Electric Circuit NOTE - Lecture notes 1-5 PDF

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Summary

ELECTRIC CIRCUITS Series and Parallel Circuits The current at any point in a series circuit is the same The current splits at each branch in a parallel circuit so the total current is always greater than the current in one branch Combining resistors o In Series: 𝑹𝑻𝒐𝒕𝒂𝒍 𝑹𝟏 𝑹𝟐 𝟏 o In Parallel: 𝑹𝑻𝒐𝒕𝒂𝒍 ...

Description

ELECTRIC CIRCUITS Series and Parallel Circuits • The current at any point in a series circuit is the same • The current splits at each branch in a parallel circuit so the total current is always greater than the current in one branch  Combining resistors o In Series: 𝑹𝑻𝒐𝒕𝒂𝒍 = 𝑹𝟏 + 𝑹𝟐 𝟏 o In Parallel: 𝑹𝑻𝒐𝒕𝒂𝒍 = 𝟏 𝟏

⁄𝑹 + ⁄𝑹 𝟏 𝟐

o The combined resistance of 2 resistors in parallel is less than that of either resistor by itself

Series and Parallel Circuits: •

• • •

There are two different ways of circuits, of connecting resistors (e.g. lamps) to the same battery. These circuits are called series and parallel circuits. A series circuit is a circuit in which resistors are arranged in a chain, so the current has only one path to take. The current is same through each resistor in series circuit. The total resistance of the circuit is, R = R1 + R2 + ...

•

In series circuit the total Voltage (V) of individual resistor is the sum of individual potential differences. That is V = V1 + V2 + ...

•

•

•

The current (I) remains the same in each resistance therefore the ammeter is connected in series with the other resistances.

A parallel circuit is a circuit in which the resistors are arranged with one end of each resistor connected together, and other end of each resistor connected together and then each end connected directly to the battery. The current in a parallel circuit breaks up and take the easiest path first, with some flowing along each parallel branch and re-combining when the branches meet again. The voltage across each resistor in parallel circuit is same. Total resistance of resistors in parallel circuit can be expressed by: 1

1

=

1

+



1

=

1 × 2 1 + 2

2

+ ⋯

or



In parallel circuit the total current is the sum of individual currents in each resistance.

I = I1 + I2 + …

• Advantages of putting lamps in parallel are: o If one lamp breaks, the other still works o Each lamp gets maximum PD

17.3 Potential Divider • A potential divider divides the voltage into smaller parts.

• To find the voltage (at VOUT) we use the following formula: 𝑹𝟐 ) 𝑹𝑻𝒐𝒕𝒂𝒍 • A variable potential divider (potentiometer) is the same as the one above but using a variable resistor; it acts like a potential divider, but you can change output voltage. 𝑽𝑶𝑼𝑻 = 𝑽𝑰𝑵 × (

17.4 Input Transducer (transducer is a device which changes properties with change in environment) • Thermistor: iA thermistor is a heat sensor (resistor) which changes its resistance with the change • of temperature (heat) around it. Its resistance decreases as the temperature increases which is reverse to the normal conductor. For example: • Icy water 0°C has high resistance, about 12kΩ. • Room temperature 25°C has medium resistance, about 5kΩ. • Boiling water 100°C has low resistance, about 400Ω. • Thermistor is called input transducer. It means it changes resistance with the change in environment • Light dependent resistor (LDR): An LDR is a light sensor (resistor) which changes its resistance with the brightness of light around it. It is made from cadmium sulphide compound (CdS) and its resistance decreases as the brightness of light falling on the LDR increases. • Darkness: maximum resistance, about 106Ω. • Very bright light: minimum resistance, about 100Ω. • LDR is called input transducer. It means it changes resistance with the change • in environment.

diode thermistor

Rheostat as a potential divider

17.5 Relay • A switch operated by an electromagnet

NORMAL CLOSED RELAY

When coil not energized, switch is closed, completing circuit

NORMALLY OPEN RELAY

When coil energized, switch is closed, completing circuit

17.6 Diode • A device that has an extremely high resistance in one direction and a low resistance in the other, therefore it effectively only allows current to flow in one direction • Forward bias is when the diode is pointing in the direction of the conventional current and reverse bias is the opposite • It can be used in a rectifier; turns AC current into DC current.

*** the pointer symbol in a diode indicates the direction of electric current...