Unit 1 - Electricity

Question 0

Four identical cells, A and B in parallel, C and D in series. Which will go flat first? (3)

Answer 0

• Cells in series (C and D)
• Current through series double through parallel
• Energy supplied to charge passing through cells PER SECOND is double parallel

Question 1

Conducting putty is reshaped into a cylinder with half the radius and a length which if four times as great. Determine how many times greater the resistance now is? (2)

Answer 1

• halving diameter increases resistance 4x, length 4x longer increases resistance 4x
• Resistance will be 16x higher

Question 2

How does voltmeter reading across resistor change if battery now has internal resistance? (2)

Answer 2

• Terminal potential difference lower

* Potential difference across resistor decreases

Question 3

What is meant by a non-oh mic conductor (1)

Answer 3

does not have a constant resistance

Question 4

How does the resistance of lamp change as pd changes? (1)

Answer 4

Resistance increases as pd increases

Question 5

Define the emf of a battery (1)

Answer 5

Work done per unit charge by the source/ battery/ source

Question 6

The emf of a battery in a circuit it is 6v. When the switch is open the voltmeter reads 6v, when closed it reads 5.8v. Explain the difference in readings (2)

Answer 6

• When switch closed current flows

* hence pd develops across the internal resistance

Question 7

State and explain why it is important for car batteries to have a very low internal resistance (2)

Answer 7

• Need slather current to start the car

* Internal resistance limits the current

Question 8

State what is meant by root mean square voltage (1)

Answer 8

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

Question 9

Why when a filament bulb is connected to a circuit does its current rapidly increase, gradually decrease, then steady at around half the maximum current? (2)

Answer 9

• Resistance of filament increases, decreasing current

* as the temperature of the filament increases

Question 10

State and explain the change to the final current through a lamp if connected in series with another lamp (2)

Answer 10

• Voltage of supply now shared

* Hence current reduced

Question 11

State and explain the change to the final current through a lamp if connected in parallel with another lamp (2)

Answer 11

• Current through lamp unchanged

* As the voltage is the same as source in both lamps

Question 12

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

Answer 12

(Any three)
• Resistance of lamps lower when first switched on
• Initial current will be larger
• Sudden rapid change in temperature

Question 13

Why does terminal pd of a source with internal resistance decrease as current increases? (2)

Answer 13

• Pd across internal resistance/ energy loss in internal resistance
• Pd across internal resistance increases with current

Question 14

A certain metal has a critical temperature of 5k. Explain what is meant by critical temperature. (2)

Answer 14

• The temperature at or below which a material…

* …becomes a superconductor/ has zero resistance

Question 15

State and explain one reason why it is advantageous for a rechargeable battery to have low internal resistance. (2)

Answer 15

• Internal resistance limits current 
• So more current can be provided to circuit
Or
• Charges quicker
• As high current/ less energy wasted

Question 16

Explain the effect on the brightness of lamps in a circuit if the battery has a significant internal resistance. (3)

Answer 16

• Terminal of is now less
• Due to lost volts across internal resistance
• Lamps become dimmer

Question 17

Why are two bulbs in series will not be their normal brightness compared to when alone in series. (2)

Answer 17

• Pd across lamps is reduced as voltage is shared

* So lamps dimmer

Question 18

There is a resistor and thermistor in series with a voltmeter around the resistor. What happens to the voltmeter reading when temperature is increased. (3)

Answer 18

• Reading will increase
• Resistance of thermistor decreases as temp. Increases
• Current in circuit increases so of increases