Physics Unit 1 AOS 3 😁

what is AC

alternating current, electrons oscillate backwards (-) and forwards (+) in a circuit.

what is dc

direct current. electrons travel directly from one terminal of the battery to the other in one direction only. batteries provide DC

rms

root mean squared. a maths term, it averages the sine wave. Because households are supplied with AC voltage, we have to be careful about labelling the voltage: 240rms means this AC voltage has the same effect as 240V voltage

delete this shit

is a measure of frequency. this means it oscillates 50 times every second

• Household energy is AC (-340V to +340V, which gives an equivalent amount, root-mean-square, of potential as 240V at 50 Hz)

Why do powerpoints have 3 pins?

brown/red for active, blue or black for neutral, green/yellow for earth

How do we connect to mains power? Recall the different sockets

a 3 point socket. The top left point is active, right is neutral, and the bottom one is the earth connection. The switch is connected in the active wire. Pins and circuits correspond to each other

How does a circuit work if it doesn’t have a + and – end?

when an appliance is connected, current flows back and forth between the active and neutral wires through the device, supplying it with energy.

What does active wire do

it is connected to 240V whose voltage oscillates periodically between +340V and -340V (giving a root-mean-square equivalent voltage of 240V) relative to a reference voltage called ‘earth’, considered to be 0V.

What does the neutral wire do?

it ****is connected to the neutral link at the switchboard (to complete the circuit), which is also connected to the *earth* via a rod driven into the ground at the switchboard. The neutral wire is always at 0 V.

Remember what unit shows up on the electricity bill?

Kilowatt hours (kWh) (convert 1 J to kWh), because this gives a convenient number, without scientific notation, that is easy to write.

Before being _to various parts of the house, the active wires pass through _that _the amount of _ _ supplied to the household

distributed, active wires, measure, electrical energy

Every domestic circuit contains?

a fuse or circuit breaker to interrupt the flow of current if it exceeds a certain value. They are safety devices that prevent wires from getting overloaded.

Why is there a limit to how much current the wires in a house or building can safely carry?

wires heat up when current passes through them

How to prevent overload of circuits?

put appliances that draw a lot of current such as ovens, hot water systems, and air-conditioners on separate circuits to lights and power points. don’t overuse power boards

What happens if too many high-current appliances, such as heaters, kettles and irons, are plugged into a power board?

it can overheat. This may cause the insulation around the wires to melt, causing a short circuit or even a fire.

What are short circuits, and why do they happen?

they occur when an electric circuit contains a near “0” resistance conductor in parallel with another household appliance or load. Current will want to take the path of least resistance (if there is near “0” resistance”) so it will completely pass through the “short circuit” and not pass any current though the other element.

Large amounts of current means?

that wires will heat up, causing insulation to melt or catch alight.

What is another way that short circuits can occur?

occur in an electrical appliance when insulation between active and neutral wires becomes damaged and they come in direct contact.

What happens if the active wire inside an appliance with a metal case becomes loose and touches the case? How can this be prevented?

the whole case becomes electrically live. If anyone touches the case, zzzzzaaaappp the current will flow through their body, with possibly fatal consequences. To prevent this, an earth wire is permanently connected to the metal case of the appliance, which is connected via the household wiring system to the earth (recall the third prong in a power point).

What happens if the active wire touches the case?

a short circuit (very large current) ****will be created and current will immediately flow directly to *earth* and should trip the fuse or circuit breaker, alerting users of the appliance to the problem.

Is Earth wire a part of the conducting circuit?

no, it’s just in place in case of fault – it provides a low resistance path to trip the circuit breaker/ blow the fuse

What is double insulation

involves using two insulating barriers to protect users. e.g. A plastic case and plastic insulation surrounding the active wire inside the appliance. Double insulated appliances do not need an earth wire, so their electric plugs have only two pins. this is indicated by a double square idk why tho

RESIDUAL CURRENT DEVICES (RCDs)

Also known as earth leakage systems, they detect any difference between the current in the active wire and the current in the neutral wire. In a property operating circuit, these two currents should be exactly the same. The most likely reason is that some current is going to earth through a fault or through a person. If this happens, the RCD is able to switch off the supply in about 20 milliseconds, hopefully preventing any serious harm.

How can an electrical fault cause a fire?

Wires get hot & melt their insulation due to…. overload, short circuit, or a faulty device.

What is overload?

parallel circuits therefore (too many appliances on a single power circuit = current adds large current in wires)

What is a short circuit?

current flows from one wire to the next with little or no resistance. Current increases rapidly.

How does a faulty device cause a fire?

it short circuits

What do the effects of electric shock depend on?

the amount of current passing through the body, the amount of it, and the path it takes through the body, and the amount of current that will flow through the body. If the current is in good contact with a 240 V source, it is well above the level that will cause death. Anything that improves the contact, like wet hands or bare feet, lowers the resistance and increases the current, potentially to life-threatening levels.

What is electric shock

the amount of current passing through the body

Since our bodies are _ _ _, _ _ passing through our bodies is quickly _into _ and can cause terrible _ and _ _.

relatively poor conductors, electrical energy, converted, heat, internal, external, burns.

Why is external current dangerous to the body?

because our bodies use electrical impulses to control vital functions, so external current can interfere

What can external current do to breathing?

cause chest muscles to contract and breathing to stop

What happens if current flows from one arm to the other?

it can pass through the chest and interfere with breathing

What happens if current passes through the heart region?

it may disrupt heart coordination and cause the heart to stop

How much current can cause fibrillation?

about 80 mA if it flows directly through the heart

What causes most electrical fatalities?

current flowing through the heart causing fibrillation

total resistance in parallel

decreases when resistors are added in a parallel circuit

series circuit

a circuit that contains more than one electrical component connected one after another in a continuous loop

current in series circuit

the current is the same in every part of the circuit

series circuit pros

easy to construct and learn from

series circuit cons

if one component breaks or is removed, the circuit won’t work as it’s no longer a closed loop

energy in vs out

energy put into a circuit equals the energy that comes out

voltage in a loop

the sum of voltage drops in a closed loop must equal the emf (voltage) from the battery

potential gain vs drop

energy gained by charges equals energy lost by charges

energy loss in series

in a series circuit, energy loss is shared across components

independent loops

each loop in a parallel circuit acts like an independent circuit with its own current

resistance and current

current flows more to branches with less resistance

total current in parallel

splits across branches and recombines afterward

voltage in parallel

the voltage is the same across all branches

parallel circuit advantage

components can operate independently of one another

Kirchhoff's Junction Rule

the total current entering a junction equals the total current leaving the junction

friction

resistance to motion

ohm

the unit used to measure resistance

resistance (R)

the measure of how hard it is for current to flow through a material, measured in ohms

electron speed

electrons move very slowly through a wire because they are so tiny

effect of resistance on current

as resistance increases, the flow of electrons (current) decreases

current

the flow of electric charge

wire length and resistance

as the length of a wire increases, resistance increases

short circuit

occurs when there is zero resistance or no load in a circuit, causing a large current

resistors

components used to control the current and voltage drop across parts of a circuit

ohmic

describes a straight-line relationship between voltage and current

ohmic conductors

used to control the amount of current in a circuit, obey Ohm’s Law

non-ohmic conductors

do not show a straight-line relationship between voltage and current

rate

measuring a quantity over time

current

the flow of electric charge from positive to negative (conventional current)

actual electron flow

from negative to positive

current direction memory aid

"Longer is positive, shorter is negative" – conventional current flows from positive to negative, electron flow is the opposite

battery

converts conventional energy into potential energy and stores a difference in electric charge

work

energy changes from one form to another (an energy conversion)

Proton to electron ratio in a neutral atom

Number of electrons = number of protons

Proton to electron ratio in ions

Number of electrons ≠ number of protons (due to loss or gain of electrons)

Electrons in negative ions

Have an excess of electrons (electrons gained)

Electrons in positive ions

Have a deficit of electrons (electrons lost)

Elementary charge (e)

Charge of one electron = -e; charge of one proton = +e

Why metals are conductors

Electrons are free to move through the lattice of positive ions, as they are weakly attracted to the nucleus

Why non-metals are insulators

Electrons are tightly bound to their nucleus and cannot move readily between atoms

Charge of object that gains electrons

Net negative charge

Charge of object that loses electrons

Net positive charge

Net charge

Sum of charges gained/lost when electrons move between objects

Ion

An atom that has gained or lost electrons

Electric current

Flow of electric charge carried by electrons (in wires) or ions (in solutions)

What is/are the requirement/s for current

Complete conducting pathway (closed loop), so electrons can flow

All circuits require

An energy source, wires, and a circuit element (so something can happen)

Electric current (definition)

Flow of electric charge

Size of electric current is equal to?

Rate(speed) of flow of charge

Measurement for current

Amperes (A)

Conventional current

Direction of current from positive to negative terminal

What is electron flow

The movement of electrons from negative to positive terminal

What do you need for current to flow in an electrical circuit?

A continuous loop from one terminal of a power supply to the other via a conductor

What happens to electrons in a current

Energy from power source pushes electrons; they repel each other, so they transfer energy without touching, like a bicycle chain

What is an electrical circuit?

a path of conductive material forming a closed loop through which charge can flow

A battery transforms {{c1::stored chemical energy}} into {{c1::electrical potential energy}}

Transforms stored chemical energy into electrical potential energy

How batteries convert energy

converts chemical energy into electrical potential energy by maintaining a voltage (difference in electric charge)

Battery terminal charges

Negative terminal = excess electrons, Positive terminal = deficit of electrons; chemical reactions maintain this difference

How batteries run out of charge

Chemical reactions are mostly used up, so there’s not enough energy to push electrons through the circuit

Potential difference

Energy gained or lost by each unit of charge between two points

Measurement of potential difference

Measured in volts (V) using a voltmeter connected in parallel

When does charge flow?

when there the potential difference between two conductive points is not zero

Voltmeter connection

Connected in parallel, with negative lead towards negative terminal and positive lead towards positive terminal