Conservation of Energy (OCR GCSE Physics A (Gateway)): Flashcards
Exam code: J249
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State the law of conservation of energy.
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Cards in this collection (35)
State the law of conservation of energy.
The law of conservation of energy states that energy cannot be created or destroyed — it can only be transferred from one store to another.
True or False?
In a closed system, the total amount of energy changes over time.
False.
In a closed system, the total amount of energy remains constant — energy is never created or destroyed, only transferred between stores.
Why is energy sometimes described as "wasted" even though it cannot be destroyed?
Energy is described as wasted when it is dissipated to the surroundings (for example, by heating or radiation) in a way that is not useful to the system.
When a bat hits a ball, some energy from the bat's kinetic store is transferred usefully to the ball's .......... store. The remaining energy is dissipated to the .......... store of the surroundings.
When a bat hits a ball, some energy from the bat's kinetic store is transferred usefully to the ball's kinetic store. The remaining energy is dissipated to the thermal store of the surroundings.
Define the term "dissipation" in the context of energy transfers.
Dissipation is the spreading out of energy to the surroundings (typically by heating or radiation) so that it becomes less concentrated and less useful.
True or False?
When a person jumps on a trampoline, energy is transferred from the trampoline's elastic potential store back to the person's kinetic store.
True.
As the trampoline rebounds, elastic potential energy is transferred back to the kinetic store of the person, which then transfers to the gravitational potential store as they rise.
Describe the useful energy transfer sequence when a person bounces on a trampoline.
As the person lands, energy moves from their kinetic store to the elastic potential store of the trampoline. As they rebound, it transfers back to their kinetic store then to their gravitational potential store as they rise.
In a closed system, the total energy transferred into the system must be .......... to the total energy transferred out of the system.
In a closed system, the total energy transferred into the system must be equal to the total energy transferred out of the system.
Give two ways in which energy can be dissipated to the surroundings.
Energy can be dissipated to the surroundings by heating (thermal transfer) and by radiation (such as light or sound).
Identify the energy store that fills as a ball thrown upwards gains height.
As the ball rises, its gravitational potential store fills as energy is transferred from its kinetic store.
True or False?
When a car brakes, most of the energy from its kinetic store is usefully transferred to the wheels.
False.
When a car brakes, energy from the kinetic store is mostly dissipated to the thermal store of the surroundings via friction between the brakes and tyres.
Describe the energy transfers that occur when a moving car hits a wall.
The car's kinetic store empties rapidly. Energy is transferred mechanically to the thermal store of the wall and is also transferred by heating to the thermal store of the surroundings via sound.
When an electric kettle is switched on, energy is transferred .......... from the mains supply to the thermal store of the heating element, and then .......... to the thermal store of the water.
When an electric kettle is switched on, energy is transferred electrically from the mains supply to the thermal store of the heating element, and then by heating to the thermal store of the water.
State what happens to the energy in a vehicle's kinetic store when it accelerates from rest.
As the vehicle accelerates, energy from the chemical store of the fuel is transferred to the vehicle's kinetic store, which increases.
True or False?
Sound produced when a car crashes transfers energy to the thermal store of the surroundings by making air particles vibrate.
True.
Sound waves carry energy away from the collision and cause air particles to vibrate, transferring energy to the thermal store of the surroundings.
What energy store fills when a ball is thrown upwards, and what does it empty as this happens?
The gravitational potential store fills as the ball rises. At the same time, the kinetic store empties as the ball slows down.
When a vehicle slows down, energy is transferred from its .......... store to the thermal store of the surroundings, mainly by heating due to .......... between the tyres and the ground.
When a vehicle slows down, energy is transferred from its kinetic store to the thermal store of the surroundings, mainly by heating due to friction between the tyres and the ground.
Describe the energy transfer when a person throws a ball upwards (start from the person's chemical store).
Energy moves from the chemical store of the person to the kinetic store of the ball, then to the gravitational potential store of the ball as it rises.
What is a closed system?
A closed system is one in which there is no net change to the total energy of the system.
The equation for mechanical work done is W = F × .........., where F is the force in newtons and the result is measured in joules.
The equation for mechanical work done is W = F × s, where F is the force in newtons and the result is measured in joules.
What is specific heat capacity?
Specific heat capacity is the energy needed to raise the temperature of 1 kg of a substance by 1 °C.
State the equation linking energy transferred, current, potential difference and time.
Energy transferred E = I × V × t, where I is current (A), V is potential difference (V) and t is time (s).
True or False?
1 kilowatt-hour is equal to 3.6 × 106 joules.
True.
1 kW h = 1000 W × 3600 s = 3.6 × 106 J, since 1 kW = 1000 W and 1 hour = 3600 s.
The equation for the thermal energy transferred when heating an object is ΔE = m × c × .........., where c is the specific heat capacity.
The equation for the thermal energy transferred when heating an object is ΔE = m × c × Δθ, where c is the specific heat capacity and Δθ is the change in temperature.
Describe the energy transfer that occurs when a current flows through a resistor.
Energy is transferred electrically from the power supply to the thermal store of the resistor, causing it to heat up. Some energy is then dissipated to the thermal store of the surroundings.
What does a kilowatt-hour (kW h) measure?
A kilowatt-hour is a unit of energy equal to 1 kW of power used for 1 hour.
State the equation for kinetic energy.
Kinetic energy E = 1/2 × m × v², where m is mass (kg) and v is speed (m/s). The unit is joules (J).
True or False?
An object must be moving to have energy in its kinetic store.
True.
Kinetic energy depends on both mass and speed — an object only has energy in its kinetic store if it is in motion.
State the equation for gravitational potential energy.
Gravitational potential energy E = m × g × h, where m is mass (kg), g is gravitational field strength (N/kg) and h is height (m).
The equation for elastic potential energy is Ee = 1/2 × k × .......... , where k is the spring constant and the result is in joules.
The equation for elastic potential energy is Ee = 1/2 × k × x², where k is the spring constant and the result is in joules.
Describe the energy transfers that occur as a vertical spring oscillates between its highest and lowest points.
At the highest point, the spring has maximum gravitational potential energy and zero kinetic energy. At the midpoint, kinetic energy is maximum. At the lowest point, elastic potential energy is maximum and gravitational potential energy is at a minimum.
True or False?
The elastic potential energy equation Ee = ½kx² only applies when the spring is within its limit of proportionality.
True.
The equation Ee = 1/2 × k × x² assumes the spring has not been stretched beyond its limit of proportionality, beyond which the spring no longer obeys Hooke's law.
Define gravitational potential energy.
Gravitational potential energy is the energy an object has due to its height in a gravitational field. It increases when an object is lifted and decreases when it falls.
When a spring is stretched, work is done on the spring and energy is transferred to its .......... store. When the spring is released, energy is transferred away from that store.
When a spring is stretched, work is done on the spring and energy is transferred to its elastic potential store. When the spring is released, energy is transferred away from that store.
What three energy stores are involved when a vertical spring oscillates, and how do they change?
The kinetic, gravitational potential and elastic potential stores all change during oscillation. As the spring moves between its extreme positions, energy transfers between these stores while the total energy remains constant.
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