IX. Electricity · Electrical circuits: basic elements. · Current. · Potential difference. · Resistance. · Ohm's law. · Effects of electrical resistors in series and in parallel. · Effects of electricity: chemical, thermal, magnetic and mechanical. · Joule Effect. · Electrical power.

Charge and current

The charge of electricity is measured in the unit coulomb (C).

There is a link between the charge and current:

If the charge flows at this rate: Then the current is:1 coulomb per second 1 ampere2 coulombs per second 2 amperesetc.

The equation is therefore the following:

Charge = current x time (C) = (A) x (s)

An electrical cell makes electrons (e-) move. An example of an electrical cell is a battery.

CurrentA flow of charge is called a current. A charge is the flow of electrons through matter.

The SI unit to measure the strength of a current is ampere (A)

More or less 6.000.000.000.000.000.000 e- flow round a circuit per second to give a strength of 1 A.

For smaller currents milliamperes (mA) are used as a unit. 1000mA = 1A

Current directionMost circuit diagrams, like the one below, have an arrow indicating the direction of the current flow. The flow always goes from positive to negative round the circuit.

As electrons are negatively charged they are repelled by the negatively charged side of the cell and go therefore in the opposite direction as the current flow.

The potential differece (PD) or voltage

The scientific name for voltage is potential difference (PD)

The higher the voltage the more energy a cell gives to the electrons it pushes out.

The PD is measured in volt (V).

If the PD accross a cell is 1 volt, then 1 joule of potential energy are given to each coulomb of charge. So 1 volt = 1 joule per coulomb 1V=(j/C)

To produce a higher PD, cells can be put in series.

Cells in parallel and in series

When batteries are connected in series you have to sum up their voltage.

The voltage tells you how much energy is given per Coulomb.

So if you connect batteries in series, the voltage given is the sum of all.

Copy this table

Resistance

The resistance of a material indicates how well it is able to prevent the electrons from flowing through it.

The resistance is calculated with the following equation:

Resistance ( ) = ΩCurrent through conductor (A)

PD accros the conductor (V)

So the lower the resistance, the less PD is needed to give the same current

For example if a PD of 6 V is needed to make a current of 3A flow through a wire the resistance would be: 6V/3A= 2Ω

1 kilohm (kΩ) = 1000 Ω1 megaohm (MΩ) = 1.000.000 Ω

Ohm’s law1. Calculate the resistance of a conductor

through which a current of 2 A flows with a PD of 12V.

2. Calculate the PD between the extremes of a conductor with a resistance of 10 Ω and a current of 7.5 A.

3. Calculate the intensity of a current which circulates through a conductor with a resistance of 10 Ω, if the PD between its extremes 0.02 mV.

Series and Parallel circuits

Bulbs in series·The bulbs share the PD (V) from the battery, bulbs glow dimly·If one bulb would be removed, the would go out because the circuit is broken.

Bulbs in parallel·Each gets the full PD from the battery because they are both connected directly to it.·If one bulb would be removed teh other would keep on working becasue the circuit isn't broken

Circuit in series

Intensity/Current (A)IT = I1 = I2 = I3 etc

PD (V)VT = V1 + V2 + V3 etc

Resistance (Ω)RT= R1 + R2 + R3 etc

Circuits in parallel

Intensity/Current (A)IT = I1 + I2 + I3 etc

PD (V)VT = V1 = V2 = V3 etc

Resistance (Ω) 1 = 1 + 1 + 1

etc RT R1 R2 R3

For

exam

ple

9)a) The combined resistance.b) The current which flows through the circuit.c) The voltage in each resistance.

10. Given the following circuit, calculate:a) The combined resistance of the circuit.b) The current that circulated through the circuit.

Power

Power = energy transformed time taken

W= Joules/SecondsFor electrical energy we use the unit kwh (Kilowatt per hour)

1 kw= 1000 watts

1kwh= 1Kw per h

or kwh= kw h

Electrical Power

Power = PD x current

Watt = Volt x Ampere

or

P= VI

So for a battery:Power = 12V x 2A = 24W

For bulb A: 8V x 2A = 16W

For bulb B: 4V x 2A = 8W

4.- Calculate the energy consumed by an iron of 1000 W if it is switched on during 30 minutes. Sol. 0.5 kwh

5.- We have a light bulb of 75 W and 220 V. Calculate a) the current that passes through the bulb; b) the resistance of the bulb; c) the energy consumed during 30 days if it will be switched on for 4 hours a day. Sol. a)

1. Calculate the energy consumed by an iron of 1000 W if it is plugged to a 220 V socket during 30 minutes.

2. We have a light bulb of 75 W and 220 V. Calculate a) the current that passes through the bulb; b) the resistance of the bulb; c) the energy consumed during 30 days if it will be switched on for 4 hours a day.

3. Calculate how much energy it will cost to dry your hair every day if you use a hair dryer of 1100 W during 20 minutes each. The Price per KWh is €0.15.

4. Calculate the amount of energy consumed and its price per month if you use a 350 W television during 3 hours each day. The Price per KWh is €0.15.

Circuits 1

Circuits 2

Resistor

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Adjuntos

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