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In this lesson, we will learn how to calculate the efficiency of electrical devices given their useful energy output and total energy output.
A 60 W
incandescent light bulb is left
on for 30 seconds.
In that time, it is supplied with
of energy but it only
36 J of energy to light.
is the efficiency of the bulb?
An LED light has an efficiency of 29%. If it converts 5 800 J to light, what is the total energy it was supplied with?
A 1.8 kW kettle has an efficiency of
0.92. It is used to boil 1.3 kg of
water, which requires 440 kJ of
energy. How many minutes does it take the kettle to boil the water? Give your answer to the
An LCD TV has an efficiency of 0.28. Energy is supplied to it at a rate of
110 W. If the TV is left on for
90 minutes, how much energy is
wasted? Give your answer to 2 significant figures.
A machine has a useful power output of
400 W and a total power input of
W. In a given time, the
total energy supplied to the machine is
108 kJ. What is the useful output
energy transfer of the machine at this time?
A 100 W incandescent light bulb has an efficiency of
. What is the useful power output of the light bulb?
A refrigerator has a total power input of
35 W and a useful power output of
7.4 W. What is the efficiency of the
refrigerator? Give your answer to 2 significant figures.
A 120 W television has a useful power
output of 30 W. What is the efficiency of
A laptop has an energy efficiency of 0.18. The laptop is used for 1 hour and during this
time 88.6 kJ of energy is wasted in
the form of heat and sound. How much energy was the laptop supplied with over this time?
Give your answer to 3 significant figures.
A 30 W fan is turned on for 5 minutes.
Over this time, the fan is
J of energy,
and it converts
of it into kinetic energy of the air around it. The
rest of the energy is wasted as heat and sound. How much energy is wasted by the fan?
A fluorescent lamp converts much more of the energy it is supplied with to light than an
incandescent bulb does. The efficiency of a fluorescent lamp is 22%. If the fluorescent lamp
is supplied with 18 000 J
of energy, how much of this is converted to light?
A food blender is used for 1 minute. Over this time, some of the energy supplied to the blender is transferred to the blades as kinetic energy, but 4 kJ of energy is wasted as sound and heat. If the total energy supplied to the blender is 38 kJ, what is the efficiency of the blender? Give your answer to 2 significant figures.
A kettle uses 373 kJ to boil 1.1 kg of water in 3 minutes. Over this time, it was supplied with 410 kJ of energy. What is the efficiency of the kettle? Give your answer to 2 significant figures.
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