Electricity Flashcards
Equation for charge flow?
Charge = current x time
Resistance and current relationship?
Greater resistance means lower current
Equation for potential difference?
PD = current x resistance
Examples of ohmic conductors?
Wires and resistors
What is special about an ohmic conductor?
Resistance doesn’t change with current
Relationship between current flowing through an ohmic conductor and PD at constant temperature?
Directly proportional
Relationship between temperature and resistance?
Higher temperature = higher resistance
How does resistance in a filament lamp work?
- When charge flows through a filament lamp, energy is transferred to the lamp’s thermal energy store
- Resistance increases with current in a filament lamp
How does resistance in a diode work?
- They will let current flow in one direction
- They have very high resistance when current is reversed
How do light dependant resistors change?
- In bright light, resistance falls
- In darkness, resistance increases
2 uses of light dependant resistors
- Burglar detectors
- Automatic night lights
How do thermistors change?
- In hot conditions, resistance drops
- In cool conditions, resistance increases
2 uses of thermistors?
- Thermostats
- Car engine temperature sensors
How do automatic lights work?
- A bulb and an LDR are connected in parallel, so the pd across both will be high when its dark and the LDRs resistance is high
- The greater the pd across a component, the more energy it gets
- So a bulb connected in parallel to an LDR would get brighter as the room gets darker
How does PD work in a series circuit?
The total pd is shared across all the components - all components’ pd adds to make the source pd
How does current work in a series circuit?
Current is the same across all components
How does resistance work in a series circuit?
Resistance adds up across components
How does pd work across parallel circuits?
All components receive the full source pd
How does current work in parallel?
Current is shared across branches - total current going in must = total current coming out
How does resistance work in parallel?
- Two resistors in parallel have a resistance less than the smallest resistor
- Resistors in parallel have the same pd
- Adding another loop gives current more directions to go, increasing total current flow in a circuit
- Using V =IR resistance decreases
What is the UK’s mains supply pd?
230V
What is the frequency of the mains supply?
50Hz
What is the brown wire and its function?
- Live wire
- Provides the alternating pd from the mains supply at 230V
What is the blue wire and its function?
- Neutral wire
- Completes the circuit and carries current away - it has 0V
What is the green and yellow wire?
- Earth wire
- Stops the appliance from becoming live by carrying current back to Earth when there’s a fault
How does an electric shock happen?
- Your body is at 0V - so touching the live wire causes a large pd produced across your body and current flows through you
- This is because your body provide a link between the supply and the earth and current flows through you
How do kettles transfer energy?
Electrically from the mains supply to the kettle’s thermal store
How does a hand held fan transfer energy?
Electrically from the battery’s chemical store to the kinetic energy store
Equation for energy transferred and power?
Energy transferred = power x time
What does the power rating of an appliance tell you?
Maximum amount of energy transfer per second
Equation for energy transferred and charge flow?
Energy transferred = charge x pd
Equation for power using pd?
Power = pd x current
Equation for power using current?
P = I^2 x resistance
2 ways power stations can meet high demand?
- They run well below their maximum power output
- Lots of small power stations which can be set up quickly
Purpose of a step up transformer?
- To increase pd and decrease current to minimise energy loss when the current moves along the wires
Purpose of a step down transformer?
To decrease pd and increase current For maximum current to get to a home
How does a cloth become positively charged when rubbed against a polyethene rod?
- Negatively charged electrons are scraped off of the cloth and dumped onto the rod
- This leaves the cloth with less electrons and a positive charge, and the rod with a negative charge
How does too much static cause a spark?
- As electric charge build on an object, the pd between the object and the earth increases
- If the pd gets large enough, electrons can jump across the gap between the charged object and the earth
How can electrostatic attraction/ repulsionbe shown?
Suspend a rod with a known charge from a string and hold another object with the same charge near the rod will repel the rod. Opposite charges will attract.
3 points for drawing an electric field.
- Positive to negative
- Right angle to the surface
- Closer lines = stronger field
What causes electrostatic forces?
Electric fields interacting
Explain sparks in terms of electric fields?
- When pd is big enough, electrons jump across the gap between earth and the charged object
- High pd causes a strong electric field between the two
- Strong electric fields cause electrons to be removed from air particles
- Ionized air is a great conductor so a current flows through it
