Electromagnetic Effects: Physics 0625 (Cambridge O Level / IGCSE)

A long straight wire passes vertically through a small hole in a flat horizontal sheet of card, with the current in the wire flowing vertically upward. You look down at the card from directly above, so the current is flowing straight out of the card towards you. Using the right-hand grip rule, what pattern do the magnetic field lines form on the card around the wire, and in which direction do they point, as seen from your viewpoint above?

A straight horizontal wire passes between the poles of a magnet, at right angles to the magnetic field. The magnetic field points vertically upward through the wire. The current in the wire flows into the page, away from you.

Using Fleming's left-hand rule, what is the direction of the force (the motor effect) on the wire?

A simple d.c. motor has a rectangular coil of wire that can spin between the poles of a permanent magnet. The coil is connected to a d.c. supply through a split-ring commutator and carbon brushes.

(a) The two long sides of the coil are both in the magnetic field and both carry the same current. Explain, in terms of the forces on these two sides, why the coil turns when the current is switched on. [3]

(b) Explain the purpose of the split-ring commutator, and describe what would happen to the coil's motion if it were replaced by two full continuous slip rings (with no split) instead. [3]

(c) State two separate changes that could be made to the motor to increase its turning effect (torque). [2]

A power station generator produces electrical power at a primary voltage of $V_p = 400\text{ V}$. This is fed into an ideal (100% efficient) step-up transformer with $N_p = 200$ turns on the primary coil and $N_s = 20\,000$ turns on the secondary coil, before being sent along transmission cables. The generator supplies a constant power output of $P = 40\text{ kW}$.

(a) Calculate the secondary (transmission) voltage, $V_s$. [2]

(b) Assuming the transformer is 100% efficient, calculate the current flowing in the transmission cables (the secondary current, $I_s$). [2]

(c) Suppose instead the electricity were transmitted directly at the primary voltage of $400\text{ V}$, with no step-up transformer, while still delivering the same power of $40\text{ kW}$. Calculate the current that would flow in the cables in this case, and hence explain why transmitting power at high voltage instead greatly reduces the power wasted as heat in the cables. [3]

A student sets up a coil of $200$ turns of insulated wire, wound around a cardboard tube. The two ends of the coil are connected to a sensitive centre-zero meter, which can show a small current flowing in either direction. The student pushes the north pole of a bar magnet into the tube and then pulls it back out again.

(a) State what the student observes on the centre-zero meter (i) while the magnet is moving into the coil, and (ii) while the magnet is then held completely still, motionless, inside the coil. [2]

(b) Name the effect being demonstrated, and state the energy change taking place as the magnet moves. [2]

(c) The student repeats the experiment three more times, changing only one thing each time:

• Trial 1: pushing the magnet in more slowly than in the original experiment
• Trial 2: using a stronger bar magnet, pushed in at the same speed as the original
• Trial 3: using a coil of $400$ turns instead of $200$ turns, with the same magnet at the same speed

For each trial, state whether the deflection on the meter would be smaller or larger than in the original experiment, and justify each answer using a factor that affects the size of an induced e.m.f. [3]

In an experiment, a straight wire carrying a current is placed between the poles of a magnet, and the wire experiences a force (the motor effect) that pushes it vertically upward.

Without changing anything else, a student then reverses both the direction of the current in the wire and the direction of the magnetic field (by turning the magnet around so its poles are swapped).

What happens to the direction of the force on the wire?

A charger for a phone contains a small transformer. It is connected to the $230\text{ V}$ a.c. mains supply and must output a secondary voltage of $V_s = 5\text{ V}$ to charge the phone's battery. The primary coil of the transformer has $N_p = 2300$ turns.

(a) Explain, in terms of the number of turns on the primary and secondary coils, what makes a transformer a step-down transformer rather than a step-up transformer. [1]

(b) Calculate the number of turns $N_s$ required on the secondary coil, so that the transformer outputs $5\text{ V}$ from the $230\text{ V}$ primary supply. [3]

(c) State two separate reasons why this transformer would not work correctly if it were connected directly to a d.c. supply instead of an a.c. supply. [2]

Two identical rectangular coils, X and Y, are placed in the same magnetic field between the poles of a permanent magnet. Each coil is connected to a d.c. supply so that a current flows through it.

• Coil X has $50$ turns and carries a current of $2\text{ A}$.
• Coil Y has $50$ turns and carries a current of $4\text{ A}$.

Coil size and the strength of the magnetic field are the same for both coils. Which statement correctly compares the turning effect (torque) experienced by the two coils when the current is switched on?

A student investigates the motor effect using a straight wire clamped so that it can be rotated between the flat pole pieces of a strong horseshoe magnet, carrying a current $I$ from a variable d.c. supply. The horseshoe magnet itself rests on a sensitive top-pan balance, so that (by Newton's third law) any force the field exerts on the wire is matched by an equal and opposite force on the magnet, which changes the balance reading, letting the student use the balance reading as a measure of the size of the force on the wire.

(a) With the wire held at right angles ($90^\circ$) to the magnetic field, the student steadily increases the current $I$ in the wire from zero. State what happens to the size of the force on the wire as $I$ increases, and explain why. [2]

(b) The student then keeps the current constant, but slowly rotates the wire so that the angle between the wire and the magnetic field decreases from $90^\circ$ (wire at right angles to the field) towards $0^\circ$ (wire parallel to the field). Describe how the size of the force on the wire changes during this rotation, stating clearly the size of the force at each of the two extremes, $90^\circ$ and $0^\circ$. [3]

(c) State two separate changes, other than increasing the current, that the student could make to increase the maximum force on the wire. [2]

A straight horizontal metal rod lies at right angles to a magnetic field that points vertically upward. The rod is moved directly out of the page, towards you, while remaining at right angles to the field throughout.

Using Fleming's right-hand rule (the "dynamo rule") for electromagnetic induction, what is the direction of the induced current in the rod?