Energy and ElectricityEnergy and Electricity

Basic ideas…Basic ideas…Electric current is when electrons start to flow around a circuit. We use an _________ to measure it and it is measured in ____.

Potential difference (also called _______) is how big the push on the electrons is. We use a ________ to measure it and it is measured in ______, a unit named after Volta.

Resistance is anything that resists an electric current. It is measured in _____.

Words: volts, amps, ohms, voltage, ammeter, voltmeter

Current in a series circuitCurrent in a series circuit

If the current here is 2 amps…

The current here will be…

The current here will be…

And the current here will be…

In other words, the current in a series circuit is THE SAME at any

point

Current in a parallel circuitCurrent in a parallel circuit

A PARALLEL circuit is one where the current has a “choice of routes”

Here comes the current…

And the rest will go down here…

Half of the current will go down here (assuming the bulbs are the same)…

Current in a parallel circuitCurrent in a parallel circuit

If the current here is 6 amps

The current here will be…

The current here will be…

The current here will be…

And the current here will be…

Voltage in a series circuitVoltage in a series circuit

V

V V

If the voltage across the battery is 6V…

…and these bulbs are all identical…

…what will the voltage across each bulb be? 2V

Voltage in a series circuitVoltage in a series circuit

V

V

If the voltage across the battery is 6V…

…what will the voltage across two bulbs be?

4V

Voltage in a parallel circuitVoltage in a parallel circuit

If the voltage across the batteries is 4V…

What is the voltage here?

And here?

V

V4V

4V

SummarySummary

In a SERIES circuit:

Current is THE SAME at any point

Voltage SPLITS UP over each component

In a PARALLEL circuit:

Current SPLITS UP down each “strand”

Voltage is THE SAME across each”strand”

An example question:An example question:

V1

V2

6V

3A

A1

A2

V3

A3

Advantages of parallel circuits…Advantages of parallel circuits…

There are two main reasons why parallel circuits are used more commonly than series circuits:

1) Extra appliances (like bulbs) can be added without affecting the output of the others

2) If one appliance breaks it won’t affect the others either

Georg Simon Ohm 1789-1854

ResistanceResistance

Resistance is anything that will RESIST a current. It is

measured in Ohms, a unit named after me.

The resistance of a component can be calculated using Ohm’s Law:

Resistance = Voltage (in V)

(in ) Current (in A)

V

RI

An example question:An example question:

V

A

1) What is the resistance across this bulb?

2) Assuming all the bulbs are the same what is the total resistance in this circuit?

Voltmeter reads 10V

Ammeter reads 2A

More examples…More examples…

12V

3A

3A

6V

4V

2A

1A

2V

What is the resistance of these bulbs?

Two simple components:Two simple components:

2) Thermistor – resistance DECREASES when temperature INCREASES

1) Light dependant resistor – resistance DECREASES when light intensity INCREASES

Resistance

Amount of light

Resistance

Temperature

Current-voltage graphsCurrent-voltage graphsI

VI

V

I

V

1. Resistor 3. Diode

2. BulbCurrent increases in proportion to voltage

As voltage increases the bulb gets hotter and resistance increases

A diode only lets current go in one direction

DC and ACDC and AC

DC stands for “Direct Current” – the current only flows in one direction:

AC stands for “Alternating Current” – the current changes direction 50 times every second (frequency = 50Hz)

1/50th s

240V

V

V

Time

T

+

+

-

-

FusesFuses

Fuses are _______ devices. If there is a fault in an appliance which causes the ____ and neutral (or earth) wire to cross then a ______ current will flow through the _____ and cause it to _____. This will break the _______ and protect the appliance and user from further _____.

Words – large, damage, safety, melt, live, circuit, fuse

Other safety devices…Other safety devices…1) Insulation and double insulation

2) Residual Current Circuit Breaker

Insulating a wire prevents someone from touching the ____ wire.

A device can also be double insulated – this is when the metal parts inside the device are insulated from the outer case.

A double insulated device does not need an _______ wire but they still have _____.

This circuit breaker detects any difference in current between the live and neutral connectors and switches off the current when needed. They can be easily _______.

Words – earth, reset, live, fuses

The Cost of ElectricityThe Cost of ElectricityElectricity is measured in units called “kilowatt hours” (kWh). For example…

A 3kW fire left on for 1 hour uses 3kWh of energy

A 1kW toaster left on for 2 hours uses 2kWh

•A 0.5kW Vacuum Cleaner left on for 4 hours uses __kWh

•A 200W TV left on for 5 hours uses __kWh

•A 2kW kettle left on for 15 minutes uses __kWh

The Cost of ElectricityThe Cost of ElectricityTo work out how much a device costs we do the following:

Cost of electricity = Power (kW) x time (h) x cost per kWh (p)

For example, if electricity costs 8baht per unit calculate the cost of the following…

1) A 2kW fire left on for 3 hours

2) A 0.2kW TV left on for 5 hours

3) A 0.1kW light bulb left on for 10 hours

4) A 0.5kW hoover left on for 1 hour

48B

8B

8B

4B