AC

Question 1

What is an alternating curent?

Answer 1

Electric current which has a flow direction that reverses periodically with time

Question 2

What is the root-mean-square value of an alternating current?

Answer 2

value of the direct current that dissipates the same heating effect or same power as the alternating current across the same resistor

Question 3

How does a transformer work?

Answer 3

• In primary winding, current flows through, there is magnetic flux density created
• Since the direction of current reverses periodically, the magnetic flux density is constantly changing in the primary winding, the magnetic flux is also constantly changing
• In the secondary windings, it is linked by the transformer core, the magnetic domains are also constantly changing, the secondary coil experiences a change in magentic flux and magnetic flux linkage
• According to Faraday’s Law, emf is induced
• According to Lenz’s law, the induced emf and hence the current will be created in such a way to oppose the change that causes it

Question 4

What is the main function of a transformer?

Answer 4

Increasing or decreasing the potential difference of an alternating supply using the principle of electromagnetic induction

Question 5

What is principle behind how a transformer work as a source of electrical power supply?

Answer 5

changing current in primary coil gives rise to changing flux in the core, flux links to secondary coil experiences changing flux linkage, hence emf is induced

Question 6

What is an ideal transformer?

Answer 6

one where there is no energy loss

Question 7

Why the input voltage and output induced emf of a transformer have the same frequency?

Answer 7

input voltage produces periodic change of magnetic flux in the core, which is linked to the secondary coil. By Faraday’s law, emf induced in the secondary coil is proportional to the rate of change of magnetic flux linkage in the core, hence induced emf also changes periodically and the cycle is repeated in the same time

Question 8

Why is emf induced in the secondary coil of a transformer not in phase with the current in the primary coil?

Answer 8

• alternating current in primary coil gives rise to alternating magnetic flux
• magnetic flux in core is in phase with current in the primary coil
• magnetic flux cuts the secondary coil, hence emf is induced in secondary coil
• the emf induced is proportional to rate of change of magnetic flux

Question 9

What is the difference between input pd of primary coil and output pd of secondary coil in a transformer?

Answer 9

• Primary current and flux in the core are in the same phase
• rate of change of flux in the core gives rise to output emf induced
• By Lenz’s law, the induced emf produces induced current and magnetic flux which opposes the changing flux in the primary coil
• thus input pd is not in phase with output pd

Question 10

What is the process of converting AC to DC?

Answer 10

Rectification

Question 11

How is rectification achieved?

Answer 11

Through the use of rectifies (eg.diode) which conduct current in one direction only