Explain electrode potentials fully
- Not all metallic materials oxidize to form ions with the same degree of ease
- Consider two cells: Fe / Cu and Fe / Zn; the first already discussed, as for the second:
i.e. Zn is oxidized & Fe is reduced
- For the first a voltmeter measures +0.780V and for the second it measures +0.323V
- These measured cell voltages represent only differences in electrical potentials, and thus it is convenient to establish a reference point
- The standard electrode potential of hydrogen is taken arbitrary as 0V
Define elctrode potential
- Defined as thepotential of a cell consisting of the electrode in question acting as a cathode and the standard hydrogen electrode acting as an a node ( taken at 0.0 )
- Reductionalways takes place at the cathode, and oxidation at the anode.
Explain what Negative electrode potential means
- That the metalelectrodeis donating electron(s) to anotherelectrode
i.e. it is oxidized
- In other words it is under oxidation process and is reducing the otherelectrode. … Zinc
e.g. hasnegative electrode potentialvalue (-0.76 V).
Explain electric current ( I )
- Electric charge in motion through a conductor
- Si Unit: Ampere (A), where 1 A = 1 C/s
I = Q / t
Explain resistance ( R )
- A current will always experience resistance ( R ) in a conductor, similar to friction, and energy is dissipated in the conductor in the form of heat
- The amount of current flowing in a circuit with resistance R and a potential difference across is given by Ohm’s Law
- Si Unit: Ohm (Ω), where 1 Ω = 1 V/A
V = IR
Explain electric power ( P )
- Rate of energy dissipation/delivery in a device ( Power )
- SI Unit : Watt (W), where 1 W = 1 J/s
- The power dissipated in a resistor R is therefore : P = U / t = QV / t = IV
Pr = I^2R
Explain resistive loss
- Is detrimental since it means a loss in energy
- To minimize losses ( in power transport from A to B ), the current and resistance should be as low as possible
- In heating devices, the opposite applies where you want energy losses through heat.
- Electric power is also expressed in kW, MW, GW, TW
- For electricity bills, electric energy is expressed in units of kilowatt-hours, where
- Electricity is NOT a form of energy, it is a carrier of energy
1kWh = 1000 W x 3600 s = 3.6 x 10^6 J
Explain megnetic fields
- When charges flow (current) in a conductor, a magnetic field (H) is generated around the conductor
- The magnetic field strength (H) produced is proportional to the current (I) and the amount of windings in the loop (N)
- The unit of H are ampere-turns per meter, or just amperes per meters
- All magnetic fields originate from charges in motion
Explain magentic induction, or magnetic flux density
- Denote by B, represents the magnitude of the internal field strength within a substance that is subjected to an H field (external magnetic field)
- The units for B are teslas (T) or webers per square meter (Wb/m2)
- B and H are field vectors characterized by magnitude and direction. Both are related according to : B = mue x H
 - μ is called permeability, which a property of the specific medium
- The units of μ are Wb/A.m or henries per meter (H/m).
Explain magnetic susceptibility
- Chi ( X )m
- Dimensionless
- Xm = Km - 1
Analogy between dielectric and magnetic parameters
- H field ≡ E field
- Magnetic field or flux density (in medium) B ≡ Displacement or charge density (dielectric) D
permeability μ ≡ permittivity ε - Magnetization M ≡ polarization P
Faraday’s law
A change in the magnetic flux (Φ) through a loop induces a voltage (emf) in the loop
Explain Faradays law
- Determines the magnitude of the induced emf (voltage) while the direction of the induced current is given by the Lenz law
- Lenz law states that : Induced current produce magnetic field that tend to oppose to the change that induce those current
- The minus sign means that the induced emf opposes the change in the flux creating it (the induced current opposes the increase / decrease of the flux creating it ) ≡ Similarities with the 3rd law of Newton)
Meaning of Lenz law (Direction of the induced Current)
- Step 1: Magnetic field lines exit the N and enter to the S; thus flux increases towards the ring (B up increasing)
- Step 2: The induced emf / the induced current must produce lines going down (B ind in) into the ring to oppose the increasing flux up
This is also a statement of the law of conservation of energy
- Step 3: Apply the right hand rule to find the direction of the induced current
- Step 4: Easy to see that current must go clockwise
This is the same reasoning of a ring falling in a magnetic field created by the magnet shown.
Explain Electromagnetic Induction
- Mechanical energy must be supplied to turn the rotor
- A generator is therefore a device that converts mechanical energy to electric energy
- Mechanical energy is derived from steam driven turbines where heat is converted to mechanical energy
- The heat is supplied by coal, natural gas, oil, nuclear reactions, sun, etc
- The AC voltage is referred to as the root mean-square voltage (Vrms), which is related to the peak value (Vp)
In SA, the household voltage of 220 V therefore would have a peak value of 311 V
What are the 2 possible ways to change the magnetic flux and induce a voltage
- Change the effective area cut by the magnetic field (AC generator)
What is a transformer
A device that can take an AC emf as input and produce another AC emf with a different voltage amplitude
Explain transformerers fully
- Alternating voltage (VP) is applied to the primary coil → current flows through the coil setting up a magnetic field around itself
- The alternating voltage implies that the generated B is changing with time thus a changing flux
- This change in flux entering the secondary coil “of course” is a source of induced emf (voltage) in secondary (VS) which is proportional to the number of windi
How can the secondary ( output ) voltage be changed
By changing the number of windings in the secondary coil
Explain step up transformers
- Increases the output voltage, thereby decreasing the current
- This is used to obtain high voltages (low currents) for long transmission power lines
- Therefore the power loss to heat (P=I2R) is much smaller
- In addition, thicker wires are used to also reduce the resistance
What is an ideal transformer
One that does not create nor destroy energy, therefore energy or power must be conserved,
Explain step down transformers
- Decreases the output voltage, thereby increasing the currents
- This is used to obtain low voltages (high currents) for household appliances
Explain thermal generating plants fully
- Large power plant are required to provide steady power supply to society
- Electricity is produced with a generator
In large AC generators :
- Outer shell with powerful magnets rotates around a stationary armature
- Armature wound with heavy wire
- Electric current is induced by rotating magnets
Examples of thermal generating plants
- Use energy of heat to produce electricity
- Water is heated in a boiler → super-heated steam → channelled through turbine
- Turbine has many fan blades attached to a shaft
- Steam → over blades → spins shaft → connected to rotor of generator → electricity
