T2 current, potential difference and resistance Flashcards
what is electric current
flow of electrical charge
equation linking charge, current and time
Q = I t
charge (C), current (A), time (s)
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 2 factors does the current in a circuit depend on
- Potential difference (V)
2. resistance (R)
equation for potential difference if current and resistance are known
V = I R potential difference (v), current (A), resistance (ohms)
what is an ohmic conductor
state the condition required
- conductor for which current and potential difference are directly proportional
- resistance remains constant as current changes
- temperature must be constant
list 4 components for which resistance is not constant as current changes
- lamps
- diodes
- thermistors
- light dependant resistors (LDRs)
what happens to resistance of a filament lamp as the temperature increases
why
- 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
what is different about current flow through a diode
- current 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
thermistor’s resistance decreases
give 2 examples of when a thermistor may be used
- thermostat to turn a heater on below a certain temperature
- freezer to turn on a cooler when the temperature becomes too high
state what happens to the resistance of a LDR as light intensity decreases
LDR’s resistance increases
give an application for a LDR
- street lights often use LDRs
- when light levels become too low, the light gains sufficient current to turn on
