Work, Power & EfficiencyEdexcel GCSE Physics: Flashcards

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Give the equation for work done by a force.

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Give the equation for work done by a force.: E = F × d (work done = force × distance moved in the direction of the force).
How does work done relate to energy transferred?: They are the same quantity — energy transferred (J) is equal to work done (J).
Name the three ways the energy of a system can be changed.: Through work done by forces, through electrical equipment, and through heating.
What happens to the total energy in a closed system?: There is no net change to the total energy — energy transfers between stores, but the total stays constant.
When do mechanical processes become wasteful?: When they cause a rise in temperature, dissipating energy to the surroundings (e.g. through friction).
Define power.: The rate at which energy is transferred, measured in watts.
Give the equation for power.: P = E ÷ t (power = energy transferred ÷ time).
What is one watt equal to?: One joule per second.
Give the equation for efficiency.: Efficiency = useful energy transferred by the device ÷ total energy supplied to the device.
Can a real device be 100% efficient?: No — some energy is always dissipated, usually as heat to the surroundings.
A force of 15 N moves an object 4 m in the direction of the force. What is the work done?: 60 J (E = F × d = 15 × 4 = 60 J).
A device transfers 500 J of energy in 10 s. What is its power?: 50 W (P = E ÷ t = 500 ÷ 10 = 50 W).