Define alternating current (a.c.) .

An alternating current is a current that periodically varies between a positive and a negative value with time, reversing direction every half cycle .

Define the root-mean-square (rms) current .

The rms current is the square root of the mean of the squares of all the current values in one cycle. It is equivalent to the steady direct current that would deliver the same power in a resistor as the alternating current.

The rms voltage is equal to .......... × the peak voltage V 0 .

The rms voltage is equal to 0.707 × the peak voltage V 0 .

True or False? The UK mains electricity supply is quoted as 230 V, which is its peak voltage .

False. The 230 V value for the UK mains supply is its rms voltage , not its peak voltage. The peak voltage is higher, at approximately 325 V.

Why do devices such as televisions and computers contain a step-down transformer?

They run on direct current (d.c.) , so a step-down transformer converts the 230 V a.c. mains supply into a lower d.c. voltage (for example, 12 V) that the device requires.

Define cathode-ray oscilloscope (CRO) .

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

How does an alternating (a.c.) voltage appear on an oscilloscope screen, compared with a direct (d.c.) voltage?

An a.c. voltage is displayed as a transverse wave . A d.c. voltage is displayed as a horizontal line at the relevant voltage.

What do the x -axis and y -axis of an oscilloscope display represent?

The x -axis represents time *. The y -axis represents voltage (or y-gain *).

What is seen on an oscilloscope screen when the time-base is switched off?

Only a vertical line on the voltage-gain axis is seen, showing the wave's amplitude but no time period.

What is seen on an oscilloscope screen when the voltage-gain is switched off?

Only a horizontal line on the time axis is seen.

For a d.c. waveform on an oscilloscope, only a .......... is displayed at the relevant voltage, and the time-base setting is irrelevant.

For a d.c. waveform on an oscilloscope, only a horizontal line is displayed at the relevant voltage, and the time-base setting is irrelevant.

What is the peak-to-peak voltage on an oscilloscope trace?

The vertical displacement between the minimum and maximum values of voltage on the trace.

True or False? The time-base setting must be correctly adjusted to read the value of a d.c. voltage on an oscilloscope.

False. For a d.c. voltage there is no time period, so the time-base setting is irrelevant . It is the voltage-gain setting that is used to read the value of the d.c. voltage.

A transformer is a device that changes high alternating voltage at low current to low alternating voltage at high current, and vice versa.

What are the three main components of a transformer?

Primary coil Secondary coil Iron core

Why is a transformer's core made from soft iron ?

Soft iron can be easily magnetised and demagnetised , and it focuses and directs the magnetic field from the primary coil to the secondary coil.

Describe how an e.m.f. is induced in the secondary coil of a transformer.

The alternating current in the primary coil creates an alternating magnetic field, producing a changing magnetic flux linkage in the secondary coil through the iron core. By Faraday's law , this changing flux linkage induces an e.m.f. in the secondary coil.

A step-up transformer has .......... turns in the secondary coil than in the primary coil.

A step-up transformer has more turns in the secondary coil than in the primary coil.

True or False? The output voltage from a transformer's secondary coil has a different frequency to the input voltage.

False. The output alternating voltage from the secondary coil is at the same frequency as the input voltage.

Where are step-up and step-down transformers used in the National Grid?

A step-up transformer is used between the power station and the transmission wires. A step-down transformer is used between the transmission wires and buildings.

Define eddy currents in a transformer core.

Eddy currents are currents induced in the core material by the changing magnetic field from the alternating current; the core is a conductor, and these currents dissipate energy as heat.

How is the efficiency of a transformer calculated?

Efficiency is the ratio of power out to power in, given as a decimal. Multiply by 100 to give the efficiency as a percentage.

State two ways of reducing eddy current losses in a transformer core.

Laminating the core with layers of insulation Using a core made from a high-resistivity metal

Laminating a transformer core means building it from .......... layers of metal that are insulated from each other.

Laminating a transformer core means building it from thin layers of metal that are insulated from each other.

True or False? Eddy current losses are reduced by making the transformer core from a low-resistivity metal.

False. Eddy current losses are reduced by using a high-resistivity core metal, which limits the induced current.

Why does laminating the core reduce eddy currents?

The insulated , thin layers prevent current from flowing between laminations, which significantly reduces the induced eddy currents.

Why are real transformers not 100% efficient?

Their power output is less than their power input, due to energy losses such as eddy currents in the core.

Why do the coils of wire in a transformer cause energy loss?

The coils have resistance , so current flowing through them causes heat energy to be lost. The larger the current , the greater the amount of heat energy lost.

State three sources of inefficiency in a transformer's core.

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

Why is a transformer core made from soft iron or iron alloys?

To allow easy magnetisation and demagnetisation , which reduces hysteresis loss.

Why are energy losses due to the heating of transmission lines significant?

Because electrical energy is transmitted across long distances , from power stations to buildings, both within the UK and to other countries.

Since power = voltage × current, stepping up the voltage transmitted through cables .......... the current.

Since power = voltage × current, stepping up the voltage transmitted through cables reduces the current.

True or False? Step-up transformers are used between transmission lines and buildings to reduce the voltage delivered to homes.

False. Step-up transformers are used between power stations and transmission lines to increase the voltage and reduce the current. Step-down transformers are used near buildings to decrease the voltage to that required.

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Alternating Currents & Transformers (AQA A Level Physics): Flashcards

Exam code: 7408

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FrontAlternating Current & Voltage
Define alternating current (a.c.).

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Define alternating current (a.c.).
An alternating current is a current that periodically varies between a positive and a negative value with time, reversing direction every half cycle.
Define the root-mean-square (rms) current.
The rms current is the square root of the mean of the squares of all the current values in one cycle. It is equivalent to the steady direct current that would deliver the same power in a resistor as the alternating current.
What is the relationship between the time period T and frequency f of an alternating current?
$T = \frac{1}{f}$
How is the peak voltage V₀ related to the peak-to-peak voltage?
$V_{0} = peak-to-peak voltage \div 2$
The rms voltage is equal to .......... × the peak voltage V₀.
The rms voltage is equal to 0.707 × the peak voltage V₀.
What is the equation for the average power delivered by an a.c. supply?
$Average power = I_{r m s} \times V_{r m s}$
True or False?
The UK mains electricity supply is quoted as 230 V, which is its peak voltage.
False.
The 230 V value for the UK mains supply is its rms voltage, not its peak voltage. The peak voltage is higher, at approximately 325 V.
Why do devices such as televisions and computers contain a step-down transformer?
They run on direct current (d.c.), so a step-down transformer converts the 230 V a.c. mains supply into a lower d.c. voltage (for example, 12 V) that the device requires.
Define cathode-ray oscilloscope (CRO).
A CRO is a laboratory instrument used to display, measure and analyse waveforms of electrical circuits. It can be used as an a.c. and d.c. voltmeter.
How does an alternating (a.c.) voltage appear on an oscilloscope screen, compared with a direct (d.c.) voltage?
An a.c. voltage is displayed as a transverse wave. A d.c. voltage is displayed as a horizontal line at the relevant voltage.
What do the x-axis and y-axis of an oscilloscope display represent?
The x-axis represents time*. The y-axis represents voltage (or y-gain*).
What is seen on an oscilloscope screen when the time-base is switched off?
Only a vertical line on the voltage-gain axis is seen, showing the wave's amplitude but no time period.
What is seen on an oscilloscope screen when the voltage-gain is switched off?
Only a horizontal line on the time axis is seen.
For a d.c. waveform on an oscilloscope, only a .......... is displayed at the relevant voltage, and the time-base setting is irrelevant.
For a d.c. waveform on an oscilloscope, only a horizontal line is displayed at the relevant voltage, and the time-base setting is irrelevant.
What is the peak-to-peak voltage on an oscilloscope trace?
The vertical displacement between the minimum and maximum values of voltage on the trace.
True or False?
The time-base setting must be correctly adjusted to read the value of a d.c. voltage on an oscilloscope.
False.
For a d.c. voltage there is no time period, so the time-base setting is irrelevant. It is the voltage-gain setting that is used to read the value of the d.c. voltage.
Define transformer.
A transformer is a device that changes high alternating voltage at low current to low alternating voltage at high current, and vice versa.
What are the three main components of a transformer?
- Primary coil
- Secondary coil
- Iron core
Why is a transformer's core made from soft iron?
Soft iron can be easily magnetised and demagnetised, and it focuses and directs the magnetic field from the primary coil to the secondary coil.
Describe how an e.m.f. is induced in the secondary coil of a transformer.
The alternating current in the primary coil creates an alternating magnetic field, producing a changing magnetic flux linkage in the secondary coil through the iron core. By Faraday's law, this changing flux linkage induces an e.m.f. in the secondary coil.
State the transformer equation relating turns and voltage.
$\frac{N_{s}}{N_{p}} = \frac{V_{s}}{V_{p}}$
A step-up transformer has .......... turns in the secondary coil than in the primary coil.
A step-up transformer has more turns in the secondary coil than in the primary coil.
True or False?
The output voltage from a transformer's secondary coil has a different frequency to the input voltage.
False.
The output alternating voltage from the secondary coil is at the same frequency as the input voltage.
Where are step-up and step-down transformers used in the National Grid?
A step-up transformer is used between the power station and the transmission wires. A step-down transformer is used between the transmission wires and buildings.
Define eddy currents in a transformer core.
Eddy currents are currents induced in the core material by the changing magnetic field from the alternating current; the core is a conductor, and these currents dissipate energy as heat.
For an ideal (100% efficient) transformer, what is the relationship between input and output power?
$I_{p} V_{p} = I_{s} V_{s}$
Power in = power out
How is the efficiency of a transformer calculated?
Efficiency is the ratio of power out to power in, given as a decimal. Multiply by 100 to give the efficiency as a percentage.
State two ways of reducing eddy current losses in a transformer core.
- Laminating the core with layers of insulation
- Using a core made from a high-resistivity metal
Laminating a transformer core means building it from .......... layers of metal that are insulated from each other.
Laminating a transformer core means building it from thin layers of metal that are insulated from each other.
True or False?
Eddy current losses are reduced by making the transformer core from a low-resistivity metal.
False.
Eddy current losses are reduced by using a high-resistivity core metal, which limits the induced current.
Why does laminating the core reduce eddy currents?
The insulated, thin layers prevent current from flowing between laminations, which significantly reduces the induced eddy currents.
Why are real transformers not 100% efficient?
Their power output is less than their power input, due to energy losses such as eddy currents in the core.
Why do the coils of wire in a transformer cause energy loss?
The coils have resistance, so current flowing through them causes heat energy to be lost. The larger the current, the greater the amount of heat energy lost.
State three sources of inefficiency in a transformer's core.
- Induced eddy currents
- The reversal of magnetism
- Poor insulation between the primary and secondary coil
Why is a transformer core made from soft iron or iron alloys?
To allow easy magnetisation and demagnetisation, which reduces hysteresis loss.
Why are energy losses due to the heating of transmission lines significant?
Because electrical energy is transmitted across long distances, from power stations to buildings, both within the UK and to other countries.
What is the equation for power loss in a transmission line?
$P = I^{2} R$
Since power = voltage × current, stepping up the voltage transmitted through cables .......... the current.
Since power = voltage × current, stepping up the voltage transmitted through cables reduces the current.
What effect does doubling the current in a transmission line have on the power lost?
It produces four times the power loss, since power lost is proportional to the square of the current ( $P \propto I^{2}$ ).
True or False?
Step-up transformers are used between transmission lines and buildings to reduce the voltage delivered to homes.
False.
Step-up transformers are used between power stations and transmission lines to increase the voltage and reduce the current. Step-down transformers are used near buildings to decrease the voltage to that required.

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