Static Electricity

- The study of charges that are not moving or stationary, as opposed to current
electricity, in which charges flow eg. In a wire.

Structure of an Atom

A Neutral Atom – Same number of protons and electrons

A positive charged object -

-Less electrons

A negatively charged object -

-More electrons

Like charges Repel –

Unlike charges attract –

A charged object always attracts a neutral object

Only Repulsion confirms that an object is charged

“Only electrons can be gained or lost by atoms using simple techniques”

Conductors and Insulators

Conductors- Metals, carbon, water (share electrons)

Insulators – Wood, plastic, glass (do not share electrons)

Charging

1) By Friction
2) By induction (conductors)
Electrostatic induction – separates charges in the neutral object in such a way that
the opposite charge comes close to the charged object.

In insulators such as paper, electrostatic induction still takes place because the
electrons of the atoms are pulled by the charged rod and so the atoms are slightly
distorted and the paper is attracted to the rod.

Distribution of charge on the surface of a conductor

- Charge on a spherical conductor is evenly distributed all over.

- Charge on the pear shaped conductor is greatest at the point
- If a conductor is so pointed that it is sharp, charges are actually expelled.
- Charges are only found on the outside of a conductor (they repel each other)

Lightning Conductors

- A long, thick copper strip on the outside of a tall building

- Connecting metal spikes on the top of the building to a metal plate in the
ground.
- Action at points occurs
Practical Uses of Static Electricity

An Electrostatic Precipitator

- Removes smoke and dust from waste gases going up

- Wire grid is highly charged
- Metal plates are earthed
- Smoke and dust particle go up the chimney become charged when they reach
the wire grid
- They are repelled from the wire grid and are attracted to the earthed plates.

Spray Painting

- Electrostatics
- Paint spray produces paint droplets all are positively charged.
- Car’s body is given a negative charge
- When the paint droplets come out they are attracted to the body
- No paint is wasted and the entire car is covered evenly.

Crop Spraying

- Sprayer and spray droplets are given a positive charge

- They induce a negative charge on the plant and the soil
- Charged droplets reach the plant they will immediately be attracted to the
plant and cover the entire surface.
Current Electricity

- Copper is the best conductor of electricity and heat

- Electric current – a flow of charges
- For a current to flow it needs a complete circuit/ no gaps
- A current also needs a source of energy pushing it around the circuit – a cell or
battery.

3 Effects on Electric Current

- Heating effect – current passes through wire, temp rises

- Chemical effect- If wire circuit is cut in 2 its ends are replaced in acid, bubbles
of gas are given off when a current passes the circuit.
- Magnetic effect – Plotting compass is placed in vicinity of a wire carrying a
current, it is deflected. This shows that the current produces a magnetic field.

- Magnetic field around the wire is circular

- Direction right hand grip rule
- Electrons really go from – to +
- Current was imagined to go from + to – and rules still follow this.
Circuit Symbols Meaning
Open Switch

Circuit is open

Does not flow, lamp does not light.

Closed Switch

Circuit is closed

Charge flows and the lamp lights

Short Circuit

Most charges flow through XY has less resistance than the thin wire in the bulb and
electricity travels by the easiest path.

Lamp does not light.

Conventional Current and Electron Flow

- Only negative charges ca flow from one point to another

Series Circuit

One lamp goes out; makes a gap in the circuit and no other lamp will fit.

Parallel Circuits

Current splits up

One lamp goes out; charge still flows through the other bulb which still lights
Electric Charge (Q)

Count ‘packets’ of electrons called Coulombs, each containing 6x10 18 electrons.

Electric Current (I)

- Flow of electrons per second (speed)

- Measures in Amps

Formula:

- Current flowing in a circuit can be measured by means of an ammeter.

- Always placed in series
Direct Current

- Charge is flowing in one direction only

- Steady direct current – current of constant value in one direction
- Varying direct current – current in one direction only but value keeps changing

Alternating Current (a.c)

- Direction of flow reverses regularly

- Eg. Household electricity flow
- Every time it changes direction and changes back is called a CYCLE
- Frequency is measured in Hertz (Hz)
- Malta = 50 Hz 100 times a sec

Potential Difference

-whatever happens in the whole universe

A cell or battery changes chemical energy to electric energy.

“The amount of energy converted to electrical form by the battery per Coulomb of
charge passing through it is called the Electromotive force (e.m.f) of the battery”

 Energy (W) per charge (Q)

 The e.m.f of a battery is measured in volts (the VOLTAGE)

Voltage is the energy required to move from one pole to the other

Current is the amount of charge per second (flow of electrons)

Resistance

Fixed resistor –

Variable Resistor (rheostat)-

Resistors in Series –

Total Resistance =

- Tries to decrease/varies the current

- Good conductors have a low resistance and allow a lot of current to pass
through.

Resistance of a wire depends on:

- Length
- Cross-sectional area
- Material
 -

Resistance (R) and the Ohm ( )

“The ohm is the resistance of a conductor through which the current is 1 ampere
when the p.d across it is 1 volt”

OHM’S LAW

“The current through a conductor is directly proportional to the P.d across its ends,
provided it is maintained at constant temperature”

The voltage is directly Proportional to current keeping the resistance constant

Characteristics of Different Conductors

1. The apparatus set up as shown.

2. Switch is closed and voltage across the resistor is changed using potential
divider, P.
3. The value of the voltage is read off the voltmeter.
4. The current flowing through the resistor is read of ammeter
5. The switch is opened and circuit is allowed to cool, resistor is kept at a
constant temperature.
6. The procedure is repeated for a number of times
7. Each time the resistance of the conductor can be found using :

a) Ohmic or Linear Conductors

- obey ohm’s law perfectly.

- As V increases, I increases and the R remains the same

- Straight line passing from origin (I-V characteristics)

b) Non-Ohmic Conductors

- obey ohm’s law not only at low currents.

- V increases, I increases and temp rises, R increases.

- At a particular V the corresponding I is lower than the current

c) Diodes

- Device which has a small resistance when connected one way but a large resistance
when connected the other way.

- Allows current to flow only when the voltage is applied in one direction.

d) Thermistors

- Number of electrons available for conduction increases when its temp increases.

- Resistance decreases when the temperature rises.

e) Light- Dependent Resistors (LDR)

- resistance of certain materials decreases when the intensity of light falling on them
increases.

- I-V characteristics similar to that of the thermistor

Resistors in Series

Resistance given by the sum of their individual resistances.

Resistors in Parallel

Easier for the current to pass through because they offer more routes.

If 2 resistors are connected in series that have a total resistance of 4 but if

connected in parallel they have a resistance of 1

Circuit Calculations
Electric Power

“The electric power in a circuit is defined as the amount of electrical energy changed
to other formed of energy per unit time”

Paying Electricity Bills

- Electrical energy -> W = P x t

- 1 kWh = 1 kW x 1h
- 1 kWh = 1000 W x 3600 s
- 1 kWh = 1000 J/s x 3600 s
- 1 kWh – 3600000 J or 3.6 MJ

Heating Elements

- Nichrome does not break or become brittle when the current makes it red
hot.
- Radiant electric fires, heat radiated by the red hot elements (900 C) is
directed into the room by radiation and also reflected by a polished reflector
at the back.
- Convector fires, elements are at a much lower temp(450 C) cause of air that is
blown on them to disperse the heat faster. (convection ovens)
- Storage heaters, off peak electricity in the night is used to heat fireclay bricks.
During the day these bricks cool off very slowly, emitting heat into the room.
Fuses

- Thin, short length of wire with a low melting point.

- When current circuit exceeds a certain value, the fuse overheats, melts and
breaks the circuit.
- A fise ensures that the current carried by a wire is not higher than its capacity.

House Circuits

- Electricity delivered by a cable containing 2 wires the LIVE (L) and the NEUTRAL (N)

- The neutral is earthed

- The live supplies all the energy in the form of an alternating current.

- Every cicuit is connected in parallel

- switches connected to live wire

- Live and Neutral wires each run in a complete ring

- The earth wire does not normally carry a current.

- Double insulated- the body of the appliance is already insulated (2 pin plugs)

The Plug
Safety in Electricity

- Switch off the main before starting repairs

- Use plugs that have a plastic case
- Do not overload circuits
- Do not have long cables all over the place
- Do not let wires or cables get in contact with water
- Do not switch on/off appliances with wet hands
- Do not connect appliances that use large amounts of power to a lighting
circuit.