11.2 Power Generation & Transmission

The sinusoidal current shown below flows through an 90 Ω resistor. It has an rms value of 2 A.

Which graph correctly shows the power dissipated in this resistor?

A cathode-ray oscilloscope screen with a grid of 1cm squares displays an alternating potential difference waveform. The gain and time base dials are set to show the full waveform on the screen.

Which mathematical expression correctly describes the waveform of the potential difference?

A coil, P, is connected to an alternating supply of potential difference. Another coil, Q, is placed nearby and connected to an oscilloscope. 

A thick iron loop is now introduced, such that it passes through the centre of coil P and the centre of coil Q. 

Which of the following shows the effect on the oscilloscope trace of introducing this new component?

A step-up transformer is being tested before its use in a power distribution system that passes underwater. 

The secondary coil has 200 turns and is connected to a sample section of insulated power cable. The four metre-long cable has a resistance of 1050 Ω m^-1. This is passed under a tank of water containing 5 L of water. The primary coil has 10 turns and is connected to a supply of alternating current. 

In 16 seconds, the temperature of the water increases from 23.5 °C to 24.5 °C. The specific heat capacity of water is 4200 J kg^-1 °C^-1

Assuming  of the average power dissipated in the cable is transferred to the water's thermal store, what is the rms current of the alternating supply connected to the primary coil?

Which of the following graphs best represents the variation of current I with time t through the segment AB in the circuit below?

A power station generates power P to be delivered to a city distance L away using cables with a total resistance of R. This power is generated at voltage V_p and then stepped up by an ideal transformer before travelling along the cables.

The turns of the coils are in a ratio of N : 1, where N > 1. The station spends £0.10 per kW h of energy produced. 

How much money is lost by the station in one hour due to power loss in the cables?

The graph shows the output voltage from a half-wave rectifier. The load resistor has a resistance of 2.0 mΩ. An engineer wishes to smooth the output voltage by placing a capacitor in parallel across the load resistor.

Which capacitor would be best for this task?

The voltage of an a.c. source varying with time t is shown in the graph below. 

Which of the following gives the correct expression for the root-mean-square voltage, V_rms, if the peak voltage is V₀?

Power is generated in a power station, stepped up and then transmitted around the country via transmission cables before being stepped down again to be used in homes. 

The total power incident on one transmission cable is 1000 MW. The cable is 100 km long and has a resistance of 0.02 Ω per m. As the electricity is transported along the cable 50% of the power is lost by the time it gets to the transformer. The primary coil has 3000 turns and the secondary coil 300 turns. 

How much current will be present in the transmission wire as it comes out of the transformer?

A square loop of conducting wire is rotated at a constant rate in a region of magnetic field. The graph shows the variation with time t of the induced emf in the loop during one cycle.

The resistance of the coil is 10.0 Ω. Which of the following values gives the average power dissipated in the loop?

What is the maximum instantaneous power delivered by a sinusoidal ac power supply with rms voltage V  supplying rms current 2I?

An ac generator produces a root mean squared emf ε at frequency f. The rotational speed of the coil in the generator is increased by a factor of three. Which of the following correctly identifies the new values of frequency and output emf_rms?

	emf	frequency
A.
B.
C.
D.

An ideal transformer is supplied with power P. The transformer has N_p turns on the primary coil and N_s turns on the secondary coil. Select the correct power output from the secondary coil.

The graph shows the variation with time t of the output voltage V of an ac generator.

Which graph, with identical scales on the axes, shows the output when the speed of rotation is doubled?

A power station produces ac voltage which is stepped up by a factor of 10⁴. This reduces the power loss in the transmission cables by a factor of

The secondary coil of an ac transformer is connected to two diodes as shown. 

Which graph correctly shows the variation with time of the potential difference V_PQ between P and Q?

A resistor of 3.0 kΩ is connected to an alternating current (ac) power supply of root mean square voltage 120 V. The graph shows the power dissipated in the resistor.

Which row correctly shows the frequency of the ac power supply and the average power dissipated in the resistor?

The diagram shows a diode bridge rectification circuit connected to a load resistor.

Which change to the circuit will produce an output signal showing the most smoothing?