Static Electricity

Static electricity is caused by a build up of electric charges that are not free
to move. This can often result in an electric shock or a spark when the built-up
charges eventually move.
● Positive and negative charges

# Like charges (+ and +, or – and –) repel, while unlike charges (+ and –) attract.
# insulators and conductors
# In an insulator all electrons are bound firmly to their atoms; in a conductor some
electrons can move freely from atom to atom. An insulator can be charged by
rubbing because the charge produced cannot move from where the rubbing occurs,
i.e. the electric charge is static.

● Electrostatic induction

● Electric fields
When an electric charge is placed near to another electric charge it experiences a
force.
The electric force does not require contact between the two charges.
# The region of space where an electric charge experiences a force due to other
charges is called an electric field.

If the electric force felt by a charge is the same everywhere in a region, the field is
uniform; a uniform electric field is produced between two oppositely charged
parallel metal plates (Figure 35.10).

It can be represented by evenly spaced parallel lines drawn perpendicular to the

metal surfaces.
The direction of the field, denoted by arrows, is the direction of the force on a
small positive charge placed in the field (negative charges experience a force in the
opposite direction to the field).
Non-uniform Electric Field
Current

• Electric current is defined as the rate of flow charge.

o In other words, the size of an electric current is the amount of charge
passing through a component per second.
• Current flows from the positive terminal to the negative terminal of a cell.

Charge

• The wires in an electric circuit are made of metal, because metal is a

good conductor of electric current.
• In the wires, the current is a flow of negatively charged electrons.
 • current and time are related by the following equation:

• Where the symbols:

o Q stands for charge (measured in coulombs, C)
o I stands for current (measured in amps, A)

● Effects of a current
(a) Heating and lighting
The lamp lights because the small wire inside (the filament) is made white hot by
the current.
(b) Magnetic
The plotting compass is deflected when it is placed near the wire because a
magnetic field is produced around any wire carrying a current.
(c) Chemical
Bubbles of gas are given off at the wires in the acid because of the chemical action
of the current.
● The ampere and the coulomb
# The unit of current is the ampere (A).
# Current is measured by an ammeter.
# The unit of charge is the coulomb (C).
(One coulomb is the charge passing any point in a circuit when a steady current of
1 ampere flows for 1 second).
1 C = 1 A s.
# Electrons flow in the opposite direction to the conventional current.
The signs or symbols used for various parts of an electric circuit are shown in
Figure 36.3
● Series and parallel circuits
Current in Series Circuits

• In a circuit that is a closed-loop, such as a series circuit, the current is the same
value at any point.
•
o This is because the number of electrons per second that passes through
one part of the circuit is the same number that passes through any other
part.
• This means that all components in a closed-loop have the same current.

Current in Parallel Circuits

• At a junction in a parallel circuit (where two or more wires meet) the current
is conserved
o This means the amount of current flowing into the junction is equal to the
amount of current flowing out of it
The sum of the currents in the branches of a parallel circuit equals the current
entering or leaving the parallel section.
● Direct and alternating current
In direct current, the electrons flow in one direction only. Graphs for steady and
varying d.c.

In an alternating current (a.c.) ,the direction of flow reverses regularly, as shown in

the graph in Figure 36.6. The circuit sign for a.c. is given in Figure 36.7
The measure of the strength of an electric current is the purpose of this instrument,
the ammeter.

Note: The ammeter is always connected in series to the rest of the circuit.
Electromotive force (e.m.f):

In simple words, the electromotive force is the force which is required to move the
electric charge along a circuit.

In physics, the electromotive force is the work done (energy provided) by a source
(such as a battery or a generator) to drive a unit charge (Q) around a circuit.

The SI unit of electromotive force is Volt (V).

Note: The e.m.f is not same in series and parallel circuits.

The difference is that in a series circuit, the resultant e.m.f will be the sum of all
e.m.f’s along the circuit.
In Parallel, if e.m.f in both cells is 1 Volt then, the resultant e.m.f will also be 1
Volt because, in a parallel circuit, the resultant e.m.f is equal to the e.m.f of one
single cell.

Potential difference

# A battery transforms chemical energy to electrical energy.

# The battery is said to have a potential difference (p.d. for short) at its terminals.
# Potential difference is measured in volts (V) and the term voltage is sometimes
used instead of p.d.

# Potential difference refers to the fact that electrical energy is converted into other
forms of energy such as thermal energy.

#In other words, potential difference is the difference in the amount of energy
between two points in a circuit.

Note: The unit of electromotive force and the potential difference is the same but
these two terms are different.

, 1 volt = 1 joule per coulomb (1 V = 1 J/C).

In general, if E (joules) is the energy transferred (i.e. the work done) when charge
Q (coulombs) passes between two points, the p.d. V (volts) between the points is
given by

V = E/Q or E = Q × V

If Q is in the form of a steady current I (amperes) flowing for time t (seconds) then
Q = I × t and E = I × t × V

e.m.f. = ‘lost’ volts + terminal p.d.

Measuring voltage

A voltmeter is an instrument for measuring voltage or p.d. It looks like an

ammeter but has a scale marked in volts.

Whereas an ammeter is inserted in series in a circuit to measure the current, a

voltmeter is connected across that part of the circuit where the voltage is required,
i.e. in parallel. (We will see later that a voltmeter should have a high resistance and
an ammeter a low resistance.)

● Voltages round a circuit

(a) in Series, V = V1 + V2 + V3

(b) Parallel V1 =V2

# Potential difference is measured using a voltmeter.

# Voltmeters are connected in parallel with the component being tested.

# The potential difference is the difference in electrical potential between two

points, therefore the voltmeter has to be connected to two points in the circuit.
Resistance
● Resistors: Conductors intended to have resistance are called resistors and are
made either from wires of special alloys or from carbon.

Those used in radio and television sets have values from a few ohms up to millions
of ohms.

Ohm's Law
• Resistance is the opposition to current
o For a given potential difference, the higher the resistance, the lower
the current
o Therefore resistors are used in circuits to control the current
o The unit of resistance is the ohm, represented by the Greek symbol
omega Ω
 The ohm is the resistance of a conductor in which the current is 1 ampere when
a voltage of 1 volt is applied across it.

# Variable resistors are used in electronics.

There are two ways of using such a variable resistor. It may be used as a rheostat
for changing the current in a circuit; as a potential divider for changing the p.d.

Ohm's Law

• The definition of resistance can be given using the equation,

• Where
o R = resistance (ohms, Ω)
o V = potential difference (volts, V)
o I = current (amperes, A)

Consequences of Ohm's Law

o The current in an electrical conductor decreases as its resistance

increases (for a constant p.d.).
o The p.d. across an electrical conductor increases as its resistance
increases (for a constant current).
o
I-V Graphs for Ohmic Resistors, Non-Ohmic Filament Lamps

• The IV graph for a resistor is very simple:

The current is proportional to the potential difference.

• This is because the resistor has a constant resistance.

• For a lamp the relationship is more complicated:

The current increases at a proportionally slower rate than the potential

difference.

• This is because:
o The current causes the filament in the lamp to heat up.
o As the filament gets hot, its resistance increases.
o This opposes the current, causing it to increase at a slower rate.
# The relationship between resistance, length and cross-sectional area can be
represented mathematically,

Resistance is directly proportional to length.

Resistance is inversely proportional to cross-sectional area (width, or thickness).

𝜌𝐿
𝑅=
𝐴

Resistors in series ● Resistors in parallel

● Resistors in parallel

•
● Power in electric circuits

where if E is in joules (J) and t in seconds (s) then P is in J/s or watts (W).
 𝑉2
𝑃= ,𝑃 = 𝐼 2 𝑅
𝑅
# Fuses

A fuse protects a circuit. It is a short length of wire of material with a low melting
point, often ‘tinned copper’, which melts and breaks the circuit when the current in
it exceeds a certain value.

Two reasons for excessive currents are ‘short circuits’ due to worn insulation on
connecting wires and overloaded circuits. Without a fuse the wiring would become
hot in these cases and could cause a fire.

A fuse should ensure that the current-carrying capacity of the wiring is not
exceeded.