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Physics 8463 - 4.1 Energy

4.1.1 Energy changes in a system, and the ways energy is stored before and after such changes

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4.1.1.1 Energy stores and systems

A system is an object or group of objects.

There are changes in the way energy is stored when a system changes.

You should be able to describe all the changes involved in the way energy is stored when a system changes, for common situations. For example:

an object projected upwards

a moving object hitting an obstacle

an object accelerated by a constant force

a vehicle slowing down

bringing water to a boil in an electric kettle.

Throughout this section on Energy you should be able to calculate the changes in energy involved when a system is changed by:

heating

work done by forces

work done when a current flows

use calculations to show on a common scale how the overall energy in a system is redistributed when the system is changed.

See Energy Types and Conservation of Energy

 

 

 

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4.1.1.2 Changes in energy

You should be able to calculate the amount of energy associated with:

a moving object,

a stretched spring and

an object raised above ground level.

  

The kinetic energy of a moving object can be calculated using the equation:

kinetic energy = ½ × mass × speed 2

Ek = ½ mv2

This equation is NOT given on the Physics equation sheet - you need to know it, what the letters mean and are measured in!

Ek = kinetic energy, in joules, J

m = mass, in kilograms, kg

v = speed, in metres per second, m/s

The amount of elastic potential energy stored in a stretched spring can be calculated using the equation:

elastic potential energy = ½ × spring constant × extension2

Ee = ½ke2

(assuming the limit of proportionality has not been exceeded)

This equation is given on the Physics equation sheet - but you need to know what the letters mean and are measured in - so I suggest learn it anyway!

Ee = elastic potential energy,in joules, J

k = spring constant, in newtons per metre, N/m

e = extension, in metres, m

The amount of gravitational potential energy gained by an object raised above ground level can be calculated using the equation:

GPE = mass × gravitational field strength × height

ΔEp = mgΔh

This equation is NOT given on the Physics equation sheet - you need to know it, what the letters mean and are measured in!

ΔEp = change in gravitational potential energy, in joules, J

m = mass, in kilograms, kg

g = gravitational field strength, in newtons per kilogram, N/kg (In any calculation the value of the gravitational field strength (g) will be given to you).

Δh = difference in vertical height, in metres, m

  
4.1.1.3 Energy changes in systems

The amount of energy stored in or released from a system as its temperature changes can be calculated using the equation:

change in thermal energy = mass × specific heat capacity × temperature change

ΔE = mcΔθ

This equation is NOT given on the Physics equation sheet - you need to know it, what the letters mean and are measured in!

ΔE = change in thermal energy, in joules, J

m = mass, in kilograms, kg

c = specific heat capacity, in joules per kilogram per degree Celsius, J/kg °C

Δθ = temperature change, in degrees Celsius, °C

The specific heat capacity of a substance is the amount of energy required to raise the temperature of one kilogram of the substance by one degree Celsius.

Required practical activity 1:

An investigation to determine the specific heat capacity of one or more materials.

The investigation will involve linking the decrease of one energy store (or work done) to the increase in temperature and subsequent increase in thermal energy stored.

Also met in 4.32
4.1.1.4 Power

Power is defined as the rate at which energy is transferred or the rate at which work is done.

power = energy transferred/time

P = E/t

power = work done/time

P = W/t

These two equations are NOT given on the Physics equation sheet - you need to know them, what the letters mean and are measured in!

P = power in watts, W

E = energy transferred, in joules, J

t = time, t, in seconds, s work done, W, in joules, J

An energy transfer of 1 joule per second is equal to a power of 1 watt.

You should be able to give examples that illustrate the definition of power eg comparing two electric motors that both lift the same weight through the same height but one does it faster than the other.