P4/P5 - Electricity

Positive and negative terminals

Single cell

provides cell with source of potential difference

Battery of cells

Open switch

Turns circuit on

Closed switch

Turns circuit off

Lamp

Emits light when electric current passes through it

Resistor (Fixed)

limits the flow of current. A fixed resistor has a resistance it cannot change

Resistor (Variable)

resistor with a slider that can be used to change its resistance. These are often used in dimmer switches and volume controls

A resistor that changes its resistance with temperature. The resistance of a thermistor depends on its temperature as its temperature increases, its resistance decreases and vice versa

resistance of an LDR depends on the light intensity as the light intensity increases, its resistance decreases and vice versa

A diode allows current to flow in one direction only. They are used to convert AC to DC current

Rectifier

A rectifier is a component that turns an alternating current into a direct current

Examples of recitifer

Diode

How do diodes act as recitifers

The diode blocks the alternating current when it changes direction

How does a rectifier work

The AC current goes into rectifier → Diode blocks the AC current and changes it → DC current is an output of the rectifier

This is equivalent to a diode and emits light when a current passes through it. These are used for aviation lighting and displays

breaks the circuit if a fault in an appliance causes too much current to flow. This protects the wiring and the appliance if something goes wrong

Used to measure the current in a circuit. Connected in series with other components

Used to measure the potential difference. Connected in parallel with other components

Direction of current flow

Positive → Negative terminal of the power supply

Components in an electrical circuit diagram

Energy source → Source of potential difference such as : Power supply/Cell/Battery

Closed path or complete circuit → Electrons need to flow in a complete loop for a current to flow

Electrical components → They act as sensors to respond to the environment such as: LDR/Thermistor/Voltmeter x Ammeter/LED/Lamp

Potential difference

energy transferred per unit charge flowing from one point to another

Sources of potential difference

• Cell

• Batteries

• Electrical generators

Formula of potential difference (V)

V = E or W (J)/Q (C)

What measures potential difference

Voltmeter in parallel

Current

Flow of electric charge

Rate of flow of electric charge

How much charge passes through a point per second

Unit for current (I)

Amperes (A)

Unit of charge (Q)

Coulombs (C)

1C

1A s (1 ampere each second)

Charge flow formula

Q = I (A) x t (s)

Direction of electron flow

To the positive terminal

Conventional current

the flow of positive charge from the positive terminal of a cell to the negative terminal. Opposite to electron flow

Measuring current

Ammeter in series

Current in series

Current value is same as the circuit is in a closed loop

Resistance

Opposition to current

Higher resistance

Lower current

Resistance in conductors

Low

Resistance in insulators

High

Unit for resistance (R)

Ohms (Ω)

Ways to increase resistance

Adding resistors

Formula for potential difference using resistance and current

V= I(A) x R(Ω)

Ohms law

current through a conductor is directly proportional to the potential difference across it

Ohmic conductors

Electrical conductors obeying ohm’s law

Examples of ohmic conductors

• Fixed resistors

• Wires

• Heating elements

Relationship between current and potential difference

If they are directly proportional, resistance remains constant

I-V graph of an ohmic conductor

• Reversing potential difference won’t have an effect on shape of line

• I -V are directly proportional

I-V graph of filament lamp

• Non ohmic conductor as I-V are not directly proportional

• Resistance of filament lamp increases as temp increases

• As current increases, temp increases increasing resistance as high temp causes atoms in metal lattice of filament vibrates more.

• Resistance is created as it is difficult for free electrons to pass through

• Resistance increases as graph curves

Why resistance increases as temperature increases

Higher temperature makes atoms in the materials vibrate really quick which collide with electrons flowing through therefore slowing down their flow which decreases current

I-V graph in a semiconductor diode

• Diode is a non ohmic conductor letting current flow in one direction only → Forward bias

• In reverse direction, diode has very high resistance so no current flows → Reverse bias

• Forward bias in graph → sharp increase in potential difference in right side of graph

• Reverse bias in graph (When diode is switched around) → shown by a zero reading of current or potential difference on left side of graph

Thermistor

non ohmic conductor and temperature dependent resistor

Low temperature in a thermistor

High resistance as electrons will move slower

High temperature in a thermistor

Low resistance as more KE given to the electrons to move more freely and faster

Uses of thermistors

• Thermostats → It regulates temperature

• Ovens

• Fire alarms

• Refrigerators

• Boilers

• Digital thermometers

  These are commonly used to regulate and monitor temperature in environments

LDR

Non ohmic conductor in which the resistance changes depending on its light intensity. LDR’s regulate amount of light intensity on it or activates a device when the light intensity reaches above or below a certain point

High light intensity of an LDR

Low resistance

Low light intensity of an LDR

High resistance

Uses of LDR’S

• Lights that switch on when it gets dark (lights, streetlights)'

• Alarm clocks

• Burglar alarm circuits

• Light intensity meters

• Security lights

• Street lights:

  • Daytime has high light intensity → LDR keeps light turned off

  • Nigh has low light intensity → LDR switches light on

Advantages of LDR

these circuits are automatic therefore not needing any human time and intervention to function correctly everyday

Investigating resistance in LDR circuit

1. Lamp turned off in dark room

2. Record reading on voltmeter and ammeter

3. Slowly increase light intensity of lamp using dimmer switch

4. Record reading on voltmeter and ammeter for each increase in light intensity

5. Calculate resistance using R=V/I

Investigating resistance in thermistor circuit

1. Heater turned off

2. Record reading on voltmeter and ammeter

3. Slowly increase heat of heater using dimmer switch

4. Record reading on voltmeter and ammeter for each increase in temperature of heater

5. Calculate resistance using R=V/I

Series circuits

Circuit consisting of a string of two or more components, connected end to end

Parallel circuits

Circuit consisting of two or more components attached along separate branches of the circuit

Current/PD in series circuits

• Current → Same at all points (I= I₁ = I₂)

• PD → Shared between the components (If given potential difference, halve the PD)

Current/PD in series circuits

• Current → Splits at junction with some going one way or the other

• PD → Same at all points

Resistance in series circuits

• Total resistance is sum of resistance in each component

• Two resistors in series will have larger overall resistance than just one because charge has to push through multiple components when flowing around circuit

• The more components that charge has to travel through, higher number collisions that occur

Resistance in parallel circuits

• total resistance decreases and is less than the resistance of any of the individual components

• two resistors in parallel will have smaller overall resistance than just one as charge has more pathways to take so only some charge will flow through each path

• The more pathways, the lesser resistance

Voltage in series formula

V(in) = V₁ + V₂ ….

Voltage in parallel formula

V(in) = V₁ = V₂ ….

Current in series formula

I(in) = I₁ = I₂ …

Current in parallel formula

I(in) = I₁ + I₂ ….

Resistance in parallel formula

1/R(total) = 1/R₁ + 1/R₂ ….

Resistance in series formula

R(Total) = R₁ + R₂ …..

Direct current

Current that is steady, constantly flowing in one same direction in a circuit from positive to negative

Sources producing DC

Electric cells and electric batteries

Potential difference in DC circuits

One direction only

DC power supply

Has fixed positive terminal and fixed negative terminal

Advantages of series circuits

• Cells connected in series give a greater resultant voltage than individual cells.

• Voltage increases if the number of cells increases.

• Series circuits do not overheat easily.

Disadvantages of series circuits

• If one of component breaks, then all of others will stop working

• Components can’t be controlled separately

Alternating current

current that continuously changes its direction, going back and forth around a circuit

How many terminals in AC circuit

2 identical terminals

Potential difference in AC

Varies and represented as a sine curve

Frequency of AC

number of times the current changes direction back and forth each second

Mains electricity of UK

Frequency → 50Hz

PD → 230V

Mains electricity

electricity generated by power stations and transported around the country through the National Grid. It is AC current

Parts of a three core cable

• Neutral wire

• Live wire

• Earth wire

• Fuse

• Cable grip

Insulation colour coding for the three wires

• Neutral → Blue

• Live → Brown

• Earth → Yellow/Green

Live wire

• Carries alternating potential difference from mains supply to circuit

• Most dangerous wire

• If it touches appliance without earth wire, it causes electrocution

Neutral wire

• Forms opposite end of circuit to live wire to complete the circuit

• Lower voltage hence less dangerous than live wire

Earth wire

• Acts as safety wire to stop appliance from being live

• This prevents electric shocks if appliance malfunctions or if live wire touches casing of plug

• It carries current to the ground if there is a fault by providing low resistance path

Power

The rate of energy transfer or the amount of energy transferred per second

Power formula (P) (with PD/Current)

P(W) = V(V) x I(A)

Power formula (P) (with Current/Resistance)

P(W) = I² (A) x R (Ω)

Power formula (P) (With Resistance/Voltage)

P(W) = V² (V)/R (Ω)

Amount of energy transferred to and from appliance depends on

• How long appliance is switched on for

• Power of appliance

Energy transfers in a circuit

Power source → Energy released → Passed to Electrons → Electrons pass energy to components

Energy transfer from AC to appliances

KE → Electrical energy of motor

Uses of motors

• Vacuum cleaners

• Washing machines

• Refrigerators

Energy formula with Power and time

E (J) = P(W) x T(s) or E = I(A) x V(v) x T

Energy formula with charge and Pd

E = Q (C) x V