P2 Electricity 1

Question 1

See goodnotes for symbols for a cell, battery,lamp,fuse,voltmeter,ammeter,diode,resistor ,thermistor,variable resistor ,LDR,LED

Question 2

What is electric current ?

Answer 2

The flow of electrical charge

Question 3

State the equation linking charge ,current and time . Give the units for the quantities involved

Answer 3

Q = l t

Charge (coulombs) , Current (Amperes) , Time (seconds)

Question 4

What can be said about the value of current at any point in a single closed loop ?

Answer 4

Current is the same at all points in a closed loop

Question 5

What two factors does the current in a circuit depend on ?

Answer 5

1. Potential Difference (V)
2. Resistance (R)

Question 6

What equation should be used to calculate potential difference if current and resistance are known ? State the units for all 3 quantities

Answer 6

V = I R

Potential Difference (V) , Current (A), Resistance (ohm)

Question 7

What is an ‘Ohmic Conductor’? State the condition required

Answer 7

• A conductor for which current and potential difference are directly proportional
• Resistance remains constant as current changes
• Temperatures must be constant

Question 8

List four components for which resistance is not constant as current changes

Answer 8

1. Lamps
2. Diodes
3. Thermistors
4. Light dependant Resistors (LDRs)

Question 9

What happens to the resistance of a filament lamp as the temperature increases ? Why ?

Answer 9

• Resistance increases
• Ions in metal have more energy , so vibrate more , causing more collisions with electrons as they flow through the metal , creating greater resistance to current flow

Question 10

What is different about current flow through a diode?

Answer 10

• The current only flows in one direction
• Resistance is very high in the other direction , preventing current flow

Question 11

State what happens to the resistance of a thermistor as temperature increases

Answer 11

The thermistor’s resistance decreases

Question 12

Give two examples of when a thermistor may be used

Answer 12

1. In a thermostat to turn a heater on below a certain temperature
2. In a freezer to turn on a cooler when the temperature becomes too high

Question 13

State what happens to the resistance to a LDR as light intensity decreases

Answer 13

The LDR’s resistance increases

Question 14

Give an application for a LDR

Answer 14

• Street lights often use LDRs
• When light levels become too low , the light gains sufficient current to turn on