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Summary AQA GCSE Complete Physics Notes
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4.1 Energy4.1.1 Energy changes in a system, and the ways energy is
stored before and after such changes
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.
4.1.1.2 Changes in energy
The kinetic energy of a moving object can be calculated using the equation:
kinetic energy = 0.5 × mass × speed2
Ek = ½ m v2
kinetic energy, Ek, in joules, J
mass, m, in kilograms, kg
speed, v, 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 = 0.5 × spring constant × extension^2
Ee = ½ k e^2
(assuming the limit of proportionality has not been exceeded)
elastic potential energy, Ee, in joules, J
spring constant, k, in newtons per metre, N/m
extension, e, in metres, m
The amount of gravitational potential energy gained by an object raised above ground level
can be calculated using the equation:
g . p . e . = mass × gravitational field strength × height
Ep = m g h
gravitational potential energy, Ep, in joules, J
mass, m, in kilograms, kg
gravitational field strength, g, in newtons per kilogram, N/kg (In any calculation the value of
the gravitational field strength (g) will be given).
height, h, 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
∆E=mc∆θ
change in thermal energy, ∆E, in joules, J
mass, m, in kilograms, kg
specific heat capacity, c, in joules per kilogram per degree Celsius, J/kg °C
temperature change, ∆θ, 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.
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
power, P, in watts, W
energy transferred, E, in joules, J
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.
Of two motors (or other devices) that do a task, the faster one requires more power.
4.1.2 Conservation and dissipation of energy
4.1.2.1 Energy transfers in a system
Energy can be transferred usefully, stored or dissipated, but cannot
be created or destroyed.
Where there are energy transfers in a closed system, there is no net change to the total
energy (e.g. heating a kettle).
In all system changes energy is dissipated, so that it is stored in less useful ways. This
energy is often described as being ‘wasted’.
,There are ways of reducing unwanted energy transfers, for example through lubrication and
the use of thermal insulation.
The higher the thermal conductivity of a material the higher the rate of energy transfer by
conduction across the material.
The rate of cooling of a building is affected by the thickness and thermal conductivity of its
walls.
4.1.2.2 Efficiency
The energy efficiency for any energy transfer can be calculated
using the equation:
efficiency = useful output energy transfer /total input energy transfer
Efficiency may also be calculated using the equation:
efficiency = useful power output /total power input
The way to increase efficiency is to reduce the amount of energy shifted to the thermal store
of surroundings (i.e. wasted as heat). This is either the lube again or insulation.
4.1.3 National and global energy resources
The main energy resources available for use on Earth include: fossil fuels (coal, oil and gas),
nuclear fuel, biofuel, wind, hydro- electricity, geothermal, the tides, the Sun and water waves.
A renewable energy resource is one that is being (or can be) replenished as it is used.
The uses of energy resources include: transport, electricity generation and heating.
4.2 Electricity
4.2.1 Current, potential difference and resistance
4.2.1.1 Standard circuit diagram symbols
Circuit diagrams use standard symbols.
, 4.2.1.2 Electrical charge and current
Electric current is a flow of electrical charge. The size of the electric current is the rate of flow
of electrical charge.
Charge flow, current and time are linked by the equation:
charge flow = current × time
Q=It
charge flow, Q, in coulombs, C
current, I, in amperes, A (amp is acceptable for amperes
time, t, in seconds, s
A current has the same value at any point in a single closed loop.
4.2.1.3 Current, resistance and potential difference
The current (I) through a component depends on both the resistance (R) of the component
and the potential difference (V) across the component. The greater the resistance of the
component the smaller the current for a given potential difference (pd) across the
component.
Current, potential difference or resistance can be calculated using the equation:
potential difference = current × resistance
V=IR
potential difference, V, in volts, V
current, I, in amperes, A (amp is acceptable for ampere)
resistance, R, in ohms, Ω
4.2.1.4 Resistors
For some resistors, the value of R remains constant but that in others it can change as the
current changes.
The current through an ohmic conductor (at a constant temperature) is directly proportional
to the potential difference across the resistor. This means that the resistance remains
constant as the current changes.
The resistance of components such as lamps, diodes, thermistors and LDRs is not constant;
it changes with the current through the component.
The resistance of a filament lamp increases as the temperature of the filament increases.
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