AQA GCSE Physics -Paper 2

Scalar

quantities with only magnitude such as speed, distance, time and mass.

Vector

quantities with a magnitude and direction e.g. acceleration, force momentum and weight. A vector quantity can be represented as an arrow, the size representing the magnitude and the direction, the direction.

Non-contact force

the objects are physically separate e.g. magnetism/gravity/electrostatic.

3 non-contact forces

magnetism

gravity

electrostatic

Contact force

objects are physically touching .

3 contact forces

friction

resistance

tension

Gravity

Weight is the force acting on an object due to gravity, it acts on an objects 'centre of mass' and is directly proportional to mass

Weigh Equation

Weight = mass x grav. field strength

Resultant forces

A number of forces acting on an object may be replaced by a single force that has the same effect as all the original forces acting together, this single force is the called the resultant force.

In a tug of war, 1000N to the left and 800N to the right. The resultant force is 200N to the left

Free-Body force diagrams

When an object is acted on by more than one force you can draw a free-body force diagram to work out the resultant force on the object. It shows the forces acting on an object without any other objects or other forces shown. Each force is shown on the diagram by a vector, which is an arrow pointing in the direction of the force. ( the normal force is that component of the contact force that is perpendicular to the surface that an object contacts)

How to resolve forces (force diagram steps)

1. Decide on a scale of N per cm.

2. Draw the 2 given forces, giving respect to their length (more cm if more N) and the angle that you are given

3. Draw a line connecting the end of the 2 other lines to form a triangle

4. Measure the length of this line, then convert it to N by using the scale

Relationship between Joules and work done

When a force causes and object to move, work is done on the object so when the force causes displacement. One joule of work is done when a force of 1 newton displaces an object 1 metre. 1 Joule = 1 newton-metre.

Equation for word done

work done = force x distance

Elastic Deformation

When an elastic object is stretched, bent, twisted or compressed it will return to its original form

Inelastic Deformation

Polyethene bags will not return to their original shape after being deformed, this is inelastic.

Hooke's Law

The extension of an elastic object is directly proportional to the force applied as long as the limit of proportionality is not exceeded:

Equation for force (Hooke's law)

force = spring constant x extention

Extension and compression's relationship with elastic potential energy explanation

A force that stretches or compresses a spring does work and elastic potential energy is stored in the spring, provided the spring is not elastically deformed, the work done and elastic potential energy are equal. Before the limit of proportionality is breeched the relationship is linear (directly proportional) afterwards it is non-linear

Elastic potential energy equation (given)

Elastic potential energy = 1/2 x spring constant x extension^2

Moment meaning

The turning effect of a force is called its 'moment'

Moment equation

moment = force x distance

Balancing

If an object is balanced the clockwise moment is equal to the anticlockwise moment.

Levers

A lever consists of a load, effort and pivot, levers use moments to multiply a force, they allow a larger force to act on the load that is supplied by the effort.

Engine Gears

Engine gears have two main jobs, producing torque and speed, however they are inversely proportional so gears are made to make a compromise between the two in any given situation.

When starting a car we want high torque, which means being able to carry a greater load , and consequentially less speed. In low gears this is achieved as...

1. The engine causes a small cog on the engine axle to rotate.

2. The small cog on the engine axle is connected to a larger cog on the wheel axle, which increases the torque (moment) exerted.

3.A high turning force but low speed is exerted on the wheel (low gear)

When a car is already moving, we want a high speed which is achieved as...

1.The engine causes a larger cog on the engine axle to turn.

2.The larger cog on the engine axle is connected to a smaller cog on the wheel axle, causing a high rate of rotation, but a low turning force.

3. The wheel rotates quickly, causing a high speed

Unit for pressure

Pressure is a measure of force per unit of area, the si unit is Pascals (Pa) which is one N/m^2

Pressure Equation

Pressure = force normal to the surface/area

Manometer with oil and water

The pressure is equal at each end of the tube, so the less dense oil is higher.(p=hdg)(on sheet)

Buoyancy

The partially or fully submerged object experiences more pressure at the bottom that on top, this creates a resultant up thrust force called buoyancy.

Why objects float

An object will rise or float if it is less dense than the substance it is in or if it displaces an amount of the substance that it is floating in greater than it weights, steel ships have a lot of air in them and therefore weigh less than the equivalent amount of water that the ship displaces, so it floats this is because the up thrust from the pressure is greater than the down force of the weight.

Atmospheric pressure and how it varies by altitude

Air pressure is created by air molecules colliding with a surface, if we consider the density of these particles being same at all attitudes, the fact the volume of the earth's atmosphere increases as we rise in altitude then we can tell that the air pressure will decrease as there is more space for the same amount of particles to bump into each other. Also air is naturally less dense further away from the ground, reducing collisions and therefore pressure.

Distance

How far an object moved regardless of direction (scalar).

Displacement

The length of a straight line from the starting point to the finish point with its direction (vector).

Walking speed estimate

1.5 m/s

Running speed estimate

3 m/s

Cycling speed estimate

6 m/s

Car speed estimate

13-30 m/s

Train speed estimate

50 m/s

Aeroplane speed estimate

250 m/s

Equation for distance

Distance travelled = speed x time

Velocity

Velocity is a vector quantity representing speed in a certain direction. This means that a circular motion at a constant rate has a constant speed but a constantly changing velocity.

Characteristics of a distance-time graph

- a flat line means no movement, a change in time but not in distance.

- a straight positive line represents a constant movement.

- a concave curve represents an acceleration, a convex represents deceleration.

- the speed at any point during an acceleration or deceleration can be determined by drawing a tangent

Acceleration equation

chance in velocity/time

Velocity-Time graph characteristics

-a flat line means no acceleration, a constant speed.

- a straight positive line represents a constant acceleration, concave and exponential acceleration and convex a decreasing acceleration.

- no movement is represented by a flat line on the x axis.

- the area under the line equals the distance travelled.

Terminal velocity explanation

Near the Earth's surface any object free falling has an acceleration of 9.8m/s due to gravity. An object falling through a fluid will fall at 9.8m/s until the resultant force (from weight and resistance) is 0 and the object moves at its terminal velocity

Process of reaching terminal velocity

1.Accelerates due to weight.

2.Air resistance increases with velocity, weight does not.

3.Air resistance becomes equal to weight, the forces become equal

Newton's First Law

If a resultant force acting on an object is zero, the object will remain stationary (this is inertia) or if it is moving, it will continue to move at the same velocity.

Newton's Second Law

The acceleration of an object is proportional to the resultant force acting on the object and inversely proportional to its mass.

Resultant force equation

Resultant force = mass x acceleration

Inertial mass

Inertial mass is a measure how difficult it is to change the velocity of an object. It is the ratio of force over acceleration (f=ma rearranged)

Newton's Third Law

Whenever two objects interact, the forces they exert on each other are equal and opposite. This is explains the result of equilibrium situations.

How to find stopping distance

stopping distance = thinking distance + braking distance

Factors affecting reaction time

- tired.

- drunk.

- high.

- driving fast.

- influenced by any distractions.

Factors affecting breaking distance

Breaking distance can be affected by adverse conditions of the road (icy/wet conditions) and/or the vehicle (this is limited to the conditions of the breaks and tyres).

How brakes work

Breaking distance can be affected by adverse conditions of the road (icy/wet conditions) and/or the vehicle (this is limited to the conditions of the breaks and tyres).

A higher speed requires a greater braking force and a greater breaking force means a greater deceleration (this can lead to brakes overheating and loss of control). F=ma shows that if acceleration is high, then force increases, showing that in a car crash with extreme deceleration the force exerted on the victim will be high and therefore dangerous.

How to find distance in a collision

s=-u^2/2a

How to find deceleration in a collision

a=-u^2/2s

How to find force in a collosion

F=m(-u^2/2s)

Unit for momentum

kg m/s

Momentum equation

momentum = mass x velocity

Conservation of momentum

In a closed system the total momentum before an event is equivalent to that after it, this is the conservation of momentum

Force equation (using momentum)

Force = change in momentum/time

Safety features to reduce rate of change of momentum

-seat belts

-air bags

-crumple zones

-crash mats

-cycle helmets

-cushioned grounds in play area

Require Practical 6- Force and Extension

In the practical, students place known masses on a spring, measure the total resultant length of the

spring and calculate its extension.

Practical 6 (Extension) Materials

• a suitable spring capable of extending more than 1 cm under a load of 1 N with loops at each

end

• metre ruler

• suitable pointer (eg splint and tape)

• weight stack appropriate for the spring (eg 10 N in steps of 1 N)

• clamp stand

• two clamps and bosses

• G-clamp or weight to prevent the apparatus tipping over the edge

Practical 6 (Extension) Method

1. Set up spring and a ruler on a clamp stand so that the 0mm point is at the bottom of the unextended spring

2.Add masses and then measure the extension

3. Convert the masses to weight with weight=mass x grav. field strength

4.Plot. Should be a straight line. Gradient is 1/spring constant (if force is x and extension is y)

Practical 7- Acceleration

-the effect of varying the force on the acceleration of an object of constant

mass

-the effect of varying the mass of an object on the acceleration produced by a

constant force

Practical 7 (Acceleration) Materials

-a 1 m ruler

-toy car

- bench pulley, string and small weight stack (eg 1 N in steps of 0.2 N)

- two clamp stands, clamps and bosses

- Blu-Tac or similar to attach weights to the car.

Acceleration Practical Method Part 1 (Measuring the effect of force on acceleration at constant mass)

1.Use the ruler to measure intervals on the bench and draw straight lines or place tape across the bench at these intervals.

2.Attach the bench pulley to the end of the bench.

3.Tie a length of string to the toy car or trolley. Pass the string over the pulley and attach the weight stack to the other end of the string.

4.Make sure the string is horizontal and is in line with the toy car or trolley.

5.Hold the toy car or trolley at the start point.

6.Attach the full weight stack (1.0 N) to the end of the string.

7.Release the toy car or trolley at the same time as you start the stopwatch, press the stop watch (lap mode) at each measured interval on the bench and for the final time at 100 cm.

8.Record the results in the table.

9.Repeat steps 5−8 for decreasing weights on the stack for example, 0.8 N, 0.6 N, 0.4 N, 0.2 N. Make sure you place the masses that you remove from the weight stack onto the top of the car each time you decrease the weight.

F=ma, a=F/m, a=1/m x F:

As force increases, so does acceleration

Acceleration Practical Method Part 2 (Measuring the effect of mass on acceleration with a constant force)

1.Setup the bench, pulley, weight stack and car as in steps 1-5 of activity 1

2.Use your results from activity 1 to select a weight for the weight stack that will just accelerate the car along the bench.

3.Put a 200g mass on the car.

4.Hold the car at the start point.

5.Attach your chosen weight stack to the end of the string

6.Release the car at the same time as you start the stopwatch, press the stopwatch (lap mode) at each measured interval on the bench and for the final time at 100 cm.

7.Record the results in a table with distances travelled for each mass

8.Repeat steps 5−8 for increasing more masses on the car

F=ma, a=F/m, a=1/m x F:

As mass increases, acceleration decreases

Action between 2 like poles

Repulsion

Action between 2 opposite poles

Attraction

How to test if a substance is magnetic

If it repels

Characteristics of a permanent magnet

•A material that produces its own magnetic field.

•Made from ferrous materials or alloys with them (e.g. iron, cobalt, nickel and steel).

•Substances that are permanently magnetised and often their alloys are called "magnetically hard".

•Alloys with less concentration of the ferrous material are often weaker, temporarily magnetised and therefore magnetically soft.

Characteristics of a induced magnet

•A material that becomes a magnet when placed in a magnetic field.

•It loses most/all its properties instantly after being moved away.

•It will always attract to the inducing magnet, as it forms poles opposite to the permanent magnet (e.g. north forms next to south).

Characteristics of a magnetic field

•The 'non-contact' forces of a magnet are strongest at the poles, like poles repel, unlike attract. The overall strength of the magnetic field depends on the distance from the magnet.

•The direction of the magnetic field at any point is given buy the direction of the force that would act on another north pole placed at that point.

•The direction of a magnetic field line is always from the north to the South Pole.

•A magnetic compass contains a small bar magnet that points towards the earth's magnetic poles which form as if the core were a magnet itself.

When a current flows through a conducting wire...

a magnetic field is produced around the wire. The strength of the magnetic field depends on:

- Size of current.

- The distance from the wire.

Corkscrew rule

The direction of the magnetic field around a current carrying wire follows the direction you turn a corkscrew.

Solenoid

-A solenoid is a long coil of insulated wire.

-Shaping a wire to form a solenoid increases the strength of the magnetic field created by a current through the wire.

-The field inside is strong and uniform.

Properties of a solenoid

-increases in strength as current increases.

- reverses if current is reversed.

- increases in strength as you get closer to the wire.

- increases in strength with an iron core.

- outside the solenoid, has a similar shape to a bar magnet.

Electromagnet

A solenoid with a ferrous metal core.

Polarity of Solenoids

To work out the north and south end of a Solenoid you look at the end of the solenoid, and see which direction the current is going round.

If it is clockwise it is south, if it is going anticlockwise it is north.

Motor effect

When a conductor carrying a current is placed in a magnetic field (produced by a different magnet), they exert a force on each other called the motor effect.

Factors affecting size of force in motor effect

-Size of current.

- Strength of magnet.

- Angle between the wire and the magnetic field lines. The force is greatest when the wire is perpendicular to the magnetic field. There is no force if they are parallel.

Fleming's left hand rule

Electric motors (motor effect)

1. Coil placed in magnetic field

2. Coil experiences a force, rotating it

3.To keep the coil moving in one direction, a split ring commutator reconnects the two sides of the coil every half-turn

Headphones/Speakers

1.a current in the coil creates an electromagnetic field

2.the electromagnetic field interacts with the permanent magnet generating a force, which pushes the cone outwards

3.the current is made to flow in the opposite direction

4.the direction of the electromagnetic field reverses

5.the force on the cone now pulls it back in

6.repeatedly alternating the current direction makes the cone vibrate in and out (the electric current must vary in the same way as the desired sound.)

7.the cone vibrations cause pressure variations in the air, which are sound waves

Generator effect

If an electrical conductor moves relative to a magnetic field (cuts the magnetic field lines) a potential difference is induced across the ends of the conductor. If this conductor is part of a circuit, a current is induced.

This induced current forms its own magnetic field that will resist the change that induced it

Requirements for generator effect

1.Relative Motion between field and conductor (K.E. Input)

2.Magnetic Field

3.Conductor(s)

How to increase induced current generator effect

1.More Conductors/Faster Relative Motion

2. Greater Flux Density

3. More coils, each one generates the same current, adding to the total.

4. Having an iron core inside the coil.

Uses of generator effect

-Alternator

-Dynamo

Alternator

A simple alternator consists of one side of a coil moving up through the magnetic field.

As it cuts the field lines it produces potential difference, as it starts moving round (separate power source) and the coil moves down the induced potential difference reverses direction, this means the alternator produces an alternating current. (sine curve)

Cars use alternators to maintain the battery's charge, using the engine to produce an input of motion.

Dynamo

-Direct current generator

-A simple dynamo consists of a coil on a wire rotating in a magnetic field. However instead of split rings connecting to the coil, it uses a split ring commutator and conductive brushes.

-This means after each half rotation, the current reverses, meaning that current to the external circuit always flows in the same direction.

-Current and p.d are never negative

Microphones

1.Pressure variations in air, cause the diaphragm to vibrate with the same frequency as the sound.

2.The diaphragm's vibrations vibrate the coil causing it to move relative to the

permanent magnet.

3.This causes a potential difference to induce in the coil.

4.The induced p.d. causes a current to flow round the circuit.

5.The size and direction of the current reflects the initial sound waves frequency, meaning the electrical signals generated match the initial pressure variations.

6.This can be amplified to drive a loudspeaker or recorded.

Transformers

A basic transformer involves a primary coil and a secondary coil wound around an iron (as it is easily magnetised) core. The primary coils induce an alternating magnetic field in the core, which in turn induces ac current in the secondary coils

Step-Up Transformer:

More turns on secondary than primary, raising voltage.Ised at power stations to transfer energy to the national grid

Step-Down Transformer

More turns on primary than secondary, lowering voltage.Used to supply electricity from the national grid to consumers.

Voltage at power station (showing why transformers are used)

25,000V

Explaining why a higher voltage is preferred for transport

1. P=IV

2. P is a constant (as we want to transfer a specific amount of power) so V and I are inversely proportional

4. Therefore, increasing V will decrease I

5.P=I^2 x R

6.Power loss(due to heating) = I^2 x R

7.P∝I^2 (R is constant)

8. Therefore, increasing voltage decreases and decreases the power loss.