Quantities and power of ten prefixes

… What do the prefixes ‘micro’, ‘milli’ and ‘kilo’ stand for?

… The ENG key on the calculator is a useful tool for putting numbers into ‘engineering prefix’ form. For example, the distance 123456 m gives us 123.456 x10^3 m = 123.456 km

… What are the physical quantity names which go with the units of:

A = …?…

V = …?…

D = …?…

Pa = …?…

X-Rays

… X-rays are part of the …?… spectrum and have a very short w…?… about the same size as…?

… CCDs (…?… …?… Devices) are electronic devices that can d…?… x-rays and form i…?… for medical diagnosis.

… How x-rays are used to diagnose bone fractures.

… Other uses of x-Ray’s for medical diagnosis and treatment include…?

… X-rays carry a lot of e…?… and can i…?… atoms. This can cause …?… in living cells which can lead to cancer.

… People who work with x-rays (such as…?), should take precautions to m…?… their exposure. Some examples of precautions include…?

… Advantages and disadvantages of ultrasound, x-rays and CT scans for medical diagnosis

… People who work with x-rays (such as…?), should take precautions to minimise exposure. So examples of precautions include…?

Ultrasound

… What is the range of audible sound frequencies for a human?

… Medical uses of ultrasound for imaging (diagnosis) and treatment are…?

… Ultrasound waves are partially …?… when they meet a b…?… between two different m…?… (materials). The time delay between a transmitted and reflected pulse can be used to calculate the …?… travelled (if the speed of the ultrasound is known).

This information then be used by a computer to build up an ultrasound scan picture for diagnostic purposes.

… Ultrasound distance calculation using distance = speed × time. Take care that you …?… the distance you calculate between transmission and detection of the ultrasound pulse.

Also remember to convert milliseconds or microseconds into seconds.

Light

… Refraction of light waves when they cross a boundary between two different materials. Remember that the wave …?… stays the same, but the ….?… and the …?… change.

… The formula for refractive index of a material is…?

… The refractive index tells us how much a light ray will …?… and is also the ratio of the speed of light in a v…?… to the speed of light in the substance.

… Total internal reflection occurs when the angle of …?… is …?… than the critical angle for the transparent material.

… Total internal reflection only occurs when light moves from a …?… refractive index material to a …?… refractive index material.

… Diamond has a very large refractive index. This means that diamond’s critical angle is quite small and so lots of total internal reflection takes place within the diamond, making it appear to sparkle!

… What is the formula that links the Refractive index with the critical angle?

… What is the formula which links the refractive index with the angles of incidence and refraction (called Snell’s law)?

… When waves pass through a gap or around an object, they …?… and spread out. What conditions are needed for this effect to be most noticeable?

Lenses

… Convex lenses ray diagrams: always draw two rays from a single point on the object. One ray is parallel to the …?… axis. The other ray goes straight through the …?… of the lens and is not refracted.

… Converging rays create a …?… image (that can be …?… onto a screen) if the object-lens distance is …?… than focal length of the lens.

… Converging rays create a …?… image (that cannot be …?… onto a screen) if the object-lens distance is …?… than the focal length of the lens.

… Diverging rays create a …?… image behind the lens (e.g. In a telescope or magnifying glass)

… Angular magnification in terms of focal lengths of a telescope is given by the formula…?

… Parallel rays from distant objects are converged by a convex lens to a focus at the p…?… focus. The distance from this point to the lens is called the …?… length.

… What’s does the power of a lens mean? What is the formula linking focal length and power of a lens? What are the units of lens power?

… How does the refractive index of a material affect the shape of a lens (for example, as used in glasses)?

… A magnifying glass is a …?… lens and forms a …?… image.

… A diverging lens (concave) refracts light away from the principle axis. The refracted ray can be traced back so that it passes through the principle focus on the other side of the lens.

… The power of a lens in measured in units of …?…

… When using the formula P = 1/f, the focal length, f, of a lens is measured in units of …?…

Diverging lenses have a negative lens power (i.e. so that f is also negative)

… How far from a convex (converging) lens must an object be positioned to produce:

- a magnified, real and inverted image?
- a magnified, virtual, upright image?
- a diminished, real and inverted image?

… The ‘Real Is Positive’ convention for distances u (‘ubject’) and v (‘vimage’) means that for a convex lens and real image, both u and v are positive. If the image appears on the same side as the object, then v will be negative.

Telescopes

… Angular magnification in terms of focal lengths of a telescope is given by the formula…?

… When drawing a ray diagram for a converging (convex) lens, what two rules can you use for rays coming into the lens?

… Parallel rays from distant objects are converged by a convex lens to a focus at the …?… length.

… What’s does the power of a lens mean? What is the formula linking focal length and power of a lens? What are the units of lens power?

… A simple telescope can be made from two …?… lenses: the first is called an …?… lens which collects light from a distant object into the second lens, called the …?… lens. Which lens is more powerful?

… Explain why most telescopes have concave mirrors rather than converging lenses as their ‘objectives’?

… When waves pass through a gap, they …?… which means that they spread out. What conditions are needed for this effect to be most noticeable?

… Telescopes should have a …?… objective lens or mirror so that …?… is minimised and the image produced is sharp.

The Eye

… The anatomy of the eye and roles of the cornea, lens, iris and retina. Note that when the ciliary muscles relax, this STRETCHES the lens via the suspensory ligaments making it flatter/thinner. This changes the lens power and focal length.

… Light entering the eye is focused by the …?… and the eye …?… onto the light sensitive cells of the …?.. The amount of light let into the eye is controlled by the coloured ring of muscle called the …?…, which adjusts the size of the …?… (The eye’s central hole). …?… muscles stretch or relax the eye …?… to change its shape. This allows light to be …?… for near or far objects.

… What is the range of vision of a normal human eye?

… What does it mean if a person is short, or long sighted? How can these defects be corrected?

… Ray diagrams for short and long sight, uncorrected and corrected.

… Comparing the structure of the eye with the camera. Note that the eye has a lens of fixed position but variable shape, whereas the camera has a lens of …?… position and …?… shape.

Magnetism

… Magnets have a m…?… field around them (an area where another magnet experiences a force)

… Magnets have two opposite p…?…, a north and a south, which exert …?… on other magnets.

… Like poles …?…, whereas unlike poles …?…

… Magnets can a…?… magnetic materials by i..?… magnetism in them (this can be p…?… or t…?…). However, magnets exert no (or very little) f…?… on non-magnetic materials.

… Describe what happens when either pole of a magnet is brought near to a magnetic material (remember to mention ‘induction’)

… Magnetic materials that contain iron are called ‘f…?…’. These materials can be magnetised.

… Explain the difference between the magnetic properties of IRON and STEEL. Explain which of these you would use for:

1) an electromagnet in a scrapyard?

2) a fridge magnet?

… Describe some methods of magnetising and demagnetising a material.

… Describe an experiment to investigate the magnetic field pattern around a bar magnet. What would this pattern look like?

Electromagnetic effects:

… A current is a flow of …? . The symbol of charge is…? and it has units of …?

… Charge has the symbol…? and is measured in units of…?

… A current carrying conductor has a magnetic field around it in the shape of a c…?… . Using the ‘right hand grip rule’.

… What experiment could you perform to investigate the magnetic field around a solenoid?

… What does the direction of the magnetic field lines tell us?

… How a magnetic field of a solenoid coil resembles a bar magnet’s magnetic field.

… The magnetic field is strongest where the magnetic field lines are…?

… Practical applications of the electromagnetic effect include…?

… How could you increase the strength of an electromagnet?

… A relay is a s…?… operated by an e…?…. It is used to protect a sensitive l…?… current circuit as it controls a separate h…?… current circuit.

What is an example of an application which uses a relay?

… Describing the operation of an electric bell.

The Motor Effect

… When a c…?… carrying conductor is placed within a magnetic field, the two m…?… fields INTERACT creating a ‘catapult’ effect. We can use Fleming’s Left Hand rule to work out the direction of the force on the conductor (remember ‘motor cars drive on the left’ – hence the left hand rule)

… A current carrying conductor which is placed p…?… to magnetic field lines experiences NO force.

… The force on a current carrying conductor in a magnetic field is given by:

F = B I L

B is the magnetic flux density (Tesla, T). This is a measure of how strong the magnetic field is (how close field lines are together).

I is the current flowing in the conductor (A)

L is the length of the conductor (m)

… The motor effect can create a turning force on a rectangular coil placed in a magnetic field. This is used in a DC (direct current) motor.

… A split ring commutator REVERSES the direction of current every 180° so that the coil keeps being rotated in the same direction by the motor effect.

… Electric motors are used in many applications including hard drives, electric cars, washing machines, DVD players…

… Charged particles also experience a force when they move perpendicular to a magnetic field.

Electromagnetic Induction

… When a magnetic field CUTS through a metallic conductor a p…?… d…?… is induced (sometimes called an “EMF” or ElectroMotive Force) which can cause a …?… to flow in a complete circuit.

… If the ‘cutting stops’, then the induced potential difference becomes …?….

… You can increase the size of the induced potential difference by increasing the …?… at which the magnetic field cuts through the conductor, have more c…?…, or use a stronger …?… …?….

… Generators and dynamos use electromagnetic induction to induce a voltage by spinning a magnet (or electromagnet) within a secondary coil.

… A simple AC generator (an alternator) can be made by spinning a coil within a magnetic field.

… In a moving coil microphone, sound waves make a diaphragm and a coil v…?…. As the coil is in a magnetic field, this ‘cutting’ motion i…?… an alternating potential difference in the coil. Therefore an AC electrical signal is produced which represents the original sound wave.

… The magnetic field in a transformer is contained within the iron core and is constantly growing and collapsing due to the alternating current supply. This oscillating magnetic field cuts the secondary coil inducing a potential difference (sometimes called an EMF, or electromotive force).

… The equation which links the voltages and turns of a transformer is…?

… The electrical power in and the power out of a transformer is given by the equation…? What assumption is made in this equation?

… Switch mode transformers operate at a much …?… frequency than tradition iron core transformers. They are also much more efficient, lighter weight and smaller. This makes switch mode transformers very useful for mobile devices for example laptop power suppliers, phone chargers etc.

Centre of Mass

… The centre of mass is the point through which all the object’s mass seems to act. It is the same point as the object’s centre of gravity.

… Describe an experiment to determine the position of the centre of mass of a plane lamina (a flat object)

… The centre of mass of a symmetric object is found along the object’s a…?… of s..?….

… For a simple pendulum, the equation linking its time period and frequency is…?

… How does the time period of a pendulum depend on its length? On its mass?

… Explain how you would measure a simple pendulum’s time period of oscillations and its amplitude.

Moments and Stability

… A moment is the turning effect of a force.

… Moment = force x p…?… distance of the ‘line of a…?…’ of the force to the …?….

… The units of moments can be Nm, Ncm, kNm etc. Just take care that the units ‘match’, e.g. F = 20N, d = 3cm so the moment, M, will have units of …?

… The ‘Principle of Moments’… For any object which is balanced (in rotational equilibrium): the anticlockwise moments = the clockwise moments.

Gears

… If a gear A with 12 teeth makes 6 revolutions per second, and turns a second gear B with 9 teeth.

- How many revolutions per second does the second gear make?
- How does the turning effect of each gear compare?

Here we use the idea of ‘counting teeth’

Gear A engages 12 x 6 = 72 teeth in one second.

Gear B therefore rotates by 72 / 9 = 8 turns

In this example we have a larger radius gear driving a smaller radius gear, making the smaller gear rotate faster – this is an example of a ‘distance’ multiplier.

The contact force acting between the gear teeth is the same (Newton’s 3rd law). However, the turning effect (moment) of gear B will now be less because M = Fd where d is a now smaller distance because gear B has a smaller radius.

… The centre of mass is the point through which all the object’s mass seems to act. It is the same point as the object’s centre of gravity.

… Stable objects have a …?… base and a …?… centre of gravity. The Line Of Action of the Weight (LOAW) weight force acts within the …?… This means the centre of gravity will …?… if you try to topple it, so when released it will return to its equilibrium position.

… Unstable objects have a …?… base and a …?… centre of gravity. This means that their centre of gravity will …?… if they are pushed a little bit. The Line Of Action of the Weight (LOAW) force then acts …?… the object’s base, causing a resultant m…?… and so the object will …?…

… Neutral equilibrium is when the centre of gravity neither rises or falls when pushed a little bit. For example, a snooker ball. The object cannot ‘topple’. The line of action of the object’s weight is always directly …?… the pivot.

… Note that moments and work done share the same units, but are completely different quantities! (Work done = force x distance travelled IN THE DIRECTION of the force)

Pressure, Force and Hydraulics

… High pressure is caused when a …?… is concentrated on a …?… area.

… Liquids are almost i…?… because the liquid particles are…?

… The pressure in a fluid acts equally in …?… directions.

… Pressure is measured in units of …?… and is given by the equation…?

… Hydraulic systems with different areas on the E…?… and L…?… sides can be used as Force M…?…

Any object that is in a fluid will experience an upthrust force. The reason for this is…

Force = Pressure x Area (upon which the pressure acts)

When you go deeper down in a fluid, the pressure will increase. The pressure in a fluid is given by p = density of fluid x gravitational field strength x depth beneath the surface of the fluid (p = ρgh)

Now imagine a submerged cube of metal which has faces of area A.

The top surface will experience a pressure due to the fluid above it given by p = ρgh. This pressure creates a downwards force on the cube = p x A

The bottom surface of the cube will experience a GREATER pressure because it is at a greater depth. So this pressure creates a greater upwards force (as pressure in a fluid acts in all directions).

The difference in the two forces acting on the cube creates a resultant upwards force, which we call ‘upthrust’.

If the object is floating (on the surface or just staying at a constant depth without sinking or rising), then the upthrust = weight of the object.

It just so happens that for a floating object, upthrust is equal to the weight of water displaced by the object (this can be shown mathematically). This is known as Archimedes Principle.

… Atmospheric pressure (1 atmosphere) is about 100,000 Pa ! We saw a video of how this huge pressure can crush a metal oil barrel in which the air has been removed. Here’s the video: https://youtu.be/JsoE4F2Pb20

… Archimedes principle states that:

“The weight of a floating object = The weight of fluid displaced”

(Floating bodies experience an ‘upthrust’ force due to the fluid they displace)

So if

ρ = object’s density

V = object’s volume

V’ = volume of water displaced

ρ’ = density of water

g = gravitational field strength

then:

W(object) = W(water displaced)

ρ V g = ρ’ V’ g

ρ V = ρ’ V’

Note that g cancels from both sides of the equation, so gravity doesn’t matter. This means that you could find the density of an object without having to know the value of g!

… This can also be stated as:

“The upthrust on a floating or submerged object = the weight of fluid displaced”

So we can say:

Upthrust = ρ’ V’ g

… Alternatively use an “MVD” table with the object and fluid as the column headings. Enter the information given in the question:

Object Fluid

Mass:

Volume:

Density:

If an object is floating, then we can say that it is submerged by a distance h. We can then use ‘h’ in the calculations to fill in the MVD table remembering that ‘Down’ is ‘Divide’ and ‘UP’ is ‘mUltiply’.

The final stage is to use Archimedes principle to create an equation which we can solve for ‘h’.

object mass = fluid mass

(Note we are using mass here instead of weight… It works out the same and simplifies the calculations!)

Circular Motion

… When an object is moving in a circle its …?… and therefore its …?… are continuously changing. However, the object’s …?… does NOT change because…?

… An object moving in a circle has an acceleration called its …?… acceleration.

… The centripetal acceleration needed by an object to move in a circular path is provided by a centripetal …?… Some practical examples of this are…?

… Which direction does the centripetal force and resulting acceleration always act?

… What force acts as a centripetal force to keep natural satellites (like the Moon) and artificial satellites in orbit around the Earth?