What physical principle makes a DC motor rotate?

A rotating magnetic field in the stator induces eddy currents in a squirrel-cage rotor A current-carrying coil in a magnetic field experiences a force (F = BIL), creating a turning moment Alternating current in the coil repeatedly attracts and repels the fixed magnets Thermal expansion of the coil under current causes it to push against the magnet poles

What is the role of the commutator in a DC motor?

It converts AC supply voltage to DC before it enters the motor coil It stores energy between pulses to keep the motor running smoothly It reverses the current direction in the coil every half-turn so the torque always acts in the same rotational direction It increases the current supplied to the coil as the motor speed increases

A DC motor is running normally when its shaft is suddenly stalled (held stationary). What happens to the current drawn?

Current drops to zero because the motor is no longer rotating Current stays the same because the supply voltage is unchanged Current increases significantly because back-EMF drops to zero and the full supply voltage drives current through the coil resistance alone Current reverses direction to oppose the stalling force

How does an AC induction motor produce rotation without a physical electrical connection to the rotor?

Permanent magnets in the rotor are attracted to the rotating stator field and follow it A rotating magnetic field in the stator induces currents in the squirrel-cage rotor; the interaction between these currents and the stator field produces torque The AC supply alternately charges and discharges capacitors in the rotor, creating pulsed torque Brush contacts deliver AC directly to rotor coils, which then repel the stator magnets

What is 'slip' in an AC induction motor?

The electrical losses in the stator resistance that reduce efficiency The mechanical slipping of the shaft coupling under high torque The small difference in speed between the stator's rotating magnetic field and the rotor's actual rotation speed The lag between the AC supply frequency and the actual motor frequency

What is power factor, and what type of load causes a lagging power factor?

Power factor is the ratio of voltage to current; resistive loads cause a lagging power factor Power factor is the ratio of real power to apparent power; inductive loads such as motors cause a lagging power factor Power factor is always 1.0 in well-designed circuits; only poorly designed circuits have a different value Power factor is the efficiency of a motor; capacitive loads always improve it to above 1.0

A 132 kV transmission line carries 500 A. What is the power loss in a 10 Ω section of line?

66,000,000 W (66 MW) 5,000 W (5 kW) 2,500,000 W (2.5 MW) 660,000,000 W (660 MW)

Why is electricity transmitted at very high voltages (e.g. 400 kV) across long distances?

High voltage prevents corona discharge from forming around the cables High voltage allows thinner, lighter cables to be used, reducing tower costs High voltage means low current for the same power; lower current means far less power wasted as heat in the line resistance (P = I²R) High voltage is needed because transformers cannot work at low voltages

At what level of current through the human body does the risk of ventricular fibrillation (potentially fatal cardiac arrest) begin?

Around 1 mA Around 20 mA Around 100 mA Around 10 A

A 13 A fuse is installed in the plug of a 3 kW electric shower connected to 230 V. Is this an appropriate fuse rating?

Yes — a 13 A fuse is exactly right because the shower draws exactly 13 A No — a 3 A fuse should be used because the shower uses low power No — the shower draws about 13 A and a fuse should be rated just above the normal operating current, so a 13 A fuse is marginal; a 16 A or dedicated circuit breaker is typical for a 3 kW shower Yes — as long as the fuse rating equals the current drawn, the fuse is correctly chosen

What is the key difference between a fuse and a circuit breaker?

A fuse protects against voltage surges; a circuit breaker protects against overcurrent only A fuse is a sacrificial wire that melts and must be replaced; a circuit breaker is a reusable mechanical switch that can be reset after tripping A circuit breaker is slower to react than a fuse, making fuses better for motor protection A fuse is rated in Volts; a circuit breaker is rated in Amperes

What makes an RCBO (Residual Current Breaker with Overcurrent protection) particularly important for electrical safety compared with a standard circuit breaker?

An RCBO can handle higher currents than a circuit breaker before tripping, protecting motors from burnout An RCBO trips at exactly 13 A to match standard plug fuses, providing coordinated protection An RCBO detects very small leakage currents to earth (as low as 30 mA) and trips within milliseconds, protecting against electrocution even when the fault current is below the overcurrent trip threshold An RCBO provides surge voltage protection that standard circuit breakers lack