Studied by 5 people
Current
The rate of flow of charge, measured in Amperes (Amps).
Potential difference is applied across a conductor which establishes an electric field.
This electric field causes electrons to drift through the circuit, creating a flow of charge, referred to as current.
Charge
Physical property like mass or volume.
Term= I * t
Term = n * e
Millikan’s Experiment
In 1909 Robert Millikan carried out the oil-drop experiment.
He analysed the motion of electrically charged oil droplets between two oppositely charged parallel plates.
The occasional oil droplet was held stationary and hence he was able to determine the charge on many droplets.
Using his data he calculated e (elementary charge) to be 1.6×10^-19
What can cause a larger current ?
A greater number of electrons moving past a point each second.
The same number of electrons moving faster through the metal.
Conventional Current
The flow of current from a positive terminal to a negative terminal.
Electron Flow
From the negative terminal to the positive terminal.
Measuring electric current
Ammeter must be placed in series in the circuit.
They should have the lowest possible resistance.
An ammeter with a high resistance would significantly decrease the current it should be measuring.
Kirchoff’s first law
For any point in an electrical circuit, the sum of the currents into that point is equal to the sum of the currents out of that point.
∑Iin=∑Iout
conservation of charge
Drift Velocity
Free electrons repeatedly collide with positive metal ions as they drift through the wire towards the positive terminal. This causes, despite popular misconception, the electrons to move quite slowly.
I = Anev ———> where I is the current, A is the cross sectional area of the wire, n is the free electron density of the material, e is 1.60×10^-19 (elementary charge) and v is the mean drift velocity.
Effect of Changing Cross-Sectional Area
If the cross-sectional area of a wire changes, so must the drift velocity. The narrower the wire the greater the drift velocity must be in order for the current to be the same.
The same rate of flow of charge (current) is due to the electrons moving faster through the narrower wire.
Mean Drift Velocity is inversely proportional to the Cross-Sectional Area of a wire.
Potential Difference
Measured in volts, named after Alessandro Volta, an Italian who invented the first battery.
1 Volt is the p.d. across a component when 1J of energy is transferred per unit charge passing through the component.
1V = 1JC^-1
V = W/Q
Measuring Potential Difference
voltmeter is connected in parallel across a particular component.
An ideal voltmeter will have infinite resistance, so that when connected no current passes through the voltmeter itself.
Electromotive Force
Work is done on the charge carriers and are gaining energy as they pass through a component such as a cell.
ϵ = W/Q
The Electron Gun
How it works:
Small metal filament is heated
electrons gain enough kinetic energy to escape from surface of the metal (thermionic emission)
heated filament is placed inside a vacuum
high p.d. applied between filament and anode
free electrons accelerate towards the anode
Anode has a small hole which electrons that are in line with can pass through
This creates a beam of electrons with a specific kinetic energy.
What is resistance
opposition that a material offers to the flow of electric current through it.
term = p.d across a component / current in component
Measured in ohms.
1Ω = 1VA^-1
Ohm’s Law
For a metallic conductor kept a constant temperature, the current in the wire is directly proportional to the p.d. across its ends.
V ∝ I
Kirchoff’s Second Law
The sum of the electromotive forces is equal to the sum of the potential difference around a closed loop.
Conservation of energy
Resistors in Series Vs Parallel
When resistors are connected in series, each additional resistor effectively increases the length of the path taken by charges and therefore increases the total resistance:
Rt = R1 + R2 + …
When resistors are connected in parallel, the additional resistor provides another path for the current, effectively increasing the cross-sectional area and lowering the resistance:
Rt = (1/R1 + 1/R2 + …)^-1
What is Internal Resistance
Whenever there is a current in a power source, work has to be done by the charges as they move through the power source.
The p.d. measured at the power source (terminal p.d.) is less than the actual e.m.f. This difference is known as lost volts.
E.M.F. = I(R +r) ——> where r is the internal resistance.
Potential Dividers
Divide the p.d. to give any value you require up to the maximum supplied from the power source.
V out = (R2 / (R1 + R2)) * V in
The potentiometer
Many low-voltage circuits that need a varying p.d. use a potentiometer rather than a potential divider.
A variable resistor with 3 terminals and a sliding contact. Adjusting this contact varies the p.d. between two of the terminals, giving a variable V out.
