Topic 2 - Electricity

Circuit Symbols

What is needed for electrical charge to flow in a circuit

• Circuit must be closed (no open switches)

• There must be a source of potential difference (battery/cell)

Electric current definition

Electric current is a measure of the rate of flow of charge in a circuit

What is electric charge measured in

Coulombs

What two factors does current in a circuit depend on

• Potential difference

• Resistance

What does the size of current tell us about electrical charge

• The size of current is the rate of flow of charge

• More charge passes around the circuit when a bigger current flows

Equation to calculate charge flow

charge flow = current × time

Q = It

Compare the value of the current at any point in a single closed loop

In a single, closed loop the current has the same value everywhere in the circuit

How the resistance and potential difference affect the current through a component

• The current (I) through a component depends on both the resistance (R) of the component and the potential difference (V) across the component

• The greater the resistance of the component the smaller the current for a given potential difference (pd) across the component

Equation to calculate the potential difference across a component

Potential difference = current × resistance

V = IR

What is potential difference

The product of a component’s resistance and the magnitude of current flow through it

What is resistance

A measure of the opposition to current flow

Factors affecting resistance

• Long wires have more resistance than short wires

• Hot wires have more resistance than cold wires

• Thin wires have more resistance than thick wires

Investigate the factors affecting the resistance of electrical circuits

To investigate how the length of a wire affects resistance:

1. One crocodile clip is attached to the wire at the 0cm position on the ruler. It should remain fixed at this point throughout the investigation. The second crocodile clip can be moved left and right along the test wire. The value written on the ruler at the point you attach this clip gives the length of the wire connected in the circuit

2. Now that you have your test circuit set up, you can begin your investigation. Attach the second crocodile clip to the wire and record the length of the wire between the two crocodile clips. Close the switch and record the current shown on the ammeter and the potential difference shown on the voltmeter

3. Move the second crocodile clip, close the switch and record the new length, current and potential difference. Repeat this for a number of different lengths of wire

4. Use the data you have recorded to work out the resistance for each length of wire, using R = V/I. Plot a graph of resistance against wire length and draw a line of best fit

Whats an ohmic conductor

A conductor whose current flow is directly proportional to the potential difference across it, when held at a constant temperature.

Describe how the current through an ohmic conductor is related to the potential difference across the resistor

The current through an ohmic conductor is directly proportional to potential difference, so you get a straight line I-V characteristic

Describe what happens to the resistance of an ohmic conductor as the current through it changes

If the resistance of components such as lamps, diodes, thermistors, and LDRs is not constant, it changes with the current through the component

What is a filament Lamp

A light emitting component consisting of an enclosed metal filament. Its resistance increases as the filament’s temperature increases.

Describe how the current through a filament lamp is related to the potential difference across the resistor

It is not directly proportional; the I-V characteristic is non linear

Describe what happens to the resistance of a filament lamp as the temperature of the filament increases

The resistance of a filament lamp increases as the temperature of the filament increases

Why? - Ions in metal have more energy, so vibrate more, causing more collisions with electrons as they flow through the metal, creating greater resistance to current flow

What is a diode

A component that only allows current to flow through in the forward direction. They have very large resistances in the reverse direction.

The relationship between the current through a diode and the potential difference across the resistor on a graph

Describe how the current flows through a diode

Current through a diode flows in one direction only

Describe what happens to the resistance of a diode when current flows in the reverse direction

It will increase if the current is reversed

What is a thermistor

A temperature dependent component, whose resistance increases as its temperature decreases.

Describe how the resistance of a thermistor changes as the temperature increases

Causes the resistance to decrease

I can describe how thermistors can be used to control a circuit

Thermistors can be used to control a circuit to provide temperature measurement or regulation, as their resistance changes with temperature, allowing them to act as sensors or control devices.

Explain how the resistance of an LDR changes as light intensity increases

The resistance will decrease

Describe how LDRs can be used to control a circuit

LDRs can be used to control a circuit by decreasing their resistance as light intensity increases, allowing them to act as automatic light sensors

Graphs that represent the relationship between the current and potential difference for: 

• an ohmic conductor

graphs that represent  the relationship between the current and potential  difference for: 

• a filament lamp

graphs that represent  the relationship between the current and potential  difference for: 

• a diode

A circuit to measure the resistance of a component by measuring current  through, and potential difference across, a component

Investigate the I-V characteristics of a variety of circuit elements, including a filament lamp, a diode, and a resistor, at constant temperature

• To find a component’s I-V characteristic, begin to vary the resistance of the variable resistor. This alters the current flowing through the circuit and the potential difference across the component

• Each time you use the variable resistor to alter the current, record the potential difference across the component for that value of current. Repeat each reading twice more to get an average pd for each measurement

• Swap over the wires connected to the battery to reserve the direction of the current. Measure negative values of current and potential difference using the same method as described above

• Now you can use your measurements to plot a graph of current against potential difference of the component. This is your I-V characteristic.

State the two ways of joining electrical components in a circuit

• Series

• Parallel

Describe the current through components connected in series

Current is the same at all position since the charge only has on path to flow through

Describe the potential difference across components connected in series

Potential difference is shared across the whole circuit

Describe the total resistance of two components connected in series and represent this as an equation

Resistance of two components is bigger than just one of them, because the charge has to push through both of them when flowing round the circuit

Describe the current through components connected in parallel

The current is shared between the different branches. When the charge reaches a junction it splits

Describe the potential difference across components connected in parallel

Potential difference is the same across all branches

Describe the total resistance of two components connected in parallel

• Two resistors in parallel will have a smaller overall resistance than just one

• Because charge has more than one branch to take, so only some charge will flow along each branch

How should you connect an ammeter and a voltmeter in a circuit to measure current and potential difference

• Ammeters should be connected in series with the component that they are measuring current through

• Voltmeters should be connected in parallel to the component that they are measuring the potential difference of

Why is it an advantage to connect lamps in parallel

If one lamp blows, the rest will be unaffected and can still receive current

Describe the difference between series and parallel circuits.

Series:

• Components are connected end to end

• All the current flows through all the components

• Can only switch them all off at once

• Potential difference is shared across the whole circuit

• PD of power supply = sum of PD across each component

• Current is the same through all parts of the circuit

• Current at one point = current at any other point

• Total Resistance is the sum of the resistance in each component

• Resistance of two components is bigger than just one of them, because the charge has to push through both of them when flowing round the circuit

Parallel:

• Components are connected separately to the power supply

• Current flows through each one separately

• You can switch each component off individually

• Potential difference is the same across all branches

• PD of power supply = PD of each branch • Because charge can only pass through any one branch

• Current is shared between each of the branches

• Current through source = sum of current through each branch

• Total resistance is less than the branch with the smallest resistance

• Two resistors in parallel will have a smaller overall resistance than just one, because charge has more than one branch to take, so only some charge will flow along each branch

Why adding resistors in series increases the total resistance while adding resistors in parallel decreases the total resistance

• If we add more components in series we increase the total resistance, as the total resistance is the sum of the individual restistances

• If we add more components in parallel we decrease the total resistance, as there are more paths for the electrons to take

Explain the design and use of dc circuits for measurement and testing purposes

What does ac and dc stand for and what they do

• Ac - alternating current, current that is constantly changing direction, they are produced by alternating voltages in which the positive and negative ends keep alternating

• Dc - direct current, current that is always flowing in the same direction. It’s created by a direct voltage where the positive and negative ends of the source are fixed

state whether mains electricity is an ac or dc supply

ac supply

What is the frequency and voltage of the UK mains electricity supply

Frequency - 50Hz

Voltage - 230V

What are the colours and the purposes of the:

• Live wire

• Earth wire

• Neutral wire

Live wire:

• Brown

• Provides the alternating potential difference from the mains supply

Earth wire:

• Green and yellow

• For protecting the wiring and for safety, it stops the appliance casing from becoming live. It doesn’t usually carry a current, only when there is a fault. It is at 0V

Neutral wire:

• Blue

• Completes the circuit by connecting the appliance back to the mains supply and carries away current. It is around 0V

Explain that a live wire may be dangerous even when a switch in the mains circuit is open.

• Your body, just like the earth is at 0V. This means that if you touch the live wire, a large potential difference is produced across your body and a current flows through you. This causes a large electric shock which could injure or even kill you

• Even if a plug socket or a light switch is turned off (the switch is open) there is still a danger of an electric shock. A current isn’t flowing, but there is still a pd in the live wire. If you made contact with the live wire, your body would provide a link between the supply and the earth, so a current would flow through you

• Any connection between live and the earth can be dangerous. If the link creates a low resistance path to the earth, a huge current will flow, which could result in a fire

Power defintion

The rate at which energy is transferred or work is done

Equations to calculate power

Equations to calculate energy transferred

Describe what affects the amount of energy an appliance transfers

• How long the appliance is being used for

• The power of the appliance

Describe how different domestic appliances transfer energy from

• batteries

• Ac mains to electric motors

• heating devices

• Battery converts chemical energy into electrical energy

• Motor converts electrical energy into kinetic energy as well as waste energy in the form of heating due to friction

Explain how the power transfer in a circuit device is related to the

• Potential difference across it

• The current through it

• The energy transferred over a given time

Describe how charge causes work to be done in a circuit

• Whenever something moves, something else is providing effort to move it

• The thing putting the effort in needs a source of energy

• It then does work by moving the object, energy is transferred from one store to another

• Whenever this energy is transferred usefully or is wasted, work is still done

Describe, with examples, the relationship between power ratings for domestic electrical appliances and the  changes in stored energy when they are in use

Describe the purpose of the National Grid

Its a system of cables and transformers which links power stations to consumers so that they have access to a source of electricity

Step up transformers and Step down transformers

Step up:

• Used just after the power station

• Increases volume and decreases current

Step down:

• Used just before houses and factories

• Decreases volume and increases current

Name the parts that make up the National Grid and there voltages

• Power station - 25KV

• Step up transformers

• Power lines - 132KV

• Step down transformers

• Factories - 33KV

• Homes - 230V

Explain why the National Grid system is an efficient way to transfer energy

Why must we reduce the potential difference before the electricity reaches homes

Appliances are designed to be 230V of pd, therefore its a bigger risk to us and our appliances

Why do we increase the pd on the national grid

• Because a high voltage will deliver more energy down the cables

• Because increasing the Pd will decrease the current

• Because the same power is delivered in a more efficient way

• So that less energy is wasted as heat in the cables

• Because the electricity will travel faster down the cables

• Because a smaller current will mean less collisions between electrons and metal ions so the cables don’t get as hot

Describe how some insulating materials can become electrically charged

By being rubbed together

Why? -

• Electrons are rubbed from one material onto the other

• The material gaining electrons becomes negatively charged

• The material loosing electrons becomes equally positively charges

Examples of materials which are electrical conductors and insulators

• Plastic and wool = insulators

• Metal and graphite = conductors

What is it that makes a material able to conduct electricity

Has delocalised electrons

If electrons move from object A to object B, what can we say about the size of the charge on both objects

Would be the same but oppsoitely charged

When charged objects are placed near each other, what two things affect the size of the force the objects experience

• The amount of charge on the object

• How close together they are

Why does the Van der Graff generator make your hair stand on end

Because they have the same charge and like charges repel

Explain the concept of an electric field

A region in which a charged object will experience a non-contact electrical force

Describe how the strength of an electric field around  a charged object varies with distance from the object

The field decreases in strength the further away you get making it a force

Where can electric fields be found

Surrounding any charged object

The electric field pattern for an isolated  charged sphere

Explain how the concept of an electric field helps to  explain the non-contact force between charged objects

Describe what happens to a second charged object placed in the electric field created around the initial  charged object

it experiences a non-contact electrostatic force