WTS TUTORING

2020 ELECTROSTATICS & ELECTRICITY

GRADE : 12

COMPILED BY : PROF KHANGELANI SIBIYA

CELL NO. : 082 672 7928

EMAIL : kwvsibiya@gmail.com

FACEBOOK P. : WTS MATHS & SCEINCE TUTORING

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TWITER : WTSTUTOR

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WHERE TO START MATHS & SCIENCE IS FOR THE NATION

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  ELECTROSTATICS

 QUANTISATION OF CHARGE

 The charge on a single electron is = − 1,6 × 10−19 C

 Principle of charge quantisation: all charges in the universe consist of an integer
multiple of the charge of an electron

 PRINCIPLE OF CONSERVATION OF CHARGE

 Principle of conservation of charge: the net charge of an isolated system remains

constant during any physical process.
 When identical objects with different charges touch, charges will be transferred
between conductors.

New charge on each :

Amount of charge transferred: :

 COULOMB’S LAW

 Coulomb’s Law describes the force that arises between charged objects.
 Electrostatic forces can be forces of attraction or repulsion.
 Coulomb’s Law: The force between two charges is directly proportional to the
product of the charges and inversely proportional to the distance between the charges
squared.
 NOTE: do not use the signs of the charges when substituting into this equation.
 A positive and negative sign in a vector implies a direction, but with the scalar
charges, it simply implies ‘the opposite.’
 This section also involves changing values into SI units.
 Charges are often very small, and can be expressed as m C; μ C, n C or p C.
 These should be changed as follows:
m C = 10 3C; n C = 10 6C; n C = 10 9C ; p C = 10 12C.

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KUTHI HUUU!!!!

F = force of attraction between objects (N)

k = Coulomb’s constant (9×109 N·m2·C−2)
Q = object charge (C)
r = distance between objects (m)

KUTHI HUUUU!!!
• Substitute charge magnitude only.
• Direction determined by charge (like repel, unlike attract).
• Both objects experience the same force (Newton’s Third Law of Motion).

 ELECTRIC FIELD

 An electric field is a region in space where a charge experiences an electric force.

 The direction of the field is determined by the direction of the force that a positive test
(+1 C) charge would experience.

 SINGLE POINT CHARGES

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  UNLIKE CHARGES

 LIKE CHARGES

 EQUATIONS:

OR
E → electric field strength (N·C−1)
F → force (N)
q → charge (C) → q is the charge that experiences the force.
Q → object charge (C) → Q is the charge that creates the electric field.

 ELECTRIC FIELD LINES AND ELECTRIC FIELD STRENGTH

 Electric field lines are diagrammatic representations of the field.

 The field does not exist just along the line.
 The fields are 3-dimensional, although our representations are limited to two
dimensions.
 To represent a stronger field, we draw the lines closer.
 The electric field is a vector quantity because it has magnitude and direction.
 The direction is that in which a positive test charge experiences electrostatic force.

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  ELECTRIC CIRCUITS

 Electricity is the process of giving energy to a charge and then using that energy to do
work.
 The most basic circuit is made of a power source, a load and the conducting wires
carrying the charge around the circuit.
 OHM’S LAW: Current through a conductor is directly proportional to the potential
difference across the conductor at constant temperature.
 Resistance is defined by V = R I.
 OHMIC CONDUCTOR

 An Ohmic conductor is a conductor that obeys Ohm’s law at all temperatures.

 It has a constant ratio of V to I
 Example: Nichrome wire
 NON-OHMIC CONDUCTOR

 A Non-ohmic conductor is a conductor that does not obey Ohm’s law at all
temperatures.
 Ratio of V to I change with change in temperature.
 Example: Light bulb
 Many resistors do not obey Ohm’s law either because the conditions are not
maintained (e.g. the temperature of a filament bulb or the heating element of a kettle
changes) or because the material just doesn’t obey Ohm’s law.

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  SEIES COMBINATION

 Often the learners have to use the fact that the current is the same everywhere in a
series circuit to determine the current through a resistor.
 A series circuit divides the voltage but the current is the same everywhere

 In series, resistances are added up as Rtotal = R1 + R2 + R3 +…

 If resistors are added in series, the total resistance will increase and the total current
will decrease provided the emf remains constant.

 PARALLEL COMBINATION

 The voltage across a set of parallel resistors is the same as the voltage across each of
the parallel resistors.
 The voltage is the same across parallel resistors, but the current divides between them.
 In parallel, resistors are added up as

 The internal resistance of the cells or battery is treated as zero in grade 11.
 If resistors are added in parallel, the total resistance will decrease and the total current
will increase, provided the emf remains constant.

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  CURRENT

 It is the rate of flow of charge.

 I → the current strength, SI unit is ampere (A).

 Q → the charge in coulombs
 T → the time in seconds.

 RESISTANCE

 It is the material’s opposition to the flow of electric current.

 R is the electrical resistance of the conducting material, resisting the flow of charge
through it.
 Resistance (R) is the quotient of the potential difference (V) across a conductor and
the current (I) in it.
 The unit of resistance is called the ohm (Ω).

 POTENTIAL DIFFERENCE

 It is the work done per unit positive charge to move the charge from one point to
another.

 It is often referred to as voltage.

 V → Potential difference in V (volts).
 W → Work done or energy transferred in J (joules).
 Q → Charge in C (coulombs).

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  POWER AND ENERGY

 POWER

 Power is the rate of conversion of (electrical) energy.

 Power is the rate at which work is done.
 There are four equations which can be used here, and simple choose the equation
according to the information given.
P → power (W)
W → work (J)
Δt → time (s)
I → current (A)
V → potential difference (V)
R → resistance (Ω)

 ELECTRICAL ENERGY

 COST OF ELECTRICITY

 Electricity is paid for in terms of the amount of energy used by the consumer.

Cost of electricity = power × time × cost per unit

 Cost of electricity and the kilowatt hour: Energy supplied to consumers is normally
measured in the units of kilowatt hours (kWh).
 This comes from E = Pt where power is measured in kW and time in hours.
 One kilowatt hour is equal to 3 600 000 J.

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  GRADE 12

 INTERNAL RESISTANCE (R)

 A real battery has internal resistance (r).

 Draw the following diagram on the board of a battery with internal resistance (r):

 The battery has an emf (ε) and internal resistance (r).

 emf is defined as the total energy supplied by the battery per coulomb of charge
moved through the battery.
 When a battery is connected to a circuit, with charge flowing through the battery,
electrical energy is dissipated because of the internal resistance (r) of the battery.
 A small amount of the electrical energy supplied by the battery is converted into heat
energy because of the internal resistance of the battery.
 A battery of emf (ε) is connected to an external circuit which has resistance.
 The resistance in the external circuit (R) is called the load.
 When connected in a circuit the potential difference drops due the internal resistance
of the cells.

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KUTHI HUUUU!!!

 VARIABLES

Independent variable → Current (I)

Dependent variable → Potential difference (V)
Controlled variable → Temperature

 WHEN THE SWITCH S IS CLOSED

 Charge now flows through the battery.

 With a current (I) through the battery, there will be a potential difference across the
internal resistance (r) of the battery.
 Vinternal resistance = I. r
 The voltmeter V will no longer read the emf of the battery.
 The voltmeter V now reads the terminal potential difference (Vload).
 In other words, the voltmeter V now reads the potential difference across the external
circuit.
 The internal resistance (r) is in series with the external resistance (r).
 Remember: potential difference is divided among resistors in series.
 therefore, the potential difference across the external circuit ( Vload ) plus the
potential difference across the internal resistance ( Vinternal resistance ) is equal to
the emf of the battery:
emf = Vload + Vinternal resistance

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  If the current in the circuit is I, then:
 The reading on the voltmeter (V) is:
V = Vload = IR (Where R is the resistance of the external circuit)
emf = Vload + Vinternal resistance
emf = IR + Ir
emf = I(R + r)
 All the resistors outside the battery make up the external resistance.

 CALCULATE THE CURRENT IN THE BATTERY

 The circuit current can be calculated using the emf of the battery and the total
resistance in the entire circuit.
 The total resistance in the entire circuit is the SUM of the external resistance (R) and
the internal resistance (r) of the battery:

KUTHI HUUUU!!!

 If the resistance (R) in the external circuit is INCREASED

 Then the circuit current (I) will DECREASE.
 There are FEWER volts lost across the internal resistance (r)

Vinternal resistance = Ir
 There are MORE volts available across the external resistance.

Vload = emf - Vinternal resistance

KUTHI HUUUU!!!
 If the resistance (R) in the external circuit is DECREASED.
 Then the circuit current (I) will INCREASE.
 There are MORE volts lost across the internal resistance (r)
Vinternal resistance = Ir
 There are FEWER volts available across the external resistance.
Vload = emf - Vinternal resistance

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 MERCY!!!!!

WHERE TO START MATHS AND SCIENCE TUTORING

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