P4 And P5 - Electricity And Circuits Flashcards
Time - Key Facts
Term symbol - t
Unit of measure - seconds
Unit symbol - s
Potential Difference - Definition and Key Facts
Definition - how much push an electron needs to jump the gap so it can move atom to atom.
Term symbol - V
Unit of measure - voltage (volts)
Unit symbol - V
Energy - Key Facts
Term symbol - E
Unit of measure - Joules
Unit symbol - J
Resistance - Definition and Key Facts
Term Symbol - R
Unit of measure - Ohms
Define - How easy it is for the electrons to move atom to atom
Power - Definition and Key Facts
Define - how quickly energy is transferred.
Term symbol - P
Unit of measure - Watts
Unit symbol - W
Calculations linking charge, current and time
Charge = current x time
Current = charge / time
Time = charge / current
Calculations linking potential difference, energy and charge
Potential difference = energy / charge
Energy = charge x potential difference
Charge = energy / potential difference
Calculations linking resistance, potential difference and current
Resistance = potential difference / current
Potential difference = current x resistance
Current = potential difference / resistance
Calculations linking energy, power and time
Energy = power x time
Power = energy / time
Time = energy / power
Calculations linking current, resistance and power
Power = current squared x resistance
Current squared = power / resistance
Resistance = power / current squared
What is a bulb?
Glows when the circuit is complete
Line with circle and a cross in the middle
What is a voltmeter?
Measures the voltage produced by a power supply.
Line with circle and a V in the middle.
What is an ammeter?
A device that measures current in (Amps).
Line with a circle and an A in the middle
What is a resistor?
It slows down the flow of electrons in a circuit.
Line with a rectangle in the middle.
What is a variable resistor?
It slows down the flow of electrons in a circuit. The resistance can be changed.
Line with a rectangle in the middle, arrow from BL to TR going through.
What is a cell?
Transforms chemical energy into electrical energy.
What is a battery?
Two or more cells in series.
Two cells
Closed Switch
It connects the components in a circuit.
Line with two joined dots
What is a light dependant resistor?
The resistance depends on the light. More light means less resistance. In light, current will flow through the circuit.
What is an open switch?
Current cannot flow through
What is a thermistor?
The resistance depends on the temperature.
What is a fuse?
Designed to melt / break circuit if current exceeds an amount.
Ohmic Resistors
• Ohmic resistors do not heat up
If it obeys Ohms Law, current is directly proportional to the voltage
• Ohms Law states that the current through a resistor at constant temperature is directly proportional to the potential difference across the resistor.
• Reversing the potential difference across a resistor reverses the current going through it.
UK Mains Electricity
Uses alternating current so range of motion is doubled
Alternates 50 times per second (50 Hz) at around 230V
Alternates positive and negative every cycle
What is a heater?
Designed to transfer the energy from an electric current to heat surroundings.
What is an LED?
Emits light when a current passes through it.
Component characteristics
The resistance of an appliance is
R= V/I
A filament lamp’s resistance increases as the filament’s temperature increases.
Diode forward resistance is low. Diode reverse resistance is high.
A thermistors resistance decreases if it’s temperature increases.
An LDRs resistance decreases if the light intensity on it increases.
Series Circuits
For components in series:
The current is the same in each component
The total potential difference is shared between the components
Adding their resistances gives the total resistance
For cells in series, acting in the same direction, the total potential difference is the sum of their individual potential differences.
Total resistance - Rtotal = R1 + R2
Adding more resistors in series increases the total resistance because the current through the resistors is reduced and the total potential difference across them is unchanged.
Parallel Circuit
For components in parallel:
The total current is the sum of the currents through the separate components.
The bigger the resistance of a component, the smaller the current that will pass through that component.
The current through a resistor in a parallel circuit is:
I = V / R
Adding more resistors in a parallel circuit decreases the total resistance because the total current through the resistors is increased and the total potential difference across them in unchanged
1/Rt = 1/R1+ 1/R2 + 1/R3
Filament Bulbs
Not an Ohmic resistor
As the temp of the filament lamp increases, the resistance also increases.
Filament bulbs get hot, and therefore waste energy- banned in UK to reduce emissions.
Graph has a straight line which begins to slope as the extra resistance comes from heat.
Three core cables
Electrical appliances are connected to the mains supply by three core cables. They have 3 wires inside with a core of copper and a colours plastic coating. Always the same for every appliance - easy to tell apart
Brown (BR) is live wire from the power station, provides alternating current.
Blue (BL) neutral wire, taking energy back to the power station.
Yellow/green stripes - Earth wire
Alternating current (AC)
Repeatedly reverses direction.
Doubles the amount of potential difference, so more energy is transferred.
Produced by alternating voltages in which the positive and negative ends keep alternating.
UK mains supply uses this
National Grid
Typical power station generates electricity at an alternating voltage of 25,000 volts.
Step up transformers are uses at power stations to transfer electricity to the National Grid. These transformers are used to make the size of the alternating PD much bigger, typically from 25,000V to about 132000V
Step down transformers are used to supply electricity from the national grid to consumers. Homes and offices in the UK are supplied with mains electricity that provides the same power as a 230V direct current supply. Factories use much more power than homes, so they are supplied with a p.d. of 100kV or 33kV
Direct current
Electrons only moving in one direction.
Batteries/cells can only supply direct current (d.c.)
Created by a direct voltage, where P and N ends of the source are fixed.
Cables and Plugs
Socket cases are made from stiff plastic materials that enclose the electrical connections. Plastic is used because it is a god electrical insulator.
Mains cable made up of two/three copper wires surrounded by an outer layer of flexible plastic material.
In a three pin plug the Earth wire is connected to the longest pin in a plug and is used to earth the metal case of a mains appliance.
Electrical resistance
• material (conductivity)
• thickness of wire - thicker = less resistance
• length
• temperature - hotter = more resistance
More current - more friction - temp increase - more resistance/loss of energy
reduce current by increasing voltage, and still have the same power
Total resistance in parallel
• total resistance is less than the resistance of the smallest resistor
Total resistance in parallel
• total resistance is less than the resistance of the smallest resistor
Total resistance in parallel
• total resistance is less than the resistance of the smallest resistor
