Electricity

Hazards associated with electricity

Damaged insulation, cables overheating, damp conditions

Explain how damaged insulation is a hazard

Contact with the wire due to gaps in the insulation can cause an electric shock or pose a fire hazard by creating a short circuit

Explain how cables overheating is a hazard

Too high currents through too small wires cause wires to heat up to very high temperatures which could melt the insulation exposing live wires and cause a fire

Explain how damp conditions is a hazard

If moisture comes into contact with live wires, it could conduct electricity which could create a short circuit or electric shock

When is double insulation used

When there is no earth connection. The earth wire is not needed as you cannot get a shock from the plastic

What is double insulation

Plastic casing rather than metal case completely covering electrical components or designed so that live wire cannot touch metal casing

Why is an earth wire not needed for a double-insulated device

You can’t get a shock from the plastic

What is normal insulation

Cover wire w insulating material eg rubber

What colour is the earth wire

Green and yellow

What does the earth wire do when the appliance is working correctly

Doesn’t carry current

What happens to the earth wire when a wire touches the metal case

Current surges through low resistance path of earth wire + hence fuse

What happens when current flows through the earth wire

Surge of current also thru fuse; fuse melts; current flows to ground

What happens when the fuse breaks/melts

Cuts off electricity to appliance

Why is there a surge of current in the earth wire when a wire touches the metal casing

Earth wire has very low resistance

What is a fuse

Thin piece of wire which overheats and melts if current is too high

What are the most common current ratings

3, 5, 15A

What must you do when choosing current rating

Pick slightly higher than current used by device

What happens if the current rating is too low

Fuse will break circuit even when acceptable current flows thru

What happens if circuit rating is too high

Won’t break circuit in enough time before damage occurs

What is a circuit breaker

Automatic electromagnet switch

How does a circuit breaker protect circuit

Breaks circuit if current rises over certain value

Advantage of circuit breakers over fuses

Can be reset and reused; operate faster than fuses

Why does current in a resistor result in electrical transfer of energy

Work done when charge flows through a circuit; work done = e transferred => electrical transfer of energy

How does current passing through a resistor/wire result in an increase in temperature?

Collisions of free electrons within wire; transfer Ek to ions of resistor; some energy dissipated to surroundings by heating

Where can electrical transfer of energy + inc temperature be used in domestic contexts

Electric heaters, toasters, kettles

What happens when a current passes through a resistor / wire?

Electrical transfer of energy and increase in temperature

What is the relationship between power, current and voltage?

P = IV

Where might you rearrange equation to be I = P/V?

Selecting an appropriate fuse by calculating the current the device uses

What is the relationship between energy transferred, current, voltage and time?

E = VIt

What is the relationship between power, energy transferred and time

E = Pt

What kind of current is mains electricity?

A.c.

What kind of current is supplied by a cell or battery

D.c.

What is d.c. current

Current only flows in one direction

Which direction does current flow in a dc circuit

From positive to negative terminal

What is alternating current

Current continuously changes direction around circuit

How come alternating current still always flows from the positive to negative terminal

AC power supplies have 2 identical terminals which change between +ve and -ve

What is frequency of UK mains electricity

50Hz

What voltage/pd is uk mains electricity

230V

What is frequency of an a.c. Power supply

Number of times current changes direction back/forth each second

What is the relationship between voltage, current and resistance?

V = IR

What is current?

Rate of flow of charge (at a point in the circuit)

What is the relationship between charge, current and time?

Q = It

What is electric current in solid metallic conductors?

Flow of negatively charged electrons

Why is current conserved at a junction in a circuit?

Charge is always conserved; total number of e- must stay same; sum of current in branches = total before

What is conventional current

Rate of flow of positive charge

What direction is conventional current

Opposite direction to flow of electrons

Will current in each branch out of a junction always be identical?

No, only if resistance along each branch is identical

What is current measured with

Ammeter in series with component

What is voltage

Energy transferred per unit charge

What is the relationship between energy transferred, charge and voltage

E = QV

What is voltage measured with

Voltmeter in parallel across component

Why is a parallel circuit more appropriate for connecting lamps

If one breaks, current can still pass thru others

Why is a series circuit more appropriate for domestic lighting

Components all controlled by one switch; fewer wires required

Why might parallel circuits be a bad choice for some situations

More wires = more complicated; harder to control V across individual components

Why is it harder to control V across individual components in a parallel circuit?

All branches have same voltage as supply

Current in a series circuit

Same at all points + components

Current in parallel circuit

Shared between branches (sum in branches = supply)

Voltage in series circuit

Shared across each component (sum of pd = supply)

Which components in a series circuit might have greater pd across them

Components with higher resistance

Resistance in a series circuit

Sum of resistance of each component

Total resistance in parallel circuit

Total resistance < that of each component

What does current in a series circuit depend on

Applied voltage + number/nature of other components

If you have higher voltage in a series circuit what happens to current?

Current increases if number of components stays same

If you have more components in a series circuit what happens to current?

Current decreases if voltage stays same

The voltage across two components connected in parallel is

The same

How does current vary with voltage in a wire/fixed resistor

Straight line through origin

Relationship between i and V for a wire/fixed resistor is

Directly proportional

IV graph for a filament bulb

As current increases, resistance increases

Why does resistance increase as current through a filament lamp increase

Higher current = hotter filament = ions vibrate more = more difficult for electrons to pass through = current increases at a slower rate

IV graph for a diode

Only allows current in one direction

Why doesn’t diode let current flow in the other direction

Has very high resistance in the other direction

What do you need for an I-V graph practical

Ammeter, voltmeter, variable resistor, power source, wires, component

What is the independent variable in IV graph practical

Current - changed by variable resistor

What is the voltmeter for in the IV graph practical

Measure voltage (dependent variable)

What could you use to indicate the presence of current in a circuit

Lamps, LEDs

If you increased resistance what would happen to current in a circuit

I = V/R so current would decrease

How would you calculate the total resistance of resistors in a series circuit

Add up individual resistors

How would you calculate the total voltage across a series circuit of resistors

Add up voltages of individual resistors

How do thermistors work

Low temp = high resistance

Would a thermistor be high or low resistance when it’s hot?

Low

How does a light-dependent resistor work?

More light = less resistance

If it was dark, would there be high or low resistance through the LDR?

High