Energy, Work & Power (Cambridge (CIE) IGCSE Physics)

The four energy transfer pathways used in physics are:

• mechanical

• electrical

• heating

• radiation

True.

An example of energy transfer by heating is a hot chocolate warming up cold hands.

Kinetic energy is the energy an object possesses due to its motion, determined by its mass and speed.

True.

Any object in motion possesses kinetic energy.

The equation for calculating kinetic energy is

Where:

• = kinetic energy, measured in joules (J)

• = mass, measured in kilograms (kg)

• = velocity, measured in metres per second (m/s)

False.

Kinetic energy is measured in units of joules (J).

In the kinetic energy equation, represents the speed of the object in meters per second (m/s).

In the kinetic energy equation, represents the mass of the object in kilograms (kg).

Kinetic energy can be calculated by multiplying half of the object's mass by the square of its speed.

Gravitational potential energy is the energy possessed by an object due to its height in a gravitational field.

False.

Energy is transferred to an object's gravitational potential store when it is lifted.

The equation for calculating gravitational potential energy is

Where:

• = change in gravitational potential energy

• = mass, measured in kilograms (kg)

• = gravitational field strength, measured in newtons per kilogram (N/kg)

• = change in height in metres (m)

False.

Gravitational potential energy is measured in units of joules (J).

In the gravitational potential energy equation, represents the mass of the object in kilograms (kg).

In the gravitational potential energy equation, represents the gravitational field strength in newtons per kilogram (N/kg).

False.

Energy is transferred away from an object's gravitational potential store when it falls.

False.

The total amount of energy in a closed system remains constant; it cannot change over time according to the principle of conservation of energy.

An energy flow diagram represents energy stores and transfers within a system, showing both the stores and the transfers taking place.

The equation representing the conservation of energy is:

total energy in = useful energy out + wasted energy

Sankey diagrams are graphical representations of energy transfers, characterised by splitting arrows that show the proportions of energy transfers taking place.

The left-hand side of a Sankey diagram represents the energy transferred into the system, while the straight arrow pointing to the right represents the useful energy output. The curved arrow represents wasted energy.

The equation for calculating wasted energy is:

True.

According to the principle of conservation of energy, the total energy in a closed system is equal to the useful energy out plus the wasted energy.

Work done is the product of the force applied to an object and the distance over which the force is applied, measured in joules (J) or newton-metres (N m).

False.

Work is done when a force is applied to an object and results in the object's movement over a distance in the direction of the force.

The formula for calculating work done is

Where:

• = work done, measured in joules (J) or newton metres (Nm)

• = force, measured in newtons (N)

• = distance, measured in metres (m)

Mechanical working energy transfer is one form of energy transfer when an object moves due to a force acting on it.

Electrical working energy transfer is one form of energy transfer when a charge moves through a potential difference.

False.

Work can also be done electrically, by heating or by radiation.

True.

The amount of energy transferred (in joules) is equal to the work done (in joules or newton-metres) when any mechanical work is done.

1 joule is equal to 1 newton metre

The relationship between energy transferred and work done is:

True.

Work is done on a ball when it is lifted to a height.

Power is the rate at which energy is transferred or work is done, measured in watts (W).

False.

Power refers to the rate at which energy is transferred or work is done, not the total amount of energy transferred.

False.

A car has more power if it does the same amount of work in a shorter amount of time.

The formula for calculating power is:

or

Where:

• = power, measured in watts (W)

• = work done, measured in joules (J)

• = energy transferred, measured in joules (J)

• = time, measured in seconds (s)

The rate of doing work, or power, refers to the amount of work done or energy transferred per unit time. It is measured in watts (W).

False.

Power is the rate at which energy is transferred or work is done, work done is the total amount of energy transferred.

The formula for calculating energy transferred using power is:

Where:

• = = work done or energy transferred, measured in joules (J)

• = power, measured in watts (W)

• = time, measured in seconds (s)

False.

Power is typically measured in watts (W), while energy is measured in joules (J).

Efficiency is a measure of the amount of useful power or energy output from a system compared to its total power or energy input. It is often expressed as a percentage.

True.

Efficiency measures how much power or energy is transferred usefully, compared to the total power or energy input. Therefore, it is also a measure of how much energy or power is wasted.

True.

The overall efficiency of a typical coal-fired thermal power station is 30%.

False.

The higher the efficiency of an energy transfer, the less energy is wasted.

Efficiency is calculated as the ratio of useful power or energy output to total power or energy input, usually expressed as a percentage.

The equation for calculating efficiency is:

True.

Efficiency can be expressed as a ratio between 0 and 1, or as a percentage between 0 and 100%.

False.

Efficiency is a ratio and therefore a dimensionless quantity that does not have any units.

True.

If an energy transfer is 65% efficient, then 35% of the energy is wasted.