P3

KEYWORDS: electromagnet, crane, Circuit breaker, electric bell, relay

Understand the uses of electromagnets

ALL – State how to make an electromagnet MOST – Describe the uses of electromagnets SOME – Evaluate the uses of electromagnets

Starter Make a mind map about

everything you remember about magnets and

electromagnets

Electromagnets

Electromagnets differ from normal magnets in one major way! They are made by passing an electric current through a wire that has been wrapped around iron. The current creates a

magnetic field and magnetises the iron core. When the current is turned off the iron loses its magnetism

LO: Understand the uses of electromagnets

Practical – Electromagnetic strength

1. Increasing the number of coils on an electromagnet will increase the strength of an electromagnet

2. Certain cores will make an electromagnet stronger than others

3. Increasing the voltage of the connecting battery/current passing through the wire will increase the strength of an electromagnet

LO: Understand the uses of electromagnets

Electric bell

LO: Understand the uses of electromagnets

Circuit breaker

LO: Understand the uses of electromagnets

Electrical relay

LO: Understand the uses of electromagnets

Scrapyard crane

LO: Understand the uses of electromagnets

KEYWORDS: Fleming’s left hand rule

Understand how electromagnets can be

used to make things move

ALL – State how electromagnets can be used in motors MOST – Use Fleming’s left hand rule SOME – Explain how loudspeakers work

The Motor Effect

If a wire carrying a current is placed into a magnetic field, an interesting thing happens. As part of the GCSE course, you are required to know which way a wire placed into a magnetic field moves

LO: Understand how electromagnets can be used to make things move

Fleming’s left hand rule

LO: Understand how electromagnets can be used to make things move

Electric Motor

LO: Understand how electromagnets can be used to make things move

An electric motor uses the motor effect of electromagnets to create motion. • The force on one side

of the wire causes it to move up

• The force on the other side of the wire causes it to move down

• The motor rotates!

Electric Motor

LO: Understand how electromagnets can be used to make things move

Graphite ‘brushes’ are used to connect the split-ring to the battery. This is used because: • Graphite is an

excellent conductor • It causes very little

friction on the conducting ring

The Loudspeaker

LO: Understand how electromagnets can be used to make things move

KEYWORDS: Electromagnetic induction, Generator, coil, wire, magnet

Understand how generators create

electricity

ALL – State how electricity can be created MOST – Describe how electromagnetic induction can be increased SOME – Explain generators work in detail

Electromagnetic induction

By moving the magnet within the coil of wire, current can be induced within the current. Note, that the current is only induced when the magnet or coil is moving! How can the amount of current induced be increased? • Increase the number of coils • Increase the speed at which the

magnet/coil moves • Increase the strength of the magnet

LO: Understand how generators create electricity

Electric generators

LO: Understand how generators create electricity

KEYWORDS: transformer, core, step up, Step down, primary, secondary, induced

Understand how transformers work

ALL – State the function of step up and step down transformers MOST – Explain how transformers work and perform transformer calculations SOME – Explain why values from equations are just an approximation

Step-up and Step-down

Transformers are an essential part of the national grid. They help to increase the voltage after the power station and help to decrease it again when it gets to your house.

LO: Understand how transformers work

How do transformers work?

Discuss with the people on your pod how transformers work. Remember the following: • The only work with an a.c.

current • They need to have an iron

core • They use electromagnetic

induction to work

LO: Understand how transformers work

How transformers work

1. The alternating current in the primary coil makes the iron core into an electromagnetic

2. As the current is alternating, the magnetic field ‘moves’ and also changes direction

3. This ‘moving’ magnetic field causes a current to be induced in the secondary coil

4. If the number of coils on the secondary is higher, the potential difference will increase and it is a step-up transformer

5. If the number of coils on the secondary is lower, the potential difference will decrease and it is a step-down transformer

LO: Understand how transformers work

Transformer calculations

The following equation links together the voltage and number of coils in a transformer:

LO: Understand how transformers work

𝑉𝑝

𝑉𝑠=

𝑛𝑝

𝑛𝑠

Vp = Voltage on primary coil (v) Vs = Voltage on secondary coil (v) np = number of turns on primary coil ns = number of turns on secondary coil

Transformer efficiency

Transformers are usually about 98% efficient. We can round this up and say that this is approximately 100%. Therefore, whatever power the device uses, it will output the same amount of power!

Power in = Power out

LO: Understand how transformers work

Transformer efficiency

Power in = Power out

Vp x Ip = Vs x Is

LO: Understand how transformers work

Vp = Voltage on primary coil (v) Vs = Voltage on secondary coil (v) Ip = Current on primary coil (A) Is = Current on secondary coil (A)

KEYWORDS: x-ray, CT scanner, Ultrasound

Understand how physics can be applied

to medicine ALL – State some medical applications of physics MOST – Describe how x-rays, CT scanners and ultrasound work SOME – Evaluate the use of these devices in particular situations

Starter Recap questions

Further notes on Ultrasound

Besides using ultrasound to create pictures of babies, they can also be used to treat kidney stones! Kidney stones are small lumps that can form in the kidneys. They are formed when small crystals of waste products filtered by the kidneys build up. When they are formed, your body will try to pass them out through the urine. However, they can sometimes get stuck in this process and cause immense pain!

LO: Understand how physics can be applied to medicine

Further notes on Ultrasound

Ultrasound can be used to treat kidney stones. Ultrasound is usually very high frequency (>20000Hz) outside of the range of human hearing. If it is directed at the kidney stones, the high frequency sound can break them up into smaller pieces and make it easier for them to pass out through the urine!

LO: Understand how physics can be applied to medicine

Distance between interfaces

The distance to an object can be found using information from an ultrasound. It can be found using the following equation:

S = V x T

LO: Understand how physics can be applied to medicine

S = Distance (m) V = Velocity of sound (m/s) T = Time (s)

KEYWORDS: refraction, angle of incidence, Angle of refraction, refractive index

Understand the phenomena of

refraction ALL – Describe what happens during refraction MOST – Explain why refraction occurs SOME – State the relationship between the angle of incidence and the angle of refraction

Starter Refraction mind

map

Refraction

Refraction occurs when light moves between two mediums that have a different density. (The word medium just means ‘something that you can travel through’) When light moves into a medium with a higher density, it slows down and will bend towards the normal. When it moves into a less dense medium, it speeds up and bends away from the normal.

LO: Understand the phenomena of refraction

Conclusion: Refraction

What can you conclude about the relationship between the two angles in refraction? Sin of the angle of incidence divided by Sin of the angle of refraction will always be a constant. The value of the constant will change depending on what materials are being used to do refraction (i.e. refraction with air and glass will have a different value to air and water and air an oil etc.)

LO: Understand the phenomena of refraction

Refractive Index

The refractive index of a material is a measure of how much light is refracted by it when it passes through the material. It can be calculated by using the equation:

Refractive index = Sin I / Sin R

LO: Understand the phenomena of refraction

I = angle of incidence R = angle of refraction

KEYWORDS: refraction, angle of incidence, Angle of refraction, refractive index

Understand how total internal reflection

occurs ALL – Describe total internal reflection MOST – Perform calculations for the critical angle of an object SOME – Explain how TIR is utilised through endoscopes and optical fibres

Demonstration: Critical Angle

When performing refraction with a semi-circular block, an interesting phenomena is observed. When the light leaves the block, it bends away from the normal. This is expected as it is moving from a more dense to a less dense medium.

LO: Understand the phenomena of refraction

Demonstration: Critical Angle

As we continue to increase the angle of incidence, we reach a point where the refracted light seems to run across the top of the glass block! The angle at which this happens is called the ‘critical angle’

LO: Understand the phenomena of refraction

Demonstration: Critical Angle

If we increase the angle even more, we can see that the light ray is no longer refracted. Instead, it seems to reflect off inside of the straight edge of the semi-circular block. This is known as total internal reflection We will explore this in greater detail later

LO: Understand the phenomena of refraction

Critical Angle

The critical angle of a material can be found using the following formula:

Refractive index = 1/sin(C)

LO: Understand the phenomena of refraction

Refractive index = property of a material C = Critical angle

Uses of Total Internal Reflection

Fibre optic cables are able to work using total internal

reflection. Light is sent through one end in short bursts and will

light up the other end.

These fibre optic cables are now used in high speed internet!

They are now also used in medicine…

LO: Understand how physics can be applied to medicine

Uses of Total Internal Reflection

Total internal reflection is put to good use in Endoscopes. These are cameras that can be used to

look inside patients without having to perform invasive

surgery.

Light is shone into one end, travels through the endoscope,

shines off the inside of a patient, back along the endoscope and

forms a picture!

LO: Understand how physics can be applied to medicine

KEYWORDS: converging, diverging, lenses, Focal point, virtual, real

Understand how to draw ray diagrams for

lenses

ALL – state the definition of converging and diverging MOST – Draw ray diagrams for lenses SOME – Explain when virtual and real images are formed when using lenses

Starter Make a mind map of all the objects that

you can think of that use lenses!

Converging lens

LO: understand how to draw ray diagrams for lenses

A converging lens is always convex. It makes rays that are coming in that are parallel converge onto a point. The point where the rays converge is called the principal focus, or focal point. Converging lenses are used in magnifying glasses and in cameras.

Diverging lens

LO: understand how to draw ray diagrams for lenses

A diverging lens is always concave. It makes rays that are coming in that are parallel diverge. The point where the rays seem to diverge from is called the principal focus, or focal point. Diverging lenses are used to correct short sight

Focal length

LO: understand how to draw ray diagrams for lenses

For both lenses, the distance between the centre of the lens and the focal point is called the focal length.

Real image

LO: understand how to draw ray diagrams for lenses

When an object is really far away from a converging lens, the light rays will be (almost) parallel when they reach the lens. The image that will be formed will be at the focal length. We call this image a real image.

Real image

LO: understand how to draw ray diagrams for lenses

A real image is an image that is formed by a converging lens if the object is further away than the principal focus. The real image will always be smaller than the actual object, inverted and forms after the lens.

Virtual image

LO: understand how to draw ray diagrams for lenses

When the object is close to a lens, the image that is formed will be much larger than the actual size of the object. We call this image a virtual image.

Virtual image

LO: understand how to draw ray diagrams for lenses

A virtual image is formed by a converging lens if the object is nearer to the lens than the focal length. A diverging lens will ALWAYS form a virtual image. The image is always bigger than the object, the right way up and forms before the lens.

Magnification

LO: understand how to draw ray diagrams for lenses

The magnification produced by a lens can be worked out using the following formula

Magnification = image height

object height

Formation of a real image by a converging lens

LO: understand how to draw ray diagrams for lenses

As part of P3, you are required to know how to draw ray diagrams like the one below to show

Although they look very complicated, they are quite easy to draw once you know how

Formation of a real image by a converging lens

LO: understand how to draw ray diagrams for lenses

Drawing ray diagrams for a converging lens is done in 4 easy steps: 1) Draw the principal axis and the lens (always shown as a

line with arrowheads) 2) Draw the object (drawn as a vertical arrow going upwards 3) Draw a line parallel to the principal axis that refracts at the

lens and goes through the focal point 4) Draw a line straight through the principal axis and the lens

line 5) Draw a line that goes through the focal point and refracts

to become parallel at the lens 6) Draw the object on the other side of the lens where the

three construction lines all join up

Uses of Converging lenses

LO: understand how to draw ray diagrams for lenses

This final example shows how a converging lens can be used as a magnifying glass. When an object is placed close to the magnifying glass (closer than the focal length), the object will appear bigger than the actual object. The image formed is virtual (remember the definition!) and will also be the right way up (i.e. not inverted)

Old Skool uses of converging lenses

LO: understand how to draw ray diagrams for lenses

All cameras have a converging lens. This focuses the light from distant objects onto a film at the back of the camera. When the shutter is opened, the film is exposed and a negative is made. The negative can then be developed into the actual picture!

New Skool uses of converging lenses

LO: understand how to draw ray diagrams for lenses

Newer digital cameras work in the same way. However, instead of photographic film at the back of the camera, they have a CCD imager. When the shutter is opened, the CCD is exposed to light and it forms the image.

New Skool uses of converging lenses

LO: understand how to draw ray diagrams for lenses

When an object is placed too close to a camera, it will appear fuzzy. This is because the object is closer to the lens than the focal length and the lens is not able to focus the image properly onto the CCD.

KEYWORDS: converging, diverging, lenses, Focal point, virtual, real

Understand the structure of the eye

ALL – label the different parts of the eye MOST – Describe what different parts of the eye do SOME – Explain how converging and diverging lenses can be used to correct for vision problems

How the eye works

Regardless of if you are looking at an object very close to you or very far away, your eye is able to focus and you are able to see the object clearly. How is your eye able to refocus based on where the object is?

LO: understand the structure of the eye

How the eye works

1. When light enters the eye, the ciliary muscles change the thickness of the lens

2. The light is focused by your lens onto the retina

3. The light sensitive cells in the retina send electrical impulses through the optic nerve to your brain

4. Your brain processes these impulses and shows you what the object looks like

LO: understand the structure of the eye

What happens if too much light

suddenly enters the eye?

Correcting vision

By using our understanding of how

the eye works and how lenses work, we can

design glasses to correct for sight problems

LO: understand the structure of the eye

Short sight

In a normal eye, the lens focuses the image exactly on the retina. However, in the eye of a person with Myopia (short sighted), the image is formed before the retina. This leads to a blurred image.

LO: understand the structure of the eye

Correcting short sight

Short sight can be corrected by glasses that have a concave (diverging) lens. This causes the light rays to diffract outwards slightly as they pass the lens so that they are focused exactly on the retina by the lens in the eye.

LO: understand the structure of the eye

Long sight

In a person with ‘hyperopia’ (long sight), the image is not correctly focused onto the retina by the eye lens. The image is focused behind the retina, leading to a blurry image. How can we correct this?

LO: understand the structure of the eye

Correcting long sight

Long sight can be corrected by using a convex (converging) lens. This causes the light rays to converge slightly before they hit the lens so that they are refracted perfectly onto the retina.

LO: understand the structure of the eye

Power of a lens

The power of a lens can be worked out using the following equation:

Power = 1

LO: understand the structure of the eye

Focal length

Power = Dioptre (D) Focal length = m

To understand how objects balance

ALL – State the definition of a moment MOST – Perform moment calculations SOME – Explain how objects balance using the concept of moments

What is a moment?

When the mass is placed on the left-hand side of the see-saw, it moves down. This is an anticlockwise turn

The turning effect of a force is called a moment

LO: Understand how things balance

pivot

Calculating moments

The moment of a force is calculated from:

Moment = force x distance from pivot

m = f x d

LO: Understand how things balance

Moment = Newton – Metres (Nm) Force = Newtons (N) Distance = metres (m)

Example 1

Gina weighs 500 N and stands on one end of a seesaw. She is 0.5 m from the pivot. What moment does she

exert?

LO: Understand how things balance

moment = 500 x 0.5

= 250 Nm

0.5 m

500 N pivot

Example 2

If a force of 20 N presses down at a distance of 3 m from a pivot, its moment is:

Moment = 20 N x 3 m = 60 Nm

LO: Understand how things balance

Example 3

If a force of 30 N presses down at a distance of 4 m from a pivot, its moment is:

Moment = 30 N x 4 m = 60 Nm

LO: Understand how things balance

Moments in balance

A seesaw is an example of the principle of moments. This states that for an object in equilibrium (not moving!) the sum of all the clockwise moments about any point is equal to the sum of all the anticlockwise moments about the same point.

Clockwise moments = anticlockwise moments W1 x D1 = W2 x D2

LO: Understand how things balance

Calculating moments

Task: Answer the questions on calculating moments on the worksheet. The second side is more difficult than the

first! Moment = force x distance from pivot

LO: Understand how things balance

What do these objects have in common?

LO: Understand how things balance

KEYWORDS: centre of mass, line of symmetry

Understand what is meant by centre of

mass

ALL – Define the centre of mass MOST – be able to find the centre of mass of symmetrical objects SOME – Explain why objects are designed to have a low centre of mass

Starter What do these objects have

in common?

Centre of Mass

LO: Understand what is meant by centre of mass

For all objects, their mass is spread out over

the whole object. However, this is not

useful to us as Physicists!

Centre of Mass

LO: Understand what is meant by centre of mass

The centre of mass of an object is that point at which the mass may

thought to be concentrated

Finding the centre of mass

LO: Understand what is meant by centre of mass

The centre of mass of complicated objects can be incredibly difficult to

find. However, for simple, symmetrical

objects, the COM can be easily found!

Centre of mass of symmetrical objects

LO: Understand what is meant by centre of mass

The centre of mass of symmetrical objects

ALWAYS lies along the line of symmetry of the

object. Where the object has more than one line of symmetry, the COM will be at the

point where the lines of symmetry intersect

Suspended objects

LO: Understand what is meant by centre of mass

If you suspend an object and then release it, it

will soon come to a rest. When this happens, the centre of mass will be

directly below the point of suspension. The

object can be said to be in equilibrium.

Demo

Irregular shapes

LO: Understand what is meant by centre of mass

The centre of mass of an irregular shape can be found by using the apparatus shown. The shape is hung from a point and a plumbline used to draw the region in which the COM lies.

This is done repeatedly with the mass hung from different

points. The point where all the lines intersect is the COM!

KEYWORDS: centre of mass, stability, Moment, base, tractor

Understand how to design stable objects

ALL – state features of stable objects MOST – Explain what happens when objects topple over SOME – Evaluate the design of objects based on their stability

Starter Challenge

starter!

The designers of this bus are worried that it will topple over. They are testing it to find out the

maximum angle it can go to before this happens. Using your knowledge of science and as many

scientific terms as possible, explain WHY this bus is likely to topple over easily!

Keywords: • Centre of mass • Moment • Pivot • Base • Topple

Stable objects

Although the design of cars has changed drastically over the last 100 years, a number

of things have remained constant. Amongst them is

to keep cars as low as possible. This means that the centre of mass of the

car is low and it is less likely to topple over!

LO: understand how to design stable objects

Why do objects tips over?

The weight of an object acts through the centre of mass.

As the object is initially tilted, the weight is causing an anticlockwise moment

about the pivot. If the object is let go, the moment will cause the object to go

back onto its base.

LO: understand how to design stable objects

Why do objects tips over?

As the object continues to be tilted, you will reach a

point where the weight will go exactly through the pivot.

LO: understand how to design stable objects

Why do objects tips over?

When the object has been tilted beyond a certain

point, the weight will now cause a clockwise moment

about the pivot. If the object is let go, the moment

will cause the object to topple over!

LO: understand how to design stable objects

Designing stable objects

LO: understand how to design stable objects

Farmers must be careful when driving tractors on slopes. If the slope is too

steep, the tractor may topple over. To limit the

chances of this happening, tractors usually have a

large base

Designing stable objects

LO: understand how to design stable objects

This bus is being tested to find the maximum angle it can be tilted to before it

topples over. This is important for road safety

as it will affect the maximum speed that a driver can go around a

corner.

High chairs

LO: understand how to design stable objects

A high chair has a centre of mass that is very high off the ground. This can

make the chair very unstable, particularly when there is a baby

strapped in! To make sure the chair does not topple

over, it is designed to have a wide base.

KEYWORDS: Pressure, force, area, hydraulics

Understand pressure in liquids

ALL – State the definition of pressure MOST – Perform calculations involving pressure SOME – Explain how pressure is used in hydraulic machines

Starter List as many situations as you

can where you might be under pressure!

Pressure

LO: understand pressure in liquids

Pressure is defined as the force per unit area. The unit of pressure is the pascal (Pa), which is equal to one newton per square metre (N/m²)

Calculating pressure

LO: understand pressure in liquids

Pressure can be calculated using the following equation:

Pressure = Force / Area

Pressure = pascals (Pa) Force = Newtons (N) Area = metres² (m²)

Hydraulic pressure

Hydraulic pressure is pressure that is caused by a liquid.

Pressure in a liquid is caused by the weight of the water above. Pressure in a liquid:

Acts in all directions Increases with depth

LO: understand pressure in liquids

Hydraulic pressure

Dams are built with the base of the dam considerably thicker than the top. Use your knowledge of pressure to explain why.

LO: understand pressure in liquids

Demonstration: Archimedes can

The Archimedes can shows us how the

pressure of a liquid increases with depth

LO: understand pressure in liquids

Hydraulic machines

LO: understand pressure in liquids

Hydraulic machines

LO: understand pressure in liquids

We can use hydraulic pressure in machines to make things move. Liquids are almost incompressible. This means that if a force is applied to liquid in one part of the system, it will move and transfer the force to another part of the system. This can be used to squeeze brakes or move earth in a digger!

Force multipliers

LO: understand pressure in liquids

Force multipliers

LO: understand pressure in liquids

The calculation that we have just done shows how pressure can be used in hydraulic systems to make forces bigger. However, this will only work if: • The liquid is

incompressible • A2 is bigger than A1

KEYWORDS: centripetal force, radius, Velocity, tangent

Understand circular motion

ALL – State situations where objects move in a circle MOST – Explain what is required for objects to move in a circle SOME – Explain what happens to the centripetal force due to changes in radius and velocity

Starter Make a mind map of

objects/situations where things move continuously in a circle

Circular motion

All the situations that we have just listed involve circular motion. The objects are moving in a circle of constant radius with a constant speed. Is the velocity of the object the same?

LO: understand circular motion

Requirements of circular motion

For an object to move in circular motion: • The velocity is

tangent to the circle • The velocity changes

as the object moves around the circle

• The velocity keeps the object orbiting in a circle

LO: understand circular motion

Centripetal force

When an object moves with circular motion, there must be a force acting on it. This force is called the centripetal force and causes the object to accelerate towards the centre of the circle.

LO: understand circular motion

What is the centripetal force?

LO: understand circular motion

Electrostatic force…

Friction…

Gravity…

Tension…

A car travelling around a bend

A stone whirled around on the end of a string

A planet moving around the sun

An electron orbiting the nucleus

More or less force?

Discuss with the people on your pod what effect there will be on the centripetal force if: 1. The speed of rotation

is increased 2. The radius is

decreased 3. The mass of the

object is increased

LO: understand circular motion

More or less force?

If you want to: 1. Increase the speed of

rotation 2. Decrease the radius 3. Increase the mass of

the object The centripetal force must also be increased. To do the opposite, the centripetal force must be decreased.

LO: understand circular motion

KEYWORDS: oscillating, pendulum, Frequency, time period

Understand the motion of a pendulum

ALL – Define the motion of a pendulum MOST – Explain how the time period of a pendulum can be increased SOME – Perform calculations involving time period and frequency

The pendulum

The picture shows a snapshot of a pendulum in motion. The pendulum moves backwards and forwards and always returns back to the middle, called the equilibrium position. This type of motion is called oscillating motion.

LO: understand the motion of a pendulum

Time period

The time period of a pendulum is the time it takes for a pendulum to complete one full cycle of motion. The easiest way to measure this is the time it takes for the pendulum to swing from one side of the pendulum to the other side and back again.

LO: understand the motion of a pendulum

What affects the time period?

The factors that affect the time period of a pendulum are: 1. The length of the

pendulum 2. The amplitude

(maximum displacement) of the swing

LO: understand the motion of a pendulum

Calculating time period

The time period of a pendulum can be calculated using the following formula:

T = 1 / f

LO: understand the motion of a pendulum

T = Time (s) f = frequency (Hz)