General Flashcards
How can insulators be charged by friction?
They are rubbed against each other, one becomes positive, the other negative
How does charging by induction work?
An uncharged object placed near a charged object becomes charged as electrons are transferred.
What is charging?
The gain or loss of electrons
In which direction do magnetic field lines go?
From north to south to form closed loops
What is the difference between hard and soft materials in terms of magnetism?
Soft materials gain and lose magnetisation very quickly
What can cause a magnetic material to lose its magnetism?
Time
Hitting it
Heat
What is the magnetic effect of a current?
Create magnetic fields in a surrounding space, so cause a compass to deflect from north. Degree of deflection depends on size of current. Reversing the current changes the deflection.
Explain the right hand rule
A fist with the thumb point up
Thumb = direction current
Fingers = direction of field lines
What is the magnetic field pattern of a coil and why?
Magnetic field created by each part of the coil combine to a create a strong field through the coil.
Which end of a coil is clockwise or anticlockwise?
Clockwise = south pole Anticlockwise = north pole
Why is iron used as a core in magnets?
It is ferromagnetic
When an external magnetic field from a current-carrying wire or coil passes through a ferromagnetic material, the atomic magnets can line up with the external field to create a much stronger resultant field.
How can you increase the magnetic strength of a solenoid?
Soft iron core
More turns per unit length
Increasing current
What happens to a wire carrying a current in a magnetic field?
It experiences a force (motor effect)
Explain the left hand rule
Forefinger = direction of magnetic field (N to S)
Middle finger = direction of current (+ to -)
Thumb = direction of motor effect force
Unit for strength of magnetic field
Tesla
What is F = BIL
F=BIL
F is the motor effect force in newtons (N).
B is the magnetic field strength in tesla (T).
I is the current in the wire in amps (A).
L is the length of wire at 90° to the magnetic field in metres (m).
The direction of the force is given by the left-hand rule.
What is a split ring commutator?
Reverses the direction of current every half turn, allowing the motor coil to rotate continuously in one direction
What happens when a wire cuts magnetic field lines?
A voltage is induced
What does electromagnetic induction result in?
Always a voltage, a current if there is a closed circuit
What factors affect the magnitude of an induced voltage?
The rate at which a wire cuts magnetic field lines
or
The rate at which the magnetic field through a conductor (eg a coil) changes
And the strength of the magnet
In which direction is an induced voltage?
Against the direction of the change that caused it
How can you increase the output of an AC generator?
Rotate coil more rapidly
Stronger magnetic field
Coil has greater area
More turns on coil
What does a step up transformer do?
Increases the voltage
A transformer has has more turns on the primary than secondary coil. What will the output voltage be?
Lower
What is the equation for the coils and pds in a transformer?
Vp/Vs = np/ns
What is the relationship between voltage and current in a transformer?
VpIp = VsIs
Why are high voltages used in transmission lines?
Keeps current low, so reduced loss due to heating of the cables
What is the equation for power wasted in a transmission line?
P=I^2 * R
Do transformers work with AC or DC?
AC
Why do we need to step down voltage?
Harder to insulate
What is the equation for Hooke’s law?
F=kx
Force applied = constant * change in length
Equation for energy stored in a stretched spring?
E=1/2 k x^2
What factors affect spring constant?
Greater cross sectional area = greater spring constant
Longer wire = smaller spring constant
State newtons first law
A body will remain at rest or in a state of uniform motion in a straight line unless acted on by a resultant external force
State newtons second law
force = mass × acceleration
State newtons third law
If body A exerts a force on body B then body B exerts an equal and opposite force of the same type on body A
What is the equation for momentum?
momentum = mass * velocity
How does force relate to rate of change of momentum?
force = rate of change of momentum
What is the equation for work done?
Work = force * distance moved
What is the equation for GPE?
gpe = mgh
What is the equation for kinetic energy?
1/2 m * v^2
What is the equation for power?
energy transfer/time
Why does temperature plateau?
For example at the melting point
All energy goes into breaking bonds
In what three ways can heat be transferred?
Conduction
Convection
Radiation
What is conduction?
particles pass on kinetic energy to their neighbours without actually moving
What is convection?
This occurs when higher energy particles move to cooler regions, heating them up
What is radiation?
heat transfer by the emission of infra-red electromagnetic waves
How do you work out specific heat capacity?
shc = thermal energy / (mass * temperature change)
What is the relationship between pressure volume and constant temperature?
PV = constant
What is specific latent heat
specific latent heat of a substance is the amount of energy needed to change the state of 1 kg of the substance without changing its temperature
What is specific heat capacity?
Specific heat capacity is defined as the amount of heat required to raise the temperature of 1 kilogram of a substance by 1
What is the equation for specific latent heat?
E=mL
Energy transferred = mass * slh
How do you work out hydrostatic pressure?
Hydrostatic pressure = hpg (height * density * gravity)
What is a transverse wave?
the vibration direction is perpendicular to the wave direction
What is a longitudinal wave?
the vibration direction is parallel to the wave direction
How do you work out frequency?
1/period
How do you work out wave speed?
distance/time
frequency * wavelength
What happens when a light ray slows down?
Refracts towards the normal
What happens when a light ray speeds up?
Refracts away from the normal
What is the doppler effect?
When there is relative motion between a source of waves and an observer, the wavelength and frequency of the waves detected by the observer is different from the wavelength and frequency of the waves received when there is no relative motion
Is the em spectrum longitudinal or transverse?
Transverse
