Chp 17: Alternating currents

Question 1

Assumptions for an ideal transformer

Answer 1

1. Primary & secondary coils have no resistance so no energy and no power is lost (100% efficiency)
2. No loss of magnetic flux, so both pri & sec coil have the same magnetic flux through them

Question 2

Why is the iron core important?

Answer 2

it concentrates the magnetic flux such that all the flux from the pri coil reaches the sec coil

Question 3

What are the ways to reduce power loss in transformer?

Answer 3

1. To reduce heat loss, thick cables are used to reduce resistance
2. To reduce heat loss through eddy currents, the iron core is laminated to prevent formation of large eddy currents

Question 4

Define root-mean-square current

Answer 4

the steady direct current that converts electrical energy to other forms of energy at the same average rate as the alternating current in a given resistor

Question 5

define alternating currents

Answer 5

A current in which its direction changes periodically.

Question 6

How does the lamination of the core reduce energy losses?

Answer 6

According to Faraday’s law, since the core experiences a change in magnetic flux linkage, there is unwanted induced EMF hence induced (eddy) currents. Laminating increases the resistance of the iron core which leads to smaller induced current in the iron core, less heating, less power loss.

Question 7

What is the advantage of using an alternating current for the transmission of electricity.

Answer 7

Transformers can only work with an alternating input current. At the generator the transformer is used to step up the voltage during transmission to reduce power losses in the transmission cables and used to step down voltage at consumer households to enhance safety.

Question 8

Explain why high voltage is used for the transmission of electricity.

Answer 8

For the power supply in an ideal transformer, the input power and the output power are the same. P = IV, a high voltage will lead to a low current being transmitted along the long cables. By P = I2R, a lower current leads to a lower power loss in the transmission cables.

Question 9

Ways in which power can be lost in a practical transformer:

Answer 9

1. Resistance of the windings (coils): The wires used for the windings of the coils have resistance and so heating occurs, resulting in power loss (I2R). This form of power loss is also known as Joule heating. Thicker wires made of material with low resistivity (i.e. high purity copper) are used to reduce this power loss.
2. Eddy Currents: The alternating magnetic flux induces eddy currents in the iron core and causes heating. This effect is reduced by laminating the iron core. Laminations reduce the area of circuits in the core, and thus reduce the EMF induced and current flowing within the core, which leads to a reduction in the energy lost.
3. Hysteresis: There are hysteresis losses in the core. The magnetization of the core is repeatedly reversed by the alternating magnetic field. The energy required to magnetize the core (while the current is increasing) is not entirely recovered during demagnetization. The difference in energy is lost as heat in the core. Energy loss is kept to a minimum by using a magnetic material with low hysteresis loss.

Question 10

Why we cannot use P= V2/R, where V is the voltage in the secondary coil to calculate power losses in the transmission of power.

Answer 10

Since the V here is the voltage across the secondary coil and not the pd across the cables, if we really want to use P= V2/R, to calculate for the power loss, V here should be the pd across the cables.