4.2 Electricity

Completed flashcard set

Draw the standard circuit symbols for:

• Switch (open)

• Switch (closed)

• Cell

• Battery

• Diode

• Resistor

• Variable resistor

• LED

• Lamp

• Fuse

• Voltmeter

• Ammeter

• Thermistor

• LDR

What are the conditions for electrical charge to flow through a circuit

• The circuit must be closed

• The circuit must have a source of potential difference

What is electric current?

A flow of electric charge

What is meant by the size of the current?

Rate of flow of electrical charge

Give the formula for the size of current

Size of current = rate of flow of electrical charge

charge flow = current x time

Q = I t

therefore, current = charge flow / time

I = Q/t

I - amperes, A

Q - coulombs, C

t - seconds, s

In a single closed loop, does the current have different values at different points?

No, in a single closed loop the current has the same value at any point

What is a coulomb?

A coulomb is a unit of electrical charge (measure of the amount of charge flowing through the circuit)

What do amperes (current) represent?

Amps represent the rate of electric charge passing through a point in a circuit

What does the current through a component depend on?

Current (I) is the rate of flow of charge passing through a point in a circuit; it depends on the potential difference (V) across the component and the resistance (R) of the component

• The greater the resistance of the component, the smaller the current for a given potential difference across the component

What is potential difference?

Difference in the electric potential energy between two points; it is what makes charges move

• It is the energy transferred per unit of charge as it moves between two points in a circuit

• For cells/ batteries: the amount of energy transferred to the charges in the circuit

• For components: amount of energy transferred to the components from the charges in the circuit

Give the formula that links current, potential difference and voltage

potential difference = current x resistance

V = I R

V - volts, V

I - amperes, A

R - ohms, Ω

How is voltage measured? How is it connected and why?

Using a voltmeter

• The voltmeter is connected in parallel to measure the difference in energy before the component and after the component

• Measured in volts, V

How is the current flowing through a component measured? How is it connected and why?

Using an ammeter

• An ammeter must be connected in series (as it measures the rate of flow of electric charge passing through a point in a circuit)

• Measured in amperes, A

What does resistance do in a circuit?

It slows down the rate of flow of electric charge

How is resistance of a component measured?

Resistance is measured by calculating the voltage across the component and the current flowing through the component and using the formula V = I R to calculate voltage

• Measured in ohms, Ω

Explain the two types of resistors. Give their circuit symbols

• Fixed resistors

  • Have a fixed/ constant resistance; it cannot be changed

• Variable resistors

  • Resistance can change as the current changes

What type of resistors are components in a circuit (eg. lamps) and why?

Variable resistors

• As their resistance will change depending on the current flowing through them

What is Ohm’s law?

What is the condition needed for this law to be true?

Ohm’s law states that the current through a component is directly proportional to the potential difference across the component as long as resistance is constant

• Temperature must be constant because changes in temperature can lead to changes in resistance, and therefore there would no longer be a constant resistance

What is an Ohmic conductor?

A material that obeys Ohm’s law; the current flowing through it is directly proportional to the potential difference across it, provided that temperature remains constant.

This means that the resistance remains constant as the current changes

Draw the current potential difference graph for an Ohmic conductor

Draw and explain the current potential difference graph for a filament lamp

• When an electrical charge flows through a filament lamp, it transfers some energy to the thermal energy store of the filament (which is designed to heat up)

• As the current flowing through the lamp increases, the temperature of the filament also increases, which causes the resistance of the lamp to increase

  • This is because high temperatures cause the metal atoms to vibrate more, which increases the frequency of collisions between the metal atoms and the charge carriers (electrons) flowing through, which slows the rate of flow of charge, increasing resistance

• This means that less current can flow per unit potential difference, so the graph gets shallower, hence the curve

Draw and explain the current potential difference graph for a diode

• The current through a diode only flows in one direction; it has a very high resistance in the reverse direction

What is a thermistor? Give its uses and its circuit symbol

A thermistor is a resistor that’s resistance is dependent on temperature

• When temperature increases, resistance decreases

• When temperature decreases, resistance increases

Uses:

• Digital thermostats

• Car engine temperature sensors

• Replacement fuses

What is a LDR? Give its uses and its circuit symbol

A Light Dependent Resistor is a resistor that’s resistance is dependent on the intensity of light

• When light intensity increases, resistance decreases

• When light intensity decreases, resistance increases

Uses:

• Automatic night lights

• Outdoor lighting

• Burglar detector (When light falls on the sensor, the resistance will decrease and can allow a signal or alarm to be triggered)

State the two ways of joining electrical circuits

• Series

• Parallel

What is a disadvantage of series circuits compared to parallel circuits?

• If one component breaks or is removed, the entire circuit is broken and electric charge will no longer flow

• Whereas for parallel circuits, it will only affect that branch (and the branches that it leads to) as there are multiple routes for charge to flow

Describe the current, resistance and potential difference of components connected in series

For components connected in series:

• there is the same current through each component (same rate of flow of charge)

• the total potential difference of the power supply is shared between the components

• the total resistance of a circuit is equal to the sum of the resistances of the individual components

Current is the same everywhere

Potential difference is shared

Resistance adds up

Describe the current, resistance and potential difference of components connected in parallel

For components connected in parallel:

• the total current through the whole circuit is the sum of the currents through the separate components

• the potential difference across each component is the same

• the total resistance of two resistors is less than the resistance of the smallest individual resistor

Current is shared between branches

Potential difference is the same across all branches

Adding a resistor in parallel reduces the total resistance

What happens if you add a resistor in parallel? Why?

Adding a resistor in parallel reduces the overall resistance

• This is because the potential difference across each branch is the same; the more resistors added in parallel, the more pathways the current has to flow, so it is easier for current to flow through the circuit

What happens if you add a resistor in series? Why?

Adding a resistor in series increases the overall resistance of the circuit

• When resistors are added in series at a constant potential difference, it causes the current flowing through the resistor to decrease. As in a series circuit the current is the same at all points in a circuit, it causes the overall current of the circuit to decrease

Give a summary comparison of series vs parallel circuits

Describe the key features of the UK’s mains electricity

• Alternating current supply

• Frequency of 50Hz

• 230V

Describe the two types of electricity supplies

• Alternating current (ac), the current is constantly changing direction. Alternating currents are produced by alternating voltages in which the positive and negative ends keep alternating

• Direct current (dc), the current is always flowing in the same direction; created by direct voltage

Give an example of an alternating current supply and a direct current supply

• Ac: mains electricity

• Dc: cells and batteries (as the positive and negative ends cannot be switched)

How are most electrical appliances connected to the mains?

Using a three core cable; this means that they have three wires inside of them, each with a core of copper and a coloured plastic coating

Give the names of the three types of wire in a three-core cable and give their colours and potential differences

• Live: brown (think living worms, in mud), 230V

• Earth: green and yellow stripes (trees), 0V

• Neutral: blue (water is neutral), 0V

Give the purposes of the three types of wire in a three-core cable

Live wire - brown, 230V

• The live wire carries the alternating potential difference from the mains supply; electricity flows in through this wire

Neutral wire - blue, 0V:

• The neutral wire completes the circuit; electricity flows out back to the mains through this wire

Earth wire - green and yellow stripes, 0V:

• The earth wire is a safety wire to stop the appliance becoming live by providing a low resistance pathway to the ground; it only carries a current if there is a fault

Why is the live wire dangerous?

• The live wire has an alternating potential difference of 230V

• The human body has a potential difference of 0V

• This means that if a human touches the live wire, a large potential difference will be produced, causing a current to flow through the body, causing a large electric shock, which could kill or injure

Why is the live wire dangerous even when a switch in the mains circuit is open?

• If a switch is open it means that a current is not flowing

• However, the live wire still has an alternating potential difference of 230V

• If you made contact with the live wire, your body would provide a link between the supply and the Earth, allowing the current to flow through your body and into the Earth

Why are there dangers of providing any connection between the live wire and Earth?

• If the link creates a low resistance path to earth, a huge current will flow

• This can cause large electric shocks (if touched by a human), or even fires

Explain how the the energy transferred by a device is related to the potential difference across it and the current through it

• When an electric charge goes through a change in potential difference, energy is transferred

• Energy is supplied to the charge at the power source to raise it’s potential energy

• This charge transfers energy to components in the circuit, losing its potential energy

Therefore, the energy transferred by a device is equal to its potential difference (energy transferred by unit of charge) multiplied by current (rate of charge flow)

Explain how the power transfer in any circuit device is related to the potential difference across it and the current through it

Power = potential difference x current

P = V I

power = (current)² x resistance

P = I² R

P - watts, W

V - volts, V

I - amperes, A

R - ohms, Ω

Power = energy transferred / time

Potential difference = energy transferred per unit of charge

Current = charge flow / time

What does the amount of energy a device transfers depend on?

• How long the appliance is switched on for

• The power of the appliance

Describe how a kettle uses energy from the mains

• Kettles transfer energy electrically from the mains ac supply

• converting electrical energy

• to the thermal energy of the heating element inside of the kettle

Describe how a handheld fan uses energy from batteries

• Energy is transferred electrically from the chemical energy store of the battery of a handheld fan

• converting electrical energy

• to the kinetic energy of the fan’s motor

When is work done in a circuit?

Work is done when charge flows through a circuit

Give the two equations to calculate the amount of energy transferred by electrical work

energy transferred = power x time

E = P t

energy transferred = charge flow x potential difference

E = Q V

E - joules, J

P - watts, W

t - seconds, s

Q - coulombs, C

V - volts, v

What does the power rating of a device represent?

• The maximum value that the device can be used at safely

• It tells you the maximum amount of energy transferred between stores per second when the appliance is in use

Define the national grid

The national grid is a system of cables and transformers linking power stations to consumers

• Electrical power is transferred from power stations to consumers on the national grid

What do transformers do?

Transformers change the voltage of the circuit using electromagnetic induction

• Step up transformers are used to increase the potential difference from the power station to transmission cables

• Step down transformers are used to decrease the potential difference to a much lower value for domestic use

Explain why the national grid is an efficient way to transfer energy

• When transferring such large amounts of energy across the country, there is a huge risk of energy waste because power is so high

• As soon as electricity leaves the power station, a step-up transformer increases the voltage

• As resistance is constant, this decreases the current

• When the current is low, this reduces energy losses via heat (as using high currents transfers energy to the thermal energy stores of the cables, which transfers the energy to the surroundings, wasting the energy)

• When the electricity reaches its place of use, a step down transformer decreases the voltage and thus increases the current as resistance is constant

• The electricity can now be used for domestic use; the current is high enough to give a good energy supply

How may materials become electrically charged?

• When certain insulating materials are rubbed together, negatively charged electrons are rubbed off of one material and onto the other

• The material that gains electrons becomes negatively charged

• The material that loses electrons is left with an equal positive charge

What is static electricity?

The build up of an electrical charge on the surface of an object

What happens when two electrically charged objects are brought close to each other

• They exert a force on each other

These forces can attract or repel and cause movement

What does a charged object create?

A charged object creates an electric field around itself; the electric field is strongest closest to the object and weakest further away from the object

What happens if a charged object is placed in the electric field of another charged object?

It will experience a force

Define electric field

• The area around a charged object where other charged objects will experience an electrostatic force

Draw the electric field pattern for an isolated charged sphere

Electric field lines always go from positive to negative (the movement of an electric charge)

Describe how sparking may occur due to electric fields

• Sparks are caused when there is a high enough potential difference between a charged object and the earth/ an earthed object

• A high potential difference creates a strong electric field between the charged object and the earthed object

• When the two objects are brought closer, the strength of the electric field becomes stronger

• When the electric field strength becomes high enough, it can cause electrons in air particles to be removed through the process of ionisation

• Air is normally an insulator, but when it is ionised it is much more conductive, allowing a current to flow through it/ electrons to jump across the gap between the charged object and the earthed object

• This leads to the production of a spark

• Torch with an LED

• When the batteries were put back in, they were put in the wrong way around

• Why did the torch not work?

• When the direction of the batteries are switched, the direction of the current is switched

• A diode has a very high resistance in the reverse direction

• So current will not flow through a diode in the reverse direction

Explain why adding resistors in parallel decreases the total resistance

• Provides multiple pathways for charge to flow

• Total current is the sum of the current in all the branches

• Therefore, current increases (for the same potential difference when more resistors are added)