Electrical Forces and Energy

Topic 5

Atoms

• Electrons (negative), Protons (+) P+E equal in magnitude, Neutrons (0)

• S.I unit of electric charge = the coulomb (C)

• Charge on one proton = 1.60 × 10^-19C

• Charge on one neutron =-1.6 × 19^019 C

Neutron Atom

has equal numbers of electrons and protons ∴charge = 0

Electrostatic Charge

Outer electrons may be added or removed, e.g; by rubbing, leaving the atom charged

Basic Law of Electrostatics

Like charges repel

Unlike charges attract

Conductor + insulator

• a conductor: an object that readily allows electron charge to flow through it or over it surface

• an insulator: an object that does not allow charge to flow through it or over its surface

Voltage

If the positive terminal of a battery (a device that separates charge chemically) is connected to the negative terminal by a wire conductor, electrons will flow from -ve to +ve terminal

Current

• it is a convention to say that current flows from +ve to -ve (the other way)

• 

Voltage

• The voltage (EMF) of a battery is the energy in J supplied to each coulomb of charge passing round the circuit and through the battery (i.e. the work one in moving 1C round the circuit)

V=w(energy)/q (charge)

The voltage of a battery may be measured by a voltmeter, connected in parallel. We say that the separation of charge in the battery sets up an electric potential or PD across the circuit - which drives the current

Electric Current

The rate of flow of electric charge (usually electrons in a circuit)

I = q/t

Current is conserved at all points in a circuit, i.e. no current is lost or gained

Current may be measured by an ammeter, connected in series

• sometimes we use non-SI unit: Amp-hour (Ah)

Direct Current

always flows in one direction (as in a battery-powered circuit)

Alternating current

keeps alternating directions (about 50 times each second) as the voltage changes polarity (the domestic mains supply is AC)

Conductivity

Metals are typically better conductors than non-metals

Rainwater is a good conductor because it contains dissolved salts that allow the passage of electrons, but distilled water is a poor conductor

Electrical Energy and Power

W=qV → the amount of work done (energy used) by a voltage V moving charge q through an electric field (through the circuit)

Most of the energy gained by the electrons as they move through the battery is given up as they pass through the various components of the circuit, but a little is given up as heat as the electrons pass through the circuit wires

Electrical circuits

enable electrical energy to be transferred and transformed into a range of other useful forms of energy, including thermal and kinetic energy and light. Heat in the wires is not useful energy

**Energy is conserved in the energy transfers that occur in an electrical circuit

Electrical circu

If the EMF of the battery is 12V, then each coulomb of charge will pick up 12J of energy in the battery and lose it all in the circuit (mostly in the light globe)

This gives the rate at which the circuit uses up energy = rate at which the battery gives energy to the electrons

The commercial unit of electrical energy = kWh (a unit)

Electric Shock

The energy imparted to the body when electric current passes through it is what does the damage

• even a small current, if flowing long enough, can be dangerous

  • E=IVt

Fibrillation

Exposure to electric shock can cause fibrillation: a rapid and uncontrolled beating of the heart that can starve the brain of oxygen, causing brain damage or death.

The current that flows trough the body depends on:

• the voltage, the path taken through the body (most dangerous = across chest), the skin resistance (lower resistance and greater current = higher danger)(lower when wet)

Three Wires

• two for incoming and outgoing current (active and neutral wires)

• earth = takes away any excess charge that may build up as a result of any malfunctioning electrical component

  • potential of earth = 0 V

• International colour code: A = brown, N= blue, E = green and yellow stripes

Fuses

• fuse = a piece of thin wire designed to melt if the current gets too great, thereby breaking the circuit and preventing the danger of fire

Circuit-Breakers

Electromagnetic switch that automatically turns off the current when it gets too high. This protects against household fires caused by unusually high currents, but does not prevent electrocution (the current does not need to be high to electrocute someone)

RCD’s

Residual Current (Circuit) Device

• a device that disconnects a circuit whenever it detects that the electric current is not balanced between active and neutral wires

• balance = current leakage through the body of a person who is grounded and accidentally touching the energised part of the circuit (a lethal shock can occur)

• designed to disconnect quickly enough to prevent injury caused by shocks.

Modern buildings have both circuit breakers and RCDs

Earthing

Metal parts in a mains appliance are connected to the earth wire so that if the metal becomes alive due to a fault, current will flow to the ground rather than through the user (thick, carries large current)

Double insulation

• two separate layers between live parts and any external metal, both of which would have to be bypassed in order to create a hazard (inner layer = surrounds live parts, second layer only layer exposed to the user)

EMF (DC single cell)

EMF (battery of cells)

EMF (AC Supply)

resistor

variable resistor

light globe

earth

switch

fuse

ammeter

voltmeter

Ohm’s Law

• All substances resist the flow of electrons to some extent

• Unit of resistance = Ohm

• For a given conductor under constant conditions (E.g. temperature), the current through it is proportional to the potential difference between its ends

  • V∝I

• Constant of proportionality is resistance

• V=IR

Resistivity

• the degree to which a substance resists electric current

• The resistivity of a material actually increases when it is heated, but we usually regard it as a constant

Resistor

an object that may be placed in a circuit to resist the current

Resistance

The property of a resistor by virtue of which it resists the flow of current through it (converting electrical energy into heat); i.e. “resistance” is the strength of the ‘resistor’

Formula Resistance

• Depends on the nature and dimensions of the resistor.

• Length (L), cross sectional area (A), resistivity (p) has resistance given by

• R=pl/A

The current in a circuit depends on the PD (V) provided by the battery and the total resistance (R) in the circuit

Ohmic conductor

Non-ohmic conductor

• In practice, resistance increases as the circuit heats up, so the graph is curved

E.g. light globe - its resistivity increases as it heats up, so higher voltages do not produce proportionally higher current

Non-ohmic property

• the principle behind light detection devices (substances whose resitivity is affected by light) and modern thermostats (affected by heat)

• Conversely when some metals and ceramics are cooled sufficiently, their resistivity drops to almost zero and they become superconductors. The main application of superconductivity has been the production of very strong magnets (easily sustained electric currents produce a magnetic field)

V=IR in ohmic and non-ohmic

• the only difference is that, for latter value of R is not constant (R increases as the conductor heats up).

Series Circuits

All the current flows through each resistor

EMF does not work to move the charge through each successive resistor - te total PD across the circuit equals the sum of the potential drops across the resistors in the circuit

Resistance in the circuit is found by simply adding the separate resistances

**connecting wires also have a slight resistance (ignored)

PD across each resistor work done per charge to go through the resistor) cna be found using V=IR

Parallel Circuits

When resistors are connected in parallel, the potential difference is the same across each resistor, but only some of the current passes through a given resistor.

Parallel Circuit Formulas

It = I1 + I2 + I3 + …. (Parallel)
1/Rt = 1/R1 + 1/R2 + 1/R3 + …

Current in each resistor can be found using I = V/R since V = same for each resistor

Series vs Parallel

• When components are connected in series, a failure of one component breaks the circuit (no component breaks the circuit. Parallel: failure of one component has no effect on the others

• Placing resistors in series reduces total current. Placing more resistors in parallel increases the total current.

• The total power consumed by a circuit is found by ADDING power consumed by each component (S+P)

Series vs Parallel II

• Charge is conserved at all points in an electrical circuit (i.e. the total charge entering and leaving any junction is always the same)

• Energy is conserved in the energy transfers that occur in an electrical circuit. (e.g. total energy supplied by the EMF is equal to the sum of heat energy produced in the wires + various forms of energy activated in the load → Kinetic, heat, light, sound energy)

Circuits in the home

All main electrical circuits have following:

• source of EMF

• form of protection

• switch

• load (e.g. lights, TV, fan,) which converts electrical energy to another form

• low-resistance cables joining the parts together

Circuits are made up of a combination of series and parallel connections

Circuits in the home II

The supply enters the house through the meter and then breaks into separate parallel circuits

for ‘power’ and lights. ‘Power’ outlets and light fittings are connected in parallel so that each

one works off the full 240 V. However, each of the two main circuits (‘power’ and lights) has

its own protection device (e.g. fuse or RCD) and switch in series so that the lights and ‘power’

outlets will fail to work if the switch is off or there is an unduly large current because of a

malfunction.

Electric Blankets

Lights switches

Electricity in remote areas

• not always possible to obtain a 240V supply - electrical devices must be modified to draw a much larger current to compensate for lower voltage to keep power rating at required level (P=IV).

• Resistance → important, current through them is larger, energy wasted is greater (P=I²R) → wires are kept short and thick.

Solar photovoltaic cells

convert heat energy from the Sun into electrical energy. Each cell produces up to 36 W of power (e.g. 3A current @ 12V). The modules connected in different ways for different requirements

AC voltage

varies from -340V to +340V as polarity changes direction. The value of 240V is a kind of average, indicating the equivalent DC supply that would provide the same energy.

Safety in the Home - General Precautions

Appliances

Power points, plugs and cords