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Z A Level Physics 2 > Alternating currents and transformers > Flashcards

Alternating currents and transformers Flashcards

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1
Q

What are the mains electricity?

A
  1. Electricity is generated and transmitted around the country in the form of alternating currents (ac) and voltages
  2. These are used because they can be transformed to high voltages and very low currents in order to minimise the thermal energy lost as current travels through the wires of the National Grid
  3. Only about 2-3% of the electrical energy from the generator is lost as heat, saving energy, carbon emissions and money
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2
Q

How is alternating current delivered by the National Grid?

A
  • As a sinusoidally varying supply with a frequency of 50Hz and a range of different voltages depending on the consumer
  • Mains electricity comes in a. sinusoidally changing pattern with the magnitude of the current or the voltage continuously varying between maximum positive and negative values
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3
Q

What is the nominal voltage?

A
  • Household mains have a nominal voltage of 230V although this value varies throughout the day depending on the demand and supply of electricity
    1. The maximum current that can be drawn by a single domestic supply is about 6.5A and the electrical socket ring main in uni house has a maximum current of 13A protected by a fuse or a circuit breaker
    2. However it is only lamps, heaters, cooker and devices such as vacuum cleaner and mowers with larger electrical motors that use ac directly off the mains
    3. Most other devices work at much lower voltages and use direct currents (dc) this means that devices such as televisions computers all require a separate (or built in) step down transformer that converts 230V into e.g. 12V dc
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4
Q

How do the alternating currents and voltage move?

A

They move in one direction for half of their cycle and in the opposite direction for the other half

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5
Q

What is the peak value of the voltage (or pd)?

A

The maximum value in either the positive or negative direction, with respect to zero

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6
Q

How is the peak-to-peak value of voltage measured?

A

From one peak in the positive direction to the other peak (called a trough) int he negative direction

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7
Q

What is peak voltage?

A
  • The peak voltage, Vo, of the alternating waveform is half the peak to peak voltage and is equivalent to the amplitude of the waveform
  • For a given component such as a resistor, the peak current Io and the peak voltage Vo are related to each other by Vo = IoR
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8
Q

What is an alternating current?

A
  1. An alternating current or voltage is one that changes direction with time
  2. This means the voltage across the resistance goes up and down in a regular pattern - some of the time it is positive and some of the time it is negative
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9
Q

How can you display the voltage of an alternating current?

A
  • You can use an oscilloscope to display the voltage of an alternating current (and direct current too)
  • The vertical height of the trace at any point shows the unpin voltage at that point
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10
Q

What is on the oscilloscope screen?

A

-The oscilloscope screen has a grid on it and you can select how many volts per division you want the y axis scale to represent using the Y gain control dial e.g. 5V per division

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11
Q

What are the different shapes on an oscilloscope?

A
  1. An alternating current (ac) source gives a regularly repeating sinusoidal waveform
  2. A direct current (dc) source is always at the same voltage so you get a horizontal line
  3. Oscilloscopes can display ac voltage as a vertical line and dc voltage as a dot if you turn off the time base
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12
Q

As alternating currents and voltages vary continuously, what value is used in calculations that gives the same effect as the equivalent direct current or voltage?

A
  • The average values cannot be used because the average values are both zero as there is the same amount of single above zero as there is below zero
  • The values chosen are the root mean square (rms) voltage and current
  • When multiplies together these quantities produce the same power in a resistor as would be produced by the same ac values
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13
Q

How do you calculate power?

A

P=VdcIdc=VrmsIrms

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14
Q

How can a sinusoidal alternating voltage V varying with time t be represented?

A

V=Vosin(2pift)

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15
Q

What happens to the power if this voltage is applied across a fixed resistor?

A

P=V^2/R=Vo^2sin^2(2pift)/R

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16
Q

How do you work out Vrms?

A

Vdc=Vrms= Vo/root2

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17
Q

As the alternating current varies in phase with the voltage what is Irms?

A

Irms = Io/root2

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18
Q

What is the mean alternating power, which is equivalent to dc power?

A

Pmean=VrmsIrms
Pmean = Ppeak/2
-The mean power dissipated through a fixed resistor by an alternating current and voltage is equal to half the peak power dissipated

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19
Q

How do you work out Ppeak?

A

Ppeak=VoIo

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20
Q

What information can you get from an ac oscilloscope trace?

A
  1. Time period, T
  2. Peak voltage, Vo
  3. Peak-to-Peak voltage
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21
Q

How do you calculate the rms from an oscilloscope?

A

Peak voltage / root2

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22
Q

How do you calculate the time period from an oscilloscope?

A
  • Distance between successive peaks along the time axis (horizontal axis) gives you the time period (as long as you know the tine base setting)
  • Frequency = 1/t
23
Q

Why is an oscilloscope useful?

A
  1. Analysing alternating waveforms is best done by displaying the waveform using an oscilloscope
  2. Oscilloscope are a form of visual, calibrated voltmeter where the operator is able to alter how the wave form is displayed
24
Q

How do you control the time taken for the signal to move across the screen of an oscilloscope?

A

Adjust the time base (often labelled time/div)

25
Q

How can you control the amplitude of the signal displayed on the calibrated screen?

A

Adjust the y-sensitivity (this is also know as the vertical sensitivity, y gain or simply volts/div)

26
Q

What does the time base do?

A

The time base (in seconds) provides a scale for the x axis of the screen and indicates the time taken for the signal to move horizontally across one square on the screen

27
Q

How do you use the square gird on an oscilloscope?

A

Using the square grid on the screen to measure the number of horizontal squares between two successive peaks (or troughs) allows the period of the waveform to be determined and hence the frequency

28
Q

What else can an oscilloscope do?

A

The oscilloscope also makes it very easy to measure the peak-to-peak value of the wave by counting vertical squares and then using the y-sensitivity (usually calibrated in volts, millivolts or microvolts) to apply a scale

29
Q

What are transformers used for?

A
  1. Many devices use transformers to increase of reduce the voltage of an alternating voltage supply
  2. Transformers are used in the National Grid to increase the voltage generated in power stations up to 400,000V so that energy can be saved as electricity is transmitted around the country
  3. Then transformers are used to reduce this high voltage for safe use in the home
30
Q

What is the structure of a transformer?

A
  1. Consists of two coils of wire linked by a soft iron core
  2. An alternating current in the primary coil creates a changing magnetic field in the core, which is made of a oft magnetic material such as iron
  3. The secondary coil is also wound around the core
  4. As the magnetic flux in the core changes, the magnetic flux linkage to the secondary coil changes and an emf is induced in the secondary coil
  5. Because transformers use electromagnetic induced, they only work with an ac supply
31
Q

How does a transformer work?

A
  1. An alternating current in the primary coil creates an alternating magnetic field
  2. This cuts through the secondary coil
  3. Inducing an alternating potential difference
  4. Which if it is in a circuit creates an ac
32
Q

What do transformers make use of?

A
  1. Transformers are devices that make use of electromagnetic induction to change the size of the voltage for an alternating current
  2. An alternating current flowing in the primary (or input) coil produces magnetic flux
  3. The changing magnetic field is passed through the iron core to the secondary (or output) coil where it induces an alternating voltage of the same freely as the input voltage
33
Q

How do you calculate the induced emf in the coils?

A

Vp = Np delatfi/delta t

34
Q

What is the transformer equation for an ideal transfer with no power loses?

A

Vs/Vp = Ns/Np

35
Q

What does a step up transformer do?

A

Increase the voltage by having more turns on the secondary coil than the primary coil (Ns/Np more than 1)

36
Q

What does a step down transformer do?

A

Reduce the voltage by having fewer turns on the secondary coil (Ns/Np less than 1)

37
Q

What does the emf depend on?

A
  • Depends on the number fo turns on the coil
  • The rate of flux change in the core of the transformer is the same for both coils, but the number of turns N is different, so the induced emf is different in the secondary coil and depends on the ratio of Ns to Np
38
Q

Why is ac used and not dc?

A

If dc then no rate of change so no voltage and emf generated

39
Q

What is the power in a transformer?

A

-Transformers cannot increase the power output of the supply and in an ideal transformer with no power loses, the power input to the transformer must be equal to the power output therefore:
VpIp=VsIs
-This means that a transformer that reduces the output voltage compared to the input voltage has a larger current in the secondary coil compared to the primary coil

40
Q

What are transformers?

A

Not 100% efficient

41
Q

How do you work out the efficacy of a transformer?

A

Efficiency = VsIs / VpIp

42
Q

How is there heat energy loss in a transformer? How can it be avoided?

A
  1. Heat is produced in the copper wires of the primary coil and secondary coil when current flows
  2. Using low resistance wires reduces these losses e.g. thick copper wire
  3. This is particular important for the secondary coil of a step down transformer because the current is larger in the secondary coil compared to the primary coil
  4. A thicker wire is often used in the secondary coil of a step down transformer
43
Q

How is there energy loss in a transformer related to magnetic flux and how can this be reduced?

A
  1. Some magnetic flux produced by the primary coil does not pass through the iron core which means the flux linkage to the secondary coil is not 100%
  2. This can be reduced bu designing the transformer with coils close to each other or wound on top of each other, which improves the flux linkage
44
Q

How is energy loss in a transformer with hysteresis? How can it be reduced?

A
  1. There is an effect called hysteresis and some energy is lost as heat energy every time the direction for eh magnetic field changes because energy is needed to reassign the magnetic domains int he core
  2. This is reduced by using a soft magnetic material such as iron, rather than steel, which needs more energy to demagnetise and magnetise
45
Q

How is energy loss in a transformer related to eddy currents? How is this reduced?

A
  1. Eddy currents form in the iron core due to the copiously changing flux and these currents heat the core up increasing energy losses
  2. Eddy currents are looping currents induced by the changing magnetic flux in the core. They create a magnetic field that acts against the field that induced them, reducing the field strength. They also dissipate energy by generating heat
  3. The effect of eddy currents can be reduced by laminating the core with layers of insulation
46
Q

How are transformers important part of the National Grid?

A
  1. Electricity from power stations is sent round the country in the national grid at the lowest possible current because the power losses due to the resistance of the cables is equal to P-I^2R is if you double the transmitted current, you quadruple the power lost
  2. Since Power= current x voltage , a low current means a high voltage
  3. Transformers allow us to step up the voltage to around 400,000V for transmission through the national grid and then reduce it again to 230V for domestic use
47
Q

What are eddy currents?

A
  1. Eddy currents are created in metal sheets when there is a change in magnetic flux
  2. In the core of a transfer, the alternating supply creates alternating magnetic flux changes and these create eddy currents
  3. Eddy currents flow in loops, in a direction that opposes the magnetic flux changes that cause them
  4. The result is that eddy currents in the iron core will reduce the emf induced in the secondary coil
  5. In a core made from solid iron, eddy currents could come large enough to melt the core, because the resistance of the iron core is very low
48
Q

How are the effects of eddy currents reduced?

A
  1. To prevent these problems, the core is built from very thin laminations or layers of metal
  2. The eddy currents are smaller when there are thin laminations, because the induced voltage drives the current rounder longer paths, so the resistance to flow increases
  3. The laminations are insulated from each other .e.g using layers of insulating varnish
49
Q

How are transformers used?

A
  1. Transformers are used to step up the voltage generated in power stations
  2. Since power transmitted is equal to the product IV stepping up the voltage in transmission lines reduces the current
  3. Smaller currents have a smaller heating effect on the power lines, so reducing the current in transmission lines reduces energy losses to the surroundings
50
Q

How do power stations used transformers?

A
  • Power stations generate electrical energy at a potential of about 25kV, this voltage is stepped up using transformers shortly after it leaves the power stations is transmitted using transmission lines operating at 275kV and 400kV
  • Overhead transmission lines are supported using the familiar larger steel pylons
  • Transformers in substations step down the voltage for distribution fo electricity to the end user
  • Distribution lines operate at 132kV, with cables supported on smaller steel pylons, wooden poles are used to support power lines operating at 11kV and 33kV
51
Q

Where are power losses in the NG?

A
  1. Power losses in the national grid total about 3% of the demand and mainly occur in the regenerator transformers, overhead lines, underground cables and grid supply transformers
  2. 2/3 of the losses in the NG occur in the overhead lines of the transmission system. However, the percentage losses in power lines in the distribution system are bigger than in transmission lines because the voltage is stepped down, so currents in the power lines are larger
    - Losses in the distribution system can reach as much as 15%
52
Q

How do you calculate power losses in transmission lines?

A
  1. Power losses are calculated using p=I^2R
    - Because the power losses are proportional to the square of the current, doubling the current quadruples the power losses
  2. Power cables are made from aluminium supported by steel cores and the low resistance of these cables reduces losses in power lines, since losses are proportional to R
53
Q

How are step-down transformers in distribution systems made more efficient?

A
  • By using thicker wire in the secondary coil
  • The current is higher in the secondary coil of step-down transformers, so I^2R losses due to the heating of the secondary coil can be significant
  • Reducing the resistance of the secondary coil reduces I^2R losses

Z A Level Physics 2 (16 decks)

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