Conservation of Energy Flashcards

All 25 cards in this deck

What is the equation for the change in gravitational potential energy of an object raised above the ground?

$\Delta GPE = m \times g \times \Delta h$ (change in GPE in J = mass in kg × gravitational field strength in N/kg × change in vertical height in m)

What value of gravitational field strength $g$ is used for the Earth in GCSE energy calculations, and what are its units?

$10$ N/kg (a value of $9.8$ N/kg is also accepted).

What are the steps to calculate the change in gravitational potential energy?

e.g. a $2$ kg book is lifted $3$ m, $g = 10$ N/kg

1. Write the equation: $\Delta GPE = m \times g \times \Delta h$
2. Substitute: $2 \times 10 \times 3$
3. Evaluate: $60$ J

What are the steps to find the height an object must fall to gain a given amount of energy?

e.g. $6$ kg of water gains $300$ J, $g = 10$ N/kg

1. Write the equation: $\Delta GPE = m \times g \times \Delta h$
2. Rearrange: $\Delta h = \dfrac{\Delta GPE}{m \times g} = \dfrac{300}{6 \times 10}$
3. Evaluate: $5$ m

True or false? The change in gravitational potential energy of an object pushed up a ramp depends on the distance travelled along the ramp.

False. It depends only on the change in vertical height.

What must you do with a mass given in grams before using it in an energy equation?

e.g. $500$ g

Convert it to kilograms by dividing by $1000$: $500$ g $= 0.5$ kg.

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

$KE = \frac{1}{2} \times m \times v^2$ (kinetic energy in J, mass in kg, speed in m/s)

What are the steps to calculate the kinetic energy of a moving object?

e.g. mass $4$ kg moving at $3$ m/s

1. Write the equation: $KE = \frac{1}{2} \times m \times v^2$
2. Substitute: $\frac{1}{2} \times 4 \times 3^2$
3. Evaluate: $18$ J

What are the steps to find the speed of an object from its kinetic energy?

e.g. $KE = 100$ J, mass $8$ kg

1. Write the equation: $KE = \frac{1}{2} \times m \times v^2$
2. Rearrange: $v = \sqrt{\dfrac{2 \times KE}{m}} = \sqrt{\dfrac{2 \times 100}{8}}$
3. Evaluate: $5$ m/s

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

False. Kinetic energy is proportional to (speed)$^2$, so doubling the speed makes the kinetic energy four times larger.

How many joules are there in $1$ kJ?

$1000$ J.

State the principle of conservation of energy.

Energy cannot be created or destroyed, only transferred between stores, so the total energy always stays the same.

What happens to the total energy of a closed system when energy transfers take place inside it?

There is no net change — the total energy stays the same.

What is a Sankey diagram?

An energy transfer diagram in which the width of each arrow is proportional to the amount of energy transferred.

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

False. The energy is transferred to other stores; the total amount of energy does not change.

True or false? In a Sankey diagram the total width of the arrows coming out equals the width of the arrow going in.

True. Energy is conserved, so the total energy out equals the total energy in.

What are the steps to find the speed of an object that has fallen from rest, using energy?

e.g. dropped from $5$ m, $g = 10$ N/kg

1. GPE lost = KE gained: $m g \Delta h = \frac{1}{2} m v^2$
2. Cancel $m$ and rearrange: $v^2 = 2 g \Delta h = 2 \times 10 \times 5 = 100$
3. Square root: $v = 10$ m/s

Which energy stores change, and how, as a ball thrown vertically upwards rises?

The kinetic energy store decreases and the gravitational potential energy store increases.

Which energy stores change, and how, when a moving object hits an obstacle?

The kinetic energy store of the object decreases; the thermal energy store of the object, obstacle and surroundings increases (some energy is also transferred by sound).

Which energy store increases when an object is accelerated by a constant force?

The kinetic energy store increases, as energy is transferred mechanically by the force doing work on the object.

Describe the energy transfers that happen when a cyclist uses the brakes to stop.

The kinetic energy store of the cyclist and bicycle decreases; the thermal energy store of the brakes and surroundings increases.

Describe the energy transfers when water is brought to the boil in an electric kettle.

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

True or false? Because energy is conserved, a dropped ball must bounce back to its original height.

False. Some energy is transferred to the thermal store of the ball, ground and surroundings (and by sound), so less is left in the gravitational potential store.

What shape is a graph of kinetic energy against speed for a fixed mass?

An upward curve through the origin (getting steeper), because $KE \propto v^2$ — not a straight line.

True or false? On a graph of kinetic energy against speed for a fixed mass, the kinetic energy read off at $20$ m/s is four times the value read off at $10$ m/s.

True. Kinetic energy is proportional to (speed)$^2$, so tripling the speed instead would give nine times the energy.