3b) Forces And Electromagnetism Flashcards
Where is the centre of mass located
Directly below the point of suspension
It is where the whole mass is concentrated
Equation to find a moment
Moment = force x perpendicular distance from the line of action of the force to the pivot
How do ypu work out the centre of mass?
Suspend the shape and a plumb line from the same point until they stop moving
Draw a line along the plumb line
Do the same again in a different pivot point
The com is where the lines meet
How do you work out if moments are equal?
Total anticlockwise moment = total clockwise moment
Why are levers useful?
They increase the distance from the pivot at which the force is applied so less force is needed to get the same moment
Why do objects turn?
The total anticlockwise moments do not equal the total clockwise moments - so there is a resultant force
What makes an object stable?
Low wide bases with low centres of mass
The centre of mass must be within the edge of its base so the line of action of the weight remains inside and there is no resultant force
How do you calculate the time it takes for a pendulum to swing?
Time = 1 divided by frequency
How do you work out pressure
Pressure = force over cross sectional area
How do hydraulic systems operate?
They have 2 pistons (one has a smaller c-sectional area). Pressure is transmitted equally through the liquid so the pressure is the same at both pistons
P=f/a so at piston 1 there is a small force and small area transmitted to piston 2
P2 has a large area and large force
How does circular motion work in terms of velocity, acceleration and forces?
Because a circles direction is constantly changing the velocity is also. So the object is accelerating towards the centre. This is due to a resultant force towards the centre.
The force keeping it moving is centripetal force
What things can effect the centripetal force
The faster the larger force
The larger the mass the bigger force
The larger the force the smaller the circle (more turning needed)
Define magnetic field
A region where magnetic materials and wires carting currents experience a force acting upon them
Why are magnetically soft materials useful?
It magnetises and demagnetises easily. When you turn off the current the mag field disappears
WHY does the left hand rule work?
It can be used to predict directions.
If the direction of the current or magnetic field Is reversed the the direction of the force is reversed too
How to remember left hand rule:
thuMb - Motion
First - Feild
seCond - Current
Factors which speed up electric motors are:
More current
Stronger magnetic field
Define electromagnetic induction
The creation of potential difference across a conductor which is experiencing a change in magnetic field
How do dynamos work?
Ps I know you know this one Aish
The cog wheel at the top is positioned so it touches on the the wheels (on the bike) as the wheel turns the cog attached to the magnet turns too. Creating an AC current
What’s the difference between step up and step down transformers
Step-ups have more coils on the secondary coil than primary and visa versa.
How do transformers work
The primary coil produces a mag field within the iron core so nearly all of the voltage passes to the secondary coil.
There is an AC in the primary so the iron core has a changing mag field which is felt by the secondary coil.
It induces alternating potential difference over the 2nd coil with the same frequency as the 1st.
The number of turns in the coils determines if the P.D. Is bigger or smaller (step up/ down part).
Transformer equation
PD across 1st coil over PD across 2nd coil = turns on 1st coil over turns on 2nd coil
Power in equals power out equations
Power = current x PD
So input = output
Explain why switch mode transformers are good
They operate at higher frequencies (50-200Hz) so they are lighter and smaller so good for mobile and laptop chargers.
More efficient as they use little power but no load is applied.
