Physics
Forces
Velocity and Acceleration
Energy transfer and efficiency
Heat Transfer
Kinetic energy and Gravitational Potential energy
Momentum
Electrical Circuits
Distance / Time graphs
• Horizontal lines mean the object is stationary.
• Straight sloping lines mean the object is travelling at a constant speed.
• The steeper the slope, the faster the object is travelling. • To work out the speed, you need to calculate the gradient. • Gradient = change in distance (m) / change in time (s)
Velocity/Time Graph part 1
• Velocity is speed in a given direction
• Acceleration is the change in velocity per second when and object speeds up. The units are m/s2
• Deceleration is the change in velocity per second when an object slows down.
Where v = the final velocity (m/s)
u = the initial velocity (m/s)
t = time taken (s) Velocity/Time Graph part 2
• Horizontal lines mean the object is travelling at a constant velocity.
• Straight sloping lines mean the object is accelerating or decelerating.
• The steeper the slope, the faster the acceleration or deceleration. • A curved line means the acceleration is changing. • The area under the graph is the distance travelled.
Using Graphs
•The acceleration or deceleration of an object can be calculated from the gradient on a velocity – time graph
•The speed of an object can be calculated from the gradient on a distance – time graph
•The area underneath a velocity – time graph tells you the distance that an object has travelled
Vectors and Velocity
Quantities which have a direction and size are known as VECTOR QUANTITIES. 4 Examples
• Displacement – distance travelled in a particular direction. • Velocity – speed in a particular direction.
• Force – always has a size and direction.
• Acceleration – it has size and direction
Speed (m/s) = distance (m) ÷ time (s)
Acceleration (m/s2) = change in velocity (m/s) ÷ time (s)
Conduction and Convection
Convection Currents
Definition
Convection is the transfer of heat in liquids and
gases.
The hotter the liquid/gas the particles move
faster and spread out. This means the
gas/liquid becomes less dense.
The less dense gas/liquid rises and the more
dense gas/liquid sinks.
Model Question (3)
Explain how heat is
transferred by the
process of convection
from the gas flame at
the bottom of the oven
to the potatoes at the
top of the oven.
Model Answer
The air particles are heated by the gas
flame and gain energy. This causes the
particles to move faster and spread out.
Because the particles are spread out the
hot air becomes less dense and rises.
Because the particles have to rise and fall for
convection to occur it can only happen in liquids and
gases, where the particles are free to move.
P1.1.3a/P1.1.1
Real world Examples
A lava lamp – The
light bulb heats the
wax causing it heat
up and rise. The wax
cools at the top and
sinks again.
Conduction and Convection
Conduction
Definition
Conduction is how heat energy is transferred
through solids when they are heated. Heat
energy can also be passed from one solid to
another by conduction.
Conductors are materials which transfer energy
more easily. Insulators are materials which don’t
transfer heat as well for example, Glass and
plastics.
As the conductor is heated the atoms gain more
energy and vibrate more. This causes them to
collide with other atoms transferring the heat
energy.
Model Question
Explain in terms of
particles how heat
is transferred through
the base of the ban?
Model Answer
Atoms in the base of the saucepan gain
thermal energy from the hob.
This causes the atoms in the saucepan to
vibrate. Due to the vibration the atoms will
collide with other atoms and pass on their
thermal energy.
Why are metals good conductors?
In a metal lattice electrons from the outer
shell are freed from their atoms. This causes
those atoms to become positively charged
ions. Heating the metal causes these ions to
vibrate more. This extra energy is
transferred from hotter to colder parts of
the metal by the free electrons colliding with
other ions and transferring their energy
P1.1.3a/P1.1.1
Infra Red and Rate of Heat transfer
Definition
All objects emit (give out) and absorb (take in) infra red radiation. The hotter an object is the more
infra red radiation it gives out.
Absorption, emission and reflection
Black (Dark, matt) surfaces are both
good absorbers and good emitters of
infra red radiation.
Light shiny surfaces are both poor
absorbers and poor emitters of infra red
radiation.
Light, Shiny Surfaces are good reflectors
of infra red radiation
Infra Red emission and surface area
The larger a surface area an object has the
more infra red radiation it will emit.
Examples of this are car engines having
cooling fins to allow for a better cooling
system. In nature you can see the application
of this with African elephants having larger
ears than Indian elephants to help with
cooling and arctic foxes having smaller ears
to prevent heat loss.
Water from the tank is slowly
pumped
through copper pipes inside the
solar panel where the water is
heated by energy from the Sun.
Question — Explain why the copper pipes inside the
solar panel are painted black.
Model Answer — Black is a good absorber of
radiation therefore, more of the energy from the
Sun is transferred into heating the water.
Difference in temperature
The bigger the temperature
difference between an object and
its surroundings the faster the
rate at which heat is transferred
P1.1.3/P1.1.1
Energy transfers and efficiency
Energy can be transferred usefully, stored or dissipated but cannot be created or destroyed.
When energy is transferred only part of it may be usefully transferred, the rest is “wasted”.
Wasted energy is eventually transferred to the surroundings which become warmer. The wasted
energy becomes increasingly spread out so becomes less useful.
Efficiency
Efficiency is a measure of how much wasted energy is produced compared to useful energy. An
efficient object is one which produces more useful energy than wasted.
Efficiency = Useful energy out
Total energy in
Efficiency = Useful power out
Total power in
P1.2.1
Energy transfers and efficiency
P1.2.1
Transferring electrical energy
The amount of energy an appliance transfers
depends on how long the appliance is switched on
for and the power it draws.
E
P
t
Energy transferred = Power x Time
E P t
kWh kW hrs
J W s The units may be given as kilowatthours, kilowatts and hours, or Joules, watts and seconds
P1.3.1
Forces between objects
• A force can change the shape of an object or change its state of rest (stop an object) or its motion (change its velocity)
• All forces are measured using the unit Newton (N)
•A force is a push or a pull.
•When two bodies interact, the forces they exert on each other are equal in size and opposite in direction. •For every action force there is an equal and opposite reaction force
Resultant forces
• Whenever two objects interact, the forces they exert on each other are equal and opposite
• A number of forces acting at a point may be replaced by a single force that has the same effect on the motion as the original forces all acting together. This single force is the resultant force
The resultant force acting on an object can cause a change in its state of rest or motion.
Force and acceleration
Force (N) = Mass (kg) x acceleration (m/s2)
•The size of acceleration depends on:
• Size of the force
• Mass of the object
• The larger the resultant
force on an object the greater its acceleration.
• The greater the mass of an object, the smaller its acceleration will be for a given force.
On the road
Stopping distance = thinking distance + breaking distance
Factors affecting thinking distance:
1. Poor reaction times of the driver caused by
1. Age of driver
2. Drugs e.g. alcohol
3. Tiredness
4. Distractions
2. Visibility
3. Speed
Investigating friction. How much force is needed to move weights on different surfaces?
Factors affecting breaking distance:
1. Mass of vehicle
2. Speed of vehicle
3. Poor maintenance
4. Poor weather conditions
5. State of the road
6. Amount of friction
between the tyre and the road surface. Falling objects
Weight and mass are not the same thing
•The weight of an object is the force of gravity on it. Weight is measured in Newtons
(N)
•The mass of an object is the quantity (amount) of matter in it. Mass is measured in Kilograms (Kg)
Weight (N) = Mass (kg) x gravity (N/kg)
In a vacuum • All falling bodies accelerate at the same rate. In the atmosphere • Air resistance increases with increasing speed. • Air resistance will increase until it is equal in size to the weight of a falling object.
• When the two forces are balanced, acceleration is zero and TERMINAL VELOCITY is achieved. • An object acted on only by the Earths gravity accelerates at about 10 m/s2
Stretching and squashing
A force applied to an elastic object such as a spring will result in
the object stretching and storing elastic potential energy
Weight (N)
Length (mm)
Extension (mm)
Hooke’s Law states:
0
120
0
The extension of a 1.0
152
32
spring is directly proportional to the 2.0
190
70
force applied, provided 3.0
250
105
that its limit of proportionality is not The extension of a material is its
current length minus it original length.
exceeded.
Force applied (N) = spring constant (N/m) x extension (m)
F = K x e
Energy and work
Key definitions
Energy transferred = work done
• Work – the amount of energy transferred. Measured in Joules (J)
• Power – The rate of doing work. Measured in Watts (W). 1 joule per second is 1 watt.
Power (W) = When a force causes an object to move a distance, work is done
Use this formula:
Work Done (J) = Force (N) x distance moved (m)
Or W = F x D
Example – if a 1kg mass (10N) is moved through a distance of 2 metres the work done is 20J.
Work Done (J) Time taken (s)
Example – if a 24J of work is done over a 30 second period, the Power would be 24 ÷ 30 = 0.8W Could you work out how much work you have done climbing a flight of stairs?
Electrical power and energy (extension)
A current in a wire is a flow of electrons. As the electrons move in a metal they collide with the ions in the lattice and transfer some energy to them.
This is why a resistor heats up when a current flows through. Electrical power (watt, W) = current (ampere, A) x potential difference (volt, V) P = I x V
Energy transferred (joule, J) = current (ampere, A) x potential difference (volt, V) x time (second, s) E = I x V x t
Distinguish between the advantages and disadvantages of the heating effect of an electric current Advantages
Disadvantages
Useful Heating a kettle
Wasted energy
Useful in Fires
Cause burns
Gravitational potential energy (GPE)
Gravitational Potential Energy – The energy that an object has by virtue of its position in a gravitational field
When an object is moved up, its gravitational potential energy increases.
When an object is moved down, its gravitational potential energy decreases
Change in gradational potential energy (J) =weight (N) x change in height(m)
Change in gravitational potential energy = mass (kg) x gravitational field strength (N / kg) x change in height (m)
E = m x g x h
Kinetic energy
When an object speeds up or slows down. Its kinetic energy
increases or decreases.
The forces which cause the change in speed do so by doing work.
The momentum of an object is produced by the object’s mass and
velocity.
The kinetic energy of an object depends on its mass and speed
Kinetic energy (J) = ½ x mass (kg) x speed2 (m/s)2
Elastic potential energy (the energy stored in an elastic object when work is done) can be transferred into kinetic energy. Momentum
Momentum is a property of moving objects
In a closed system the total momentum before an event is equal to the total momentum after the event. This is called conservation of momentum.
p = momentum (Kg m/s)
m = mass (Kg)
p = m x v v = velocity (m/s)
• Can you calculate the momentum of an athlete running at a velocity of 5 m/s with a mass of 75 Kg?
• If a train is 1200 Kg and is moving at a velocity of 5.0 m/s and collides with a stationary train with a mass of 1500 kg. The trains will move together after the collision.
Can you calculate the momentum of both trains before the collision? And show the velocity of the wagons after the collisions?
P2 4.2 Electric circuits
Electric circuits are assembled from components. Each component has an internationally‐agreed symbol. A circuit diagram shows how components are connected using the standard symbols.
Exam tip: you need to learn a set of symbols so that you can say what a symbol represents or sketch the symbol for a named component.
Symbol sets can be found on most GCSE physics revision sites or a set of flashcards can be found at:
https://www.examtime.com/en‐US/p/289885 When components are connected in a complete circuit, an electric current flows. An electric current is a flow of charge. The charge is carried by a very large number (millions of millions) of electrons, each of which has a negative charge. The unit of current is the ampere (A) and the unit of charge is the coulomb (C). current = charge
time
I = Q
t
I = current in amperes (A)
Q = charge in coulombs (C)
t = time in seconds (s)
So one ampere is one coulomb per second.
P2 4.2 Electric circuits – ammeters and voltmeters
Ammeters and voltmeters look very similar, which can cause confusion. But they measure different things and must be placed in different positions in the circuit.
Ammeter
Voltmeter
Measures
current
potential difference (PD)
Units of measurement
amperes (A)
volts (V)
Position in circuit
in series – the current flows through it
in parallel – the current flows past it
This diagram shows the ammeter (A) connected in series – the current flows through it. If the ammeter was removed, the circuit would be incomplete and would not work. The current is the same wherever the ammeter is placed in the circuit.
But the voltmeter (V) is connected in parallel – the current flows past it, through the resistor in this case. If the voltmeter was removed, the circuit would still work. If the voltmeter was placed in a different position in the circuit, such as across the battery or fuse, the readings would be different.
So what is potential difference? Although each electron moving when an electric current flows has the same charge, each charge can carry a different amount of energy. It’s rather like supermarket lorries. Although each lorry can carry the same amount of food, different foods have different amounts of energy. So the same size lorries can carry different amounts of energy. In electric circuits, the same amount of charge can carry different amounts of energy. P2 4.2/3 Electric circuits – potential difference and resistance
On the last slide we said that potential difference measures how much energy a certain amount of charge carries. The unit of potential difference is the volt (V) – you will have previously called this voltage, the correct term is now potential difference. The equation to calculate potential difference is …
potential difference = energy or work done
charge
V = W
Q
V = potential difference in volts (V)
W = energy or work done in joules (J)
Q = charge in coulombs (C)
So one volt is one joule per coulomb.
Resistance is a measure of how difficult it is for an electric current to pass through a component. In general, the thinner a wire is, the more difficult it is for an electric current to pass through it when given the same amount of energy (which you now know is the potential difference). But different materials produce different resistances too. So the resistance depends on the material and its size. The unit of resistance is the ohm (Ω). The equation is …
resistance = potential difference
current
R = V
I
R = resistance in ohms (Ω)
V = potential difference in volts (V)
So one ohm is one volt per ampere.
I= current in amperes (A)
P2 4.3/4 Current – potential difference graphs
When we experiment to measure the current that flows through different components as we change the potential difference, we find that each component produces a different shape graph that is characteristic of that component. The simplest is for a resistor with a low resistance, such as a piece of wire, which is shown by the blue line in the graph. It is a straight line that goes through the origin – when the potential difference is zero, the current is also zero. The straight line means that the current is directly proportional to the potential difference – as the potential difference is doubled, the current also doubles. This type of component obeys Ohm’s law. We say the component is ohmic.
“The current through a resistor at constant temperature is
directly proportional to the potential difference applied.”
The red line shows the characteristic flattened S‐shape for a filament lamp (an old‐fashioned type of light bulb). A filament lamp does not obey Ohm’s law because the graph is not a straight line. This shape means that the resistance increases as the potential different increases. This is explained by what happens in the filament, which is a thin piece of wire. As the current flows, the wire gets hot and glows (which is why the filament bulb produces light). As the wire gets hot, the metal ions vibrate more making it more difficult for the electrons to move through the filament. So the resistance increases as the temperature rises.
The bottom graph shows the shape for a diode. A diode is a component that only allows an electric current to flow in one direction. So when a potential difference is applied in the reverse direction, the current is always zero. At first, the current stays zero even when the potential difference is applied in the forward direction, but a current starts to flow once a certain potential difference has been reached. P2 4.5/6 Series and parallel circuits
In a series circuit (on the left), there is only one way for the current to flow round the circuit. If one lamp breaks, neither lamp will light because there is no longer a complete circuit. In a series circuit, each lamp will be dimmer than a single lamp.
In a parallel circuit (on the right), the current splits and flows two (or more) ways. If one bulb breaks, the other will still light because there is still a complete circuit. In a parallel circuit, each lamp will be as bright as a single lamp. Parallel circuits are used in homes, offices and cars so that a single failure does not cause all the lights to go out.
Rules for SERIES circuits
Rules for PARALLEL circuits
The current in a series circuit is the same wherever you measure it. Wherever you place an ammeter, the reading will be the same.
The total current in a series circuit is the sum of the currents in each branch. If you connect an ammeter before the circuit splits, the reading will equal the total of the readings taken in each branch.
The total potential difference in a series circuit is the sum of the individual potential differences. If you connect a voltmeter across both lamps, the reading will equal the total of the readings taken across each lamp.
Similarly, if two or more cells or batteries are connected in the same direction, the total potential difference is the sum of the individual potential difference. For example, two 1.5 batteries connected in the same direction will give a total potential difference of 3.0V.
The potential difference in a parallel circuit is the same in each branch, and in each component if there is only one in each branch. If you connect a voltmeter across each lamp, the readings would be the same.
Similarly, if two or more identical cells or batteries are connected in parallel in the same direction, the potential difference is the same as each cell or battery. So two 1.5 batteries in parallel would still give 1.5V.