Topic 2- Electricity Flashcards
Equation for electric charge flow
charge flow= current x time
What can be said about the current in a single closed loop?
current is the same at all points
what two factors does a current in a circuit depend on?
resistance (R) potential difference (V)
Explain why the resistance stays the same when the current changes:
The current through an ohmic conductor is directly proportional to the potential difference across the resistor. (meaning the resistance stays the same at the current changes)
Explain the changes in resistance through lamps, diodes, thermistors
filament lamp: as temperature of the filament increases, so does the resistance.
diodes: current through a diode flows in one direction only, diode has a very high resistance in the reverse direction.
State the conditions required for an ohmic conductor
- a conductor where the current is directly proportional to the potential difference.
- resistance remains constant as the current changes
- temperature must be constant.
Ohms law:
ohms law states that the current through a conductor between two points is directly proportional to the voltage across two points.
formula: voltage= resistance x current
Examples of ohmic conductors
Copper or constantan wire
examples of non-ohmic conductors (4)
-Filament lamp/bulb
-diode
-LDR
-Thermistors
(resistance changes when the current changes)
How is resistance caused?
When electrons collide with ions
Explain how the resistance changes when the current changes in a filament lamp
The resistance increases as the temperature increases. resistance is caused when electrons collide with the ions, as the filament gets hotter the ions move and vibrate more or faster. This causes more collisions with the flowing electrons, hence an increased resistance.
Explain how the resistance changes when the current changes in a diode
The current through a diode flows in one direction only. The diode has a very high resistance in the reverse direction.
Explain how the resistance changes when the current changes in an LDR
You can see from the graph that the resistance falls as the light intensity increases.
LDRs can be used for switching lights on automatically when it gets dark.
There’s also a symbol, remember
Explain how the resistance changes when the current changes in a thermistor
You can see from the graph that the resistance falls as the temperature increases.
Thermistors can be used in the home for thermostats to control the central heating.
There’s also a symbol, remember it.
When can a thermistor be used
- in a thermostat, to turn a heater on
- in a freezer, to lower temperature
Give an application for an LDR
- street lights turn on when it gets dark
What two main factors does the amount of energy transferred by an appliance depend on?
- how long the appliance is being used for
- power of the appliance
describe the energy transfers in a battery powered torch
- battery converts chemical energy into electrical energy
- bulb converts electrical energy into light as well as waste energy in the form of heat
Describe the energy transfer in a battery powered motor
- battery converts chemical energy into electrical energy
- motor coverts electric energy into kinetic energy, forming waste energy in the form of heat due to friction.
what determines the power of a circuit device?
- potential difference across the circuit
- the current through the circuit
- amount of energy transferred in a specific amount of time.
What is the national grid?
Electrical power transferred from power stations to consumers using the national grid
Two types of transformers used in the national grid:
- step-up transformers
- step-down transformers
Where are step-up transformers found in the national grid?
what do they do?
- they are used when connecting power stations to transmission cables
- they increase the potential difference
Where are step-down transformers found in the national grid?
what do they do?
- used to connect transmission cables to domestic buildings
- they decrease the potential difference
