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Physics Term 1 (Static electricity (Definitions (Discharge (Something…
Physics Term 1
Static electricity
Charging
Induction
- two metallic objects
- charged rod brought near
- electrons attracted/repelled to the further/nearer object
- separate objects
- spheres have an equal and opposite charge
Conduction
- Contact between objects
- electron flows from the more negative object to the more positive object
Friction (for insulators)
- energy from friction removes outer electrons from wool
- extra electrons on shoes causes a net negative charge
- charge conserved > the wool gets a net positive charge
Determining charge
Electroscopes
Metal knob connected to two thin lightweight pieces of metal foil, called leaves
1.charge electroscope
2.if leaves remain, object is neutral
3.if leaves open further, object is of the same kind of charge of elctroscope
4.if leaves close, its the opposite
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Discharging (insulators)
Heating
- heat causes air around object to be ionised,
- the ions in the air can neutralise the charges in the object
HUMID CONDITIONS water vapour, a polar molecule, removes excess charges on the insulator
(negative charges "leak off" as they as attracted to the positive end of water molecules)
Electrostatic Hazards
Lightning
- friction builds up between water molecules and air in thunderclouds
- thunderclouds become charged
- the air becomes ionised
- provides conducting oath down to nearest/sharpest object on ground
Electrostatic Discharge
- friction (tires on trucks)
- rubbing (rugs)
Electric Fields
- lines are denser in regions of stronger electric field (near the charges)
- NO LINE SHOULD INTERSECT
- number of lines is proportional to magnitude of charge
- +ve -> -ve
Current of electricity
Current
- rate of flow of electrical charges (ie how many electrons flow per second)
I = Q/t
- Q in Coulombs
- t in seconds
Ammeter
- Connected in series
- No/Very little resistance
- Measures current
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Electromotive Force (EMF)
- work done by source per unit charge driven around a complete circuit (ie J/C)
ℰ = W/Q
- ℰ in Volts (V)
- W is work done is Joules (J)
- Q is Coulombs (C)
Potential Difference
- work done to drive a unit charge through two points
V = W/Q
Voltmeter
- Infinite/Very high resistance
- Connected in parallel
- Measures potential difference
Resistance
- ratio of potential difference across a component to the current that flows through it
R = V/I
VICTORY FOR RI
Resistors
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Variable Resistor
- move slider
- change length
- change resistance
Factors affecting resistance
- length (as length increases, resistance increases)
- cross sectional area (as cross sectional area increases, resistance decreases)
R = ρ(l/A)
- ρ is resistivity of material in Ω m
- l is length
- A is cross sectional area
Ohm's Law
- current passing through a metallic conductor is directly proportional to the potential difference across the ends provided physical conditions are the same
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Practical Electricity
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Household electricity
Parts
Earth wire(Green yellow)
- No potential no voltage
- Touches metal casing
- Safety mechanism
When the circuit breaks, the electricity goes via the earth wire, causing the fuse to break (safety)
Alternative : double insulation
- has insulation other than wire covering to prevent any exposed metal
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Live wire(Brown)
- high potential and voltage
- Contains fuse(melts and opens circuit after certain Current)