electricity

meow

What is insulation and why is it used?

What is insulation and why is it used?

- Wires or device surrounded with insulating material (e.g. plastic)

- User cannot touch live wire → no electric shock

What is double insulation and why is it used?

- Both the wires inside a device and the outer case of the device are insulated

- No chance of electrocution

- Necessary when earth wire not present

What are the various wires in a plug?

- Green and yellow - earth

- Brown - live

- Blue - neutral

Describe how an earth wire acts as a safety feature

- Earth connects to metal casing

- Provides low resistance path for current to earth if casing becomes live

- Large current in earth wire melts fuse

- Circuit breaks and turns off

Why do metal objects connected to mains electricity require earth wires?

- Metal cases conducts electricity

- Earth wires prevent user getting electric shock

What is a fuse?

- A wire which melts if the current is too high

- Connected to live wire

- Circuit breaks and turns off if there is a fault

- Must be replaced after fuse has melted

How should the value of a fuse by chosen?

Fuse value should only be a little bigger than the current flowing through

What is a circuit breaker?

- A device which stops the current flowing in a circuit when the current is too high

- Circuit cannot overheat if there is a fault

- Can be reset

State the advantages of using a circuit breaker instead of a fuse

- Resettable (fuses must be replaced)

- Work instantly (fuses do not)

- Don’t require earth wire

- More sensitive

Why are fraying cables, long cables and water dangerous when using mains electricity in the home?

- Fraying cables: potential electrocution risk

- Long cables: tripping hazard

- Water (and metal objects): conduct electricity and could electrocute you

What is an electric current?

- A flow of charge

- In a metal wire, it is the flow of negatively charged electrons

Why does a current in a resistor result in an increase in temperature?

- Resistance causes transfer of electrical energy to heat energy

- Used in devices with heating elements, e.g. toasters, electric heaters

Define electrical power

- The transfer of electrical energy per second

What are the units of power?

- Watts (W)

- Or Joules per second (J/s)

What is the equation linking power, current and voltage?

- Power = current x voltage

- P = I x V

What is the equation linking energy, power and time?

- Energy = power x time

- E = P x t

What are the units of energy?

Joules

Give the equation that links energy, current, voltage and time

- Energy transferred = current x voltage x time

- E = I x V x t

- As E = P x t, and P = I x V, we can substitute I x V into the equation for energy transferred

Define alternating current

- A current which changes direction continuously

- e.g. mains electricity

Define direct current

- A current which flows in one direction only

- e.g. from a battery or cell

Which way do electrons flow in a circuit?

- From negative terminal to positive terminal

Define electric current

- Rate of flow of electrons carrying a negative charge

- Unit = amp (A)

Why can current flow through metals?

- Metals have delocalised electrons

- Can move freely throughout metal

Why can current not flow through plastics?

- No delocalised electrons

- Charge cannot be carried

- Plastics are insulators

Give the equation linking charge, current and time

- Charge = current x time

- Q = I x t

- Unit for charge = Coulomb (C)

Which device is used to measure the size of a current?

- Ammeter

How are ammeters connected in a circuit?

- In series

Define voltage (also known as potential difference)

- Amount of energy transferred per coulomb of charge passed

- Unit = volt (V)

- 1 volt = 1 joule per coulomb

Give the equation linking voltage, current and resistance

- Voltage = current x resistance

- V = I x R

Which device is used to measure voltage?

- Voltmeter

How are voltmeters connected in a circuit?

- In parallel

- Around component being measured

Give the equation linking energy transferred, charge and voltage

- Energy transferred = charge x voltage

- E = Q x V

- Unit for energy = joules

Define resistance

- Ratio of voltage and current

- Unit = ohm (Ω)

Describe a series circuit

- All components connected in a single loop

- Only one path for current to follow

Describe a parallel circuit

- Circuit has branches

- Multiple paths for current to follow

Give the advantage of connecting light bulbs in series

• One switch can turn all bulbs off/on

• Uses fewer wires

Give the disadvantages of connecting light bulbs in series

• If one bulb breaks, the circuit breaks. All bulbs stop working

• Voltage shared between all bulbs. More bulbs = less bright

Give an example of when a series circuit should be used

- Decorative lights (e.g. fairy lights)

- Each bulb requires low voltage

- Can all be controlled by one switch

Explain why lights in a house are connected in parallel

- Lights can be switched on and off separately

- Brightness of each light does not change if another is switched on

- All receive mains voltage (230V)

- If bulb breaks/is removed, other lights keep working

Why are lamps and LEDs frequently incorporated into circuits?

- Indicate presence of a current in the circuit

- Lamp/LED on = current flowing

What are the current and voltage rules in a series circuit?

- Current is the same everywhere

- If additional component added, current reduces through all components

- Total voltage is the sum of all the individual components’ voltage

- If voltage doubled, current also doubles

What are the current and voltage rules in a parallel circuit?

- Voltage is the same everywhere

- Total current is the sum of the individual components’ current

Why is current conserved at a junction?

- In parallel circuit, current is shared between each component

- Total number of electrons flowing into the junction must equal total number of electrons leaving

- Current entering junction therefore equals current leaving junction

How is the total resistance in a series circuit found?

- Sum of the individual resistors

Why does resistance increase in a filament when the temperature increases?

- Positive ions vibrate faster

- Inhibits flow of electrons

What is Ohm’s law?

- The current through a wire or resistor at constant temperature is directly proportional to the potential difference across the wire or resistor

What do current/voltage graphs show?

- Gradient = 1 / resistance

- Steeper gradient → smaller the resistance

What is a thermistor?

- A temperature dependent resistor

- Resistance decreases when temperature increases

- Therefore current increases when temperature increases

Explain what happens to the voltage of a neighbouring component in series when a thermistor is cooled

- Resistance of thermistor increases

- Current through circuit decreases

- Voltage decreases

What can thermistors be used for?

- Thermostats

- Keep engines at optimum temperatures

What is an LDR?

- Light dependent resistor

- Its resistance decreases when light intensity increases

What can LDRs be used for?

- Automatically switching on lights when it gets dark

What is a diode?

- A device which allows current to flow in one direction only

Explain how to calculate the resistance of a component in a circuit

- Connect ammeter in series

- Measure current

- Connect voltmeter in parallel with component

- Divide voltage reading by current reading

What is the difference between a standard resistor and variable resistor?

- Standard resistors have a fixed resistance

- Variable resistors can have their resistance changed

Describe how to investigate how the resistance of a piece of wire changes with voltage across the wire

- Connect ammeter in series with wire and take reading

- Connect voltmeter in parallel with wire and take reading

- Substitute readings into R = V/I

- Include variable resistor in circuit

- Repeat and take average

- Repeat for different voltages

- Control variables = length of wire, type of metal, temperature of wire

Explain why the temperature of the wire should be kept constant in the above investigation

- Resistance increases with temperature

- Wire could get hot and melt

- Control temperature by taking readings quickly and switching off between readings