Physics energy tranfers

Define system.

A system is an object or group of objects.

What happens to a system when it is in equilibrium?

Nothing changes, so nothing happens.

What is an open system?

A system that allows the exchange of both energy and matter to or from its surroundings.

What is a closed system?

A system that can exchange energy but not matter with its surroundings.

What is an isolated system?

A system that does not allow the transfer of matter or energy to or from its surroundings.

Define the principle of conservation of energy.

Energy cannot be created or destroyed, it can only be transferred from one store to another.

Name the eight energy stores that an object may have.

Kinetic, gravitational, elastic, magnetic, electrostatic, chemical, nuclear and thermal.

True or False?

The colder an object is, the more energy it has in its thermal store. False. All objects have energy in their thermal store; the hotter the object, the more energy it has in this store.

Name the four transfer pathways by which energy can be transferred between stores.

Mechanically, electrically, by heating and by radiation.

A charge moving through a potential difference transfers energy ______.

A charge moving through a potential difference transfers energy electrically.

Describe the energy transfers that occur when a ball is thrown upwards.

Energy is transferred from the chemical store of the person to the kinetic store of the ball as it is thrown. As the ball's height increases, energy is transferred from its kinetic store to its gravitational potential store.

When a moving car crashes into a wall, what happens to the energy in its kinetic store?

The car's speed decreases very quickly, so the energy in its kinetic store decreases. Most of this energy is transferred to the thermal store of the surroundings (dissipated).

When a car hits a wall, energy is transferred ______ to the thermal store of the wall by the force of the car on the wall.

When a car hits a wall, energy is transferred mechanically to the thermal store of the wall by the force of the car on the wall.

Besides the mechanical transfer to the wall, how else is energy transferred away when a car crashes into an obstacle?

Energy is transferred mechanically to the thermal store of the car, the wall and the air, as sound waves cause the air particles to vibrate.

What happens to the energy in the chemical store of a vehicle's fuel when the vehicle accelerates?

It is transferred to the kinetic store of the vehicle.

Name two causes of friction that transfer energy to the thermal store of the surroundings when a vehicle slows down.

Friction between the tyres and the ground, and friction between the brakes and the brake pads.

True or False?
When a kettle boils water, energy is transferred by heating directly from the mains electricity supply to the water.

When a kettle boils water, energy is transferred by heating directly from the mains electricity supply to the water. False. Energy is transferred electrically from the mains to the thermal store of the heating element, then by heating to the thermal store of the water.

Define kinetic energy.

Kinetic energy is the amount of energy an object has as a result of its mass and speed.

What happens to the energy in an object's kinetic store when it speeds up?

Energy is transferred to its kinetic store.

What happens to the energy in an object's kinetic store when it slows down?

Energy is transferred away from its kinetic store.

Kinetic energy can be calculated using the equation Ek = ½ × m × v², where v is the object's ______.

Kinetic energy can be calculated using the equation Ek = ½ × m × v², where v is the object's speed.

What are the SI units of mass and speed used in the kinetic energy equation?

Mass in kilograms (kg); speed in metres per second (m/s).

If the values used in a kinetic energy calculation are given to two significant figures, to what precision should the final answer be rounded?

To the same degree of accuracy — two significant figures.

True or False?

To calculate kinetic energy, you multiply the mass by the speed, then square the result. False. You square the speed first, then multiply by the mass and by one half: Ek = ½ × m × v².

Define gravitational potential energy.

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

What happens to an object's gravitational potential energy store when it is lifted up?

Energy is transferred to its gravitational potential store.

What happens to an object's gravitational potential energy store when it falls?

Energy is transferred away from its gravitational potential store.

The gravitational potential energy equation is Ep = m × g × h, where h is the object's ______ in metres.

The gravitational potential energy equation is Ep = m × g × h, where h is the object's height in metres.

What is the approximate value of gravitational field strength, g, on the surface of the Earth?

9.8 N/kg.

Why would it be easier to lift a mass on the Moon than on the Earth?

Because the Moon's gravitational field strength is less than the Earth's.

True or False?
Gravitational field strength on the surface of gas giants, such as Jupiter and Saturn, is less than on Earth, so it would be easier to lift a mass there.

Gravitational field strength on the surface of gas giants, such as Jupiter and Saturn, is less than on Earth, so it would be easier to lift a mass there. False. Gravitational field strength on gas giants is more than on Earth, so it would be harder to lift a mass there.

Define elastic potential energy.

Elastic potential energy is the energy stored in an elastic object when work is done on the object.

What happens to the energy in a spring's elastic potential store when the spring is released?

Energy is transferred away from its elastic potential store.

The elastic potential energy equation is Ee = ½ × k × e², where k is the ______.

The elastic potential energy equation is Ee = ½ × k × e², where k is the spring constant.

What are the units of the spring constant, k, and the extension, e, in the elastic potential energy equation?

Spring constant in newtons per metre (N/m); extension in metres (m).

Under what condition does the equation Ee = ½ke² apply?

Only when the spring has not been stretched beyond its limit of proportionality.

Name three ways an object's shape can change so that it stores elastic potential energy.

By stretching, bending or compressing (e.g. springs, rubber bands).

True or False?When using the elastic potential energy equation, an extension given in centimetres can be used directly without converting.

When using the elastic potential energy equation, an extension given in centimetres can be used directly without converting. False. An extension given in cm must be converted to metres before it can be used in the equation.

In a mass-spring system oscillating vertically, does the total energy of the system change as it oscillates?

No — the total energy remains constant, although the amounts in each individual store (elastic, kinetic, gravitational) change.

At the highest point of the oscillation, what is the kinetic energy of the mass?

Zero — the mass is stationary at this point.

At the highest point of the oscillation, is the gravitational potential energy store at a maximum or a minimum?

Maximum — the mass is at its highest point.

As the mass passes through its resting (equilibrium) position, what is true about its kinetic energy?

It is at a maximum, as the mass is moving at its maximum speed.

At the point of maximum extension, which energy store is at its maximum?

The elastic potential store, because the spring is at its maximum extension.

At the point of maximum extension, the mass is ______ and so the energy in its kinetic store is zero.

At the point of maximum extension, the mass is stationary and so the energy in its kinetic store is zero.

True or False?At the point of maximum extension, the gravitational potential energy store of the mass is at a maximum.

At the point of maximum extension, the gravitational potential energy store of the mass is at a maximum. False. At this point, the mass is at its lowest point, so its gravitational potential energy store is at a minimum.

Define specific heat capacity.

Specific heat capacity is the amount of energy required to raise the temperature of 1 kg of a substance by 1 °C.

What determines the temperature of an object?

The amount of energy in the object's thermal store.

State the equation used to calculate the change in thermal energy of a substance.

ΔE = mcΔθ

State the unit of specific heat capacity.

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

A substance has a low specific heat capacity. Does it heat up and cool down quickly or slowly, and why?

It heats up and cools down quickly, because it takes less energy to change its temperature.

Water has a very ______ specific heat capacity, making it ideal for heating homes.

Water has a very high specific heat capacity, making it ideal for heating homes.

True or False?

Good electrical conductors, such as copper, are also good thermal conductors. True. This is because of their free electrons.

What is the aim of this required practical?

To determine the specific heat capacity of a substance by linking the energy transferred to it with its rise in temperature.

In this experiment, state the independent variable and the dependent variable.

Independent variable: time. Dependent variable: temperature.

Name three variables that should be controlled in this experiment.

The mass of the block, the current supplied, and the potential difference supplied.

Combining the equations E = QV and Q = It gives an equation for the thermal energy supplied, E = ______.

Combining the equations E = QV and Q = It gives an equation for the thermal energy supplied, E = IVt.

State the equation used to calculate specific heat capacity, c, from the results of this experiment.

c = ΔE/(mΔθ)

Why is the specific heat capacity value calculated in this experiment likely to be higher than the true value?

Because not all the energy transferred by the heater reaches the block — some is dissipated to the surroundings and some is transferred to the thermometer.

How can a joulemeter improve the accuracy of this experiment?

It measures the energy transferred directly, eliminating errors from the voltmeter, ammeter, and stopwatch.

True or False?

Insulating the block reduces the effect of energy dissipated to the surroundings. True. Insulating the block reduces the amount of energy dissipated, so the measured specific heat capacity is closer to the true value.

Name the three ways energy can be transferred within a system.

Heating, work done by forces (mechanically), and work done when a current flows (electrically).

What two things can an energy transfer by heating cause in a substance?

A rise in temperature, or a change of state.

When is mechanical work done?

When a force acts over a distance.

Electrical work is done when a ______ flows in a circuit.

Electrical work is done when a current flows in a circuit.

What is the unit of work done, and why?

The joule (J), because work done is a measure of energy transferred.

True or False?

Whenever you see the phrase "work is done", it means that energy has been transferred. True.

A box is pushed across a floor. If the system is defined as the box and the floor, describe the energy transfer that takes place due to friction.

Energy is transferred by heating from the kinetic store of the box to the thermal store of the floor.

Define power.

Power is the energy transferred (or work done) per unit time.

State the equation linking power, energy transferred and time.

P = E/t

State the unit of power and its symbol.

The watt (W).

One watt is equal to one ______ per second.

One watt is equal to one joule per second.

Two electric motors lift the same weight by the same height, but motor A does this faster than motor B. Which motor has the greater power?

Motor A.

Two cars accelerate to the same final speed and transfer the same amount of energy. What can be said about the car that reaches that speed sooner?

It has a greater power.

1 kilowatt is equal to 100 watts True or False?

1 kilowatt is equal to 100 watts. False. 1 kilowatt (kW) is equal to 1000 watts.

When calculating power using P = E/t, what unit must time be in, and why?

Seconds, because one watt is equivalent to one joule per second.

Define the law of conservation of energy.

Energy cannot be created or destroyed, it can only be transferred from one store to another.

What does the law of conservation of energy imply about the total energy in a closed system?

It remains constant.

A bat hits a ball. State the useful energy transfer that takes place.

Energy is transferred usefully from the kinetic store of the bat to the kinetic store of the ball.

As a rollercoaster travels from a high point to a low point on the track, describe the useful energy transfer.

Energy is transferred usefully from the gravitational potential store to the kinetic store.

As a person bounces on a trampoline, energy is transferred usefully: elastic potential store → ______ → gravitational potential store.

As a person bounces on a trampoline, energy is transferred usefully: elastic potential store → kinetic store → gravitational potential store.

In an electric heater, state one way energy is usefully transferred to the surroundings.

By heating the air particles in the room.

True or False?
In a rollercoaster, energy is dissipated to the surroundings due to friction and sound.

In a rollercoaster, energy is dissipated to the surroundings due to friction and sound. True.

Energy is transferred mechanically from the elastic potential store of a spring. Where is it transferred to?

The kinetic store of the mass.

Define useful energy.

The energy that is transferred from store to store and used for an intended purpose.

Define wasted energy.

The energy that is not useful for the intended purpose and is dissipated to the surroundings.

What does "dissipated" mean?

Spread out.

An electric motor lifts a load. State the wasted energy transfer taking place.

Energy is transferred from the kinetic store of the motor to the thermal store of the motor and surroundings, due to friction.

Lubrication reduces wasted energy in machines by reducing ______.

Lubrication reduces wasted energy in machines by reducing friction.

Name two factors that affect the effectiveness of insulation.

How well the insulation conducts heat, and how thick the insulation is.

True or False?When describing wasted energy in an exam, it is acceptable to say the energy is "lost".

When describing wasted energy in an exam, it is acceptable to say the energy is "lost". False. Saying energy is "lost" implies it is not conserved; you should say it is dissipated to the surroundings.

Why does friction in bike gears cause wasted energy?

Friction transfers energy from the kinetic energy store of the bike to the thermal energy store of the gears and chain, heating them.

Define thermal conduction.

Thermal conduction is the process where energy is transferred by vibrating particles in a substance, from hot to cold.

How do vibrating particles transfer energy during conduction?

Vibrating particles transfer energy from their kinetic store to the kinetic store of neighbouring particles.

True or False?
Materials with high thermal conductivity heat up more slowly than materials with low thermal conductivity.

Materials with high thermal conductivity heat up more slowly than materials with low thermal conductivity. False. Materials with high thermal conductivity heat up faster than materials with low thermal conductivity.

Give three examples of substances with high thermal conductivity.

Copper, aluminium and other metals.

Define insulator.

An insulator is a substance that is a poor thermal conductor, for example wool, plastic or wood.

Good insulators which keep the energy transfer through them as low as possible have a low thermal conductivity and layers that are as ______ as possible.

Good insulators which keep the energy transfer through them as low as possible have a low thermal conductivity and layers that are as thick as possible.

List the three factors that affect the energy transfer through a layer of insulating material.

The temperature difference across the material, the thickness of the material and the thermal conductivity of the material.

Why does trapped air make loft insulation a good insulator?

Air trapped between the fibres of the insulation has a much lower thermal conductivity than the roof material, reducing energy transfer by conduction.

What is cavity wall insulation made from, and what effect does it have?

Cavity wall insulation is made from blown mineral fibre filled with gas. It lowers the conduction of heat through the walls from the inside to the outside.