DC Circuits

Topics Circuit Symbols 5 principles of DC Circuits 2 tips for solving DC Circuits Putting it all together – Electricity Problem Solving Toolkit

Circuit Symbols

Circuit Symbols (pg 324) Sources of E.M.F.: Resistor Meters Switch Cell & Battery D.C. Power Supply A.C. Power Supply Resistor Fixed Resistor Variable Resistor (Rheostat) Light Dependent Resistor (LDR) Thermistor Light Emitting Diode (LED) Meters Ammeter Voltmeter Switch Light Bulb (Lamp) Fuse Bells Variable Potential Divider (Potentiometer)

Cell (Battery)

D.C. Supply

A/C Supply

Fixed Resistor

Variable Resistor (Rheostat)

Light Dependent Resistor (LDR) LDRs decrease their resistance when exposed to light Often used as components for light-sensitive circuits (e.g. turn on lamps when it is dark)

Thermistor Thermistor’s decrease resistance when exposed to heat

Semiconductor Diode Allows current to flow in one direction Blocks current in opposite direction

Light Emitting Diode (LED) Same as a diode, except now it emits light when current is flowing in the allowed direction

Potentiometer Also called “variable potential divider” Is actually just a wire attached to a metre rule Comes with an apparatus called a “jockey” Will use this in the lab to prepare for SPA 2

Ammeter

Voltmeter

Galvanometer

Switch

Light Bulb

Fuse

Bell

Earth

Transformer

5 principles of dc circuits

Current in Series In a series circuit (i.e. no parallel circuits), the current is the same at all points of the circuit

Worked Example 1 What is the reading of Ammeter X? Ammeter Reading = 0.2 A A A Ammeter X

Current in Parallel In a parallel circuit, there must be branches Current follows the “what goes in must come out” rule

Worked Example 2 What is the value of I? 0.3 A I 0.2 A

Worked Example 3 What is the value and direction of current in wire X?

Potential Difference in Series Total p.d. is equals to the sum of the individual p.d. components across the series This is similar to calculating resistance of resistors in series Note that p.d. across wire (without resistors) is zero

Worked Example 4 What are the readings of voltmeters X and Y? V V V Voltmeter X 1.0 V V Voltmeter Y

Worked Example 5 What is the reading of voltmeter X? V V 1.0 V

Potential Difference in Parallel p.d. is the same across parallel branches

Worked Example 6 What is the reading of voltmeter X? V 4.0 V V

Worked Example 7 What is the reading of voltmeter X? V 4.0 V V V 1.0 V

Potential Divider Principle The ratio of the resistances is the ratio of the p.d. Equation form (not recommended to memorize): V1 = [R1/(R1 + R2)]Vɛ

Worked Example 8 What is the reading of the voltmeter? 3 V 1 Ω 1 Ω V

Worked Example 9 What is the reading of the voltmeter? 3 V 1 Ω 2 Ω V

Worked Example 10 What is the reading of the voltmeter? 5 V 3 Ω 4 Ω V

How is a potential divider useful? Let’s say I only have a 10 V battery, but I only need 5 V of emf for a circuit. I can use a potential divider to “divide up” my 10 V battery into just 5 V. 10 V R R

Potentiometer However, using resistors to divide up emf is inflexible. We cannot change the ratio easily (need to change the resistors manually). An easier method is to use a potentiometer (or variable potential divider)

Potentiometer 50 cm 50 cm R V R/2 R/2 V

Worked Example 11 What is the reading of the voltmeter? 5 V 30 cm

Worked Example 12 State and explain what will happen to the lamp as the jockey slides from the 0 cm mark to the 100 cm mark. 5 V 0 cm 100 cm

Worked Example 13 Potential divider circuits may also involve the use of LDRs and Thermistors Design a circuit which switches on a lamp automatically when it turns dark (hint: when bright, p.d. across lamp is low. when dark, p.d. across lamp is high)

2 tips for dc circuits

Redraw Circuit Diagrams

Replace Resistors in a Cluster

Electricity Problem Solving Toolkit 4 equations Definition of Current Definition of Resistance Electrical Power Electrical Energy 2 arrangements of Resistors In series In parallel 5 Principles Current in Series Current in Parallel P.d. in series P.d. in parallel Potential Divider 2 tips redraw diagram replace parallel resistors