7.1 - Alternating Currents & Transformers

Question 1

What is the definition of an alternating current?

Answer 1

A current which periodically varies between a positive to a negative value with time.

Question 2

What does A.C look like plotted on a graph?

Answer 2

Sinusoidal as the current varies every half cycle.

Question 3

What is peak current/ peak voltage defined as?

Answer 3

The maximum value of the alternating current or voltage.

Question 4

What is the rms value of an alternating current defined as?

Answer 4

The square root of the mean of the squares of all the values of the current in one cycle.

Question 4

In an A.C circuit, how do electrons move in the current carrying wire?

Answer 4

They move back and forth with simple harmonic motion.

Question 5

What is the rms of A.C equivalent to?

Answer 5

The direct current that produces the same power.

Question 6

What is the equation to calculate the rms value of A.C?

Answer 6

Iᵣₘₛ = I₀/√ 2
I₀ = peak current

Question 7

What is the definition of the rms value of an alternating voltage?

Answer 7

The square root of the mean of the squares of all the values of the voltage in one cycle.

Question 8

What is the rms value of alternating voltage equivalent to?

Answer 8

The d.c voltage that produces the same power.

Question 9

What is the equation to calculate the rms value of an alternating voltage?

Answer 9

Vᵣₘₛ = V₀/√ 2
V₀ = peak voltage

Question 10

What is the rms value defined as?

Answer 10

The steady direct current, or voltage, that delivers the same average power in a resistor as the alternating current, or voltage.

Question 11

How do you calculate the average power of a A.C circuit?

Answer 11

Average power = Iᵣₘₛ x Vᵣₘₛ

Question 12

What are the applications of alternating current/voltage?

Answer 12

• Mains electricity is supplied as alternating current by the National Grid

Question 13

What is the voltage and frequency produced by the mains electricity that goes into households?

Answer 13

• 230V (rms value)
• 50Hz

Question 14

What does CRO stand for?

Answer 14

Cathode-Ray Oscilloscope.

Question 15

What is a CRO used for?

Answer 15

It is a laboratory instrument used to display, measure and analyse waveforms of electrical circuits (can be used as an a.c and d.c voltmeter).

Question 16

What is A.C voltage on an oscilloscope represented as? (what does it look like on the screen)

Answer 16

Transverse wave.

Question 17

Since A.C voltage is displayed as a transverse wave on an oscilloscope, what can properties be measured?

Answer 17

• frequency, time period and peak voltage.

Question 18

What is D.C voltage represented as on an oscilloscope? (what does it look like on the screen)

Answer 18

As a horizontal line at the relevant voltage.

Question 19

Since D.C voltage is displayed as a horizontal line on an oscilloscope, what can properties be measured?

Answer 19

-x-axis is time, y-axis is voltage
- therefore voltage level is shown

Question 20

What is the time-base on an CRO?

Answer 20

how many seconds each division represents.
(s/div, s/cm)

Question 21

What happens when the time-base is turned off on a CRO for A.C?

Answer 21

Only a vertical line on the voltage-gain axis is seen with its relevant amplitude.

Question 22

What does voltage-gain mean on an CRO?

Answer 22

The amount of voltage per division (per square).

Question 23

What is a transformer

Answer 23

A device that changes high alternating voltage at low current to low alternating voltage at high current, (vice versa).

Question 24

Why were transformers designed, what do they help with?

Answer 24

They reduce heat energy lost whilst electricity is transmitted down electrical power lines from power stations to the national grid.

Question 25

What are transformers made up of?

Answer 25

• A primary coil
• A secondary coil
• An iron core.

Question 26

What is the structure of a transformer?

Answer 26

The primary and secondary coils are wound around the soft iron core.
There is a magnetic field in the iron core.

Question 27

Why is the soft iron core of a transformer necessary?

Answer 27

• It focuses and directs the magnetic field from the primary and secondary coil.
• It can easily be magnetised and demagnetised

Question 28

What happens in the primary coil of a transformer?

Answer 28

• An alternating current producing an alternative voltage is applied.
• this creates an alternating magnetic field inside the iron core and therefore a changing magnetic flux linkage

Question 29

What happens in the secondary coil of a transformer?

Answer 29

• A changing magnetic field passes through the coil through the iron core
• This results in a changing magnetic flux linkage in the coil and therefore an emf is induced
• This emf produces an alternating output voltage from the secondary coil (same frequency as input voltage)

Question 30

What is the transformer equation?

Answer 30

Nₛ/Nₚ = Vₛ/Vₚ
Nₛ = number of turns in the secondary coil
Nₚ = number of turns in the primary coil
Vₛ = output voltage from the secondary coil
Vₚ = input voltage in the primary coil

Question 31

What is a step up transformer?

Answer 31

A transformer that increases the voltage of the power source.
Nₛ>Nₚ

Question 32

What is a step down transformer?

Answer 32

A transformer that decreases the voltage of the power source.
Nₚ>Nₛ

Question 33

When is a step up transformer used?

Answer 33

Between power stations and transmission wires.

Question 34

When is a step down transformer used?

Answer 34

Between the transmission wire and buildings.

Question 35

What is the transformer efficiency equation?

Answer 35

efficiency = Iₛvₛ/IₚVₚ
Iₛ = output current from the secondary coil
vₛ = output voltage from the secondary coil
Iₚ = current in the primary coil
Vₚ = voltage in the primary coil

Question 36

What can the ideal transformer equation also be written as? (ratios)

Answer 36

Iₛ/Iₚ = Vₚ/vₛ = Nₚ/Nₛ

Question 37

What creates eddy currents?

Answer 37

• A changing magnetic field (and flux) from the alternating current/voltage
• This creates a changing magnetic field in the core that acts against the field that induced them
• An emf is therefore induced
• A current flows, as the core is made from a conducting material

Question 38

How are eddy currents reduced?

Answer 38

• Laminating the iron core with layers of insulation
• Having a core made from a high resistivity material

Question 39

How does laminating the core reduce eddy currents?

Answer 39

The core is built of thin layers of metal instead of solid metal, reducing the eddy currents.
The laminations are insulated from each other, so the current doesn’t flow between them.

Question 40

How do the wires in a transformer contribute to inefficiencies?

Answer 40

• Coils of wire have resistance
• This causes heat energy to be lost from the current flowing through the coils
• The larger the current, the greater the amount of heat energy lost

Question 41

What are 3 things that create inefficiencies in transformers?

Answer 41

• Induced eddy currents
• The reversal of magnetism
• Poor insulation between the primary and secondary coil

Question 42

What are 4 ways to reduce energy loses in transformers?

Answer 42

• Making the core from soft iron
• Laminate the core
• Use thick wires - reduces resistance
• Using a core that allows all the flux due to the primary coil to be linked to the secondary coil

Question 43

Why are step up transformers used to increase the voltage generated in power stations?

Answer 43

• voltage increases and current decreases (through transmission lines)
• decreased current decreases heat energy loses