30° to the field direction, at the same speed. What is the magnitude of the magnetic force on the electron?
[1 mark]
A1.4 × 10⁻¹⁴ N
B1.6 × 10⁻¹⁴ N
C8.0 × 10⁻¹⁵ N
D0 N
(c)
The electron enters the field with its velocity perpendicular to the field. Describe the path of the electron in the field and explain why the path has this shape.
[2 marks]
Total for question 5: 4 marks
6
A circular coil of 80 turns is placed in a magnetic field. The magnetic flux through each turn changes uniformly from 6.0 × 10⁻⁴ Wb to 2.0 × 10⁻⁴ Wb in 0.050 s.
(a)
What is the change in flux linkage of the coil?
[1 mark]
A4.0 × 10⁻⁴ Wb
B4.8 × 10⁻² Wb
C1.6 × 10⁻² Wb
D3.2 × 10⁻² Wb
(b)
What is the average e.m.f. induced in the coil?
[1 mark]
A0.64 V
B8.0 × 10⁻³ V
C0.96 V
D3.2 × 10⁻² V
(c)
The same change of flux takes place in 0.025 s instead of 0.050 s. State the new average e.m.f. and explain your answer.
[2 marks]
Total for question 6: 4 marks
7
Two small charged spheres in air carry charges of +3.0 nC and −3.0 nC. Their centres are 8.0 cm apart, and each sphere may be treated as a point charge. Take 1/(4πε₀) = 8.99 × 10⁹ N m² C⁻².
(a)
Calculate the magnitude and state the direction of the electric field strength at the midpoint of the line joining the centres of the two spheres.
[3 marks]
(b)
Calculate the electric potential at the midpoint. Explain why the electric field strength at the midpoint is not zero even though the potential there is zero, and calculate the force on a charge of +2.0 nC placed at the midpoint.
[4 marks]
Total for question 7: 7 marks
8
A 4700 μF capacitor is charged to a p.d. of 12 V and is then discharged through a fixed resistor of resistance 8.2 kΩ.
(a)
Determine the initial charge on the capacitor, the initial discharge current, and the time taken for the energy stored in the capacitor to fall to 25% of its initial value.
[6 marks]
(b)
A student says: 'If the capacitor is discharged through a resistor of half the resistance, it will lose its energy twice as fast, and only half as much energy will be dissipated in the resistor.' Evaluate the student's statement.
[6 marks]
Total for question 8: 12 marks

End of questions

Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).