Electrical Scientific Principles Level 3

The following resistances are connected together in parallel: 2ohm, 5ohm, 10ohm and 5ohm. What is the total resistance?

• 22 Ohms

• 1 Ohm

• 0.22 Ohms

• 10 Ohms

Answer explanation

To find the total resistance in parallel, use the formula 1/R_total = 1/R1 + 1/R2 + 1/R3 + 1/R4. For 2, 5, 10, and 5 ohms, this gives 1/R_total = 1/2 + 1/5 + 1/10 + 1/5 = 1. Therefore, R_total = 1 Ohm.

What is the 'constant' for RMS values?

• 0.707

• 0.642

• 1.707

• 1.642

Answer explanation

The 'constant' for RMS (Root Mean Square) values is derived from the relationship between RMS and peak values in sinusoidal waveforms. The correct value is 0.707, which represents the ratio of RMS to peak value.

What is the SI unit for power?

• Coulomb

• Volt

• Watt

• Amps

Answer explanation

The SI unit for power is the Watt. It measures the rate at which energy is transferred or converted. Other options like Coulomb, Volt, and Amps refer to charge, voltage, and current, respectively, not power.

What is the RMS value of Three Phase voltage in the UK?

• 230V

• 400V

• 240V

• 415V

Answer explanation

The RMS value of three-phase voltage in the UK is 400V. This is the standard line-to-line voltage used in three-phase systems, while 230V is the line-to-neutral voltage.

A length of electrical conductor has a length resistance of 1 ohm. If its diameter was doubled what would happen to its resistance if the length remained the same?

• 2 Ohms

• 0.5 Ohms

Answer explanation

Doubling the diameter of the conductor increases its cross-sectional area by a factor of four. Since resistance is inversely proportional to area, the new resistance becomes 1 ohm / 4 = 0.25 ohms. However, since the length remains the same, the resistance is halved to 0.5 ohms.

A motor supplies 12kW of power, but is only operating at 80% efficiency. How much power does the motor take from the supply?

• 15kW

• 14Kw

• 9kW

• 10kW

Answer explanation

To find the power taken from the supply, divide the output power by efficiency: 12kW / 0.8 = 15kW. Thus, the motor takes 15kW from the supply.

A 30 ohm resistance and a coil with inductance of 40 ohms are connected in series to a 100v 50Hz supply. Calculate the total current for the circuit.

• 4.8A

• 2.4A

• 2A

• 4A

Answer explanation

The total impedance Z = √(R² + (XL)²) = √(30² + 40²) = 50 ohms. Using Ohm's law, I = V/Z = 100V/50Ω = 2A. Thus, the total current is 2A.

An AC voltage has a peak voltage of 325v. Calculate the AVERAGE voltage.

• 206.9V

• 165.6V

• 229.8V

• 189.8V

Answer explanation

The average voltage (V_avg) for a sinusoidal AC voltage is calculated as V_peak / √2. Thus, V_avg = 325V / √2 ≈ 206.9V. Therefore, the correct answer is 206.9V.

What type of machine is a capacitor start, capacitor run motor?

• 3 - Phase

• Single Phase

• Wound

• Synchronous

Answer explanation

A capacitor start, capacitor run motor is a type of single phase motor. It uses capacitors to improve starting torque and efficiency during operation, distinguishing it from other motor types like 3-phase or synchronous motors.

By what method are small 3-phase cage rotor induction motors started?

• Star/Delta starter

• Autotransformer starter

• Rotor resistance starter

• DOL starter

Answer explanation

Small 3-phase cage rotor induction motors are typically started using a DOL (Direct On Line) starter, which connects the motor directly to the power supply, providing full voltage and current for immediate startup.