Electricity Questions

Question 1

An oscilloscope has a screen of eight vertical and ten horizontal divisions. Describe how you would use the oscilloscope to display an alternating waveform of time period 10ms so that two complete cycles are visible. (3)

Answer 1

connect to y-input

adjust/change time base

so that each division is 2.0 ms OR 20 ms across screen

reference to y-gain/sensitivity

Question 2

Define RMS voltage? (1)

Answer 2

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

or the equivalent dc/steady/constant voltage that produces the same heating effect/power

Question 3

State and explain the change, if any, to the final current through the lamp if it is connected to the same supply with another similar lamp in series (2)

Answer 3

voltage of supply now shared by lamps or resistance increased

hence current reduced

Question 4

State and explain the change, if any, to the final current through the lamp if it is connected to the same supply with another similar lamp in parallel (2)

Answer 4

current through the lamps unchanged/stays the same as both connected directly to the supply

Question 5

State and explain why a filament lamp is most likely to fail as it is switched on. (2)

Answer 5

resistance of lamps will be lower when first switched on

hence initial current will be larger

sudden rapid change in temperature

Question 6

The student connects another lamp E in the circuit as shown in Figure 2.

Lamps A and C are identical, B and D are identical.

Lamp E is identical to lamps A and C.

Explain what the student would observe regarding the brightness of the lamps.

Refer to potential differences across lamp E in your answer. (3)

Answer 6

the pd across new lamp = 0 / E does not light ✔

no current in E ✔

other lamps are not affected ✔

because the current in the lamps/pd across lamps does not change ✔

Question 7

What is a potential divider? (1)

Answer 7

a combination of resistors in series connected to a voltage source, that are used to divide voltage

Question 8

State and explain what happens to the voltmeter reading when the resistance of R is increased while the temperature is kept constant. (3)

Answer 8

• when R increases the PD across it increases
• Because total PD = PD over R + PD over T, this means that the PD over T must decrease
• Therefore the voltmeter reading decreases
• Resistance over R is an increased share of the total reistance

Question 9

With switch S closed (in the on position) and no resistors connected between X and Y the voltmeter reading V is 1.62 V.

The student concludes that this voltmeter reading equals the emf ε of the power supply.

State why the student’s conclusion that ε = 1.62 V was correct. (1)

Answer 9

Power supply is on open circuit (so current is zero)

OR

Voltmeter has a (very) large resistance (so current is zero)

OR

No current (load) (so no lost volts)

OR

(Current is zero) so no lost volts