Electrical energy is derived from electrical potential energy or energy that has been changed from electrical potential energy.
In
order for Electrical energy to be materialized a circuit is needed. The
function of the circuit is to deliver the combination of electric
current and electric potential, which is the driving force of electrical
potential energy. After the electrical potential energy has been
converted to another type of energy it stops behaving as electrical
potential energy. Therefore, it can be said that electrical energy is
actually a potential energy before it is being changed to other type of
energy or energies.
1. Electrical
energy is very important because this form of energy can be converted
easily into other forms of energy, such as heat, light, mechanical and
electromagnetic energy.
2. The
energy carried by electrical charges can be transformed to other form
of energy by using different electrical appliances. The table shows the
transformation of energy for a few household appliances.
Appliances
|
Energy transformed
|
Rice cooker
|
Electrical to heat
|
Radio
|
Electrical to sound
|
Light
|
Electrical to heat and light
|
Washing machine
|
Electrical to mechanical
|
3. Electrical
energy can be defined as the energy carried by electrical charges which
can be transformed to other forms of energy by using of an electrical
device or appliances.
Energy, Voltage, Current and Time
1. The potential difference or voltage,V, across two points is defined
as energy,E, dissipated or transferred by coulomb of charge,Q, that
moves through the two points.
Therefore:
Potential difference = Electrical energy dissipated / Charge
V= E / Q
2. Current is the rate of charge flow. Therefore, the total charge flows through the two points is given as:
Q = It I = current, t = time in second
3. Since the energy dissipated or transferred is given by:
E= VQ
Therefore, the relationship between E, V,I and t can be written as:
E = VIt
4. From Ohm’s law, V =IR, therefore,
E = IR × It
E = I²Rt
5. From I = V / R = > ; E = V
Therefore:
Potential difference = Electrical energy dissipated / Charge
V= E / Q
2. Current is the rate of charge flow. Therefore, the total charge flows through the two points is given as:
Q = It I = current, t = time in second
3. Since the energy dissipated or transferred is given by:
E= VQ
Therefore, the relationship between E, V,I and t can be written as:
E = VIt
4. From Ohm’s law, V =IR, therefore,
E = IR × It
E = I²Rt
5. From I = V / R = > ; E = V
Electromotive Force and Internal Resistance
.
.
3. If a resistor, R is then connected to the terminals of the cell, the voltmeter reading is the potential difference, V across the terminals of the cell.
4. The value of potential difference, V is less than e.m.f. of the cell. The difference between E and V is due to the potential difference needed to drive the current, I through the internal resistor, r of the cell.
Hence,
E-V = Ir ═> E = V + Ir
5. The internal resistance, r is given by:
r = E-V / I
Internal resistance is caused by the resistance of the supplier of
electrical energy - usually battery and power supply. The current that
flows through the circuit also passes through the battery. Internal
resistance of the battery causes the loss of energy. SOme of the energy
per charge the battery provides will be used to overcome the internal
resistance of the battery.
An electrical cell is made from materials (metal or chemicals, for example). All materials have some resistance. Therefore, a cell must have resistance. This resistance is called the internal resistance of the cell.
A cell can be thought of as a source of e.m.f. with a resistor connected in series.
When current flows through the cell a voltage develops across the internal resistance. This voltage is not available to the circuit so it is called the lost volts, (VL).
VL can also be written as Ir
The voltage across the ends of the cell is called the terminal potential difference, (Vt.p.d).
Vt.p.d can also be written as IR
Because voltage is a measure of energy, and energy is always conserved, the e.m.f. of a cell is equal to the sum of its terminal potential difference, (Vt.p.d), and the lost volts, (VL).
This gives rise to the equation E = Vt.p.d.+ VL
This equation can be written in different forms, e.g. E = I (R + r).
It is always to be reminded that the internal resistance is considered to be in series with the cell.
The e.m.f is the work done by a source in driving one coulomb of charge
around a complete circuit. No device is a perfect energy converter. Some
of the energy produced by the chemical reaction inside the battery is
lost when it is converted to electrical energy. The useful energy
produced by the battery is the potential difference between its
terminal.
The electromotive force of a battery is equal to the potential difference between its terminals in an "open circuit", when there is no current being drawn. The potential difference between the terminals generally drops when the current is being drawn.
You cannot measure the e.m.f of a battery directly because whenever you use a voltmeter to measure the potential difference between the terminals of a battery, a small current will flow from the battery through the voltmeter.
This means that a small potential drop has already occurred inside the battery before the current reaches the external circuit. The reading you get on the voltmeter is therefore the potential difference between the terminals of the battery and not the e.m.f of the battery.
However, for practical purposes, the difference between the terminal potential difference and e.m.f of battery is negligible. Thus the reading of a voltmeter across the terminals of the battery is normally taken as the e.m.f of the battery.
The other way to measure the e.m.f of the battery is to get a series of values for the potential difference between its terminals for different magnitudes of current and then plot a graph of potential difference against current. E.m.f can be determined buy extrapolating the graph. When the current is zero, the potential difference is equal to the e.m.f of the battery.
In parallel connection, the resistors are connected as shown below:
In such a case, the total or effective resistance between terminals A and B is given by:
1/Rab = 1/R1 + 1/R2 + 1/R3
i.e. the reciprocal of the effective resistance is equal to sum of the reciprocals of the individual resistances.
If there are only two resistors, the above formula reduces to:
Rab = (R1*R2) / ( R1+ R2)
e.g., if two 1K resistors are connected in parallel, the effective resistance is (1*1)/(1+1)= 1/2 = 0.5K=500 ohm.
In Parallel circuit, the Voltage that pass through each resistor is the same.
V1 = V2 = V3
Series Connection:
In series connection, the resistors are connected end to end as shown below:
In such a connection, the total resistance between the terminals A and B is the sum of individual resistances.
Rab = R1 + R2 + R3
Rab = total effective resistance in the circuit
For example, if a 2.2K ohm and 3.3K ohm resistors are connected in series, the total resistance is 2.2+3.3 = 5.5Kohm.
The Current through each resistor is the same. I1 = I2 = I3 = I
1. The cell functions as the source of energy and the light bulb is the energy consuming device.
2. The light bulb converts electrical energy into heat and light energy.
3. The electrical charges that flow round the circuit transfer energy from the source (the cell) to the device.
4. In a
cell, chemical reaction converts chemical energy into electrical energy.
This energy pushes the free electrons to move them from the negative
terminal to the positive terminal of the cell.
5. Work done by the source in driving the charges around a complete circuit. This work done is known as electromotive force.
6. The
electromotive force (e.m.f.) is the work done by one source in driving a
unit charge around a complete circuit. <-------------- i="">need
to remember this term
Electromotive Force and Potential Difference
1. The
definition for electromotive force (e.m.f.) is similar to that of
potential difference (p.d.). However, there is a distinction between
e.m.f. and p.d.
2. The e.m.f. of a cell is the energy supplied to a unit of charge within the cell.
3. The
p.d. across a component in a circuit is the conversion of electrical
energy into others forms of energy when a unit of charge passes through
the components.
Internal Resistance
1. In an open circuit when there is no current flow, the potential difference, V across the cell is the electromotive force, E of the cell.
2. In a closed circuit when there is a current flow, the potential difference, V across the cell is smaller than the electromotive force, E of the cell.
3. This drop in potential difference across the cell is caused by the internal resistance of the cell.
4. The internal resistance of a source or a cell is the resistance against the moving charge due to the electrolyte in a source or the cell.
5. Work is needed to drive a charge against the internal resistance.
6. This causes a drop in potential difference across the cell as the charge flows through it
.
Electromotive force and Internal Resistance
1. A cell can be modeled as an e.m.f., E connected in series with an internal resistor, r.
1. A cell can be modeled as an e.m.f., E connected in series with an internal resistor, r.
2. When a high
resistance voltmeter is connected across the terminals of the cell, the
reading of the voltmeter gives the e.m.f., E of the cell.
3. If a resistor, R is then connected to the terminals of the cell, the voltmeter reading is the potential difference, V across the terminals of the cell.
4. The value of potential difference, V is less than e.m.f. of the cell. The difference between E and V is due to the potential difference needed to drive the current, I through the internal resistor, r of the cell.
Hence,
E-V = Ir ═> E = V + Ir
5. The internal resistance, r is given by:
r = E-V / I
Internal resistance
An electrical cell is made from materials (metal or chemicals, for example). All materials have some resistance. Therefore, a cell must have resistance. This resistance is called the internal resistance of the cell.
A cell can be thought of as a source of e.m.f. with a resistor connected in series.
When current flows through the cell a voltage develops across the internal resistance. This voltage is not available to the circuit so it is called the lost volts, (VL).
VL can also be written as Ir
The voltage across the ends of the cell is called the terminal potential difference, (Vt.p.d).
Vt.p.d can also be written as IR
Because voltage is a measure of energy, and energy is always conserved, the e.m.f. of a cell is equal to the sum of its terminal potential difference, (Vt.p.d), and the lost volts, (VL).
This gives rise to the equation E = Vt.p.d.+ VL
This equation can be written in different forms, e.g. E = I (R + r).
It is always to be reminded that the internal resistance is considered to be in series with the cell.
Difference Between E.M.F and Potential Difference
The e.m.f is the work done by a source in driving one coulomb of charge
around a complete circuit. No device is a perfect energy converter. Some
of the energy produced by the chemical reaction inside the battery is
lost when it is converted to electrical energy. The useful energy
produced by the battery is the potential difference between its
terminal.The electromotive force of a battery is equal to the potential difference between its terminals in an "open circuit", when there is no current being drawn. The potential difference between the terminals generally drops when the current is being drawn.
You cannot measure the e.m.f of a battery directly because whenever you use a voltmeter to measure the potential difference between the terminals of a battery, a small current will flow from the battery through the voltmeter.
This means that a small potential drop has already occurred inside the battery before the current reaches the external circuit. The reading you get on the voltmeter is therefore the potential difference between the terminals of the battery and not the e.m.f of the battery.
However, for practical purposes, the difference between the terminal potential difference and e.m.f of battery is negligible. Thus the reading of a voltmeter across the terminals of the battery is normally taken as the e.m.f of the battery.
The other way to measure the e.m.f of the battery is to get a series of values for the potential difference between its terminals for different magnitudes of current and then plot a graph of potential difference against current. E.m.f can be determined buy extrapolating the graph. When the current is zero, the potential difference is equal to the e.m.f of the battery.
The Effective Resistance of resistors connected in Parallel
In such a case, the total or effective resistance between terminals A and B is given by:
1/Rab = 1/R1 + 1/R2 + 1/R3
i.e. the reciprocal of the effective resistance is equal to sum of the reciprocals of the individual resistances.
If there are only two resistors, the above formula reduces to:
Rab = (R1*R2) / ( R1+ R2)
e.g., if two 1K resistors are connected in parallel, the effective resistance is (1*1)/(1+1)= 1/2 = 0.5K=500 ohm.
In Parallel circuit, the Voltage that pass through each resistor is the same.
V1 = V2 = V3
The Effective Resistance of resistors connected in Series
In series connection, the resistors are connected end to end as shown below:
In such a connection, the total resistance between the terminals A and B is the sum of individual resistances.
Rab = R1 + R2 + R3
Rab = total effective resistance in the circuit
For example, if a 2.2K ohm and 3.3K ohm resistors are connected in series, the total resistance is 2.2+3.3 = 5.5Kohm.
The Current through each resistor is the same. I1 = I2 = I3 = I
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