SP13: electromagnetic induction Flashcards
what is needed to make electric current flow in a circuit?
voltage or potential difference
how can a potential difference be induced in a conductor?
-when there is movement between the conductor and a magnetic field
- a coil of wire is moved in a magnetic field
- a magnet is moved into a coil of wire
electromagnetic induction/the generator effect
when motion between a conductor and a magnetic field creates electricity
what does an induced voltage produce if the conductor is connected in a complete circuit?
it produces an induced current, this creates a magnetic field around itself & the magnetic field opposes the original change
what does the direction of the induced potential difference or induced current depend on?
the direction of movement
the current is reversed when:
-the magnet is moved out of the coil
-the other pole of the magnet is moved into the coil
an induced potential difference or induced current will increase if:
-the speed of movement is increased
-the magnetic field strength is increased
-the number of turns on the coil is increased
what is a generator?
device that converts kinetic energy into electrical energy
what is an alternator? (AC generator)
-a coil of wire rotating in a magnetic field, which induces current as it rotates
-the coil is connected to two metal slip rings (commutators), which allow the current to pass out of the coil
-each side of the wire always connects to the same commutator
alternator output on a voltage-time graph
-alternating sine curve
coil at 0° degrees (alternator)
coil is moving parallel to the direction of the magnetic field, no potential difference is induced
coil at 90° (alternator)
the coil is moving at 90° to the direction of the magnetic field, so the induced potential difference is at its maximum
coil at 180° (alternator)
coil is moving parallel to the direction of the magnetic field, no potential difference is induced
coil at 270° (alternator)
-the coil is moving at 90° to the direction of the magnetic field, so the induced potential difference is at its maximum
-here, the induced potential difference travels in the opposite direction to what it did at 90°
coil at 360° (alternator)
-back at its starting point, having done a full rotation
-coil is moving parallel to the direction of the magnetic field, no potential difference is induced
the maximum potential difference or current produced by an alternator or dynamo can be increased by:
-increasing the rate of rotation
-increasing the strength of the magnetic field
what is a dynamo? (DC generator)
-a coil of wire rotating in a magnetic field, which induces current and voltage as it rotates
-the coil is connected to a split ring commutator
-the side of the coil that is moving down is connected to one part of the commutator, the side that’s moving down is connected to another part
-the split ring commutator changes the coil connections every half turn
-as the induced potential difference is about to change direction, the connections are reversed
-this means that the current to the external circuit always flows in the same direction
dynamo output on a voltage time graph
sine curve that stays in the same direction all the time
coil at 0° (dynamo)
coil is moving parallel to the direction of the magnetic field, so no potential difference is induced
coil at 90° (dynamo)
coil is moving at 90° to the direction of the magnetic field, so the induced potential difference is at its maximum
coil at 180° (dynamo)
coil is moving parallel to the direction of the magnetic field, no potential difference is induced
coil at 270° (dynamo)
-moving at 90° to the direction of the magnetic field, so the induced potential difference is at its maximum
-the induced potential difference travels in the same direction as at 90°
uses of alternators
to charge batteries of ships, cars & motorbikes
uses of dynamos
-toys
-power tools
-bicycle lights
steps of a moving-coil microphone
- pressure variations in sound waves cause the flexible diaphragm to vibrate
- the vibrations of the diaphragm cause vibrations in the coil
- the coil moves relative to a permanent magnet, so a potential difference is induced in the coil
- the coil is part of a complete circuit, so the induced potential difference causes a current to flow around the circuit
- the changing size and direction of the induced current matches the vibrations of the coil
- the electrical signals generated match the pressure variations in the sound waves
headphones
- variations in an electric current cause variations in the magnetic field produced by an electromagnet
- this causes a cone to move, which creates pressure variations in the air and forms sound waves
how to produce sound waves from the alternating current provided to a loudspeaker:
- a current in the coil creates a magnetic field
- the magnetic field interacts with the permanent magnet generating a force, which pushes the cone outwards
- the current is made to flow in the opposite direction
- the direction of the magnetic field reverses
- the force on the cone now pulls it back in
- repeatedly alternating the current direction makes the cone vibrate in and out
- the cone vibrations cause pressure variations in the air - which are sound waves
how do you make a loudspeaker cone vibrate correctly?
elastic current must vary in the same way as the desired sound
what is a transformer?
a device that can change the potential difference or voltage of an alternating current
step up transformer
increases the voltage
step-down transformer
reduces the voltage
structure of a transformer
-a basic transformer is made from two coils of wire, a primary coil from the alternating current (ac) input and a secondary coil leading to the ac output
-coils are wound around an iron core
-the core is easily magnetised and can carry magnetic fields from the primary coil to the secondary coil
can transformers use direct current?
no, they can only work with alternating current
when a transformer is working:
- a primary voltage drives an alternating current through the primary coil
2, the primary coil current produces a magnetic field, which changes as the current changes
- the iron core increases the strength of the magnetic field
- the magnetic field passes through the secondary coil
- the changing magnetic field induces a changing potential difference in the secondary coil
- the induced potential difference produces an alternating current in the external circuit
the ratio of potential differences on the transformer coils matches the ratio of…
the numbers of turns on the coils
equation connecting primary and secondary transformers
V(p)/V(s) = N(p)/N(s)
V(p)
the potential difference in the primary (input) coil in volts
V(s)
the potential difference in the secondary (output) coil in volts
N(p)
the number of turns on the primary coil
N(s)
the number of turns on the secondary coil
equation for power output of a transformer
V(s) x I(s) = V(p) x I(p)
(potential difference across primary coil × current in primary coil = potential difference across secondary coil × current in secondary coil)
what is the assumption when calculating the power output of a transformer
a transformer is 100% effective
how does the national grid transport electricity?
-the higher the current in a cable, the greater the energy transferred to the surroundings by heating
-high currents waste more energy than low currents
-to reduce energy transfers to the environment, the national grid uses step-up transformers to increase the voltage from power stations to thousands of volts, which lowers the current in the transmission cables
-step-down transformers are then used to decrease the voltage from the transmission cables, so it is safer to distribute to homes and factories
