physics

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An electrical circuit is a closed loop that contains a power source such as a cell and something for the electrons to flow through, like a wire

Key terms:

Current (I)

A measure of the flow of electrons around the circuit (similar to the flow of water around a pipe)

Measured in Amperes/Amps (A)

Resistance (R)

Everything that resists or opposes the flow of electrons (similar to a partial pipe blockage)

Measured in Ohms

Potential Difference (V) aka Voltage/Volts (V)

the force driving the flow of electrons and is provided by the cell/battery

(can think of this as the pump that forces the water along the pipe

Conventional current: Current flows from a positive terminal to a negative terminal

In a cell, the longer line is positive and shorter line is negative

V=IR

potential difference (voltage) = current times resistance

Relationship:

As long as the resistance stays constant, as we increase the voltage, the current should increase proportionally, or vice versa

For wires and resistors, increasing the temperature will increase the resistance. As temperature increases, all of the ions in the metal vibrate faster, which makes it harder for electrons to pass along the wire (or in other words, the resistance increases).

diodes: devices that only allow current to flow in one direction. They have a high resistance in the reverse direction so now current can flow there.

Q=IT

I is measured in Amps

T in seconds

Charge (Q)= a measure of the to a; current that flowed within a certain period of time.

- measured in Coulombs (C)

What are the properties of series and parallel circuits?

A series circuit has all its components connected on the same line as each other. The charge flows through the circuit like water flows through pipes. The current is the same at all points of the circuit.

A parallel circuit has components connected on separate branches of wires to the energy source. The current is split between these branches.

Static electricity

- build up of charge on insulating materials

- causes sparks when discharged

- All materials contain charge because they contain positive protons and negative electrons. However, they usually cancel out meaning most materials are neutral.

If materials are rubbed together (eg hat and bag), friction causes electrons to be rubbed off one and onto the other one

On conducting materials like metals where electrons can move, electrons flow straight back so no charge builds up

On insulating materials like polythene rod and cloth, electrons can't flow back so transfer caused by friction gives positive static charge on material that lost electrons in this case cloth and the negative static charge on the one that gained them.

Which way the electrons get transferred each time depends on the specific materials involved.

Key point: Only negative electrons are being transferred. The positive static charges occur because they've lost negative charge and not because they gained positive protons.

What happens if an object keeps on gaining negative electrons?

As more and more electrons are transferred and the size of the charge increases, a potential difference will develop between charged material and the earth/any earthed object (always at 0 volts). if the difference between the charged and earthed object is large enough then electrons can jump across the gap and the jumping is what a spark is.

Charges can sometimes build up on conductors too, it's just much less common.

- example: cars build up charge because metal frame is in contact with wind which can transfer electrons. Might have experienced this if you touched car and received a small electric shock. This is a small spark from the car, discharging to the earth through your body.

Fuses and Earthing

- Always risk that something will go wrong

one of the most common problems is a sudden increase in current (surges)

-Can happen when something changes in the circuit (on and off)

-Fault in circuit or appliance

Surges can damage appliance, cause fires and electric shocks

Fuses and circuit breakers break circuit whenever current gets too high

fuse:

a thin piece of wire that's connected to live wire, if there's a surge, a huge amount of Current willl flow through the live wire and then the fuse. this causes the wire to heat up really quickly and because of its thinner, it'll melt. this breaks the circuit meaning no more current can flow through.

In real life they come in little cases with the thin wire inside

Fuses come in range of different ratings depending on what current you want to break the circuit.

Benefits: simple and cheap

downside: permanently broken after a single surge, they have to be replaced each time

Circuit breakers

Similar to fuses in that they break circuit whenever there a surge. However they don't get permanently damages, they just get tripped and turn off the circuit. They can easily be reset instead of having to be replaced

Downside: more expensive than fuses

one of the main ways a surge can cause harm

if live wire comes loose, it can touch the casing. if a person touches the casing anywhere on the appliance they get an electric shock. One way to prevent this is by EARTHING.

An earth wire provides an alternative pathway for current to flow away so that any electricity is diverted through earth wire instead of coming through us

Double insulation

The entire appliance is covered in a plastic casing, meaning there are no exposed metal parts. Plastic is an insulator/ doesn't conduct electricity we can't get an electric chock from touching it. In general, if an appliance has double insulation, it doesn't need an earth wire, it only needs a live and neutral wire.

SOME FORMULAS:

V=IR

voltage (volts/V) = current (amperes/amps/A) x resistance (ohms)

Q=IT

charge (coulombs/C) = current (amps/A) x Time (seconds/s)

The three energy formulas:

E=Pt

energy = power x time

E = VIt

energy = voltage x current x time

E= QV

energy = charge x voltage

2 power formulas:

P=IV

power=current x voltage

P=I^2 x R

power = current squared x resistance