2.1 Current, potential difference and resistance
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What is electric current?
The flow of electrical charge
State the equation linking charge, current and time. Give the units for the quantities involved.
Q = It Charge (Coulombs), Current (Amperes), Time (Seconds)
What can be said about the value of current at any point in a single closed loop?
Current is the same at all points in a closed loop
What two factors does the current of a circuit depend on?
• Potential Difference (V) • Resistance (R)
What equation should be used to calculate potential difference if current and resistance are known? State the units for all 3 quantities.
V = IR Potential Difference (Volts), Current (Amperes), Resistance (Ohms)
What is an ‘Ohmic Conductor’? State the condition required.
A conductor for which current and potential difference are directly proportional. Resistance remains constant as current changes. Temperature must be constant.
List four components for which resistance is not constant as current changes
• Lamps • Diodes • Thermistors • Light Dependent Resistors (LDR)
What happens to the resistance of a filament lamp as the temperature increases?
Resistance increases - ions in the metal have more energy, so vibrate more, causing more collisions with electrons as they flow through the metal, creating greater resistance to current flow.
What is different about current flow through a diode?
The resistance only flows In one direction. Resistance is very high in the other direction, preventing current flow.
State what happens to the resistance of a thermistor as temperature increases
The thermistor’s resistance decreases
Give two examples of when a thermistor may be used
In a thermostat, to turn a heater on below a certain temperature. In a freezer, to turn on a cooler when the temperature becomes too high.
State what happens to the resistance of a LDR as light intensity increases
The LDR’s resistance increases
Give an application of a LDR
Street lights often use LDRs. When the light levels become too low, the light gains sufficient current to turn on.