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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.

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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)

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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%

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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)

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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%.

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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 %.

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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

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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.

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Measuring current

This is how we draw an ammeter in a circuit.

A A

SERIES CIRCUIT PARALLEL CIRCUIT

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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

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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.

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V V

6V4A

A

A

a))

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Measuring current & voltage

V

V

6V4A A

A

A

b)b)

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