Power and Energy

Energy, Work and Power

Work

• Energy is the capacity to do work

• Work is done when force is acting on an object that moves

Rememeber!

Work done= Force(N) x Distance (m)

The greater the force, the greater the work.

The larger the distance moved, the larger the work

When work is done, energy is transferred from one form to another.

Energy Transferred (J) = Work Done (J)

Multiple Select

What is needed to calculate work?

• force

• distance

• time

• mass

Multiple Choice

A student needs 10 Newtons of force to lift a box to the top of a 2 meter shelf. How much work do they do?

• 12 J

• 8 J

• 20 J

• 5 J

Multiple Choice

If 150 Joules of work is needed to move a box 10 meters, what force was used?

• 1500 N

• 15 N

• 1.5 N

• 150 N

Multiple Choice

In which of these cases is work being done?

• A student sleeps

• A student pushes a locked door that does not move

• a student carries his chair into another room

• A student sits on the carpet

Power

Rate of transfers energy

Unit is watt (W), which is the same as a joule per second (J/s)

Multiple Choice

What is being transferred as you do work?

• Energy

• Heat

• Power

• Gravitational Potential Energy

Multiple Choice

What is the equation for power and energy?

• P = E/t

• E = P/t

• T = P x E

• P = W/t

Multiple Choice

Four people of equal weight on a beach use different routes to get to the top of a sea wall. Which person produces the greatest average power?

• A

• B

• C

• D

Multiple Choice

Three boxes each weigh 100N. A man lifts all the boxes together from the ground on to a shelf that is 1.5m above the ground. The man takes 2.0s to do this.

• 75W

• 225W

• 300W

• 900W

Multiple Choice

A weight-lifter raises a 2000N weight through a vertical height of 2.0m in 0.80s. What useful power does he develop in doing this?

• 800W

• 3200W

• 4000W

• 5000W

Efficiency

When there are energy being transferred, there are some percentage that will ends up in the intended form.

Calculating Efficiency

In exams, make sure you are clear whether they are asking for useful energy output (Efficiency) or wasteful energy output (inefficiency).

Multiple Choice

Some processes are more efficient than others. Which expression gives the efficiency of a process?

• A

• B

• C

• D

Multiple Choice

This is the Sankey diagram for a typical energy-saving lamp. How efficient was it?

• 2.5%

• 25%

• 7.5%

• 75%

Multiple Choice

An electric current delivers a power of 60 watts to a light bulb. 3 watts of electrical energy are transferred to light energy. Calculate the efficiency of the bulb.

• 5%

• 10%

• 45%

• 90%

Multiple Choice

Which of the following is true about an inefficient machine?

• it produce a large amount of energy

• it uses very little energy

• it wastes a large amount of energy

• It works very quickly

Energy resources

Renewable Vs Non-Renewable

Advantage Vs Disadvantage

Originated from the Sun or not

Need to know

• Renewable energy resource is one that is replenished at a faster rate than the rate at which it is being use

• Reliable energy resource is one that can produce energy at any time

• Geothermal, Nuclear and Tidal are energy resources that do not come from the sun

• The Sun’s energy is produced by through the process of nuclear fusion in its core

• Reliability, Availability, Pollution (noise, radioactive waste, greenhouse gases), Environemental impact ( wildlife habitat) are common reasons to justify pros and cons of energy resources

Poll

Self Evaluation: Level of Understanding?

• Below proficient

• Approaching proficient

• Proficient

• Highly Proficinet