Physics Chapter 11 Flashcards
Define magnetic flux.
The product of magnetic flux density, B, normal to a surface and surface area.
State Faraday’s law.
Induced emf is proportional to the rate of change of flux linkage
Define capacitance.
ppt: Charge stored per unit voltage applied
exam notes: Ratio of charge to potential difference
State Lenz’s law. [1]
It states that emf (or current) is induced so as to oppose the change that causes it.
A small coil is placed near to a cable carrying an alternating current, an emf is induced. Use Faraday’s law to explain why an emf is induced in the small coil. [3]
The varying a.c. in the cable produces a changing magnetic field. [1]
It produces a varying magnetic flux cutting through the small coil. [1]
Since induced emf is directly proportional to the rate of change of magnetic flux linkage [1] , an emf is induced.
A coil is moved at a constant speed towards a wire carrying a constant current. Explain, by reference to Faraday’s and Lenz’s laws of electromagnetic induction, why work must be done on the loop. [3]
When the coil is moved towards the wire, there is a change of magnetic flux cutting the coil. [1]
A current is induced in the coil to oppose the change. [1]
A magnetic force is produced and exerted on the coil to oppose the motion. [1]
Hence, work must be done to maintain the speed of the coil.
With reference to the forces acting on the electrons in a conducting rod moving in a magnetic field,
describe how an emf is induced in the rod. [3]
The electrons are moving perpendicularly to the magnetic field. [1]
A magnetic force F = qvB is exerted on the electrons. [1]
The work done by the magnetic force per unit charge leads to the induced emf. [1]
Explain, using Faraday’s law of induction, how a transformer steps down the voltage. [4]
The varying current (or voltage) produces a varying magnetic field in the primary coil. [1]
It produces a change of magnetic flux linkage in the secondary coil. [1]
An emf is then induced in the secondary coil. [1]
When the number of turns in the secondary coil is smaller than that in the primary coil, the voltage is stepped down. [1]
Outline how energy losses are reduced in the core of a practical transformer. [2]
Laminated core is used to reduce eddy current. [1]
So thermal energy loss to the surroundings is reduced. [1]
Outline why electrical energy is transmitted at high voltages. [3]
When the voltage across power transmission is very high, the current flowing through the transmission cables is low. [1]
Since the power loss due to resistance is P = I^2 R, [1]
the thermal energy loss in the cable can be reduced with a smaller current. [1]
Outline the features of an ideal step-down transformer. [2]
Number of turns in the secondary coil is smaller than that in the primary coil. [1]
Laminated iron core is used. [1]
Outline what is meant by the root mean square value of a time-varying voltage. [2]
Vrms (in an a.c.) is the voltage that gives the average power dissipation [1] same as the power dissipation of a constant d.c. voltage. [1]
Note: formula is not allowed to answer this question
