DC Circuits DC Circuits • Kirchoff’s laws • Simple series ...
DC Circuits. Topics 1.Circuit Symbols 2.5 principles of DC Circuits 3.2 tips for solving DC Circuits...
Transcript of DC Circuits. Topics 1.Circuit Symbols 2.5 principles of DC Circuits 3.2 tips for solving 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 (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)
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)
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
Toolkit• A handyman has a toolbox with many tools
(e.g. hammer, spanner, screwdriver, etc.) In each situation he won’t need to use ALL his tools, but different situations call for different tools.
• Similarly, when solving electricity problems there is a total of 11 tools you can use. You won’t need to use ALL your tools for any one question, but different questions call for different tools.
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
3 tips• check for short circuit• redraw diagram• replace cluster of resistors
Arrangement of Resistors
• In series• Rtotal = R1 + R2 + R3 +….
• In parallel• 1/Rtotal = 1/R1 + 1/R2 + 1/R3 + ….
• 2 parallel resistors only (optional)• Rtotal = (R1R2)/(R1 + R2)
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
3 tips• check for short circuits• redraw diagram• replace cluster of resistors
Current in Series
• In a series circuit (i.e. no parallel circuits), the current is the same at all points of the circuit
Current in Parallel
• In a parallel circuit, there must be branches• Current follows the “what goes in must come
out” rule
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
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
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)
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
Practice Task
• GLM Pg 319 Qn 1(a), 1(b)
• Potential divider circuits may also involve the use of LDRs and Thermistors
• GLM Pg 319 Qn 2(b), Pg 320 Qn 3(b)
Worked Example 13
• 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)
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
3 tips• Check for short circuits• redraw diagram• replace cluster of resistors
3 Tips
• The following are not found in most textbooks• Strictly speaking, these are tips not tools, but
they can be really helpful nevertheless
Tip 1: Checking for Short Circuits
• In theory, a wire has zero resistance• When a wire bypasses a circuit component, all
the current flows through the wire instead of the component. This is called “shorting the component”.
• If a wire shorts ALL the resistors, it is said to form a “short circuit”, which is very dangerous in real life since (current becomes very high)
Tip 2: Redraw Circuit Diagrams
• A helpful habit is to redraw circuit diagrams such that the arrangement is easy to see:
Arrangement of Resistors
Practice Task
• Redraw the following circuit diagrams– GLM Pg 312 Qn 4– Pg 313 Worked Example 2– Pg 315 Qn 2
Tip 3: Replace Resistors in a Cluster
• A short cut for some calculation questions involve replacing a cluster of resistors with one resistor of same effective resistance
3 Ω
2 Ω
2 Ω
convert to 4 Ω
Practice Task
• GLM Pg 307 Qn 1(a)• Pg 312 Qn 4(c)
[hint: you have already determined I1 and I2 from earlier parts of question]
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
3 tips• check for short circuit• redraw diagram• replace cluster of resistors