Electromotive Force (EMF) is defined as the work done (energy provided) to move a unit
charge round a complete circuit.
It is a measure of the energy provided by a source (such as a battery or generator) to move
electric charge through a circuit. EMF is the force that drives the flow of electric current in a
circuit. EMF is measured in volt (V).
EMF=Work done (W)/Charge (Q)
W= work done ( in Joules), Q = Charge ( coulomb)
Sources of EMF:
Batteries: Chemical reactions inside a battery create a potential difference, resulting
in an EMF.
Generators: Mechanical motion (like rotating a coil in a magnetic field) induces an
EMF through electromagnetic induction.
EMF is the cause of current flow. It is the "push" that drives current through a circuit and is
essential for powering electrical devices.
Electric Current is the flow of electric charge through a conductor or a circuit. It is defined
as the rate of flow of charge. The unit of electric current is the ampere (A)
Current = charge / time
I=Q/t
I – Current in Ampere, Q - Charge in coulomb, t - time in second
Direction of Current:
In conventional terms, electric current is considered to flow from the positive terminal to the
negative terminal of a power source.
In reality, electrons (which are negatively charged) flow from the negative to the positive
terminal, but the direction of current flow is defined by the movement of positive charges.
Types of Electric Current:
1. Direct Current (DC): The current flows in one direction only, such as the current
supplied by batteries.
2. Alternating Current (AC): The current periodically changes direction, as is the case
with the electrical power supplied to homes and businesses.
Current in a Circuit:
When a voltage (electromotive force, or EMF) is applied across a conductor, it causes
free electrons in the conductor to move, resulting in an electric current.
The amount of current depends on the voltage and the resistance of the circuit, as
described by Ohm's Law:
I= V / R
where:
o I = current (in amperes, A)
o V = voltage (in volts, V)
o R = resistance (in ohms, Ω)
Electric current is the flow of electric charge through a conductor, driven by a voltage or
EMF. It is essential for the operation of electronic devices and electrical systems
The current through a device is measured using an ammeter connected in series with the
device.
Potential Difference between two points in a circuit is the energy required (work done) to
move a unit charge from one point to another in a circuit. The unit of potential difference is
the volt (V).
One volt is defined as the potential difference between two points when one joule of energy is
required to move one coulomb of charge between those points.
Potential difference = Work done / time
V=W / Q
V= Voltage (in Volt), W= work done (in Joules), Q = Charge (coulomb)
Relationship with Electric Current:
When a potential difference is applied across a conductor (such as a wire), it causes
the free electrons in the conductor to move, creating an electric current.
The greater the potential difference, the greater the driving force for the current to
flow.
Ohm's Law:
Potential difference is related to current and resistance in a circuit by Ohm's Law:
V=I×R
where:
V = potential difference (volts, V)
I = current (amperes, A)
R = resistance (ohms, Ω)
Example:
If you have a battery with a 9V potential difference, it means that the battery is
providing 9 joules of energy to move each coulomb of charge through the circuit.
The potential difference across a device is measured using a voltmeter connected in parallel
with the device.
Resistance is defined as the ratio of the potential difference across a conductor to the current
flowing through it. The unit of resistance is the ohm (Ω)
It is a measure of how much a material or component opposes the flow of electric current. It
determines how much current will flow through a circuit for a given potential difference
(voltage).
One ohm (Ω) is the resistance that allows 1 ampere (A) of current to flow when a potential
difference of 1 volt (V) is applied across it.
The relationship between voltage, current, and resistance is described by Ohm's Law:
Rearranged, Ohm's law can also be expressed as:
R=V/I
Factors Affecting Resistance:
Several factors affect the resistance of a material:
1. Material: Different materials have different intrinsic resistivities. For example,
metals like copper have low resistance (good conductors), while materials like rubber
or wood have high resistance (insulators).
2. Length of the Conductor: The longer the conductor, the hi
3. gher the resistance. This is because the electrons have to travel further, facing more
collisions with atoms.
4. Cross-Sectional Area: The wider the conductor, the lower the resistance. A larger
area allows more electrons to flow through with less obstruction.
5. Temperature: In most materials, as the temperature increases, resistance increases.
This is due to increased atomic vibrations, which make it harder for electrons to
move.
Example:
Copper wire has low resistance and is widely used in electrical wiring.
Rubber has high resistance and is used as an insulating material to prevent
unintended flow of current.
Resistors in Series
When resistors are connected in series, they are arranged one after the other in a single path.
The same current flows through all the resistors, but the voltage across each resistor may be
different, depending on its resistance.
Key Features of Resistors in Series:
1. Current: The same current flows through each resistor in a series circuit.
2. Voltage: The total voltage across the series combination is the sum of the individual
voltages across each resistor.
3. Total Resistance: The total or equivalent resistance of resistors in series is the sum of
their individual resistances.
Connecting light bulbs in series results in reduced brightness due to increased total resistance
and voltage division. If one bulb fails, all bulbs will go out.
Resistors in Parallel
When resistors are connected in parallel, they are arranged such that both ends of each
resistor are connected directly to the same two points, forming multiple paths for the current
to flow. In a parallel circuit, the voltage across all resistors is the same, but the total current is
the sum of the currents through each resistor.
Key Features of Resistors in Parallel:
1. Voltage: The voltage across each resistor is the same and equal to the voltage applied
across the parallel combination.
2. Current: The total current flowing into the parallel combination is the sum of the
currents flowing through each resistor.
3. Total Resistance: The total or equivalent resistance (RtotalR_{\text{total}}Rtotal) of
resistors in parallel is always less than the smallest individual resistance.
When lights are connected in parallel:
Each light bulb gets the full voltage of the power supply.
The bulbs operate independently, so if one fails, the others stay on.
The total current increases with the number of bulbs, and the total resistance
decreases.
All bulbs will be equally bright (if they are identical) because they receive the same
voltage.
