Energy Changes in a System Flashcards

All 84 cards in this deck

What is a 'system' in physics?

An object or group of objects.

What happens to energy when a system changes?

There are changes in the way energy is stored — energy is transferred between stores.

Name four ways energy can be transferred between stores.

Mechanically (by a force doing work), electrically, by heating, and by radiation.

Name four energy stores used in GCSE Physics.

Kinetic, gravitational potential, elastic potential and thermal (also chemical, magnetic, electrostatic, nuclear).

True or false? 'Heat' is one of the named energy stores.

False. The store is the thermal store; heating is a way of transferring energy.

A ball is thrown vertically upwards. Which store decreases and which increases as it rises?

The kinetic store decreases and the gravitational potential store increases.

A car brakes to a stop. Describe the change in the way energy is stored.

The kinetic store of the car decreases and the thermal store of the brakes and surroundings increases.

Water is boiled in an electric kettle. Describe the change in the way energy is stored.

Energy is transferred electrically from the mains to the thermal store of the water (and some to the surroundings).

An object is accelerated by a constant horizontal force. Describe the energy transfer.

The force does work on the object, so its kinetic store increases.

A moving object hits an obstacle and stops. Where is the energy from its kinetic store transferred to?

To the thermal store of the object, obstacle and surroundings (and some by sound waves).

Which energy store increases as a bungee cord is stretched?

The elastic potential store of the cord.

What is the equation for the kinetic energy of a moving object?

$E_k = \frac{1}{2}mv^2$ (kinetic energy = ½ × mass × speed²)

What are the units of each quantity in $E_k = \frac{1}{2}mv^2$?

$E_k$ in joules (J), $m$ in kilograms (kg), $v$ in metres per second (m/s).

What are the steps to calculate kinetic energy?
e.g. a 4 kg ball moving at 3 m/s

1. Square the speed: $3^2 = 9$
2. Multiply by the mass: $4 \times 9 = 36$
3. Halve it: $E_k = 18$ J

What are the steps to find the speed of an object from its kinetic energy?
e.g. $E_k = 100$ J, mass 2 kg

1. Rearrange: $v^2 = \dfrac{2E_k}{m}$
2. Substitute: $v^2 = \dfrac{2 \times 100}{2} = 100$
3. Square root: $v = 10$ m/s

True or false? Doubling the speed of an object doubles its kinetic energy.

False. Kinetic energy depends on speed squared, so doubling the speed gives four times the kinetic energy.

True or false? Doubling the mass of an object at the same speed doubles its kinetic energy.

True. Kinetic energy is directly proportional to mass.

What is the equation for the gravitational potential energy gained by an object raised above ground level?

$E_p = mgh$ (mass × gravitational field strength × height)

In $E_p = mgh$, what is $g$ and what are its units?

Gravitational field strength, in newtons per kilogram (N/kg); about 9.8 N/kg on Earth.

What are the steps to calculate the gravitational potential energy gained by a lifted object?
e.g. a 2 kg mass raised 5 m, $g = 9.8$ N/kg

1. Write $E_p = mgh$
2. Substitute: $2 \times 9.8 \times 5$
3. $E_p = 98$ J

What are the steps to find the height an object was raised from its gain in gravitational potential energy?
e.g. $E_p = 196$ J, mass 2 kg, $g = 9.8$ N/kg

1. Rearrange: $h = \dfrac{E_p}{mg}$
2. Substitute: $h = \dfrac{196}{2 \times 9.8}$
3. $h = 10$ m

True or false? In $E_p = mgh$, $h$ is the distance moved along a slope.

False. $h$ is the vertical height gained or lost.

What is the equation for the elastic potential energy stored in a stretched spring?

$E_e = \frac{1}{2}ke^2$ (½ × spring constant × extension²)

What assumption must be made when using $E_e = \frac{1}{2}ke^2$?

That the limit of proportionality of the spring has not been exceeded.

In $E_e = \frac{1}{2}ke^2$, what do $k$ and $e$ stand for, and in what units?

$k$ is the spring constant in N/m and $e$ is the extension in metres (m).

What are the steps to calculate the elastic potential energy stored in a stretched spring?
e.g. spring constant 200 N/m, extension 0.10 m

1. Square the extension: $0.10^2 = 0.01$
2. Multiply by $k$: $200 \times 0.01 = 2$
3. Halve it: $E_e = 1$ J

True or false? In $E_e = \frac{1}{2}ke^2$, $e$ is the stretched length of the spring.

False. $e$ is the extension — the increase in length from its natural length.

What are the steps to find the speed of an object dropped from a height (no air resistance)?
e.g. dropped from 5 m, $g = 9.8$ N/kg

1. $mgh = \frac{1}{2}mv^2$, masses cancel so $v^2 = 2gh$
2. Substitute: $v^2 = 2 \times 9.8 \times 5 = 98$
3. $v = \sqrt{98} = 9.9$ m/s

What are the steps to find the energy dissipated when an object slides down a slope?
e.g. it loses 500 J from its gravitational potential store and has 400 J of kinetic energy at the bottom

1. Find the decrease in the GPE store: 500 J
2. Find the kinetic energy at the bottom: 400 J
3. Subtract: $500 - 400 = 100$ J dissipated

True or false? On a frictionless slide, riders of different mass reach the same speed at the bottom.

True. Masses cancel in $mgh = \frac{1}{2}mv^2$, so the final speed depends only on the vertical height (and $g$).

A baby bouncer: describe the energy stores as the baby moves down from rest to its lowest point.

The gravitational potential store decreases, transferring to the kinetic store and then to the elastic potential store of the spring; at the lowest point it is all elastic potential.

What is the equation for the energy stored in or released from a system as its temperature changes?

$\Delta E = mc\Delta\theta$ (mass × specific heat capacity × temperature change)

What is the specific heat capacity of a substance?

The amount of energy required to raise the temperature of one kilogram of the substance by one degree Celsius.

What are the units of specific heat capacity?

J/kg °C (joules per kilogram per degree Celsius).

What are the steps to calculate the energy needed to heat a substance?
e.g. 2 kg of water heated by 10 °C, $c = 4200$ J/kg °C

1. Write $\Delta E = mc\Delta\theta$
2. Substitute: $2 \times 4200 \times 10$
3. $\Delta E = 84\,000$ J

What are the steps to calculate a specific heat capacity from measurements?
e.g. 26 000 J raises the temperature of 2.0 kg of iron by 26 °C

1. Rearrange: $c = \dfrac{\Delta E}{m\Delta\theta}$
2. Substitute: $c = \dfrac{26\,000}{2.0 \times 26}$
3. $c = 500$ J/kg °C

True or false? Of two equal masses, the one with the higher specific heat capacity needs more energy for the same temperature rise.

True. More energy is needed per kilogram per degree Celsius.

True or false? In $\Delta E = mc\Delta\theta$, the mass must be in grams.

False. The mass must be in kilograms.

What are the steps of the required practical to determine the specific heat capacity of a metal block?
e.g. a 1 kg aluminium block

1. Measure the mass of the block; put an immersion heater and thermometer into it and wrap it in insulation
2. Switch on and record the temperature and the energy transferred (joulemeter, or $E = Pt$)
3. Calculate $c = \dfrac{\Delta E}{m\Delta\theta}$

In the specific heat capacity practical, why is the block wrapped in insulation?

To reduce energy transfer to the surroundings, which would otherwise make the calculated specific heat capacity too high.

In the specific heat capacity practical, how is the energy transferred to the heater found?

With a joulemeter, or by using $E = Pt$ (power of the heater × time).

In the specific heat capacity practical, why is the thermometer left in the block for a few minutes before the initial temperature is recorded?

So the thermometer is at the same temperature as the block.

Give the main source of error in the specific heat capacity practical.

Energy transferred to the surroundings rather than to the block, so not all the measured energy heats the block.

Why can a liquid-in-glass thermometer be misread in this practical, and how is it avoided?

Parallax error — read the scale at eye level, straight on.

What does the width of each arrow on a Sankey diagram represent?

The amount of energy transferred — it is drawn to scale.

What are the steps to draw a Sankey diagram for an energy transfer?
e.g. a lamp given 100 J which transfers 20 J by light

1. Draw one input arrow, width to scale for the total input: 100 J
2. Branch off the useful output: 20 J by light
3. Branch the wasted energy downwards: $100 - 20 = 80$ J to the thermal store

On an energy transfer diagram, what must the outputs add up to?

The total energy input, because energy cannot be created or destroyed.

What is the definition of power?

The rate at which energy is transferred, or the rate at which work is done.

What are the two equations for power?

$P = \dfrac{E}{t}$ and $P = \dfrac{W}{t}$ (energy transferred or work done ÷ time).

An energy transfer of 1 joule per second is equal to what power?

1 watt (W).

What are the steps to calculate the power of a device?
e.g. it transfers 600 J in 4 s

1. Write $P = \dfrac{E}{t}$
2. Substitute: $P = \dfrac{600}{4}$
3. $P = 150$ W

What are the steps to calculate the energy transferred by a device from its power?
e.g. a 50 W lamp left on for 20 s

1. Rearrange: $E = Pt$
2. Substitute: $E = 50 \times 20$
3. $E = 1000$ J

Two motors lift the same weight through the same height, but one takes less time. Which motor has the greater power output?

The faster one — it transfers the same energy in less time.

True or false? A device with a greater power output always transfers more energy in total.

False. It transfers energy at a faster rate; the total energy also depends on the time.

State the principle of conservation of energy.

Energy can be transferred usefully, stored or dissipated, but cannot be created or destroyed.

What is a closed system?

A system in which no energy transfers in or out, so the total energy stays constant.

What does it mean to say energy is 'dissipated'?

It is transferred to the surroundings and stored in less useful ways.

True or false? Energy is used up when a device is running.

False. Energy is transferred between stores — some usefully, some dissipated — but never used up.

True or false? In any system change, some energy is dissipated.

True. In all system changes energy is dissipated to less useful stores.

What is meant by 'wasted' energy?

Energy that is not transferred usefully — it is dissipated to the surroundings, usually to thermal stores.

Give two ways of reducing unwanted energy transfers.

Lubrication of moving parts and the use of thermal insulation.

Which unwanted energy transfer does lubricating a machine's moving parts reduce?

Friction between the parts, so less energy is dissipated to thermal stores.

In a car engine, where does most of the wasted energy end up?

In the thermal store of the engine and the surroundings (some by sound).

True or false? Wasted energy can be collected up and used again as easily as useful energy.

False. It is dissipated and spread out into the surroundings, so it is stored in less useful ways.

How does the thermal conductivity of a material affect the rate of energy transfer by conduction across it?

The higher the thermal conductivity, the higher the rate of energy transfer by conduction.

How does the thickness of a building's walls affect its rate of cooling?

Thicker walls give a lower rate of energy transfer, so the building cools more slowly.

Which walls make a building cool fastest: walls of high or of low thermal conductivity?

Walls of high thermal conductivity — energy is conducted out at a greater rate.

Why does cavity wall insulation reduce the rate of energy transfer from a house?

The trapped air or foam has a low thermal conductivity, so energy is conducted out more slowly.

True or false? Thick walls of a low thermal conductivity material give the slowest rate of cooling.

True. Both greater thickness and lower thermal conductivity reduce the rate of energy transfer.

What are the steps of the required practical to compare materials as thermal insulators?
e.g. comparing newspaper, bubble wrap and felt

1. Wrap identical beakers in the different materials (same thickness) and add the same volume of hot water at the same starting temperature
2. Put a lid on and record the water temperature every minute for a set time
3. The material with the smallest temperature drop is the best insulator

In the insulation practical comparing materials, what is the independent variable?

The type of insulating material.

Give three variables that must be controlled in the insulation practical.

Volume (mass) of water, starting temperature of the water, and the thickness of the material (also identical beakers and same time).

In the insulation practical, why is a lid placed on the beaker?

To reduce energy transfer from the water surface by evaporation and convection.

In the insulation practical, how do you tell which material is the best insulator?

It gives the smallest decrease in water temperature in the same time.

Apart from the type of material, which factor affecting insulation can be investigated in this practical?

The thickness of the material (e.g. the number of layers).

What is the equation for efficiency in terms of energy?

efficiency $= \dfrac{\text{useful output energy transfer}}{\text{total input energy transfer}}$

What is the equation for efficiency in terms of power?

efficiency $= \dfrac{\text{useful power output}}{\text{total power input}}$

What are the steps to calculate the efficiency of a device?
e.g. a lamp is supplied 200 J and transfers 50 J usefully as light

1. Identify the useful output (50 J) and total input (200 J)
2. Divide: $\dfrac{50}{200} = 0.25$
3. Multiply by 100 for a percentage: 25%

What are the steps to find the useful power output from an efficiency?
e.g. efficiency 0.75, total power input 0.24 W

1. Write $0.75 = \dfrac{\text{useful power output}}{0.24}$
2. Rearrange: useful power output $= 0.75 \times 0.24$
3. $= 0.18$ W

True or false? A device can have an efficiency greater than 1 (100%).

False. The useful output can never be more than the total input, because energy cannot be created.

Give three ways to increase the efficiency of an intended energy transfer.

Lubricate moving parts, use thermal insulation, and streamline moving objects to reduce air resistance.

How does oiling the moving parts of a machine affect its efficiency?

Efficiency increases — less energy is wasted against friction, so a greater proportion is transferred usefully.

True or false? Increasing the efficiency of a device means increasing its total energy input.

False. It means a greater proportion of the same input is transferred usefully.

How does insulating the pipes and tank of a hot water system improve its efficiency?

Less energy is dissipated to the surroundings, so more of the input is transferred usefully to the water.