Electricity Flashcards
Define current
What we need to memorise:
It is the rate of flow of charge so I=Q/t
What we need to understand:
ie how much charge passes through the circuit in a given time. Think about water in a pipe as charge.
Define potential difference
What we need to memorise:
The energy transferred per coulomb between 2 points in a circuit so V=W/Q
What we need to understand:
ie it is the energy required to move the charge from one place to another
How do we measure current
Using an ammeter. The ammeter has to be in series with the component we are measuring so that the current also flows through the ammeter
How do we measure voltage
Using a voltmeter. The voltmeter has to be in parallel with the component we are measuring because when measuring voltage, we are measuring it between 2 points in the circuit, not in the whole circuit
Define 1 coulomb
The amount of charge that passes through In 1 second given that the current is 1 ampere
Define 1 volt
When 1 joule of energy is converted in order to move 1 coulomb of charge through the component
Define resistance
A components resistance is a measure of how difficult it is for current (charge per second) to flow through it at a given pd so R=V/I
Define 1 Ohm
The resistance if a potential difference of 1V can make a current of 1A
What is Ohm’s law
It states that, for an ohmic conductor, the current is directly proportional the pd across it, given that physical conditions e.g. temperature remain constant
Describe the I/V characteristic of an ohmic conductor and the difference with a V/I graph
- In both I/V and V/I graphs, the current is directly proportional to the voltage
- However, in an I/V graph, a steeper gradient means a lower resistance, because you can produce more current with a smaller increase in pd
- In a V/I graph, a shallower gradient means a lower resistance, because increasing the current more, doesn’t increase the pd as much so less work is needed to increase the current more
Describe and explain the I/V and V/I characteristics of a filament lamp
- filament lap is a metal wire so it acts as an ohmic conductor at low current
- However at higher currents, the wire heats up , so physical conditions are not constant, so ohms law stops applying, resistance is not constant
- As the temp increases, resistance increases, so it requires more pd to increase the same current.
- Therefore, for an I/V graph, the curve becomes shallower at higher currents (in either direction) but for a V/I graph, the curve becomes steeper at higher currents (in either direction)
Explain why as temperature increases, resistance increases, in the filament lamp
As temperature increases, the ions in the metal wire vibrate more vigorously, so there are more collision with the free electrons (the charge), so the current decreases at the same pd, so the resistance increases. (The ions of the metal are resisting the charge more.)
Describe and explain the I/V and V/I characteristic of a diode
- Diodes only allow current to flow in one direction, past a specific voltage, called the threshold voltage.
-This direction is the forward bias. Here the resistance is very low so current passes through easily - In the direction of the reverse bias, the resistance is extremely high, so very little current can flow
- The I/V graph has no current (y=0) below the threshold voltage (a point on the x axis and then after that the graph is very steep
- The V/I graph has no current (x=0) until the threshold voltage (a point on the y axis), then the graph is shallow
What generic circuit do we use when obtaining the I/V characteristic of a component
Power supply with ammeter, variable resistor and component in series, with voltmeter connected in parallel with component
Why is the resistance in an ammeter assumed to be 0
Because if the ammeter had resistance, it would decrease the amount of current that passes through that point in the circuit so the value on the ammeter would be wrong
Why do we assume the resistance of a voltmeter to be infinite
Because the voltmeter is in parallel so we don’t want current to be able to pass through the voltmeter, because then less current would be able to pass through the other branch of the circuit, which would affect the voltage, meaning the reading would be wrong
Define the resistivity of a material
- It is a measure of how easily a material conducts electricity
-It is defined as the resistance of a material with length 1m and cross-sectional area 1m^2 - Therefore a lower resistivity means a lower resistance, so a better conductor
Resistivity formula: p=RA/l
What are the units of resistivity
Ωm
because p=RA/l and m^2/m = m and then you need to x by Ω
