EM Flashcards
why wet hand may cause electric shock
- water is conductor
- lower resistance
- conducting path
why a bird is safe to stand on an electric cable
resistance much higher than cable
negligible current flows through its body
a student claims that R=V/I cant be used.
Argue this statement
R=V/I is the definition of resistance
its applicable to all conductors
why fuse is located on the live wire
The fuse is connected to the live wire so that the kettle is isolated from high potential when the fuse is blown/ the switch is opened.
uses of earth wire
In case the neutral wire is broken, the current will flow through the earth wire instead of the person because of its low resistance. Electric shock is prevented
A large current will be drawn due to the low resistance and fuse will be blown. Thus, the electrical appliance is separated form the live wire.
use of commutator
reverse the direction of current flowing in the coil every half turn
net torque will not change its direction after coils pass through the vertical
the coils can turn continously.
the assumption when calculating the magnetic field
earth magnetic field is negligible
How an ac current is stepped up by a transformer
its changing current in the primary coil keeps producing a changing B-field in the soft metal core.
The changing B-field will then induce an ac emf in the secondary coil
If the number of coils in the secondary coil is larger than the number of coil in the primary coil, the voltage across the secondary coil will be higher.
Why P=V2/R is not used when calculating power loss across the wire
The V is related to the rms voltage of the ac current
However, when we want to calculate the loss of energy, we should use the potential difference between the beginning and the end of the transmitting cable.
When question said X is at a higher potential
remember it is focusing on external circuit
why the chance of electric shock is high in bathrooms
Humid conditions in bathrooms and water is a conductor which provides a conducting path or lowers the resistance between our hands and the source of electricity
why a current would be induced in the coil when B field decreases
By Lenz’s law, an emf would be induced such that it opposes the change in magnetic flux by driving an induced current in the coil.
unit of magnetic flux
Wb
why an external force F is required to maintain the uniform motion of rod
By lenz’s law, a magnetic force acts on the rod such that it opposes the rod motion and so an external force F is needed to balance the force to maintain uniform motion
if two rod are moving in the same direction in the b field and connected by wires
will there be induced current
No
Both rod cut the field lines in the same way
the emf produced are equal and thus oppose each other
