IGCSE Physics
Unit 4 Part 1
Electricity
After lightning strikes count and for each three seconds it is a kilometer away
Benjamin franklin hung a key from a kite to attract lightning and to study electricity
Lightning can be as much as 1.3 billion volts
- Human influence on results
-
Static electricity
Charging the balloon
Removing or gaining electrons
Electrostatic attraction
- Charges:
Friction causes change in charge - moving of electrons
Higher electron affinity → pulls more electrons
1. Like in magnets - unlike attract , like repel
What is static electricity
Greeks first discovered - rubbing amber to attract hair and cloth
Changing charge comes from the transfer of electrons
Gains electrons: negative charge
loses electrons: positive charge
Charged objects can either attract the opposite charge or a neutral object
Grounding: Running charge into the ground to prevent buildup
Conductors vs Insulators
Conductors
Insulators
Copper
Rubber
Gold
Glass
Steel
oil
silver
diamond
Sea Water
wood
Electric Fields
An influence on the field around it
With plastic - makes negative → attracts water
Charges: away from positive - Towards negative
The curves on the outside is the edge effect
q is for charge
SI Unit of Charge: Coulomb (C)
Electron: - 1.6x10-19
Proton:+1.6x10-19
Current is the flow of electrons(ampere)
Electric quantities and circuits
Current is the flow of electrons or the flow of charge
Measured with Ammeters(A)
- must break circuit
Galvanometer (above) is a more accurate form of an ammeter
A battery is two or more cells
Cell is one part
- Works through electrons leaving negative and moving to positive
- Confusing because current goes from positive to negative
-
- C = A x s
Circuit diagram
Voltage and Potential difference
Higher voltage causes more flow
Potential difference is the work done by a unit charge passing through a
Component
Measured in Volts(V) and measured by a Voltmeter
- Difference in electric potential between two points
- Where it enters a component and where it leaves
-
Measuring Potential Difference across a cell is called the Electromotive
Force(e.m.f) and it is also measured in Volts(V)
- Electrical Work done by a source in moving a unit charge around a complete
circuit
𝑊𝑂𝑅𝐾 𝑑𝑜𝑛𝑒 𝑜𝑛 𝑡ℎ𝑒 𝐶ℎ𝑎𝑟𝑔𝑒(𝐽)
𝑊
or 𝐸 = 𝑄
𝐶ℎ𝑎𝑟𝑔𝑒(𝐶)
𝑊𝑂𝑅𝐾 𝑑𝑜𝑛𝑒 𝑏𝑦 𝑡ℎ𝑒 𝐶ℎ𝑎𝑟𝑔𝑒(𝐽)
𝑊
or 𝑉 = 𝑄
𝐶ℎ𝑎𝑟𝑔𝑒(𝐶)
𝑒. 𝑚. 𝑓. (𝑉) =
𝑝. 𝑑. (𝑉) =
Bigger EMF = the more strongly the electrons are pushed → similar to water pressure
Everything that gives Voltage must be balanced by something removing voltage
Everything must be balanced
To connect the Voltmeter to a circuit plug the Wire into one side, plug the wire connecting to the
lamp into that wire and do the same thing on the other side.
Electrical Resistance
A resistor takes energy away from the circuit
Resistors can be in many different forms
Are used to limit current flow
Colours for determining the resistance value
Eg: Dials, Stove tops
𝑅𝑒𝑠𝑖𝑠𝑡𝑎𝑛𝑐𝑒(Ω) =
𝑅=
𝑃𝑜𝑡𝑒𝑛𝑡𝑖𝑎𝑙 𝐷𝑖𝑓𝑓𝑒𝑟𝑒𝑛𝑐𝑒(𝑉)
𝑐𝑢𝑟𝑟𝑒𝑛𝑡(𝐴)
𝑉
𝐼
Bigger Resistor lets less current go through
Measuring Resitance
Measure the Voltage, Measure the Current, use formula
A longer wire has a greater resistance
A thicker wire has less resistance
Relationship between Resistance, Current and P.d.
𝑉 = 𝐼𝑅: Ohm’s law
P.d. = Current * Resistance
Voltage is proportional to current
For Ohmic Resistors/Conductors
For Non ohmic resistors
Diodes:
Made of a semiconductive Material
Symbol looks like a one way resistor:
Can flow one way but not the other(positive to negative)
LEDs: Light Emitting Diodes
Power
Can also be written as Work/time
Most devices have a power reading
Indicates maximum electrical power the appliance draws from main supply when operating at
full power
𝑃𝑜𝑤𝑒𝑟 = 𝑉𝑜𝑙𝑡𝑎𝑔𝑒(𝑝. 𝑑.) × 𝑐𝑢𝑟𝑟𝑒𝑛𝑡(𝐴)
Power is I*V
Energy transferred is I*V*t
6
kWh: 3. 6 × 10 𝐽
Electrical circuits
Resistors
Controls the amount of current flowing
Variable resistors: can change amount of resistance(Rheostat: turnable resistors)
Light dependent resistor: More light: less resistance, less light: more resistance
Thermistors: resistors dependent on temperature, Lower temperature: more resistance
Thermistor graph: NTC; Negative temperature coefficient
example
Relays
A switch that relies on an electromagnet
Can be used as a safety mechanism
Parallel vs Series:
Parallel: Voltage is always the same, current adds up
Series: Voltage adds up, current is the same
Resistor combinations
Series:
adds up together
V = V1 + V2 + V3
Combine resistance and use the voltage to find the current
Use this to find the individual voltage
For series: R = R1 + R2 +R3
- The combined resistance is equal to the sum of the resistances
- The current is the same at all points in the circuit
- The Bigger the resistance, the greater the p.d. across it
Higher resistance means higher p.d.(greater expectations)
The sum of p.d. Across each component is sum to the e.m.f
R1 / R2 = V1 / V2
Parallel:
If one breaks, the others are fine
In parallel:
- The effective resistance is less than the resistance of either resistance
- The current from the source is greater than the current through either resistor
- The pd across every component is the same
Resistance in parallel:
1/R = 1/R1 + 1/R2 …
This formula gives 1/R. On the calculator flip the result if its a fraction, If not then make it
a fraction and then inverse
When only 2 resistors are used:
𝑅 𝑡𝑜𝑡𝑎𝑙 =
𝑅 1× 𝑅 2
𝑅 1+𝑅 2
Can make sets of two with say four resistors and then another set of two out of
the answers
Can come from Ohm’s law
Potential divider
Voltage is split across the potential divider in its ratio
Voltage is in the ratio of its resistance
AC VS DC
Live & neutral VS positive and negative
AC: flows back and forth → going so quickly that it looks like it is on
Electrical safety
Water protection, insulated wires, not too many plugs on one multi adapter
Wires often have a ground/earth wire to ground the wires
Adapters need to have a fuse, if there is no fuse, it can cause a fire
- Multi plug(Bar adapter) is safer than Block
- The fuse acts as a weak point so if something breaks, it breaks in the fuse
Types of fuses
Earth Wire
Is safe to touch because it is always grounded
Is used to ground the device in case the outside is charged
The Earth wire has no resistance
Meant as a precaution and is supposed to trigger the fuse to break
To make sure that the object is safe to touch with the hands
Neutral does not mean the ground(it's the equivalent of negative)
For extra security use double insulation: Insulate the insulation
Double insulation symbol
Removes need for ground wire
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