P3 Part B Flashcards
What is a moment and what is the equation to calculate it?
A moment is the turning effect of a force
Moment(N/m) = Force(N) x perpendicular distance from the line of action of the force to the pivot(m)
How do you get the maximum moment or turning effect?
You need to push at right angles (perpendicular), any other angle means a smaller moment because the perpendicular distance between the line of action and the pivot is smaller.
Where is the centre of mass of an object?
The centre of mass hands directly below the point of suspension.
In a symmetrical shape, it is the point where lines of symmetry cross.
If the ………. moments are ……… to the clockwise moments, the object won’t turn.
If the anticlockwise moments are equal to the clockwise moments, the object won’t turn.
If someone weighs 300N and sits 2m from the pivot of a seesaw, and another person weighs 700N - where should they sit to balance the seesaw?
Anticlockwise moment = Clockwise moment
300 x 2 = 700 x y
y = 0.86 m
How do levers use balanced moments to decrease the amount of force needed to get the same moment?
Name 3 examples of force multipliers:
Levers increase the distance from the pivot at which the force is applied - so less force is needed to get the same moment. Levers are known as force multipliers.
- Long sticks/bars
- Wheelbarrows
- Scissors
Why do objects need a wide base and low centre of mass to be stable?
If the line of action of the weight of the object lies outside of the base of the object, it’ll cause a resultant moment, tipping the object over.
What is the time period of a pendulum? How does the length of the pendulum effect it?
It is the time taken for a pendulum to swing from one side to the other and back again.
The longer the pendulum, the greater the time period.
How is the time period of a pendulum calculated?
Time period (s) = 1 ÷ Frequency (Hz)
Why are liquids virtually incompressible?
A force applied to one point int he liquid will be transmitted to other poiints in the liquid. Pressure in a liquid is transmitted equally in all directions.
Give the equation for pressure
Pressure = Force ÷ Cross-sectional area
How is the pressure in liquids used in force multipliers?
In hydraulic systems, a pressure is exerted on the liquid using a small force over a small area and this force is transmitted to the second piston. The second piston has a larger area so there will be a larger force.
Where are hydraulic systems used?
In car braking systems, hydraulic car jacks, manufacturing and deployment of landing gear on some aircraft.
If an object is travelling in a circle, what will be happening to its velocity and acceleration?
The velocity is constantly changing (but not necessarily speed) so the object is accelerating - towards the centre of the circle.
What is the force which causes objects moving in a circular movement to accelerate towards the centre called?
Centripetal force
Give three different examples where centripedal force is in affect, and name what form it takes.
- In a car going round a bend, the centripetal force is friction between the cars tyres and the road.
- When a bucket is whirling round on a rope, the centripetal force comes from the tension in the rope.
- In a spinning fairground ride, the centripetal force comes from tension in the spokes of the ride.
What three factors does centripetal force depend on?
- The faster an object is moving, the bigger its centripetal force will have to be to keep it moving in a circle.
- The bigger the mass, the bigger the centripetal force.
- A larger force is needed to keep something moving in a small circle because it has more ‘turning’ to do.
What is a magnetic field?
A magnetic field is a region where magnetic materials (like iron and steel) and also wires carrying currents experience a force acting on them
Draw a magnetic field diagram.
Where do the arrows always point?
The arrows on the field lines always point from the North Pole of the magnet to the South Pole.
What does the right hand thumb rule tell you about which way a magnetic field goes?
When your thumb points in the direction of the current, and your fingers are curled round, your fingers will indicate the direction of the magnetic field.
If you extend your first two fingers out, the upper one will point in the direction of the magnetic field, whilst the second one down will indicate the direction of the electric current.
The magnetic field inside a solenoid (a coil of wire) is strong and uniform. What is the magnetic field like outside?
It’s just like the one round a bar magnet
How can you increase the strength of a magnetic field around a solenoid?
Add a magnetically ‘soft’ iron core through the middle of the coil, so it becomes an electromagnet.
Why are magnetically soft materials useful for something that needs to be able to switch its magnetism on and off?
They magnetise and demagnetise easily. When curent through the solenoid is turned off the magnetic field disappears - because the iron doesn’t stay magnetised.
Give an examle of when a magnet that can be turned on and off is useful
In cranes that lift iron and steel, electromagnets can be used. When the electromagnet is turned on, it will pick up magnetic materials and drop them when turned off.
What is the force experienced by a current-carrying wire in a magnetic field known as?
The motor effect
What position must a wire in a magnetic field be in to experience the full force?
It must be running at 90° to the magnetic field. If the wire is parallel to the field, it won’t experience any force. At angles inbetween, it will experience some.
What does Fleming’s Left Hand Rule tell us?
Thumb - motion
First finger - field
Second finger - current
How does a simple electric motor work? (4 main points)
- A simple electric motor can be built using a coil of wire that is free to rotate between two opposite magnetic poles.
- When an electric current flows through the coil, forces act upon the two side arms of the coil of wire.
- These forces are just the usual forces that act on any current in a magnetic field.
- Because the coil is on a spindle and the forces act one up and one down, it rotates.
Why is a split ring commutator used in a simple electric motor?
It swaps the contacts in every half turn to keep the motor rotating in the same direction.
How can the direction of an electric motor be reversed? How can it be sped up?
The direction of the motor can be reversed either by swapping the polarity of the direct current (DC) supply or swapping the magnetic poles over.
To speed up: increase current, make the magnetic field stronger
How are electric motors used?
The coil is linked to an axle, causing it to spin round. This motion is used in CD players, food mixers, printers, fans, hair dryers etc
What is electromagnetic induction?
It’s the creation of a potential difference across a conductor, which is experiencing a change in magnetic field.
How can electromagnetic induction be caused?
By moving a magnet in a coil of wire or moving an electrical conductor in a magnetic field (“cutting” magnetic field lines). Shifting the magnet from side to side creates a little ‘blip’ of current.
In electromagnetic induction, if you move the magnet in the ……… direction, the potential difference/current will be reversed too. If the polarity of the magnet is reversed, then the ……… will happen
In electromagnetic induction, if you move the magnet in the opposite direction, the potential difference/current will be reversed too. If the polarity of the magnet ic reversed, then the same will happen
What happens in electromagnetic induction if you keep the magnet (or the coil) moving back and forwards?
You produce a potential difference that keeps swapping direction - and this is how alternating current (AC) is created.
What does turning a magnet end to end in a coil do?
It creates AC current.
As you turn the magnet, the magnetic field through the coil changes - this change in the magnetic field introduces potential difference, which can make current flow in the wire. (After half a turn, direction of the magnetic field, potential difference and direction of the current reverses)
Give one example of an appliance that uses electromagnetic induction
- Dynamos (used in bikes to power the lights)
- The cog wheel at the top is positioned so it touches the wheels, as the wheels move round it turns the cog which is attached to the magnet. This creates an AC current.
Why can transformers only change the potential difference of AC current?
It uses electromagnetic induction to change the potential difference.
Draw and label diagrams of a step up and step down transformer
Step up - more turns in secondary coil
Step down - more turns in primary coil
Outline how transformers work in 4 main steps
- The primary coil produces a magnetic field which stays within the iron core. This means nearly all of it passes through the secondary coil and hardly any is lost.
- Due to the AC current in the primary coil, the feld in the iron core is constantly changing direction (100 times a second if its at 50Hz) - it’s changing magnetic field.
- The rapidly changing magnetic field is felt by the secondary coil.
- The changing field indues an alternating potential difference across the secondary coil (with the same frequency as the AC in the primary) - electromagnetic induction of potential difference.
Why do different transformers have more turns on one coil than another?
It determines whether the potential difference induced in the secondary coil is greater or less than in the primary coil.
In step up: PD is greater in secondary
In step down: PD is less in secondary
What would happen if you supplied DC to the primary coil in a transformer?
Nothing would happen in the secondary because the magnetic field wouldn’t be constantly changing. There’s be no induction in the second because you need a changing field to induce potential difference.
What is the transformer equation?
PD across Primary coil (V) ÷ PD across secondary coil (V) = Number of turns on primary coil ÷ Number of turns on secondary coil
This equation can be flipped (with secondary coil factors on top of the fractions)
Transformers are nearly 100% efficent so power in = power out. What is the formula for power supplied?
Power = current x potential difference
What are switch mode transformers?
They are transformers that operate at higher frequencies (between 50Hz - 200Hz). Because they work at higher frequencies they can be made lighter and smaller - useful for portable devices and power supplies. They are also more efficent than normal transformers - use little power when they’re switched on but no appliance is attached to them.
