Power

A builder carries bricks to the top of the same ladder on four mornings.

All the bricks have the same mass.

On which morning does the builder use the most power?

A teacher wants to measure the specific latent heat of ice.

The diagram shows the equipment the teacher uses.

Here are some steps in the method for this experiment. The steps are not in the correct order.

Write numbers in the boxes next to each step to show the correct order. One is already done for you.

In one set of results 0.006 kg of ice melts.

Calculate the weight of the ice that melts.

Gravitational field strength = 10 N / kg

Use the equation: gravitational force = mass × gravitational field strength

The heater is turned on for 150 seconds.

The power of the heater is 15 W.

Calculate the energy transferred by the heater.

Use the equation: energy transferred = power × time

In another set of results 0.015 kg of ice melts.

The energy transferred by the heater is 5010 J.

Calculate the specific latent heat of ice.

Use the Equation Sheet.

Specific latent heat of ice =........ J / kg

Two students do an experiment to measure their power by walking up steps.

This is the data from student P:

• Weight = 600 N

• Number of steps = 250

• Height of each step = 0.20 m

Calculate the work done by student P.

This is the data from student Q:

• Work done = 36 000 J

• Time taken = 240 s

Calculate the power of student Q.

Give your answer in kW.

Describe how the students carry out this experiment. Include any equipment they use.

Student P carries out the experiment five more times and calculates their power.

Here are student P ’s results.

How can student P ’s results be described?

Tick one (✓) box in each row.

Explain your answer to (d)(i).

Two students do an experiment to measure their power by walking up steps.

This is the data from student P:

• Weight = 600 N

• Number of steps = 250

• Height of each step = 0.20 m

Calculate the work done by student P.

This is the data from student Q:

• Work done = 36 000 J

• Time taken = 240 s

Calculate the power of student Q.

Give your answer in kW.

Describe how the students carry out this experiment. Include any equipment they need to use.

Student P carries out the experiment five more times and calculates their power.

Here are student P ’s results.

How can student P ’s results be described?

Tick one (✓) box in each row.

Explain your answer to (d)(i).

A student does an experiment to measure the power rating of a Bunsen burner.

The diagram shows their experiment.

This is the method they follow:

• Pour 0.2 kg of water into a beaker.

• Increase the temperature of the water using the Bunsen burner.

• Measure the time taken for the temperature of the water to increase by 50 °C.

Suggest a way to measure the time taken.

The mass of the water is 0.2 kg. The temperature change is 50 °C.

The specific heat capacity of water is 4200 J / kg °C.

Calculate the thermal energy transferred to the water.

Use the Equation Sheet.

The student repeats the experiment. They say that the energy transferred by the Bunsen burner is 40 000 J after 200 s.

Calculate the power of the Bunsen burner.

Another student repeats the experiment for two Bunsen burners A and B.

The graph shows the time taken for the temperature of 0.2 kg of water to increase by 50 °C.

The student says, ‘Bunsen burner A has the highest power.’

Explain how the graph shows this.

The manufacturer says, ‘The line on the graph for Bunsen burner B is not accurate. The power of the Bunsen burners should be the same.’

Both Bunsen burners are working correctly.

What did the student do wrong when testing Bunsen burner B ?

Tick (✓) one box.

Three students A, B and C do an experiment to measure the power rating of a Bunsen burner.

The diagram shows their apparatus.

This is the method they follow:

• Pour 0.2 kg of water into a beaker.

• Increase the temperature of the water using the Bunsen burner.

• Measure the temperature of the water every 30 seconds.

The graph shows student A ’s results.

Student A uses point P on the graph to calculate the gradient of the line of best fit.

Explain why student A ’s calculation is incorrect.

Calculate the gradient of the line of best fit on the graph. Give your answer to 2 significant figures.

Gradient =........ °C / s

What is the significance of the y intercept of the line on the graph?

What is the significance of the gradient of the line on the graph?

Student B repeats the experiment. The gradient of their line of best fit is 0.25 °C / s. The specific heat capacity of water is 4200 J / kg °C.

Use student B ’s value to calculate the thermal energy transferred to 0.2 kg of water after 200 s.

Use the Equation Sheet.

Student C also repeats the experiment.

They say the thermal energy transferred is 32 000 J after 200 s.

Use student C ’s value to calculate the power of the Bunsen burner.

Use the Equation Sheet.

A student does 20 “step ups” onto a wall.

• The total work done by the student is 3000 J.

• The time taken is 24 s.

Calculate the power of the student.

A 6.25 W

B 125 W

C 2500 W

D 3600 W

Which statement describes power?

A The rate of flow of charge.

B The rate of flow of current.

C The rate of stretching of a spring.

D The rate of transfer of energy.

A teacher lifts a weight of 45 N through a height of 1.8 m.

Calculate the work done by the teacher.

Use the equation: work done = force × distance

Another teacher does 1500 J of work.

They take 5 seconds to do the work.

Calculate their power.

Give the unit in your answer.

Use the equation: power = work done / time

Explain in words what is meant by power.

A weightlifter lifts a 200 pound (200 lb) mass.

1 lb = 0.454 kg.

Calculate the weight of the mass in Newtons.

Use the equation: gravitational force = mass × gravitational field strength

Gravitational field strength = 10 N / kg.

This is some data for a weightlifter.

Weight lifted = 620 N Height of the lift = 1.6 m Time taken to complete lift = 4.0 s

Calculate the power generated by the weightlifter.

Use the equations: work done = force × distance power = work done / time

A heater transfers 150 000 J of energy in 120 s.

Calculate the power of the heater.

Use the equation: energy transferred = power × time

A 0.80 W

B 1.25 W

C 1250 W

D 18 000 W

Which row of the table gives a power of 300 W?

Use the equation: power = work done ÷ time

Which of the following transfers the most energy?

A 1.0 kW over one hour

B 2.0 kW over two hours

C 3.0 kW over one hour

D 500 W over four hours

A student uses a small motor to lift a toy car through a vertical distance of 1.0 m.

The car has a weight of 0.05 N.

Calculate the work done when lifting the car through this distance.

Calculate the power of the motor if the car takes 5.0 seconds to travel the 1.0 m distance.

State the unit.

The motor uses 2 cells in series.

Each cell has a potential difference of 1.5 V.

Write down the total potential difference of the cells.

The motor has a resistance of 6.0 .

Calculate the current in the circuit when the motor is in use.

Use the equation: Potential difference = Current × Resistance

The student wishes to increase the time taken to lift this toy car vertically through the 1.0 m distance shown.

Suggest a change he could make to this experiment to achieve this.

A student uses a small motor to lift a toy car through a vertical distance of 1.0 m.

The car has a weight of 0.05 N.

Calculate the work done when lifting the car through this distance.

Calculate the power of the motor if the car takes 5.0 seconds to travel the 1.0 m distance.

State the unit.

The motor uses 2 cells in series.

Each cell has a potential difference of 1.5 V.

Write down the total potential difference of the cells.

The motor has a resistance of 6.0 .

Calculate the current in the circuit when the motor is in use.

Use the equation: Potential difference = Current × Resistance

The student wishes to increase the time taken to lift this toy car vertically through the 1.0 m distance shown.

Suggest a change he could make to this experiment to achieve this.