CP9 Electricity Flashcards
What is power measured in?
watt, w
What is a direct current and what devices supply a direct current?
a movement of charge in only 1 direction. Cells and batteries supply direct current
What is an alternating current?
the movement of charge changes direction
What frequency is UK mains electricity supplied at?
50Hz
What voltage is UK mains electricity supplied at?
230V
Is UK mains electricity d.c or a.c.?
a.c.
In a plug what is the live wire for and what voltage should it be at?
The live wire connects the appliance to the generators at the power station. It should be at 230V.
In a plug what is the neutral wire for and what voltage should it be at?
The neutral wire is the return path to the power station. It should be at 0V.
What is each end of the earth wire connected to and what voltage should it be at?
The earth wire is connected to the metal case of an appliance at one end and a metal stake pushed into the ground at the other. The earth wire wshould be at 0V.
What is the purpose of the earth wire?
To keep the metal case of an appliance at 0V so in the case of a fault it can’t cause an electric shock.
What can happen if a fault in an appliance causes a very large current to flow?
Large currents get very hot (like in the filament bulb) and can potentially start a fire.
What is the purpose of a fuse or circuit breaker?
If a current above a safe value passes through a fuse or circuit breaker it will blow or trip breaking the cicuit
What would happen if the live wire flowed through to the earth wire?
Because there would be very low resistance as there are no components in the circuit a very large current would flow which would likely cause a fire.
Whereabouts in a household circuit should fuses/circuit breakers be connected and why?
Fuses and circuit breakers should be connected to the live wire so they protect the entire circuit from that point on if there is a fault.
What is resistance measured in?
Ohm,
How can a variable resistor be used to change the current in a circuit?
Increasing the resistance will decrease the current
What is the formula for the total resistance of multiple resistors in series?
R total = R1 + R2 + R3 …
Why does the total resistance of the circuit increase when two resistors are placed in series?
The total current has to flow through both resistors and so the total current is reduced by both resistors
What happens to the resistance of a circuit when a resistor is added in parallel?
The resistance of the circuit decreases
Why does the total resistance of the circuit decrease when two resistors are placed in parallel?
The current splits so a smaller current flows through each resistor reducing the affect on the total current of each resistor. (Also think of having one corridor or two corridors to move a large number of people through.)
Describe the circuit you would use to investigate the relationship potential difference, current and resistance for a filament lamp
Build a series circuit with a power supply, a variable resistor, a filament bulb and an ammeter. Connect a voltmeter in parallel with the bulb.
How would you use a circuit containing a variable resistor to make multiple measurements of potential difference and current?
Changing the resistance of the variable resistor will alter the values of current and p.d. which can be recorded in a table.
If you have a graph of current against potential difference how do you find the resistance?
The gradient gives the resistance.
How does current vary with potential difference in a filament lamp?
Initially current is proportional to p.d. the current continues to increase with p.d. but at a decreasing rate.
