H556/02
Module 4
Module 6
Electricity
Waves
Quantum Physics
Capacitors
Electric Fields
Electromagnetism
Nuclear and Particle Physics
Medical Imaging
Current is the Rate of Flow of Charge
The current in a wire is like water flowing in a pipe. The amount of water that flows depends on the flow rate and the time. It's the same with electricity - current is the rate of flow of charge
I = Q/t
The Coulomb is the unit of charge
You can measure the current flowing througfht part cirremt f;pwomg thrpight [art pf a corciot using an ammeter. This is the circuit synmbol for an ammeter:
One coulomb (C) is defined as the amount of charge that apsses in 1 second when the current is 1 ampere*
Attach an ammeter in series with the component you're investigating
Remember that conventional current flowsa from + to -, the opposite way from electron flow
The charge in an Electron is the Smallest Unit of Charge
In electrical circuits, charge is usually carried by electrons (or sometimes by ions). Electronsall carry the same charge, -e where e is elecmentary charge
e = 1.60 x 10^-19C
Protons carry an opposite cahrge of the same magnitude, +e
The elementary charge is the smallest unit that cahrge comes in - the net charge on any particle or object will alays be a nultiple of e. We say that charge is quantised
Potential Difference is the Work Done per Unit Charge
To make elctric charge flow through a conductor, you need to do work on it.
Potential difference (p.d.), or voltage, is defined as the work done per unit charge moved
W = VQ or V = W/Q
The potential difference across a component is 1 volt (V) when you do *1 joule of work moving 1 coulomb of charge through the component. This defines the volt. 1 V = 1 J C^-1*
Back to the 'water analogy' again. The p.d. is like the pressure that's forcing water along the pipe.
You can measure the potential difference across a component using a voltmeter. This is the circuit symbol for a voltmeter
The maxmum value that a voltmeter or ammeter can measure is called trhe full scale deflection
Remember, the potentail differencea crros components in parallel is the same, so the voltmeter should be connected in parallel with the componet you're investigating
Work Done on a Charge Equals the Kinetic Energy it Gains
When a charged particle is accelerated by a potential difference, the energy transferred to it is equal to the work done on the aprtic;e, W = VQ. For an electron (with charge of size e), thus cab be wrutteb as W = Ve
The energy transferred is equal to the kinetic energy gained by the electron
Kinetic energy = 1/2mv^2
eV = 1/2mv^2
This gives you the velocity of a single electron accelerated through a potential difference - don't get it confused with mean drift velocity
The Mean Drift Velocity is the Average Velcoty of the Charge Carriers
When current flowsthrough a wire, ypu might imaginr thr electrons all moving uniformly in the same dircetion. In fact, they move randomly in all directions, but tend to drift one way. The mean drift velocity is just the average velocity and it's much, much less than the elctrons' actual speed*
The Current Depends on the MEan Drift Velocity
If you're using difference charge carriers, just replace e with the cjarge on each carrier, and let n be the number density of charge carriers
The current is given by the continuity equation
I = Anev
If you double the number of electrons, the current doubles**
If you double the are the current doubles
IF the electrons move twice as fast, the curent doubles* as twice as many ekectrons move past a point in the same amount of time
Different Materials have Different Numbers of Charge Carriers
In a metal, the charge carriers are free electrons - they're the oness from the outer shell of each atom. Thinkning abnout the fomula I = Anev, there are loads of charge carriers per unit volume, making n big. The drift velocity is small, even for a high current
Semiconductors have fewer charge carriers, si the drift velocity needs to be heighter to give the same current**
A perfect insulator wouldn't have any charge carriers, so n= 0 ub the firnyka and ger no current. Real insulators have a very small n
Charge Carriers in Liquids and Gases are Ions
Ionic crystals like sodium chloride are insulators. Once molten, though, the liquid conducts, Positive and negative ions are the charge carriers. The same thing happanes in an ionic solution like copper sulfate solution.
A substance containing ions that conducts electricity like this is called an electrolyte
Everything has Resistance
If you put a potential difference (p.d.) across an electrical component, a current will flow
How much current you get get for a particular p.d. depends on the resistance of the component
You can think of a component's resistance as a measure of how difficult it is to get a current to *flow through it
Mathematically, resistance is R = V/I This equation really dines what is mean by resistance
The is the circuit symbol for a resistor 
Resistance is measured in ohms (Ω)
A component has a resjsta ce of 1 Ω if a potential difference of 1 V makes a current of 1 A flow through it
Three Things Determine Resistance
IF you think about a nice, simple electrical com[pnent like a length of wire, its resistance depends on
Length (L). The longer the wire the more difficult it is to make a current flow