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Physics electricity revision - Coggle Diagram
Physics electricity revision
Symbols in a circuit
How a circuit works
For electrical charge to flow through a closed circuit the circuit must include a source of potential difference (voltage).
Electric current is a flow of electrical charge (specifically electrons with a negative charge in metals). The size of the electric current is the rate of flow of electrical charge. It is measured in amps (A).
Charge flow = current x time Q = I x t
Charge is measured in coulombs. Current is measured in amps. Time is measured in seconds.
A current is the same at any point in a closed loop.
Current is dependent on the resistance and potential difference of the component
Resistance in components
The current across a resistor is directly proportional to the potential difference across a resistor, assuming it remains at a constant temperature.
The resistance of a filament lamps increases as temperature increases, resulting inn a non linear relationship between current and potential difference.
Current flows through a diode in one direction. Once a certain potential difference is reached, resistance becomes almost 0. At other points, it has very high resistance.
The resistance of a thermistor decreases as temperature increases.
Use: A fridge uses a thermistor. When it warms up (when open), the resistance decreases and the current increases. This switches on the fridge so that it can cool back down. When it cools down, the resistance increases and the current decreases. This switches off the fridge so it doesn’t get too cold.
The resistance of an LDR (light dependent resitor) decreases as light intensity increases.
Use: The automatic brightness setting on mobile phones relies on an LDR. When in a dark room, the light intensity is low so the resistance is high. This provides a low current so the screen is dimmer (it doesn’t need to be bright when there isn’t much ambient light). When outside in the sun, the light intensity is high so the resistance is low. This provides a high current which makes the screen brighter (to make it stand out amongst the abundant ambient light).
Parallel or series circuits
Parallel circuits: Total current = current from branch 1 + current from branch 2 Potential difference across each component are equal. Total resistance = less resistance than smallest resistor.
Series circuit: Current same through each component. Total potential difference shared between components. Total resistance is sum of each resistor.
Determine resistance by building a circuit with an ammeter in series and a voltmeter in parallel across the component.
Plugs and wall sockets
AC vs DC
Alternating current (AC) switches direction periodically (at a frequency of 50 Hz in the UK).
Direct current (DC) continuously travels in the same direction.
Plug wiring
Plug design
The Earth pin is longest so that when plugged in, it makes contact first so that the circuit is automatically Earthed before connecting the main part of the circuit.
Brass is hard wearing and does not oxidise (rust).
Copper is a very good conductor and it bends easily (flexible).
The fuse is connected to the live wire. If too much current passes through it, the fuse melts and breaks the connection to the live wire.
Plastic is a very good insulator so if anyone touches the plug/cable it stops them from getting an electric shock.
No earth pin if in an appliance that has a plastic case that cannot become live if there is a fault.
Must earth pin when in an appliance with a metal case, if there is a fault, the case can become live and there would be a risk of
electric shock to the user.
Short-circuiting is where a contact is made between live and neutral wires unintentionally and cuts out part of the circuit. A
large current flows bypassing the intended part of the circuit.
Wire functions
Live wire – brown - Carries alternating potential difference from the supply
Neutral wire – blue - Completes the circuit providing a path back to the supply
Earth wire – green and yellow stripes - A safety wire to stop the appliance becoming live.
Potential difference of wires
The potential difference between the live and Earth wire is about 230 V.
The Neutral wire is 0 V.
The Earth wire is at 0 V, and only carries a current if there is a fault.
Danger of live wire: even if a switch in the mains circuit is open, the live wire is still dangerous. Providing any connection
between the live and the Earth results in a current flowing and this can result in an electric shock.
Equations
Power
Power = Current x voltage = I x V
Power = Current squared x Resistance = I squared x R
