Topic 3 - Electricity Flashcards
What’s current?
Current is the rate of flow of charge
What’s potential difference?
It’s the work done per unit charge
Why are voltmeters connected in parallel?
Because the potential difference across components in parallel are the same
What’s full scale deflection?
The maximum value that a voltmeter or ammeter can measure
What’s the charge of an electron?
-1.6 x 10*-19 C
What happens when you double the number of charge carriers (electrons)?
The current doubles
What happens when the charge carriers (electrons) move twice as fast?
You get twice the charge in the same time (twice the current)
What happens when you double the area?
It also doubles the current
What happens when you double the number of charge carriers (electrons)?
It gives you twice the charge in the same time (twice the current)
How do metals, semiconductors and insulators differ in terms of n (number density of charge carriers/number of delocalised electrons per unit volume)?
- metals: n are free electrons, there are per unit volume so n is big and drift velocity is small
- semiconductors: have fewer n so drift velocity needs to be higher to give the same current
- a perfect insulator doesn’t have charge carriers n=0 so there’s no current. Real insulators have a small n
What 3 things determine resistance?
- lengthy: the longer the wire more difficult it is to make a current flow
- area: the wider the wire the easier it is for current to flow
- resistivity (rho): this depends on what material the wires made of as its structure may make it easier or harder for charge to flow. It can also depend on environmental factors like temp
What are the properties of an ohmic conductor?
Provided the temperature is constant, the current through an ohmic conductor is directly proportional to the potential difference across it
What are the IV characteristics of a metallic conductor?
- at constant temperature the current through a Loïc conductor is directly promotional to the pd
- it’s a straight line
What are the IV characteristics of a filament lamp?
- it’s a curve but it gets shallower as the pd rises
- the current increases the temp in the lamp so resistance increases
What do the IV characteristics of a thermistor look like?
- as the V increases the I increases
- more current leads to an increase in temp and a decrease in resistance
- so more current can flow, so the graph curves upwards
What are NTC thermistors?
Negative temperature coefficient
So the resistance decreases as the temperature goes up
Define power
The rate of doing work
Define potential difference
The work done per coulomb
Define current
The number of coulombs transferred per second
Where does resistance come from in metals?
From electrons colliding with atoms and losing energy
What’s internal resistance?
- in a battery chemical energy is used to make electrons move
- as they move they collide creating resistance (internal resistance)
- it’s what makes cells and batteries warm up
How do you calculate the total emf for cells in series?
By adding the individual cells
