P15 Flashcards
What happens when a current flows through a conducting wire
When a current flows through a conducting wire a magnetic field is produced around the wire
How to increase the strength of the magnetic field around the wire
The strength of the magnetic field depends on the current through the wire and the distance from the wire
What does making the wire into a solenoid do and what is the magnetic field like in a solenoid
Shaping a wire to form a solenoid increases the strength of the magnetic field created by a current through the wire.
The magnetic field inside a solenoid is strong and uniform
How are solenoids similar to bar magnets
The magnetic field around a solenoid has a similar shape to that of a bar magnet.
What does adding an iron core do to a solenoid and what does this make when this happens
Adding an iron core increases the strength of the magnetic field of a solenoid.
An electromagnet is a solenoid with an iron core
What is the motor effect
When a conductor carrying a current is placed in a magnetic field the magnet producing the field and the conductor exert a force on each other and causes the conductor to move. This is called the motor effect.
What is the equation for a conductor at right angles to a magnetic field and carrying a current
force = magnetic flux density × current × length F = B x I x L
What is the basis for an electric motor
A coil of wire carrying a current in a magnetic field tends to rotate, has a split ring commutator
What is magnetic flux density
How many field lines there are in a region which shows the strength of the magnetic field
What is fleming’s left hand rule
Using your left hand:
First finger represents Field ( pointing finger )
Second finger represents the direction of the Current ( longest finger )
Thumb represents the motion
How does an electric motor work
In a DC motor forces act on two side arms of a coil of wire thats carrying a current
The coil is on an axle and the forces act one up and one down, it rotates
The direction of the motor can be reversed either by swapping the polarity of the DC supply ( reversing current ) or swapping magnetic poles ( reversing field )
Has a split ring commutator
How is the motor effect used in headphones and speakers
Loudspeakers and headphones use the motor effect to convert variations in current in electrical circuits to the pressure variations in sound waves
How does the generator create a potential difference and how is a current induced in the conductor
If an electrical conductor moves relative to a magnetic field or if there is a change in the magnetic field around a conductor a potential difference is induced
If the conductor is part of a complete circuit, a current is induced in the conductor
What does an induced current do
Generates a magnetic field that opposes the original change
Either the movement of the conductor or the change in magnetic field
How do you change the size of the induced potential difference
Increasinf the speed of the movement - cutting more magnetic fields in a given time
Increase the strength of the magnetic field
How do alternators work
Coil spins, current is induced in the coil.
Current changes direction every half turn
Alternators have slip rings and brushes so contacts don’t swap every half turn
Produce an alternating potential difference
How do dynamos work
Same as alternators
But they have a split ring commutator
This swaps the connection every half turn to keep the current flowing in the same direction
How do microphones work
Microphones use the generator effect to convert the pressure variations in sound waves into variations in current in electrical circuits
How do loudspeakers work
Alternating current sent through a coil of wire attached to the base of a paper cone
Coil surrounds one pole of a permanent magnet and is surrounded by yhe other pole so the current causes a force on the coil
When the current reverses, the force acts in the opposite direction which causes the cone to move in the opposite direction too
Variations in current make cone vibrate which makes air around the cone vibrate and creates variations in pressure that makes a sound wave
The frequency of sound wave is same as frequency of the AC so by controlling the frequency of the AC you can alter the sound wave produced
What do transformers do
Change the the size of the potential difference of an alternating current
How do transformers work
They all have 2 coils of wire, the primary and secondary, joined by an iron coil
When an alternating potential difference is applied across the primary coil, the iron core magnetises and demagnetises quickly. These changes in the magnetic field induces an alternating PD in the secondary coil
If the second coil is part of a complete circuit, this causes a current to be induced
The ratio between the primary and secondary PDs is the same as the ratio between tbe number of turns on the primary and secondary coils
Equation for voltage and number of coils for primary and secondary coils
vp/ vs = np/ ns p= primary coil s= secondary coil V= voltage N= number of coils
