A boy produces an average useful power output of \(60\,\mathrm{W}\) as he rides his bicycle for \(2.0\,\mathrm{minutes}\).
How much useful energy does he transfer?
Correct Answer: \( \boxed{\mathrm{D}} \)
Power is the rate of energy transfer:
\( P = \frac{E}{t} \)
Rearranging:
\( E = Pt \)
First convert the time into seconds:
\( t = 2.0 \times 60 = 120\,\mathrm{s} \)
Substitute the values:
\( E = 60 \times 120 \)
\( E = 7200\,\mathrm{J} \)
Therefore, the useful energy transferred by the boy is \(7200\,\mathrm{J}\).
Hence, the correct answer is (D).
A microwave oven is rated at \(900\,\mathrm{W}\).
Which statement correctly describes the meaning of this value?
Correct Answer: \( \boxed{\mathrm{A}} \)
Power is the rate at which energy is transferred.
\( P = \frac{E}{t} \)
The SI unit of power is the watt:
\( 1\,\mathrm{W} = 1\,\mathrm{J\,s^{-1}} \)
Therefore, a power rating of \(900\,\mathrm{W}\) means that the microwave transfers \(900\,\mathrm{J}\) of energy every second.
Amperes measure electric current, volts measure potential difference, and ohms measure resistance, so options (B), (C), and (D) are incorrect.
Hence, the correct answer is (A).
The engine of a motor vehicle develops a large power. Which statement is correct?
Correct Answer: \( \boxed{\mathrm{C}} \)
Power is defined as the rate at which energy is transferred or work is done.
\( P = \frac{\Delta E}{t} \)
A large power output means that a large amount of energy is transferred in a short time.
Therefore, an engine with a large power transfers a large amount of energy every second.
A large power does not necessarily mean that the driving force must be large or that the vehicle must be very fast. Power depends on both force and speed.
The physical size or volume of the engine is not the definition of power.
Hence, the correct statement is that the engine transfers large amounts of energy each second.
Therefore, the correct answer is (C).
