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OCR GCSE Physics
Topic P7: Energy
P7.2 Power and Efficiency
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One watt represents 1 joule of energy transferred per
second
.
True
Efficiency
is the ratio of useful energy output to total energy
input
Order the steps to calculate efficiency:
1️⃣ Identify the total power input
2️⃣ Identify the useful power output
3️⃣ Divide the useful power output by the total power input
4️⃣ Multiply the result by 100%
One watt is defined as the energy transferred or work done at a rate of 1 joule per
second
What does efficiency measure in a system?
How well energy is converted
Calculating efficiency helps determine how much
energy
is lost or wasted.
True
If a 100 W light bulb transfers 1000 J of energy in 10 seconds, what is its power?
100 W
One watt represents 1 joule of energy transferred per
second
.
True
What is the formula for power in terms of force and velocity?
P = F*v
What does 1 watt represent in terms of energy transfer?
1 joule per second
To convert watts to kilowatts, divide by
1000
How many kilowatts is 2500 watts equivalent to?
2.5 kW
What is the formula for calculating efficiency?
(Useful output / Total input) × 100%
What is the formula for calculating power using energy and time?
P = E/t
A 100 W light bulb that is on for 10 seconds transfers 1000 J of
energy
Efficiency is expressed as a
percentage
.
True
What does energy transformation refer to?
Conversion of energy
Burning coal to produce electricity is 100% efficient.
False
How many kilowatts is 2500 watts equivalent to?
2.5 kW
Power (P) is equal to the energy (E) transferred divided by the
time
If a 100 W light bulb transfers 1000 J of energy in 10 seconds, what is its power output?
100 W
What does efficiency measure in a device?
Energy conversion
If an engine uses 5000 W of power and delivers 3000 W of useful work, what is its efficiency?
60%
Steps to calculate efficiency
1️⃣ Identify the total power input
2️⃣ Identify the useful power output
3️⃣ Divide the useful power output by the total power input
4️⃣ Multiply the result by 100%
What is the typical efficiency of a combustion engine?
25-35%
Match the energy transformation with its typical efficiency:
Electric motor ↔️ 75-95%
Solar panel ↔️ 15-20%
Combustion engine ↔️ 25-35%
Energy transformation refers to the conversion of one form of energy into another.
True
Higher efficiency transformations are more desirable because they minimize energy losses and environmental
impact
Efficiency measures how well a system converts energy into useful
work
Optimizing the design of a device can involve improving insulation to minimize heat
loss
Devices with higher efficiency components reduce
energy waste
and environmental impact.
True
The unit of power is the
watt
Match the unit of power with its abbreviation and conversion:
Watt ↔️ W ||| 1 W = 1 joule/second
Kilowatt ↔️ kW ||| 1 kW = 1000 W
How is efficiency expressed?
As a percentage
Understanding power is crucial for designing efficient
systems
To convert kilowatts to watts, you multiply by
1000
.
True
The formula to calculate power using energy and time is
P = E/t
The unit of power is the
watt
What is the formula for power in terms of energy and time?
P = E/t
The unit of power is the
watt
See all 63 cards
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