Work done is the amount of energy transferred when an external force causes an object to move over a certain distance.

Define the joule in terms of work done.

One joule is the energy transferred to (or work done on) an object when a force of 1 N acts on that object parallel to its motion through a distance of 1 m.

If a force acts in the same direction as an object's motion, the object will .......... energy.

If a force acts in the same direction as an object's motion, the object will gain energy.

When work is done against friction as a block is pushed, what happens to the kinetic energy given to the block?

It is transferred to other forms of energy , such as heat and sound.

Define the principle of conservation of energy .

Energy cannot be created or destroyed, it can only be transferred from one form to another; the total energy in a closed system remains constant.

What is meant by energy dissipation ?

Energy dissipation describes ways in which energy is wasted — it is not transferred to a useful energy store and is lost to the surroundings, commonly as thermal, light or sound energy.

In a television, electrical energy is transferred into light, sound and thermal energy. Which of these is the wasted energy?

Thermal energy , produced by the heating up of the wires, is wasted. Light and sound are the useful energy transfers.

For an object travelling up a rough inclined surface: loss in kinetic energy = gain in gravitational potential energy + work done against ..........

For an object travelling up a rough inclined surface: loss in kinetic energy = gain in gravitational potential energy + work done against friction

State the energy transfer that occurs for a falling object in a vacuum.

Gravitational potential energy → kinetic energy

State the energy transfer that occurs for a battery connected to a bulb.

Chemical energy → electrical energy → light energy

True or False? The equation E = mc Δ T for thermal energy means the total energy in a system can increase without any energy being transferred in from elsewhere.

False. Energy cannot be created — any rise in thermal energy must come from an equal transfer from another energy store or from outside the system , in line with the conservation of energy.

Define elastic potential energy .

Elastic potential energy is the work done in stretching or compressing an object.

Define thermal energy in terms of work done.

Thermal energy is the work done on or by a system to transfer heat.

Nuclear energy is the work done by nuclei during the processes of .......... and fission.

Nuclear energy is the work done by nuclei during the processes of fusion and fission.

Define kinetic energy .

Kinetic energy is the energy an object has due to its motion (or velocity); the faster an object moves, the greater its kinetic energy.

When an object is falling, it gains kinetic energy. Where is this energy transferred from?

It is transferred from the gravitational potential energy the object is losing.

An object will maintain its kinetic energy unless its .......... changes.

An object will maintain its kinetic energy unless its speed changes.

What concept, combined with an equation of motion, is used to derive the kinetic energy equation?

Work done — a force accelerating an object does work on it, transferring energy to the object.

Define gravitational potential energy .

Gravitational potential energy is energy stored in a mass due to its position in a gravitational field.

What is taken as the reference point (zero) for gravitational potential energy?

Ground level at the Earth's surface.

The equation E p = mg Δ h is only valid for energy changes in a .......... gravitational field.

The equation E p = mg Δ h is only valid for energy changes in a uniform gravitational field.

When a mass falls, what happens to its gravitational potential energy?

It decreases (is lost) and is converted to other forms of energy, such as kinetic energy.

Give three examples of scenarios that involve an exchange between kinetic and gravitational potential energy.

A swinging pendulum , an object in free fall , and sports involving falling such as skiing or skydiving .

True or False? The equation E p = mg Δ h can be used to calculate the gravitational potential energy of a satellite in orbit.

False. This equation only applies for a uniform gravitational field , such as near the Earth's surface; in space the field is not uniform, so this equation does not apply.

Power is the rate at which a machine transfers energy, also equal to the rate of doing work (work done per unit time).

One watt is a transfer of energy of 1 J in 1 s (1 W = 1 J s -1 ).

A lightbulb rated at 60 W indicates the amount of energy transferred by an electrical .......... , rather than by a force doing work.

A lightbulb rated at 60 W indicates the amount of energy transferred by an electrical current , rather than by a force doing work.

True or False? The equation P = Fv can be used to find the power of a body that is accelerating.

False. P = Fv only applies where a constant force moves a body at constant velocity ; power is required to produce an acceleration, so this simple form does not apply to an accelerating body.

Efficiency is a measure of how successfully energy is transferred in a system; it is the ratio of the useful power output to the total power input .

Power is defined as the energy transferred per unit of .......... .

Power is defined as the energy transferred per unit of time .

True or False? Efficiency is measured in units of watts.

False. Efficiency has no units — it is a ratio of two quantities (power or energy) with the same units, so the units cancel.

For a lightbulb and an electric heater, which energy output is useful and which is wasted in each case?

In a lightbulb , light energy is useful and heat energy is wasted. In an electric heater , heat energy is useful and sound energy is wasted.

A system has an efficiency ratio of 0.40. What is this as a percentage?

40 % (multiply the ratio by 100 : 0.40 × 100 = 40 %).

A LevelPhysicsOCRFlashcards3. Forces & MotionWork, Energy & Power

Work, Energy & Power (OCR A Level Physics): Flashcards

Exam code: H556

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Cards in this collection (47)

Define work done.
Work done is the amount of energy transferred when an external force causes an object to move over a certain distance.
State the equation for work done when the force is parallel to the object's displacement, and define each symbol.
$W = F x$
W = work done (J), F = average force applied (N), x = displacement (m)
Define the joule in terms of work done.
One joule is the energy transferred to (or work done on) an object when a force of 1 N acts on that object parallel to its motion through a distance of 1 m.
State the equation for work done when the force acts at an angle θ to the object's displacement.
$W = F x \cos θ$
where θ is the angle between the direction of the force and the direction of motion.
If a force acts in the same direction as an object's motion, the object will .......... energy.
If a force acts in the same direction as an object's motion, the object will gain energy.
True or False?
Work can only be done on an object if the force acts exactly parallel to its direction of motion.
False.
Work can be done when a force acts at an angle to the motion — only the component of the force parallel to the displacement contributes, using $W = F x \cos θ$
When work is done against friction as a block is pushed, what happens to the kinetic energy given to the block?
It is transferred to other forms of energy, such as heat and sound.
Define the principle of conservation of energy.
Energy cannot be created or destroyed, it can only be transferred from one form to another; the total energy in a closed system remains constant.
What is meant by energy dissipation?
Energy dissipation describes ways in which energy is wasted — it is not transferred to a useful energy store and is lost to the surroundings, commonly as thermal, light or sound energy.
In a television, electrical energy is transferred into light, sound and thermal energy. Which of these is the wasted energy?
Thermal energy, produced by the heating up of the wires, is wasted. Light and sound are the useful energy transfers.
For an object travelling up a rough inclined surface: loss in kinetic energy = gain in gravitational potential energy + work done against ..........
For an object travelling up a rough inclined surface: loss in kinetic energy = gain in gravitational potential energy + work done against friction
State the energy transfer that occurs for a falling object in a vacuum.
Gravitational potential energy → kinetic energy
State the energy transfer that occurs for a battery connected to a bulb.
Chemical energy → electrical energy → light energy
True or False?
The equation E = mcΔT for thermal energy means the total energy in a system can increase without any energy being transferred in from elsewhere.
False.
Energy cannot be created — any rise in thermal energy must come from an equal transfer from another energy store or from outside the system, in line with the conservation of energy.
What is the relationship between the transfer of energy and the work done in a system?
Transfer of energy = work done
Define elastic potential energy.
Elastic potential energy is the work done in stretching or compressing an object.
Define thermal energy in terms of work done.
Thermal energy is the work done on or by a system to transfer heat.
Nuclear energy is the work done by nuclei during the processes of .......... and fission.
Nuclear energy is the work done by nuclei during the processes of fusion and fission.
State the equation for elastic potential energy and define each symbol.
$E = \frac{1}{2} F x = \frac{1}{2} k x^{2}$
F = stretching or compressing force (N), x = extension or compression (m), k = force constant (N m^-1)
State the equation for the energy released in a nuclear process and define Δm.
$E = Δ m c^{2}$
Δm is the mass defect (kg) and c is the speed of light.
True or False?
The work done in stretching a spring depends on the mass of the object attached to it.
False.
Elastic potential energy depends on the force (or spring constant) and extension, $E = \frac{1}{2} F x = \frac{1}{2} k x^{2}$ , not on the mass of the object.
Define kinetic energy.
Kinetic energy is the energy an object has due to its motion (or velocity); the faster an object moves, the greater its kinetic energy.
State the equation for kinetic energy and define each symbol.
$E_{k} = \frac{1}{2} m v^{2}$
m = mass (kg), v = speed or velocity (m s^-1)
When an object is falling, it gains kinetic energy. Where is this energy transferred from?
It is transferred from the gravitational potential energy the object is losing.
An object will maintain its kinetic energy unless its .......... changes.
An object will maintain its kinetic energy unless its speed changes.
True or False?
Doubling an object's speed doubles its kinetic energy.
False.
Kinetic energy is proportional to speed squared, $E_{k} = \frac{1}{2} m v^{2}$ , so doubling the speed quadruples the kinetic energy.
What concept, combined with an equation of motion, is used to derive the kinetic energy equation?
Work done — a force accelerating an object does work on it, transferring energy to the object.
Define gravitational potential energy.
Gravitational potential energy is energy stored in a mass due to its position in a gravitational field.
State the equation for gravitational potential energy and define each symbol.
$E_{p} = m g Δ h$
m = mass (kg), g = acceleration due to gravity (m s^-2), Δh = change in height (m)
What is taken as the reference point (zero) for gravitational potential energy?
Ground level at the Earth's surface.
The equation E_p = mgΔh is only valid for energy changes in a .......... gravitational field.
The equation E_p = mgΔh is only valid for energy changes in a uniform gravitational field.
When a mass falls, what happens to its gravitational potential energy?
It decreases (is lost) and is converted to other forms of energy, such as kinetic energy.
Give three examples of scenarios that involve an exchange between kinetic and gravitational potential energy.
A swinging pendulum, an object in free fall, and sports involving falling such as skiing or skydiving.
True or False?
The equation E_p = mgΔh can be used to calculate the gravitational potential energy of a satellite in orbit.
False.
This equation only applies for a uniform gravitational field, such as near the Earth's surface; in space the field is not uniform, so this equation does not apply.
Define power.
Power is the rate at which a machine transfers energy, also equal to the rate of doing work (work done per unit time).
Define the watt.
One watt is a transfer of energy of 1 J in 1 s (1 W = 1 J s^-1).
State the equation for power in terms of a constant force and constant velocity, and give the two conditions needed for it to apply.
$P = F v$
Only valid where a constant force moves a body at constant velocity, and the force is applied in the same direction as the velocity.
A lightbulb rated at 60 W indicates the amount of energy transferred by an electrical .........., rather than by a force doing work.
A lightbulb rated at 60 W indicates the amount of energy transferred by an electrical current, rather than by a force doing work.
What is the watt expressed in SI base units?
${kg m}^{2} s^{- 3}$
True or False?
The equation P = Fv can be used to find the power of a body that is accelerating.
False.
P = Fv only applies where a constant force moves a body at constant velocity; power is required to produce an acceleration, so this simple form does not apply to an accelerating body.
Define efficiency.
Efficiency is a measure of how successfully energy is transferred in a system; it is the ratio of the useful power output to the total power input.
What is the equation for efficiency in terms of power?
$efficiency = \frac{useful power output}{total power input} \times 100 %$
What is the equation for efficiency in terms of energy?
$efficiency = \frac{useful energy output}{total energy input} \times 100 %$
Power is defined as the energy transferred per unit of ...........
Power is defined as the energy transferred per unit of time.
True or False?
Efficiency is measured in units of watts.
False.
Efficiency has no units — it is a ratio of two quantities (power or energy) with the same units, so the units cancel.
For a lightbulb and an electric heater, which energy output is useful and which is wasted in each case?
In a lightbulb, light energy is useful and heat energy is wasted. In an electric heater, heat energy is useful and sound energy is wasted.
A system has an efficiency ratio of 0.40. What is this as a percentage?
40 % (multiply the ratio by 100: 0.40 × 100 = 40 %).

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