3b Flashcards
What is a moment?
The turning effect of a force
What is the equation for calculating the size of the moment of the force?
Moment= Force x perpendicular distance from the line of action of the force to the pivot
Where does the centre of mass hang in relation to he point of suspension?
Directly below
If total anticlockwise moment = total clockwise moments what will happen?
The object won’t turn
If total anticlockwise moment do not equal total clockwise moments what will happen?
There will be a resultant moment and the object will turn
When will an object tip over?
If its centre of mass moves beyond the edge of its base
What is the time taken for the pendulum to swing from one side to the other and back again called?
The time period
What formula do you need to calculate time period?
Time period= 1/ frequency
Are liquids compressible?
Liquids are virtually incompressible- you can’t squash them, their volume and density stay the same
Is pressure in a liquid transferred equally or unequally in all directions?
Equally
What is the formula for Pressure?
Pressure (pa, pascals) = force (N) / Cross-sectional area (m2)
Where can the pressure in liquids be used?
Hydraulic Systems
Explain how a hydraulic system works?
The system has two pistons, one with a smaller cross-sectional area than the other. Pressure is transmitted equally through a liquid- so the pressure at both pistons is the same. Pressure= force/area so at the 1st piston a pressure is exerted on the liquid using a small force over a small area. This pressure is transmitted to the 2nd piston. The 2nd piston has a larger area and so as force=pressure x area, there will be a larger force.
Name two things that Hydraulic systems can be used in
Car braking systems, hydraulic car jacks
What is the force that keeps something moving in a circle?
Centripetal force
In a car going round a bend, which force is actually providing the centripetal force?
Friction
In a bucket whirling round on a rope, which force is actually providing the centripetal force?
Tension in the rope
In a spinning fairground ride, which force is actually providing the centripetal force?
Tension in the spokes of the ride
What does Centripetal force depend on?
Mass, Speed, Radius
If an object is moving faster, what happens to the centripetal force?
The centripetal force has to be bigger to keep it moving in a circle
If an object has a larger mass, what happens to the centripetal force?
The centripetal force has to be bigger to keep it moving in a circle
To keep something moving in a smaller circle does the force need to be bigger or smaller?
Larger, has more ‘turning’ to do
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 currents experience a force acting on them
What shape is the magnetic field around a current-carrying wire?
Concentric circles with the wire in the centre
Describe the magnetic field inside and outside a coil of wire
Inside the magnetic field is strong and uniform, outside the coil the magnetic field is just like the one around a bar magnet
What happens to the magnetic field if the current is stopped?
It disappears
What is the magnetic field inside a coil of wire called?
A solenoid
How can you increase the strength of the magnetic field around a solenoid?
Adding a magnetically “soft” iron core through the middle of the coil
Give an example of something that can have its magnetism switched on and off
An electromagnet
What is the force experienced by a current-carrying wire in a magnetic field called?
The motor effect
What is the the motor effect?
The force experienced by a current-carrying wire in a magnetic field
How does Fleming’s left hand rule explain which way the force acts?
The thumb will point in the direction of the force (motion), the first finger will point in the direction of the field and the second finger in the direction of the current
What are the two factors that can speed up a simple electric motor?
More current, stronger magnetic field
What does a spit ring commutator do (in a simple electric motor)?
It swaps the contacts every half turn to keep the motor rotating in the same direction
How can the direction of a motor be reversed?
Swapping the polarity of the direct current supply or swapping the magnetic poles over
What is electromagnetic induction?
The creation of a potential difference across a conductor which is experiencing a change in magnetic field
What happens if you move a magnet in a coil of wire?
It induces a voltage
What will happen if you turn a magnet end to end in a coil?
As you turn the magnet, the magnetic field through the coil changes- this change in magnetic field induces a potential difference, which can make a current flow in the wire. When you’ve turned the magnet through a half turn, the direction of the magnetic field through the coil reverses. When this happens, the potential difference reverses, so the current flows in the opposite direction around the coil of wire. If you keep turning the magnet in the same direction for example always clockwise, then the potential difference will keep on reversing every half turn and you’ll get an AC current.
Give an example of an appliance that uses electromagnetic induction to generate a current
Dynamos- used of bikes to power the lights
What do Step-up Transformers do?
They step the voltage up, they have more turns on the secondary coil than the primary coil
What do Step-down Transformers do?
They step the voltage down, they have more turns on the primary coil than the secondary coil
How do transformers work?
Electromagnetic induction, the primary coil produces a magnetic field which stays within the iron core. This means that nearly all of it passes through the secondary coil and hardly any is lost. Because there is an alternating current in the primary coil, the field in the iron core is constantly changing direction. This rapidly changing magnetic field is then felt by the secondary coil. This changing field induces an alternating potential difference across the secondary coil.
What does the relative number of turns on the two coils determine?
Whether the potential difference induced in the secondary coil is greater or less than the potential difference in the primary
In a step-up transformer, the p.d. across the secondary coil is what than the p.d. across the primary coil
Greater
In a step-down transformer, the p.d. across the secondary coil is what than the p.d. across the primary coil
Less
Does electricity flow round the iron core of transformers?
No, the iron core carries magnetic field, not current
What is the equation for output potential difference from a transformer?
P.D. across primary coil/P.D. across secondary coil = No. of turns on primary coil/ No. of turns on secondary coil
In transformers, what does energy in equal?
Energy out
What is the formula for power supplied?
Power= Current x Potential difference
What are switch mode transformers? Give two examples
A type of transformer that operate at higher frequencies than traditional transformers therefore they are lighter and smaller. Used for mobile phone chargers and power supplies. More efficient.
