Unit 2 Flashcards
Picture a circuit with no cell in it. Explain why there is no current in this circuit.
Because there is no source of potential difference
A student measured the resistance of a diode using an electric current. He found the resistance to be 0.2 ohms. This time he measured the resistance to be 100 million ohms. Suggest why the students measurements were so different.
The diode was the wrong way round.
Explain the purpose of a variable resistor in a circuit to measure iv characteristics
Change the current flowing through the circuit so the potential difference can be measured for each current value
Picture a graph t hat flattens out at 3.5 on the x axis. Suggest why the lamp behaves as an ohmic conductor up to 3.5 volts.
A resistor that is ohmic when the relationship between current and potential difference is linear. The graph is linear until 3.50 v so the resistance is ohmic in this range.
Explain why adding resistors in series increases the total resistance whilst adding resistors in parallel decreases the total resistance.
In series, resistor share the potential difference from the power source. The more resistors that are in the series the lower the potential difference for each one so the lower the current each one. Current is the same all round a series circuit so adding a resistor will decrease the current for the whole circuit a decrease in total current means an increase in total resistance. In parallel or resistors have the same potential difference as the source. Adding another resistor in parallel increase is the current flowing in the circuit as there are more path for the current to flow through. An increase in total current means a decrease in total resistance because v=IR
A toaster cable has a fault such that the live wire is an electrical contact with the Neutral wire. Explain why the toaster will not work while this fault remains.
Current flows into the toaster by the live wire and out through the Neutral wire. Default means that a closed loop path has been formed before the current has reached the toaster. So very little current will flow through the toaster which means the toaster will not work.
State two purposes of the insulation which covers the live wire.
Make the wire easy to identify.
To stop an electric current flowing out and causing electric shock.
A man switches on the iron and touches the exposed live wire. He receives an electric shock. With reference to the electrical potential of the man, explain why he receives an electric shock
The man has an electric potential of 0 volts and the wire has an electric potential of 230 volts. So a potential difference exists between them. This causes a current to flow through the man, giving him the electric shock.
A socket is switched off and the iron is unplugged. Explain whether there is still a danger of the man receiving an electric shock from the plug socket.
Yes. Although there is no current flowing when it is switched off, there is still a potential difference, so the live wire in the socket can cause a current to flow through you to the Earth.
Write down the equation that links potential difference charge and energy transferred
V=EQ
Explain how a step-up transformer improves the efficiency of the National Grid
A step-up transformer increases potential difference, which decreases current. This decreasing current decreases energy lost to thermal energy stores of the cables and surroundings. By reducing the energy loss to the thermal stores it makes the electricity transfer more efficient, thus increasing efficiency.
A man walks up some carpeted stairs. The handrail is made of metal and is electrically connected to Earth. When he puts his hand near the rail, there is a spark.
Given that the spark lept from the man to the handrail, was the man positively charged or negatively charged.
Negatively charged. Only negative charges can move and they will move from an area of negative charge to an earthed area.
Explain what is meant by an electric field
A region in which a charged object will experience a force
When the potential difference is high enough and are particle begins to break apart due to two charges pulling together. Explain why this happens
The negative charges are attracted towards the positive sphere, while the positive charges are attracted towards the negative sphere. So parts of the particles are pulled away from each other.
when the potential difference is high enough the force becomes large enough to break the particle apart
