magnetism, electromagnetism, emi Flashcards
What is the direction of conventional current?
Positive to negative (Long to short in circuit diagram of cell)
Magnetic field lines point from ____ to ____
North to South
Right hand grip rule?
Curl fingers in the direction of current flow, thumb points to North
OR Curl fingers in the direction of magnetic field lines, thumb points to direction of current (into the plane / out of the plane)
What does x and . represent?
x: into the plane
.: out of the plane
Force between 2 parallel current-carrying conductors (current in OPPOSITE direction)?
Wires repel
Force between 2 parallel current-carrying conductors (current in SAME direction)?
Wires attract
Fleming’s left hand rule?
Thumb: force (father)
Second finger: magnetic field (mother)
Third finger: current (child)
How to explain the force on a current carrying conductor?
Magnetic field due to the current interacts with magnetic field due to the magnet, resulting in a resultant magnetic field which exerts force on current carrying conductor.
Why does galvanometer register a deflection when magnet is moved in and out of solenoid?
As the magnet moves towards the solenoid, its magnetic field lines ‘cut’ the coils of the solenoid. The magnetic flux linking the coils have changed.
When this happens, a galvanometer connected to the coil registers a momentary deflection (an e.m.f. is induced).
Why does galvanometer not register a deflection when magnet is stationary solenoid?
When the magnet is stationary in the solenoid, there is no deflection as there is no change in the magnetic flux linking the coils (no e.m.f. induced).
Lenz’s Law states that…
Lenz’s Law states that the direction of the induced e.m.f., and hence the induced current in a closed circuit, is always such that its magnetic effect opposes the motion or change producing it.
Faraday’s law tells us the _____ of the induced e.m.f.
Lenz’s law tells us the _____ of the induced e.m.f.
Faraday’s law tells us the magnitude of the induced e.m.f.
Lenz’s law tells us the direction of the induced e.m.f.
By Lenz’s Law, when an S pole approaches a solenoid, ___ pole will form at that end of the solenoid.
By Lenz’s Law, when an S pole approaches a solenoid, an S pole will form at that end of the solenoid.
By Lenz’s Law, when an S pole moves away from a solenoid, ___ pole will form at that end of the solenoid.
By Lenz’s Law, when an S pole moves away from a solenoid, a N pole will form at that end of the solenoid.
By Lenz’s Law, when an S pole moves away from a solenoid, ___ pole will form at that end of the solenoid.
By Lenz’s Law, when an S pole moves away from a solenoid, an N pole will form at that end of the solenoid.
Lenz’s law is a consequence of the Law of energy conservation. What is the energy conversion?
Mechanical work is transformed into electrical energy.
Differences between DC motor and AC generator?
DC motor
- requires split ring commutator (reverse current every half cycle)
- fleming’s left hand rule
AC generator
- slip rings (to transfer alternating induced e.m.f. to the external circuit)
- uses electromagnetic induction
- fleming’s right hand rule
Doubling the number of turns of the coil…
Doubling the number of turns of the coil doubles output voltage
Doubling the frequency of rotation…
Doubling the frequency of rotation doubles output voltage and frequency
How is voltage and number of coils and current in transformer related?
VS/VP = NS/NP = IP/IS
Efficiency =
output power∕input power × 100%
What are the causes of power loss?
- Heat loss due to the resistance of the coils
- Leakage of magnetic field lines between the primary and secondary coils
- Heat loss due to eddy current induced in the iron core
Electricity from power station to homes?
Electricity produced in power stations is stepped up by step-up transformers.
High voltage electricity is transmitted through transmission cables.
Step-down transformers reduce the voltage to suitable values for use.
Faraday’s law states that…
Faraday’s Law of electromagnetic induction states that the magnitude of the induced e.m.f. in a circuit is directly proportional to the rate of change of magnetic flux in the circuit.
