# Physics - P2.4 - Current Electricity

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## P2.4.1 - Electrical Charges

• When two electrically insulating materials rub together, electrons rub off one material and are deposited on the other - direction depends on materials
• Electrons have negative charge so material gaining electrons becomes negatively charged - material losing electrons becomes positively charged: charging by friction
• Two objects with opposite electrical charges attract each other, two objects with the same electrical charge repel each other
• Bigger distance between objects = weaker force between them
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## P2.4.2 - Electrical Circuits

• Every component in a circuit has a circuit symbol - put together in a circuit diagram to show how the components are conneted together
• I = Q / t
• I - current - A
• Q - charge - C
• t - time - s
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## P2.4.3 - Resistance

• Current measured by ammeter - in series - ampere (amp) - A
• Potential difference (pd) mesured by voltmeter - in parallel - volt - V
• V = W or E / Q
• V - potential difference - V
• W or E - work done or energy - J
• Q - charge - C
• Resistance - opposition to current flow - unit is ohm - Ω
• R = V / I
• R - resistance - Ω
• V - potential difference - V
• I - current - A
• Ohm's law - resistor kept at constant temperature: current-pd graph line is straight and go through origin: current ∝ pd - ohmic conductor
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## P2.4.4 - More Current-Potential Difference Graphs

• Line on current-pd graph for filament bulb is curved - current isn't ∝ to pd - resistance increases as current increases: resistance increases as temperature increases
• Reversing pd makes no difference to curve shape
• Current through a diode only flows in one direction - in reverse direction diode has very high resistance - current = 0
• Light-dependent resistor (LDR) - light falling on it gets brighter = decreased resistance
• Thermistor - increased temperature = decreased resistance
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## P2.4.5 - Series Circuits

• Series circuit - components connected one after the other: break anywhere in the circuit = charge stops flowing - no choice for route of charge: current though each component is the same
• Current depends on pd of supply and total resistance of the circuit
• Pd pf supply is shared between all components: pds across all individual components = total pd of supply
• Resistance of individual components added up = total resistance of circuit
• Bigger resistance of component = bigger share of supply pd
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## P2.4.6 - Parallel Circuits

• Parallel circuit - each component connected across supply - break in circuit: charge flows in other parts
• Each component connected across supply pd: pd on each component is the same
• Junctions in circuit: different amounts of charge flow through different components - current depends on resistance - bigger resistance = smaller current
• Total current through circuit = sum of currents through separate components
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