Electricity
➢ Charges
Charge is an inherent property of matter. An object having charge interacts only with
another object having charge via electrostatic force. Charges are of two kinds namely
positive and negative. Opposite charges attract and similar charges repel.
➢ Electric field
It’s the area of effect where if a charge is placed it will experience an electrostatic force.
➢ Electric field line
The electric field is represented using imaginary lines which help you visualise the electric
field. These lines are known as the electric field line. Along with being a visual aid they
also provide us with two other information, how strong the field is and in which direction
the force is acting on a positive test charge.
The closer the lines are the stronger the field. The arrowhead on the field lines denote
the direction of the force on a positive charge and hence are always shown to be flowing
from positive to negative. The field lines are always perpendicular to the surface they are
arising from.
Field out of a positive and negative charged sphere
Field lines between two parallel plated
Fieldline between two charged spheres
➢ Electric field strength
Electric field strength ‘E’ is defined as the force exerted on a unit charge present in the
field.
➢ Charging a material using friction
If two materials are rubbed against each other some electrons from one of the materials
may transfer to the other due to friction leaving the material accepting the electrons
negative and the one losing electrons positively charged.
Insulators are more easily charged using this method as they don’t allow charges to flow
out. On the other hand, conductors are difficult to charge with this method as the charges
easily flow through the conductor and don’t stay.
➢ Induced charge
If a charged object is brought near an uncharged object, for instance, a positive charge
rod is brought near a neutral object. The electrons in the neutral object will be attracted
towards to positively charge rod leaving behind a partial positive charge. This cause a
partial negative charge on the side near the rod and hence the object will be attracted to
the rod.
If the rod was negatively charged then the electrons will be repelled away from the
negatively charged rod leaving behind partial positive charge and making the other end
partially negative.
➢ Charging a conducting sphere using induction
Negative charge
▪ Bring a positively charged rod near a conducting sphere separated from the ground
by an insulating rod.
▪ The electrons will be attracted towards the positively charged rod leaving behind a
partial positive charge.
▪ Connect the sphere to the ground using a conducting wire. Electrons from ground
flow through the wire into the sphere neutralising the partial positive charge.
▪ Remove the ground wire first and then the rod, the electrons will redistribute
themselves evenly on the sphere and the sphere will become negatively charged.
Positive charge
▪ Bring a negatively charged rod near a conducting sphere separated from the ground
by an insulating rod.
▪ The electrons will be repelled away from the negatively charged rod leaving behind a
partial positive charge.
▪ Connect the sphere to the ground using a conducting wire. Electrons from the sphere
flow through the wire into the ground leaving in the sphere positive charge.
▪ Remove the ground wire first and then the rod, the positive will redistribute
themselves evenly on the sphere and the sphere will become positively charged.
➢ Current
Current is defined as the rate of flow of charge.
I=
q
t
Where ‘I’ is current, ‘q’ is the charge and ‘t’ is the time taken.
The S.I unit of current is Ampere (A)
➢ Electric potential difference
The potential difference ‘V’ between two points is the energy supplied or work done per
unit charge to move a charge from one point to the other:
V=
W
q
Where ‘V’ is the p.d, ‘W’ is the work done and ‘q’ is the charge.
The unit of potential is the volt, V, and 1 V = 1 J C−1.
➢ Electromotive force (e.m.f)
E.m.f is defined as energy supplied or work done by a source in driving charge round a
complete circuit.
The unit of e.m.f is the volt, V, and 1 V = 1 J C−1.
➢ Resistance (Ohms law)
It is the difficulty current faces to pass through a material. Resistance can be defined as
the voltage required to pass a current of 1 Ampere. The unit of resistance is ohms (Ω)
V = IR
Where ‘V’ is the voltage, ‘I’ is the current and ‘R’ is the resistance.
This equation is also known as ‘Ohms law’.
➢ Factors affecting resistance
▪ Length:
Greater the length greater the resistance. Resistance is directly proportional to
length.
RαL
▪
Cross-section area:
Greater the cross-section area lesser the resistance. Resistance is inversely
proportional to the cross-section area.
Rα
▪
1
A
Temperature:
For most of the material greater the temperature greater the resistance.
Resistance is directly proportional to temperature.
RαT
➢ Electric power
Power is given by the equation
W
P= t
Where ‘W’ is the work done and ‘t’ is the time taken
Multiplying and dividing by charge ‘Q’
W
Q
W
Q
P= t x Q
Interchanging the position of ‘Q’ and ‘t’
P= Q x t
W
Q
Q
= V and t = I
P=V x I
Using ohm’s law we get
V2
P=I2 R and P= R
Where ‘P’ is power, ‘V’ is voltage, ‘I’ is current and ‘R’ is resistance
➢ Circuit diagram
A circuit is a path that starts and ends at the same point. Below is an example of a
simple electric circuit. A circuit helps us to direct current and voltage as per the
requirement to the desired place at the desired time. An ammeter is used to measure
current and its always connected in series. A voltmeter is used to measure voltage and
its always connected in parallel.
cell
switch
Ammeter
Resistor(load)
Voltmeter
➢ Conventional current
Current is considered to flow from positive to negative. This is known as the conventional
flow of current. While electrons flow from negative to positive (opposite to the direction
of current). (To keep in mind nothing actually is flowing from positive to negative in case
of most of the conductors it’s just convection. Only for a few exceptions, all the movement
is done by electrons)
➢ Series circuit
In a series circuit, components are connected one after another as shown in the diagram
below.
▪
▪
▪
In a series circuit, voltage is divided amongst all the components and hence all
components will work with different intensities. Component with the greatest
resistance has the greatest potential difference(voltage).
As there is no alternate path for the current to flow, the current is the same in all
the component in a series circuit.
As there is no alternate path, if one component breaks, current supply to all the
components stops. Hence, individual switching of the appliance is not possible in
series circuit.
➢ Parallel circuit
In a parallel circuit, components are connected between the same two points as shown
below.
▪
In a parallel circuit voltage is the same in all the components and hence all
components will work with normal intensity.
▪
▪
As there is an alternate path for the current to flow, the current is divided in
between all the component in a parallel circuit. Component with the greatest
resistance has the least current.
As there is an alternate path, if one component breaks, current supply to all the
other components continues. Hence individual switching of the appliances is
possible in parallel circuit.