DC Circuits

Topics

1. Circuit Symbols2. 5 principles of DC Circuits3. 2 tips for solving DC Circuits4. Putting it all together – Electricity Problem

Solving Toolkit

CIRCUIT SYMBOLS

Circuit Symbols (pg 324)

• Sources of E.M.F.:– 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?

A

AAmmeter Reading = 0.2 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?

I0.3 A

0.2 A

Worked Example 3

• What is the value and direction of current in wire X?

0.2 A

0.3 A

0.2 A

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 VV

V

3.0 V

Voltmeter Y

1.0 VVoltmeter X

Worked Example 5

• What is the reading of voltmeter X?

V V

V

Voltmeter X

4.5 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

V

Voltmeter X

4.0 V

Worked Example 7

• What is the reading of voltmeter X?

V

V

Voltmeter X

4.0 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?

1 Ω 1 Ω

V

3 V

Worked Example 9

• What is the reading of the voltmeter?

1 Ω 2 Ω

V

3 V

Worked Example 10

• What is the reading of the voltmeter?

3 Ω 4 Ω

V

5 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.

R R

10 V

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

R

V

R/2 R/2

50 cm 50 cm

V

Worked Example 11

• What is the reading of the voltmeter?

V

30 cm 70 cm

5 V

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.

100 cm

5 V

0 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