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Topic 2 Electrictiy By Bethan Poole (2.1 Current, potential difference…
Topic 2 Electrictiy
By Bethan Poole
2.5 Static electricty (physics only)
2.5.1 Static charge
Same force repel
Opposite force attract
Non-Contact force
Electrical sparks
As Charge of isolated object increases the PD between earth and object increases
When PD becomes high enough, a spark may jump across the gap-Discharges the charged object felt as electric shock
Can serve as source of ignition-Can be dangerous e.g Spark at petrol station
Lightning example of spark when charge builds up
Friction moves negatively charged electrons from one material to another: Objects that gain electrons become negatively charged- the objects that lose electrons become positely charged
When insulating materials rub against each other they cn become electrically charged e.g when balloon is rubbed aganst a jumper
2.5.2 Electric fields
2nd object placed in field experience a force - Gets stronger the closer they are
Field lines close -stronger -greater chance of spark
Direction of arrow idicates positive or negative charge
Strength depends on... Distance from object (further from object weaker the field) Amount of charge (higher the charge, stronger the field)
Charged object creates an electric field around itself
Electrostaic Forces- Non contact forces - Forces of attraction or repulsion - Charged object near uncharged object , can attract it -i.e Rod Experiment when objects have same charge repel opposite charge attract
2.1 Current, potential difference and resistance
2.1.2 Electrical charge and current
Electric Current- Flow of electrical charge. Greater the rate of flow, the higher the current
Current measured in
amperes (A)
- Amps, Measured using ammeter Ammeter always in series
Electric Charge- measured in coulombs(C)
Calculated using equation-
Charge flow= Current x Time Q=IT
Current is the same everywhere in series circuit
2.1.3 Current, resistance and potential difference
Resistance- measure of how a component resists the flow of charge
Higher the resitance- More difficult for charge to flow, the lower the current
Resitance measured in ohms (Ω)
Potenial difference- Difference in electrical potential from one point in a circuit to another. Thought of like an electrical push
Bigger the potentail dfference across a component- greater the flow of chrge, bigger the current
measured in volts (V) - Voltmeter
Potential Difference, Current and Resistance -
Potential difference = Current x Resistance V=IR
2.1.1 Standard circuit diagram symbols
Standard circuit symbols are used to represent components:
Resistor
LED
Diode
Open switch
Closed Switch
Cell
Battery
Variable Resistor
Bulb/Lamp
Fuse
Voltmeter
Ammeter
Thermistor
LDR
2.1.4 Resistors
Potential difference- Current graphs- used to show relationship between PD (voltage) and current
Straight line through origin
- Current and PD directly proportionate-
Steep Gradiant
- Low resistance large current will flow for a small PD-
Shallow gradient
- High resistance large PD needed to product small current-- Some resistors the value of R not constant but changes as current changes- Non linear graph
Ohmic Resistor
PD= Current Indicated but linear graph
Fiament Lamp
Current increases, temp increases Indicated by curved graph
Diodes
Current only flows one direction, high resistance in reverse directiion, horozontal line on x axis shows no current flows
Thermistor
- Resistance decreases as temp increases
LDR
- Resistance decreases as light intensity increases- useful for automatic lighting control or detection is needed
2.2 Series and parallel circuits
2.2 Series and parellel circuits
Series
Same current through all components, Total PD shared between components, Total resistance of two components is the sum of the resistance of each components, adding resistors increases the toal resistance in ohms - Total R= R1+R2
Parallel
PD same across components, Total current drawn from power supply is sum of currents through seperate components, total resistance of two resistors less than resistance of the smallest indiidual resistor, adding resistors reduces total resistance
2.3 Domestic uses and safety
2.3.2 Mains electricity
230V and freq. of 50Hz
Cable contains three wires- each wire different electrical potential
Live wire (Bown)- 230v potential
Neutral wire (Blue)- at or close to 0v
Earth wire (Green and yellow stripes)- 0v potential Safety wire stops exterior wire of appliance becoming live
During operation- PD causes current to flow through live and neutral wires, live wire carries alterrnating potential from supply, neutral wire completes circuit, current only flow in earth wire if there is a fault connecting it to a non-zero potential.
Dangers
Touching live wire can create large PD across body and cause a large current flowign through body
Live wire can be dangerous even if switch is open
e.g TV may be switched off but still plugged in at the wall- live wire between wall and TV switch is still at an alternating potential , only needs path for electricity to flow, if someone touches the live wire creating PD from live to earth wire they'll get an electric shock
2.3.1 Direct and alternating potential difference
DC- PD always positive or negative- current direction constant
Type of current supplied by cells and batteries
AC- PD alternates from positive and negative
Type of current used in mains electricity
2.4 Energy transfers
2.4.3 The National Grid
System of cables and transformers linking power stations to homes and businesses
Power Station 25000V -> Step-Up Transformer-> Transmission Cables 400000V-> Step-down Transformer-> Homes, Shops etc 230V
Step-UpTransformer - Increases PD (Voltage) from power station to transmission cables - Reduces the current and reduces the heating effect caused by current following in the trnsmission cables- redcues enery loss making the transmisson efficient
Step-down Transformer- Reduce PD from the transmission cables to make safe for domestic
2.4.1 Power
Power in circuit depends on the potential difference across it and the current flowing through it
A device with a higher potential difference or current will use ore energy pper second than one with a lower PD ir current i.e it will be more powerful
Power=Potential Difference x Current P=VI
or
Power=(current)2 x Resistance P=I2R
Power (watts), PD (volts), Current (Amps) Resistance (Ohms)
2.4.2 Energy tranfers in everday appliances
The amount of energy an appliance transfers depends on how long it's switched on for and the power of the appliance
Energy transferred= Power x Time E=PT
or
Energy Tranferred= Charge folw x PD E=QV
Energy Transferred (Joules) Power (watts) Time (second) Charge flow (Coulombs) PD (volts)
