Electricity Flashcards
(19 cards)
What is current and how can you calculate charge using it
Current is the rate of flow of electrical charge around a closed circuit.
Current is caused by a source of potential difference and it has the same value at any point in a single closed loop of a circuit.
Charge (C)= Current (I) x Time (s)
Q=It
What is resistance and can you calculate the potential difference using it
Resistance is anything in the circuit which slows the flow down.
The greater the resistance across a component, the smaller the current that flows through it (for a given potential difference across the component)
PD (V)= Current (A) x Resistance (Ohm)
V=IR
What is an ohmic conductor
The resistance of ohmic conductors doesn’t change with the current, at a constant temperature, the current is directly proportional to the potential difference.
When charge flows through a filament, some energy is transferred to its thermal energy store and it will heat up, so resistance increases with temperature.
For diodes, the resistance depends on the current’s direction with low resistance one way and high resistance when reversed.
How do you investigate factors affecting resistance
Attach two crocodile to the wire, 10cm apart.
Close the switch and record the current and PD.
Open the switch and move the clips an extra 10cm apart, record each length, current and PD each time and repeat.
Calculate the resistance at each length, use R= V/I
Plot a graph, resistance against wire length.
The line of best fit should be straight to show resistance is directly proportional to length and if the 1st clip is at 0cm, it should go through the origin as well.
How does resistance change in an LDR and thermistor
LDR- a resistor dependent on light intensity so in bright light, the resistance falls and in darkness, the resistance is high; they are used in automatic night lights, outdoor lighting and burglar detectors.
Thermistor- is temperature dependent so in hot conditions, the resistance falls and when cold, the resistance is high; they make useful car engine temperature sensors and electronic thermostats.
How do all components: have the same current passing through them and have the same PD when parallel to each other
In series circuits, the same current flows through all components; the size of the current is determined by the total PD and resistance.
I1=I2=I3
In parallel circuits, all components get the full source PD, so the voltage is the same across all components; this means that identical bulbs connected in parallel will all be at the same brightness.
How do resistors change the resistance in different circuits
Rtotal=R1+R2
In series circuits, adding a resistor means they have to share the PD and across each resistor the PD is lower so the current is lower meaning the resistance increases; the bigger its resistance, the bigger its total PD share.
In a parallel circuits, both resistors have the same PD meaning the ‘pushing force’ is the same as the PD but by adding another loop, the current has several directions to go and therefore increasing current and decreasing resistance.
What is the difference between direct and alternating current and PD
In alternating current (ac) supplies the current is constantly changing directions and they are produced by alternating PD and the positive and negative ends alternate.
The UK mains supply is an ac supply of 230V
The frequency of the ac mains supply is 50hz
Cells and batteries supply direct current (dc), it’s always flowing in the same direction and is created by a direct pd.
What is the function of each wire in a three-core cable
Live wire (brown)- provides the alternating PD (230V) from the mains supply.
Neutral wire (blue)- it completes the circuit so when the appliance is operating normally, current through the live and neutral wires is around 0V.
Earth wire (green and yellow)- for protecting the wiring and stops the appliance from becoming live, it only carries a current when there is a fault and it’s also at 0V.
How is the live wire possibly dangerous
The body is at 0V, so if you touch the live wire, a large PD is produced across your body and a current flows through you, causing a large electrical shock.
If a plug socket or a light switch is off, it’s still a danger of an electric shock because there’s still a PD in the live wire.
Any connection between live and earth wire can be dangerous as if the link creates a low resistance path to earth, a huge current will flow and possibly cause a fire.
How is power related to PD, current and resistance
Power (w) = Potential difference (V) x Current (A)
P=VI
Power (w) = Resistance (ohm) x Current squared (A)
P=I2R
How do appliances transfer energy to the kinetic or thermal energy stores
Electrical appliances are designed to transfer energy to components in the circuit when a current flows.
Kettles transfer energy electrically from the mains ac supply to the thermal energy store of the heating element inside it.
Energy is transferred electrically from the battery of a handheld fan to the kinetic energy store of the fan’s motor.
What are the two equations for energy transferred
Energy transferred (J) = Power (W) x Time (s)
E=Pt
Energy transferred (J) = Charge flow (C) x Potential difference (V)
E=QV
What is the national grid and why is it efficient at transferring energy
The national grid is a huge system of cables and transformers that covers the UK and connects power stations to consumers.
To transmit lots of power, you need high current or high PD but high current loses lots of energy
Increasing the PD is cheaper and keeps the current lower; this decreases the energy lost by heating the wires and surrounding, making it more efficient at transferring energy.
How is static electricity produced
When certain insulating materials are rubbed together, negatively charged electrons will be scraped off one and dumped on the other one.
Both materials will be electrically charged, one with positive static charge and the other with equal negative static charge.
The may the electrons are transferred depends on the material involved.
How can static electricity cause sparks
As electric charge builds on an object, the PD between it and the earth increases.
If the PD gets large enough, electrons scan jump across the gap between the charged objects and the earth and this is a spark.
Electrons can also jump to any earthed conductor that’s nearby but it usually happens when the gap is small.
Why do charged objects repel or attract
2 things with opposite charge attract but 2 things with the same charge repel.
These forces get weaker the further apart they are.
Electrostatic attraction/repulsion is where these forces will cause the object to move if they can.
Negative-Negative = Repulsion
Positive-Positive = Repulsion
Positive-Negative = Attraction
What is an electrical field and what is an electric field for an isolated charged sphere
An electric field is created around any electrically charged object.
The closer to the object , the stronger the field is.
The further from the object, the weaker the field is.
How do electric fields explain the non-contact force and sparking
When a charged object is in an electrical field, it feels a force causing attraction or repulsion caused by the electrostatic fields of each charged object interacting with each other.
A high PD causes a strong electric field which causes electrons in air particles to be removed.
Air is normally and insulator, but when ionised it’s more conductive, so a current can flow through it, causing a spark.
