higher content Flashcards
What is inertia?
The idea that without an unbalanced force, nothing will change in terms of speed, direction etc.
How is inertial mass defined?
By the ratio of force over acceleration
if an object is more inert then it’s harder to move
When will a seesaw balance in terms of moments?
When the anticlockwise moment is equal to the clockwise one
What does the principle of moments state?
For a balanced object, clockwise moments = anticlockwise moments
What is the centre of mass of an object?
The point at which the mass may be thought to be concentrated
Where does the centre of mass lie in symmetrical objects?
Along the line of symmetry
Where will the COM be when an object has more than one line of symmetry?
Where the lines of symmetry intersect
Where will the centre of mass be of a suspended object?
The centre of mass will be directly below the point of suspension
How can the COM of an irregular shape be found?
- by hanging the shape from a point and a plumb line is used to draw a vertical line downwards from the pivot
- this is repeated from different points
What is the stability of an object affected by?
- the width of the base of the object
* the height of it’s centre of mass
When will an object not topple over?
When the line of action is within the base
What are the three types of equilibrium when a force is applied to objects?
- stable equilibrium - tilts and falls back to original position
- unstable equilibrium - tilts and falls
- neutral equilibrium - rolls
What happens to the centre of mass when a stable object is tilted?
It is raised
What happens to the centre of mass when an unstable object is tilted?
It is lowered
What happens to the centre of mass when a neutral object is tilted?
It stays at the same level if pushed
What is the equation for momentum?
Mass x velocity
What is momentum measured in?
kgm/s
Why will a ball be harder to stop if it has a large mass and moving at a high speed?
There will be a greater force exerted on the person’s hands by the ball. They will have to exert an equal and opposite force on the ball to stop it
What happens to momentum in a collusion of explosion?
Momentum is conserved, provided that there are no external forces having an effect
What are the external forces that could have an effect on momentum?
- friction which decreases momentum
* gravity (e.g. a collision on a slope)
What does Fleming’s left hand rule show?
The direction of movement of the wire
What does each finger represent in Fleming’s left hand rule?
- index finger - direction of field from N to S
- middle finger - direction of current from + to -
- thumb - direction of movement
How is the direction of the force in an electromagnet reversed?
- reversing the direction of the current
* reversing the direction of the magnetic field
What is 1 tesla equal to?
1 newton per amp per metre
What equation is used to calculate the force on a wire?
F=BIL
What is the motor effect?
When a wire is placed in a magnetic field and experiences a force
How does a d.c. motor work?
When a coil of wire is placed in a magnetic field and therefore the two sides of the coil experience a force due to the motor effect
Why does the coil rotate in a d.c. motor?
The current of the opposite side of the coil is in the opposite direction, so the forces are in opposite directions
Where are split ring commutators used?
In electric motors
How do split ring commutators work?
Every half turn it breaks the current so the coil of wire keeps turning in the same direction
What is an easy way to tell if a setup is a motor or a generator?
A motor will have a battery, a generator will not
What is the generator effect?
The induction of a potential difference in a wire which is moving relative to a magnetic field
How is a voltage induced in a wire using the generator effect?
• if a wire is moved downwards to cut through the magnetic field
When is a voltage not induced using the generator effect?
- if the wire is stationary
* if the wire is moved parallel to the field line
How is the voltage induced increased using the generator effect?
- move the wire faster
* use a stronger magnet
How is the voltage induced reversed in a wire?
- move the wire in the opposite direction
* reverse the magnetic field
During electromagnetic induction, why do you have to do work when you push the magnet into the coil?
The electromagnet is repelling the magnet so energy is transferred
During electromagnetic induction, why do you have to do work when you pull the magnet out of the coil?
The electromagnet is attracting the magnet so work is done against the force
What happens when a magnet is moved into a coil of wire which is a part of a complete circuit?
A current is induced in the wire - the coil is now an electromagnet
What is purpose of slip rings in an a.c. generator?
So the wire doesn’t twist as the coil rotates
What is the energy transfer in an electric generator?
Kinetic to electrical
What is the energy transfer in an electric motor?
Electrical to kinetic
What is an a.c. generator also known as?
An alternator
What is a d.c. generator also known as?
A dynamo
Why does an alternator produce an a.c. output?
- the two sides of the coil are always connected to the same side of the lamp
- when the LHS of the coil moves past the top it reverses the direction of movement
- so voltage and therefore current reverses
Why does a dynamo produce a d.c. output?
- the two sides the coil are connected to split rings so the two sides of the coil are not connected to the same side during a turn
- every half turn the connections are reversed so when induced voltage is reversed, the current flowing is not reversed
How does a microphone work?
- a magnet is moved near a coil and a current is generated so the diaphragm is attracted to coil
- diaphragm vibrates in response to incoming sound waves and coil vibrates
- this induces a current in the coil (a.c.)
What is a transformer made of?
A primary and secondary coil wrapped around an iron core
Why is the iron core of a transformer laminated?
So no induced currents flow in the iron core
What does it mean when an iron core is laminated?
It is made in layers of iron
How does a transformer work?
- a current flows in the primary coil and becomes an electromagnet
- this magnetises the iron core which is linked to the secondary coil
- a.c. voltage - magnetic field inside secondary coil will keep changing size and direction so a voltage is induced in the secondary coil
- voltage will change direction and br an a.c. current
What does the size of a secondary voltage in a transformer depend on?
- primary voltage
* number of turns on each coil
What is the equation to work out the voltage or number of turns in a transformer?
Vp/Vs = Np/Ns
