Electricity Flashcards
Define Current
Current is the rate of flow of charge
What direction does conventional current flow and what is this the opposite to?
Conventional current flows from positive to negative and this is the opposite direction to electron flow
What is the coulomb the unit of?
Charge
Define a coloumb
One coulomb is defined as the amount of charge that passes in 1 second if the current is 1 ampere
How can you measure the current flowing through a part of a circuit?
By using an ammeter
How should the ammeter be connected and why should it be connected in this way?
The ammeter should be connected in series to make sure that the current flowing through the ammeter is the same as the current flowing through the component.
What needs to be done to make electric charge flow through a conductor?
Work needs to be done on the conductor
Define potential difference
Potential difference is the work done per unit charge moved
How can you measure the potential difference across a component?
By using a voltmeter
How should a voltmeter be connected and why should it be connected in this way?
A voltmeter should be connected in parallel with the component as the potential difference across components in parallel is the same
What is the definition of the volt?
The potential difference across a component is 1 volt when you convert 1 joule of energy moving 1 coulomb of charge through the component
How can the volt be explained in terms of other units?
One volt is equal to one joule per coulomb
What happens if you put a potential difference across an electrical component?
A current will flow
What does the amount of current you get for a particular potential difference depend on?
The resistance of the component
How can you think of a component’s resistance?
You can think of a components resistance as a measure of how difficult it is to get a current to flow through it
Unless told otherwise what can you assume about the resistance of voltmeters and ammeters?
Voltmeters are infinitely resistant
Ammeters have no resistance
What unit is resistance measured in?
Ohms
Define the ohm
A component has a resistance of 1 ohm if a potential difference of 1V makes a current of 1A flow through it
What can be said about the resistance of an ohmic conductor?
It is constant
Define Ohm’s Law
Provided the physical conditions such as temperature remain constant the current through an ohmic conductor is directly proportional to the potential difference across it.
What type of graph is the I/V graph for an ohmic conductor at a constant temperature?
A straight line graph through the origin
What do I/V graphs show?
They show how resistance varies
Define the term ‘I/V Characteristics’
The term IV Characteristics refers to a graph of I against V which shows how the current flowing through a component changes as the potential difference across it is increased.
What type of line on an I/V graph for an ohmic conductor shows low resistance?
A steep line
What type of line on a V/I graph for an ohmic conductor shows low resistance?
A shallow line
What is the I/V graph for a filament lamp?
A curve that starts steep but gets shallower as the voltage rises
What is the filament in a lamp?
The filament in a lamp is just a coiled up length of metal wire. It does not have the same characteristic graph as a metallic conductor as it gets hot. Current flowing through the lamp increases its temperature.
When does the resistance of a metal increase?
The resistance of a metal increases as the temperature increases
Describe the V/I graph for a filament lamp
The V/I graph for a filament lamp is a curve that starts shallow and gets steeper as the current and voltage increase.
What are semiconductors used in?
Sensors
Why are semiconductors not as good at conducting electricity as metals?
As there are far fewer charge carriers available
Why do semiconductors make excellent sensors for detecting changes in their environment?
As if energy is supplied to the semiconductor more charge carriers can be released
Name two types of semiconductor components?
Thermistors and diodes
What is a thermistor?
A thermistor is a resistor with a resistance that depends on its temperature
What does NTC stand for in the term ‘NTC Thermistor’?
Negative temperature coefficient
What does ‘NTC’ mean for a thermistor?
This means that the resistance decreases as the temperature goes up
Describe the I/V characteristic graph for an NTC thermistor
The I/V characteristic graph for an NTC thermistor curves upwards
What effect does warming a thermistor have?
Warming the thermistor gives more electrons enough energy to escape from their atoms. This means that there are more charge carriers available so the resistance is lower.
What are diodes?
Diodes including LEDs are designed to let current flow in one direction only.
What is forward bias?
Forward bias is the direction in which the current is allowed to flow in a diode
What is the threshold voltage most diodes require in the forward direction before they will conduct?
0.6V
What is reverse bias?
In reverse bias the resistance of the diode is very high and the current that flows is very tiny
How can you tell which direction a diode will let current flow in?
Diodes will let current flow in the direction that the triangle in the circuit symbol points
What three factors does the resistance of an object depend on?
- Length
- Cross sectional area
- Resistivity