3 Domestic uses and safety + 4 Energy transfers + Static electricity

Earth wire

Green and yellow striped safety wire to stop the appliance from becoming live. This prevents electric shocks from occurring by provides a path for current to flow from the case of the device to the ground if there is a fault (at 0V unless there is a fault)

Neutral wire

Blue (left) wire at 0V that completes the circuit and carries the current away from the device

Live wire

Brown (right) wire that carries the current from the supply to the device at 230V. It is the most dangerous of the three wires

material of pin

brass - because it is hard and a good electrical conductor

material of outer case of plug

plastic - because it is a good electrical insulator

purpose of a fuse

If the current is above the value of the fuse, it will melt and break the circuit for safety

Mains electricity in the UK

is an alternating current (a.c.) supply
frequency of 50 Hz
potential difference of 230 V

alternating current

is constantly changing its direction of flow.
It is produced by an alternating voltage, where the positive and negative ends are always alternating.
varying potential difference
from a socket

direct current

current flows in a fixed direction
potential difference is constant
is produced by a direct voltage eg battery

Why is the live wire dangerous?

Since out bodies are at 0V touching a Live wire causes an electric shock due to the potential difference between the wire and your body, which causes a current to flow through you.

Connections between the Live wire and the Earth are dangerous due to

this would make a complete circuit from your mains supply to the ground

Energy

E measured in Joules (J)

Power

P measured in Watts (W)
Rate of energy transfer
P \= I ² R
P \= I V
E \= P t

Time

t measured in seconds (s)

Current

I measured in amps (A)
The rate of flow of charge
V \= I R

Potential difference

V measured in volts (V)

Resistance

R measured in ohms ()
How hard it is for current to flow

Charge

Q measured in coulombs (C)
Can only flow if there is a source of Potential Difference
E = Q V
Q = I t

P \= I x V

Power \= current x potential difference

P \= I^2 x R

Power \= current^2 x resistance

E \= P x t

Energy transferred \= power x time

E \= Q x V

Energy transferred \= charge x potential difference

A power rating of 2000 means

the device transfers 2,000 J of energy per second from one store to another.

Sankey diagram

A diagram that represents energy transfers by arrows. The width of each arrow depends on the amount of energy it represents.

Work is done when a..

Charge flows in a circuit

Insulators do not conduct electricity because

Their electrons cannot flow throughout the material, they are fixed

Conductors can conduct electricity because

Their electrons can flow, and are not fixed (they are delocalised)

Like charges \___

repel

Opposite charges \___

Attract

What is the national grid made up of?

Pylons and wires, step up and step down transformers

What does the national grid do?

Transfers electrical power from power stations to consumers (homes, industry etc)

step-up transformer

increases voltage and decreases current
located between a power stations and power lines

step-down transformer

decreases voltage and increases current
located between power lines and houses

Why do step up transformers increase voltage (p.d)?

increase the voltage to minimise energy loss. P \= I x V so increasing the voltage decreases the current. And the current heats the wire so therefore by lowering it less energy is lost and it is more efficient

what is the purpose of step down transformers?

they lower the pd so it is safe for domestic uses

electric field lines show the direction a

positive charge would go if it was placed in the field

When the plastic strip is hung over the wooden rod, the two halves of the strip move equally away from each other. What two conclusions should the student make about the forces acting on the two halves of the plastic strip?

- the forces are equal
- the forces act in opposite directions

Sparking is caused when..

there is a high enough potential difference between a charged object and an earthed object. The High pd causes a strong electric field which causes a large force. Electrons are removed from air particles and air becomes ionised which means a charge can flow through

all materials contain

charge because they contain positive protons and negative electrons. Normally though these charges cancel each other out which means that most materials are neutral overall

What is static charge?

A charge that can't move

How do insulating materials become charged?

rubbing two insulating materials together the friction causes negatively charged electrons to agin energy move from one to the other, giving both materials an equal but opposite charge.

gaining electrons

negative charge

loosing electrons

positive charge

Explain how rubbing an acetate rod with a cloth causes the rod and cloth to become charged

friction between cloth and rod causes electrons to move from the acetate rod to the cloth. Meaning the net charge on cloth is now negative and net charge on rod is now positive

Greater static charge leads to (spark)

a greater pd between the object and the Earth and if a build-up of static creates a high enough pd, a spark can jump between the object and a nearby earthed conducto