Voltage and resistance in a circuit

Level 1 Science

• Voltage is the amount of energy carried by the charged electrons (current) in the circuit.

• Thus, the voltmeter measures the change in electron’s energy between entering and leaving a component so it has to be connected in parallel to that component.

• An electrons voltage decreases as it goes through a component.

• The unit of voltage is the volt and the symbol is V

• Alessandro Volta

• Power packs and batteries give electrons energy.

• This energy is all used up getting through the circuit until the electron gets back to the power pack or battery where it gets re-energised (or recharged).

For example:•an electron comes out of a power pack with 12 volts of energy.

•It might use 11 volts getting through a light bulb and 1 volt getting through the wires.

•The electron gets back to the power pack with no energy.

•The electron then gets 12 more volts of energy which it uses up as it goes around the circuit again.

Energy Conversion

• An electron goes into a circuit component with some energy and comes out of it with less.

• Voltage measures how much energy an electron loses as it goes through a circuit component.

• Circuit components are energy converters - they might convert

electrical energy to :light if the component is a bulb,

heat (heaters),sound (speakers),

movement (motors),magnetism (solenoids).

Connecting voltmeters• Voltmeters are also connected

across components red terminal to red terminal in parallel. (Touch test)

Energy of electrons• When electrons leave their

power source, they have a certain amount of energy. They lose all their energy as they pass through components and they must return to the power source with 0 Volts.

SERIES• Voltages

across each component ADDS to total voltage

• In a series circuit the lamps are dimmer because they have less power ( ½ I and ½ V).

PARALLEL• The

voltage in any loop in a circuit will = 0

With identical components parallel paths will have the same voltage.

What goes in must come out.

12V given to each electron

12V removed from each electron

In a parallel circuit the lamps are bright because they each get the full energy of the electrons passing through them.

Resistance is a property of a substance that limits the flow of electrons through it.

Conductors have a very low resistance.

Insulators have a very high resistance.

• The bigger the resistance the lower current.

Resistance enables a component

to convert electrical energy to

another form of energy.

ie - heating coils convert electrical

E to heat E

- lamp filaments convert electrical

E to heat E and then to light E.

- electrical motors convert elec E to magnetic E to get kinetic E.

Resistance can be an advantage if we wish to create heat (as in a toaster)or it can be a disadvantage if we don’t want to create heat (as in an electric motor).

Limiting heat loss in a circuit:

- a short length of wire has less resistance than a long length of wire.

- a good conductor has less resistance than a poor conductor.

-a low current creates less resistance than a high current.

Resistance is calculated by:

R = V

I

Where R = resistance measured in ohms (),

V = voltage measured in volts (V),

I = current measured in amps (A).

Temperature and Resistance

When the temperature of a conductor is kept constant, its’ resistance stays constant = Ohms Law

As the temperature of a conductor increases, so does its resistance.

Power

Is a measure of how quickly electrical energy is converted into other forms of energy.

Power is measured in watts (W).

1W is equal to 1 joule per second.

Power Calculations

Power = Voltage X Current

P = V I

Where P is measured in watts (W),

V is measured in volts, and

I is measured in amps

Electricity at Home

Mains power is 240V AC.

AC means the current changes direction 50 times per second and is said to be 50 Hertz (50 Hz).

Fuses are used to protect people and property. They melt when the current gets too high.

3 Pin Plugs

Neutral

Blue

Phase

Red

Earth

Yellow and Green

Current flows from phase to neutral.

The Earth wire is a safety wire to protect people from any short circuit.