Electricity levels 7-9 Flashcards
Explain what happens to the electrons in an electric circuit when the cell is added
how does resistance slow the electrons down
Becuase electrons are negatively charged, when a cell is put in a circuit the negative end of the cell repells the electrons and the positive side of the cell attracts the electrons. This means that the electrons flow round the circuit from negative to positive.
Resistance is caused by collisions of the electons with the ions in the metal. Each collision slows down the current in the circuit. A resistor has a higher resistance if it causes more collisions.
Explain how to find out how long it takes for the charge to flow round a circuit.
Example: how long does it take 7 coulombs of charge to flow past a point when there is a current of 11.5A?
The time it takes for the charge to move can be found by rearranging the equation Q = It
This gives us the relationship t = Q/I
Example: if a curent of 11.5A carries 7C past a point it takes 7/11.5 = 0.61s to happen
What is the effect of adding in more parallel branches on the resistance of the circuit?
Explain why this happens
When more branches are added to a circuit the resistance of the circuit goes down. This happens even if the extra brang contains a very large resistor.
This happens becuase this branch provides an extra route for electrons to move down. This means that the same current as before flows down the pre-exisitng branches, but now the extra branch carries a bit of extra current, increasing the total current in the circuit.
Explain how half wave rectification can be used to convert an AC current into a DC current
Half wave rectification uses a diode to allow current to travel in only one direction. This effectively curs off the half of the current that passes in the other direction, creating pulses of current.
What is the difference between the movements of the electrons in an AC and a DC current.
In an AC current, the electrons move backwards and forwards. The Average postiion of the electrons over a long perid of time remains the same.
In a DC current the electrons move in only one direciton. These electrons travel round the circuit.
Describe what the term ‘double insulated’ means and explain why these appliances do not need to have an earth wire attached
Double insulated means that the case of the appliance is entirely plastic and the wires inside are insulated.
These appliances do not need an earth as it is impossible for a wire to cause the case to become live. This is becuase the case is an insulator.
Explain why the fuse in an apliance is situated right next to the live wire and not next to the neutral wire
In mains electricity the live wire carries electricity to the appliance and the neutral wire carries the electricity away from the appliance.
When there is a short circuit the electric current does not travel away from the appliance down the neutral wire, it bypasses it and travells down the earth wire instead.
A fuse next to the live wire will have current flowing through it in either case, allowing the fuse to blow when there is a short circuit. A fuse next to the neutral wire would have no current running through it in the event of a short circuit, so would not act as a safety feature
Explain what resistance heating is.
How can you calculate the power of resistance heating
Resistance heating is the transfer of electrical potential into thermal energy in a resistor. It occurs due to electrons moving through the resisor colliding with the ions in the metal and transferring kinetic energy to them.
The power of resistance heating can be found in the following ways
P=I2R
P=V2/R
P = power (W - Watts)
I = Current (A - Amps)
V = Potential difference (V - Volts)
R = Resistance (Ω - Ohms)
Explain how resistance heating can change the power rating of an appliance.
Resistance heating is when the component heats up due to the current passing through it. This causes the ions to vibrate, increasing the number of collisions and therefore the resistance.
When the resistance goes up, but the potential difference stays the same, this causes the power to go down. This is becuase P = V2/R
Explain why conservation of energy means that the current in a circuit must reduce when a step up transformer is used to increase the potential difference.
