electricity by dr. ahmed mostafa assist. prof. of anesthesia and i.c.u

ElectricityBy

Dr. Ahmed MostafaAssist. Prof. of anesthesia and I.C.U.

Basic quantities and units

Electric charge:- May be positive or negative, and is produced by the

accumulation of an excess or deficit of electrons in an object.

- Is measured in coulombs.

- One coulomb is defined from the unit of current as that charge which passes any point in a circuit in a second, when a steady current of 1 ampere is flowing. A coulomb is equal in magnitude to the electric charge possessed by 6.24 × 1018 electrons.


Electric current:

- Are produced by the movement of charge.

- Can be measured as the number of coulombs passing any given point per second.

- The unit of current is the ampere (A), where:

1 ampere (A) = 1 coulomb s−1


Electric current:

- When a current flows through a conductor it produces magnetic lines of force around the conductor.


Electric current:

• Definition of the ampere:

The current which, if flowing in two

parallel wires of infinite length, placed

1 meter apart in a vacuum, will produce

a force on each of the wires of

2 × 10−7 newtons per meter.


Potential difference (voltage):

- When a potential difference is applied across a conductor it produces an electric current.

- A current is a flow of positive charge from the higher potential to the lower.

- One volt can be defined as a potential difference producing a change in energy of 1 joule when 1 coulomb is moved across it.


Ohm’s law:

- Is the electrical property of a conductor which opposes the flow of current through it.

- Electrical resistance is measured in ohms (Ω).

- Ohm’s law states that the current flowing through a resistance is proportional to the potential difference across it.

- The potential difference across the resistance = V volts, the current = I amps and the resistance has a value of R Ω. So

V = I R volts


Direct current (DC) & alternating current (AC):

- The terms DC and AC are normally used to describe the electricity supply to a circuit or system.

- DC describes current which only flows in one direction. Generally DC is supplied by a battery (or power adaptor).



- If the current supplied is plotted against time it will give a graph as shown in following figure:



- AC describes a supply in which the current reverses direction cyclically.



- AC is the normal mains supply, and has this form because of the way in which electricity is generated and distributed.

- An AC voltage is described by its amplitude (peak value) and frequency. The amplitude of mains voltage in the UK is 340 V, and it has a frequency of 50 Hz.



- Usually mains voltage is quoted as 240 V, which is the root mean square (RMS) value.

- The RMS value for an AC voltage is the DC voltage or current which would have the same heating effect. This is used to compare AC and DC because the heating and lighting effects of a current are not dependent on the direction of flow.



- AC currents and voltages are important because they can be used to carry information. In this case they are usually referred to as signals. Often electrical currents and signals are a combination of DC and AC.


Impedance and reactance:

Resistance: is a measure of a device’s ability to resist DC current. It is represented by R and measured in ohms.

Reactance: describes a device’s ability to resist the flow of AC. The reactance of a device will be dependent on the frequency of AC applied. It is normally represented by X and is measured in ohms.

Impedance: for a device is obtained by mathematically combining its reactance and resistance. It is normally represented by Z and is measured in ohms.

Z =

Electrical circuit

• It is the basic structure unit of electronics.

• Components:

1. Source of energy: (Either DC or AC)

2. Resistor.

3. Capacitor.

4. Inductor.

5. Semiconductor.

Electrical circuit

2- Resistor:

Resistor opposes the flow of both AC and DC alike. A resistor in a circuit can be used to reduce currents or voltages. Often multiple resistors are used in a circuit. These can be combined in order to calculate the values of currents and voltages produced in different parts of the circuit.

Electrical circuit

2- Resistor:

• Series resistances:

- A series arrangement is ‘end to end.

- Combination of these values is by simple addition. i.e.

RT = R1 + R2

Electrical circuit

2- Resistor:

• Parallel resistances:

- A parallel arrangement is ‘side by side’.

- the total resistance RT is given by:

= +

Electrical circuit

Wheatstone bridge circuit:

Consists of a ring of four resistances supplied by a DC voltage across diagonally opposite corners of the ring A and C. The values of the resistances are balanced using a variable resistor R1, such that points B and C are at exactly the same potential.

Electrical circuit


This occurs when: =

When this condition is fulfilled, if a galvanometer, G, is connected between B and D no current will be detected and the bridge is balanced or zeroed.

Electrical circuit


The bridge circuit is very sensitive to any variation in the value of the resistances, and if one of them changes a current will be detected on G. The circuit is applied by using a strain gauge as one of the resistances (R2)

Electrical circuit

3.Capacitor (Condenser):

- Describes the property of a device enabling it to store electric charge.

- Consists of two conducting plates separated by a thin layer of insulating material.

- When a voltage is applied, there is an initial surge of current, but when the plates have become charged no more current flows.

Electrical circuit


- The following figure shows a capacitor charging circuit and the current and voltage changes occurring when the switch is closed.

Electrical circuit


- The amount of charge stored depends on the size of the capacitance, which is measured in farads (F). This in turn depends:

The size of the capacitor plates.

The separation of the plates.

The dielectric material used.

- In a circuit, a capacitor has the useful property of being able to pass AC signals, but to block DC, since there is no direct contact between the plates.

Electrical circuit


- The resistance of a capacitor (to DC) is therefore very high since it is effectively an open circuit. However the reactance of a capacitor (to AC) is low and decreases with frequency. This enables it to be used to bypass unwanted AC signals to earth in cases of electrical interference. The frequency dependence of capacitors also means that they are useful components in filters.

Electrical circuit

4. Inductor:

Is made by forming a conductor into coils, which are often wound around a core of ferrous material. This construction has the effect of producing a concentrated magnetic field through the axis of the inductor and around it, whenever a current flows.

Electrical circuit

4. Inductor:

- When a voltage is applied across the terminals of an inductor, current does not flow immediately but increases slowly in step with the buildup of the magnetic lines of force.

Electrical circuit

4. Inductor:- Similarly, if the voltage is switched off, the

current does not fall to zero immediately but dies down slowly, since as the magnetic field collapses it maintains the current flow for a while.

- The build-up and collapse of the magnetic field tends to slow down changes in current flow, whenever the applied potential difference varies.

Electrical circuit

4. Inductor:

- The behavior of inductances in an electrical circuit is thus analogous to the inertial effect of masses in a mechanical system.

Electrical circuit

4. Inductor:

- An inductance has a relatively low resistance to DC, simply equal to that of the coils of wire. However, when AC is applied to an inductance, the continually varying current meets a comparatively high reactance. The reactance of an inductor increases with frequency. Inductors therefore tend to block AC but pass DC.

Electrical circuit

4. Inductor:

- Inductances are used as components in filters and to ‘smooth out’ spikes and surges in power supplies.

Electrical circuit

Electrical circuit

5. Semiconductor:

- Allow conductivity intermediate between conductors and insulators.

- The outer electrons are bound to atoms less firmly than in an insulators and less loose than conductors.

- If little energy is given to these electrons, they escape form atom they are bound to and hence conduct electricity.

Electrical circuit

5. Semiconductor:

They include:

a.Thermistors.

b.Transducers: e.g. Photoelectric cell.

c. Transistors.

d.Diodes.

Electrical circuit

5. Semiconductor:

c. Transistors:

- These are used to amplify small current signals, enabling small electrical signals of a few micro-amps to be converted to much greater signals of tens of milliamps.

Electrical circuit

5. Semiconductor:

c. Transistors:

- The basic transistor consists of a tiny slice of semiconductor material with connections to three regions: base, collector and emitter.

- A common configuration allows a small signal fed into the base to produce an amplified signal in the collector circuit.

Electrical circuit

5. Semiconductor:

c. Transistors:

Electrical circuit

5. Semiconductor:

d. Diodes:

- A diode is a semiconductor (silicon or germanium) device which only enables current to flow through it in one direction.

- It is often used to convert AC to DC in order to provide a DC power supply from the AC mains.

Electrical circuit

5. Semiconductor:

d. Diodes:

- This is commonly found in the mains adaptors used as a substitute for equipment batteries.

- Diodes are also used in protective circuits and to process signals in measurement systems.

Electrical circuit

5. Semiconductor:

d. Diodes:

Circuit elements

Defibrillator circuit


• A circuit using both capacitance and inductance is the defibrillator circuit.

• Its operation consists of two phases, charging and discharging. These phases are controlled by the switch S1.

• When charging, S1 connects the capacitor to the DC power supply, which charges it to deliver the required amount of energy or number of joules set by the operator.


• On discharge, S1 connects the capacitor to the patient circuit, which enables the stored charge to be delivered to the patient via the switch (S2) on the paddles.

• The inductor in the discharge circuit has the effect of slowing down and spreading out the delivered pulse of energy to the myocardium, which makes it more effective than the shorter sharper spike waveform that would be delivered without the inductance.

Transformer

Transformer

• A transformer consists of two inductors wound around the same former.

• The close physical relationship between the two coils means that current changes in one circuit (the primary winding) will induce currents in the second coil (the secondary winding) via the coupling effect of the magnetic field.

Transformer

• The degree of coupling will depend on the number of turns in the primary winding(N1) and the secondary winding (N2 ).

• If an AC voltage (V1) is applied across the primary, the voltage produced across the secondary (V2 ) will be given by:

V2 = V1 × N2/N1

Transformer

• A transformer can thus be used to step up or step down AC voltages in circuits.

• Transformers are commonly used in distributing the electrical power supply from the national grid to domestic users.

• An alternative use for transformers is in transferring signals between circuits and in devices such as microphones or loudspeakers.

Diathermy hazards

• Diathermy uses high-frequency (0.4–1.5 MHz) currents to generate heat in the tissues during surgery.

• This is applied via a probe to produce coagulation and cutting effects.

• Hazards are: burns, electric shock, interference in monitoring equipment and possibly indwelling pacemakers.

Prevention of diathermy hazards

• Use of isolated patient circuit. • Proper application of diathermy pad.• Use of isolating capacitor.

Prevention of diathermy hazards

• Avoiding inadvertent patient contact with earthed metalwork.

• Use of bipolar diathermy: This form of diathermy uses a pair of probes, one to deliver the diathermy signal and the other to act as a return circuit. They are arranged as the arms of forceps, which restricts the current field to a small area surrounding the forceps tips.

Thank youDr. Ahmed Mostafa

electricity by dr. ahmed mostafa assist. prof. of anesthesia and i.c.u

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