Edexcel A-Level Physics: Electric and Magnetic Fields

A bar magnet is pushed, north-pole first, into one end of a stationary coil of insulated wire. The coil is connected to a sensitive centre-zero ammeter. While the magnet is moving into the coil the ammeter shows a deflection; when the magnet is then held still inside the coil the reading falls to zero.

Explain, with reference to Faraday's law and Lenz's law, why a current is induced only while the magnet is moving, the direction in which the induced current flows in the face of the coil nearest the approaching magnet, and what this implies about the force the operator must apply to keep pushing the magnet in.

(6 marks)

A capacitor is charged to a potential difference of 9.0 V and is then discharged through a fixed resistor. The circuit values are listed below. The capacitor is fully charged at the instant ($t = 0$t=0) the discharge begins.

Quantity	Symbol	Value
Capacitance	$C$C	470 μF
Discharge resistance	$R$R	22 kΩ
Initial potential difference	$V_0$V0​	9.0 V

(a) Calculate the initial charge stored on the capacitor. (2 marks)

(b) Calculate the time constant of the discharge circuit. (1 mark)

(c) Calculate the charge remaining on the capacitor 15 s after the discharge begins, and the energy stored on the capacitor at $t = 0$t=0. (3 marks)

A student charges a capacitor to 12.0 V and then discharges it through a resistor of resistance 100 kΩ. A data logger records the potential difference $V$V across the capacitor at 10-second intervals:

Time, $t$t / s	0	10	20	30	40	50
Potential difference, $V$V / V	12.0	7.3	4.4	2.7	1.6	1.0

(a) Show that these data are consistent with an exponential decay. (2 marks)

(b) Use the data to estimate the time constant of the circuit, explaining your method. (2 marks)

(c) Hence determine the capacitance of the capacitor. (1 mark)

A proton travelling at $4.0 \times 10^{6}\ \text{m s}^{-1}$4.0×106 m s−1 enters a uniform magnetic field of flux density 0.25 T. The proton's velocity is perpendicular to the field, and the field region is large enough for the proton to follow a curved path within it.

(mass of proton $= 1.67 \times 10^{-27}\ \text{kg}$=1.67×10−27 kg; charge of proton $= +1.60 \times 10^{-19}\ \text{C}$=+1.60×10−19 C)

(a) Calculate the magnitude of the magnetic force on the proton as it enters the field. (2 marks)

(b) Explain why the proton follows a circular path, and calculate the radius of that path. (3 marks)

A power station generates 25 MW of electrical power at 25 kV. Before transmission, an ideal step-up transformer raises the voltage to 400 kV for the overhead grid. The primary coil of the transformer has 2000 turns. The transmission cables have a total resistance of 8.0 Ω.

(a) Calculate the number of turns on the secondary coil of the transformer. (2 marks)

(b) Calculate the power dissipated in the transmission cables when the 25 MW is carried at 400 kV. (2 marks)

Capacitance and magnetic flux density are two defined quantities in this topic.

(a) Define the capacitance of a capacitor, and hence state what is meant by a capacitance of one farad. (2 marks)

(b) Define the magnetic flux density of a uniform field, naming the SI unit in which it is measured. (1 mark)