A defibrillator device sends an impulse of electrical energy to maintain a regular heartbeat in a person. The device is powered by an alternating current (ac) supply connected to a step-up transformer that charges a capacitor.
(a)
State two reasons for placing the diode in the circuit.
[2]
Question 1b
Marks: 3
The e.m.f. across the primary coil of the transformer is 200 V. The number of turns on the primary coil is 5 and the number of turns on the secondary coil is 50.
(b)
Calculate the e.m.f across the secondary coil.
[3]
Question 1c
Marks: 2
(c)
Calculate the fraction between the primary and secondary coils of the transformer.
[2]
Question 1d
Marks: 1
In rectification to produce a steady direct current from an alternating current a smoothing capacitor is necessary.
(d)
Define smoothing.
[1]
Question 2a
Marks: 2
The diagram shows an alternating current generator with a rectangular coil rotating at a constant frequency in a uniform magnetic field.
The graph shows how the output voltage V from the generator varies with time t.
(a)
Use the graph to determine the maximum output voltage.
[2]
Question 2b
Marks: 3
(b)
Calculate the root mean squared voltage, Vrms.
[3]
Question 2c
Marks: 4
The average power output of the generator is 4.8 × 105 W.
(c)
Calculate the root mean squared current, Irms.
[4]
Question 2d
Marks: 2
(d)
Sketch a line on the graph to show the Vrms.
[2]
Question 3a
Marks: 3
A transformer inside the charger of a household appliance has a primary coil at 230 V and a secondary coil at 80 V. The number of turns in the primary coil is 1650.
(a)
Calculate the number of turns in the secondary coil.
[3]
Question 3b
Marks: 2
(b)
Hence, state whether this is a step-up or step-down transformer. Explain your answer.
[2]
Question 3c
Marks: 3
The appliance has an output power of 30 W.
(c)
Calculate the output current for the appliance.
[3]
Question 3d
Marks: 2
(d)
Outline how eddy currents are reduced in the core of a transformer.
[2]
Question 4a
Marks: 1
(a)
State the meaning of rectification.
[1]
Question 4b
Marks: 2
The graph shows the voltage output from an alternating current supply.
(b)
Sketch how the graph changes during:
(i) Half-wave rectification
[1]
(ii) Full-wave rectification
[1]
Question 4c
Marks: 2
In rectification, a smoothing capacitor is often necessary. The resulting graph of the output current against time gives a 'ripple' shape.
(c)
State how the 'ripples' in the graph can be reduced.
[2]
Question 4d
Marks: 2
The circuit shows a diode bridge circuit.
(d)
State the diodes that will conduct when
(i) A is positive.
[1]
(ii) B is positive.
[1]
Question 5a
Marks: 5
The following paragraph explains the operation of a basic a.c. generator.
An a.c. generator consists of a coil rotating in a __________ field. The ends of the coil are attached to __________ rings that rotate along with the coil. These touch __________ that transfer the current into an external circuit. The magnetic __________ changes as the coil rotates and an __________ is induced. An ac generator converts __________ energy into __________ energy.
(a)
Complete the sentences using keywords from below.
You may use any keyword once, more than once, or not at all.
e.m.f. flux electrical field magnetic
brushes mechanical slip electric
[5]
Question 5b
Marks: 3
A generator produces an rms voltage of 35 V.
(b)
Show that the peak voltage for the generator is around 50 V.
[3]
Question 5c
Marks: 4
The average power output of the generator is 0.25 kW.
(c)
Calculate the peak current produced by the generator.
[4]
Question 5d
Marks: 1
(c)
Outline the effect of the output if the frequency of an ac generator is increased.
between the primary and secondary coils of the transformer.
