Post on 27-Dec-2015
2
Open Circuits
Current can only exist where there is a conductive path
Open circuit - When there is no conductive path
If I = 0- Ohm’s Law gives R = V/I = V/0 infinity
An open circuit has infinite resistance
Short Circuits
Short circuit - the voltage is zero
If V = 0- Ohm’s Law gives R = V/I = 0/I Zero
An short circuit has zero resistance
Power
4
Power is the rate of doing work.
Power = Work/time (dW/dt, where W is energy and t is time)
Power is measured in watts.
One watt = one joule per second
(1joule/Second)
Power in Electrical Systems
5
From V = W /Q and I = Q/t, we get
P = VI
(P = W/t)From Ohm’s Law, we can also find that
P = I 2R and P = V 2/RPower is always in watts.
6
Power in Electrical Systems
We should be able to use any of the power equations to solve for V, I, or R if P is given.
For example:
PRV
R
PI
Calculating Power: ExampleExample: What is an iPod’s power in watts if it was on for 1.30 hrs and used 210 000 J of electrical energy?
Given: t = 1.30 h x 3600t = 4700sE = 210 000 J
P = E / t
•Power (P) is measured in watts (W)•Energy (E) is measured in joules (J)•Time (t) is measured in seconds (s)
7
Required: P = ?
Analysis: P = E / t
Solution: P = 210 000 J / 4700 s
P = 45 W
Statement: Therefore, the iPod’s power is 45 watts.
Calculating Power: Example (solution)
8
Electric Power
10
Energy
Energy = Power × time
Units are watt-seconds, watt-hours, or more commonly, kilowatt-hours.
For multiple loads, the total energy is the sum of the energy of the individual loads.
Efficiency
%100in
out
P
P
Efficiency (in %) is represented by η (Greek letter eta)Ratio of power out to power input
Always less than or equal to 100%
11
To find the total efficiency of a systemObtain product of individual efficiencies of all subsystems:
Total = 1 × 2 × 3 × ∙∙∙
SAMPLE PROBLEM 1
A bulb uses 100 J of electrical energy and produces 35 J of light energy. Calculate the percent efficiency of the light bulb.
Given: E out = 35 J
E in = 100 J
Required: percent efficiency (% efficiency)
12
Analysis: % efficiency = E out x 100% E in
Solution:% efficiency = 35 J x 100% 100J% efficiency = 0.35x100%% efficiency = 35%
Statement: The efficiency of the light bulb is 35%.
13
14
• A toaster oven uses 1200 J of energy to produce 850 J of thermal energy. Calculate the percent efficiency of the toaster oven.
Given:E in = 1200 J
E out = 850 J
Required:% efficiency = ?
SAMPLE PROBLEM 2
Analysis:% efficiency = E out x 100% E in
Solution:% efficiency = 850 J x 100%
1200 J
% efficiency = 70.8 %
Statement:The efficiency of the toaster oven is 70.8 %.
15
Conductors Vs Insulators• Conductors – material through which electric current
flows easily.• Insulators – materials through which electric current
cannot move.
ConductorsConductors:MetalWater
Insulators:RubberPlasticPaper
The CELL
The cell stores chemical energy and transfers it to electrical energy when a circuit is connected.
When two or more cells are
connected together we call
this a Battery.
The cells chemical energy is
used up pushing a current
round a circuit.
Simple circuits
Here is a simple electric circuit. It has a cell, a
lamp and a switch.
To make the circuit, these components are
connected together with metal connecting wires.
cell
lamp
switch
wires
Circuit diagram
cell switch
lamp
wires
Scientists usually draw electric circuits using symbols;
Circuit diagramsIn circuit diagrams components are represented by the following symbols;
cell battery
switch
lamp
motorammeter
voltmeter
buzzer
resistor
variable resistor
Types of circuit
There are two types of electrical circuits;
SERIES CIRCUITS PARALLEL CIRCUITS
22
The components are connected end-to-end, one after the other.
They make a simple loop for the current to flow round.
SERIES CIRCUITS
23
PARALLEL CIRCUITS
The current has a choice of routes.
The components are connected side by side.
If one bulb ‘blows’ there is still be a complete circuit to the other bulb so it stays alight.
24
Measuring current
This is how we draw an ammeter in a circuit.
A A
SERIES CIRCUIT PARALLEL CIRCUIT
25
Measuring current
SERIES CIRCUIT
PARALLEL CIRCUIT
• current is the same
at all points in the
circuit.
2A 2A
2A
• current is shared between the components
2A2A
1A
1A
measuring voltage
V
This is how we draw a voltmeter in a circuit.
SERIES CIRCUIT PARALLEL CIRCUIT
V
26
Measuring current & voltage
copy the following circuits on the next two slides.
complete the missing current and voltage readings.
remember the rules for current and voltage in series and parallel circuits.
27
V V
6V4A
A
A
a))
28
Measuring current & voltage
V
V
6V4A A
A
A
b)b)
29