Unit 3B - Forces And Electromagnetism Flashcards
Is you apply a larger force on a spanner causing a turning effect would it mean a larger or smaller moment?
Larger
How do you work out the moment?
Moment = force x perpendicular distance from the line of action of the force to the pivot
To get the maximum moment you need to push at right angles to the moment why would pushing at any other angle mean a smaller moment?
Because the perpendicular distance between the line of action and pivot is smaller
What is the centre of mass?
The point at which the whole mass is concentrated
The centre of mass hangs directly below?
The point of suspension
What experiment can you do to find the centre if mass of any flat shape?
- ) suspend the shape and a plumb line from the same point and wait until they stop moving
- ) draw a line along the plumb line
- ) do the same things again but suspend the shape from a different point
- ) the centre of mass is where the two lines cross
If the anticlockwise moments equal the clockwise moments the object is?
Balanced
Total anticlockwise moments = ?
Total clockwise moments
Levers used the idea of balanced moment to make us easier to do what?
Do work e.g. lift an object
How do levers work?
Levers increase the distance from the pivot, meaning less force is needed to get the same moment meaning it is easier to lift stuff
What are levers known as and why?
Force multipliers because they reduce the amount of force that’s needed to get the same moment by increasing the distance
Name three examples of simple levers.
Long sticks, wheelbarrow and scissors
If the total anticlockwise moments do not equal the total clockwise moments there will be?
A resultant moment and so the object will turn
What do the most stable objects have?
A wide bass and a low centre of mass
When will an object begin to be unstable and tip over?
When the centre of mass and line of action of weight moves beyond the bade as it will cause a resultant moment and it will tip over
How can a simple pendulum be made?
By suspending a wight from a piece of string,when you pull back a pendulum and let it go it will swing back and forth
Why a is time period in a pendulum?
The time taken for the pendulum to swing from one side to the other and back again
What is the same for each swing or a pendulum?
The time period
How can you calculate time period?
Time period = 1 / Frequency
What does the time period of a pendulum depend on?
The length of it, the longer the pendulum the greater the time period
If the centre of mass acts beyond the pivot in a bid what will happen?
The bus will fall over
Can you compress a liquid?
No
What does it mean by liquids being virtually incompressible?
You can’t squash them, their volume and density stay the same
As liquids are in im incompressible and can flow what can happen?
A force applied to one point in the liquid is transmitted to other points in the liquid
What can you say about how much and in which direction pressure is transmitted in liquids?
Pressure in liquids is transmitted equally in all directions
How can you figure out pressure in hydraulics?
Pressure = force / area over which force is applied
What is pressure measured in?
Pascals (Pa)
What is force measured in?
Newtons (N)
What does in mean by hydraulic systems are used we force multipliers?
They use a small force to produce a bigger force
How does a hydraulic system work?
It uses two pistons one with a smaller area and one with a larger area, as pressure is transmitted equally through the liquid the pressure at both piston is equal. So at the 1st piston a pressure is exerted on the liquid using a small force over a small area, the pressure is then transmitted it the 2nd piston. The 2nd piston has a much larger area and so as force = pressure x area, there will be a larger force produced
Name four things that hydraulic systems are used in?
Car braking systems, hydraulic car jacks, manufacturing and deployment of landing gear
What happens when an object is travelling in a circular motion?
It is constantly changing direction, which means its velocity is constantly changing so the object is accelerating, the acceleration is towards the centre of the circle
I’m circular motion what must be acting on a object to cause acceleration?
A resultant force, the force acts towards the centre of the circle, the force that keeps something moving is called a centripetal force
What three things can cause of centripetal force?
Fraction, tension or even gravity
What happens is the centripetal force stops?
The object will go off at a tangent
Where does the centripetal force come from when a bucket is whirling around on a rope?
The tension in the rope
What three things does centripetal force depend on?
Mass, speed and radius
What happens to a centripetal force the faster a object is?
The centripetal forge has to be bigger to keep the object moving in a circle
What happens to the centripetal force the lather the mass of an object?
The bigger the centripetal force is as it has to be to keep the object moving in a circle
Do you need more or less forge to keep something moving in a smaller circle and why?
More because it has more turning to do
What three things cause a bigger centripetal force?
More speeds, more mass and a smaller radius
What is the definition of a magnetic field?
A magnetic field is a region where magnetic materials (like iron and steel) and also wires carrying currency experience a force acting on them
Which way to the arrows on the field limes point?
From the North Pole of the magnet to the South Pole
A magnetic field is created through what type of wire?
One carrying a current
What can you used to show the direction of the magnetic field around a current carrying wire?
Your right hand thumb, your finger represent the current and your fingers represent the direction of the magnetic field
What is acceleration?
A change in velocity
Is the magnetic field inside a coil of wire weak or strong?
Strong
What is the magnetic field like outside of a coil of wire?
Like magnetic fields around a bar magnet
What is another name for a coil of wire?
Solenoid
If you add a magnetically soft iron core through the middle of the coil what do you get?
An electromagnet
How easily can a magnetically soft material magnetise and demagnetise?
Very easily
What about their magnetism makes electromagnets really useful?
It can be switched on and off
How do you turn off an electromagnet?
Turn off the current so the magnetic field and it’s magnetism stops
What machines uses an electromagnet?
Cranes in scraps yards and steel works
Why are electromagnets useful in cranes?
Because they can be turned on and off, if you were to use a normal magnet the crane would pick stuff up but then wouldn’t put it down so it wouldn’t be very useful
Name two things that can effect the size of an electromagnet?
Number of turn of wire, the size of the current going through the wire
Passing an electric current through a wire produces?
A magnetic field around the wire
What is the force experiences by a current carrying wire in a magnetic field known as?
The motor effect
What happens when you put a wire into a magnetic field?
You will have two magnetic fields combining, which puts a force on the wire
Which direction do magnetic fields travel?
From north to south
Which direction does the force that a wire experiences when put in a magnetic field go in relation to the wire and the magnetic field?
At 90 degrees and so perpendicular to both
What happens to the force the a wire experiences in a magnetic field if the direction of current in the wire or the magnetic field is reversed?
Then the force is also reversed
What force does a wire experience if it runs along parallel to the magnetic field?
It won’t experience any force
Does wire have to be at 90 degrees to a magnetic field to experience a force?
No at 90 degrees the wire will experience full force but at angles in between 90 degrees and 0 it’ll feel some force but not it all
What can you use to tell you which way the force acts on a wire through a magnetic field?
Flemings left hand rule
How does the Fleming’s Left Hand rule work and what can it tell you?
Your first finger points in the direction of the field, your second finger points in the direction of the current and your thumb will point in the direction of the force
What do you do with your hand for the Flemings Left Hand Rule?
Point your thumb upwards and your first finger perpendicular to that like a gun and then point your second finger so that it is perpendicular to both your thumb and first finger
How can you remember what each finger tells you in Flemings Left Hand Rule?
First Finger represents Field
seCond finger represent current
thumb represents what is left and so force
What effect do electric motors use?
The motor effect
What two factors speed up an electric motor?
- ) more current
2. ) stronger magnetic field
What does a split ring commutator do and why?
It swaps the contacts every half turn to keep the motto running in the right direction
How can you reverse the direction of a motor?
Swapping the polarity of the direct current, so from positive to negative to positive to negative or by swapping the magnetic poles over
Name six things electric motors are used in?
CD players, food mixers, fans, drills, hair dryers and cement mixers
How can an electric motor be used in a fan?
You link the coil of wire to an axle, and so as the coil rotates so does the axle and so if there’s a fan attached to the axle it rotates as well
How does an electric motor work?
There is a coil of wire inside a magnetic field, and so the forces acting on each arm of the coil of wire are opposite. As the forces act one up and one down it causes the coil to rotate. After a half turn the contacts are swapped so that the motor continues to rotate in the same direction instead of rotating back
What is electromagnetic conduction?
The creation of a potential difference across a conductor which is experiencing a change in magnetic field
What is another word for induction?
Creation
What two ways can you induce a potential difference in a wire?
- ) by moving a magnet in a coil of wire
2. ) by cutting magnetic field lines with an electrical conductor
What happens to the potential difference and current if you move the magnet in the opposite direction or reverse the polarity of the magnet?
Both the potential difference and current will be reversed
How is an alternating current created with a magnet in a coil of wire?
If you keep moving the magnet backwards and forwards, you induce a potential difference that keeps swapping direction producing an alternating current
How does a generator work?
As you turn a magnet in a coil of wire the magnetic field through the coil changes, inducing a potential difference which makes a current flow in the wire, when you have turned a magnet a half turn the direction of the magnetic field reverses as the poles are turning, this creates a reversing potential difference and so the current begins to flow on the opposite direction creating an alternating current
What is an example of a device that uses electromagnetic induction to generate a current?
A dynamo
If the current flows from right to left in a coil of wire which direction are the magnetic field lines inside and outside the coil of wire?
Inside the coil they go towards where the current is coming from so left to right, and outside the coil they go opposite and so right to left
How is a dynamo used to power lights in a bike
The wheel turns it turns the cog, which is attached to a magnet in a coil. So as the cog and magnet turns it induces a potential difference and current which is used to power the lights
How does a transformer work?
An alternating current in the primary coil produces a changing magnetic field in the iron core. This produces a changing magnetic field around the secondary coil. As these field lines cut the field lines of the secondary coil it induces an alternating potential difference across the end of the secondary coil through electromagnetic induction. If there is a complete circuit an alternating current will then flow out through the secondary coil
What type of current do transformers have to be in?
Alternating current?
Why can’t transformers use direct current?
They need the alternating current to produce a changing magnetic field and in the primary coil and so in the iron core. And so in the secondary coil to induce a potential difference, a direct current wouldn’t do this
What does a transformer look like?
They have two coils of wire joined together by an iron core
What are the names for the two types of coil in a transformer?
Primary and secondary
What are the two different types of transformer?
Step up and step down
Whats the difference between the amount of coils on a step up and a step down transformer?
A step up transformer has more turns on the secondary coil where as a step down has more turns on the primary coil
Does the iron core carry the current or magnetic field?
Magnetic field not electricity flows in the iron core
How efficient are transformers?
100%
In a transformer power in = ?
Power out
What are switch mode transformers?
They’re a type of transformer that operates at higher frequency that traditional transformers
Between what frequency can switch mode transformers work at?
Between 50kHz and 200 kHz
Why are switch mode transformers made at a much higher frequency?
They can be made much lighters and smaller than traditional transformers
Name two things that switch mode transformers are used in?
Mobile phone chargers and power supplies
