P5 Quiz

Question	Answer
What is an electrostatic force?	A force caused by positive and negative charges.
How do charges behave?	Two charges which are the same will repel; different charges attract.
How are electrons held in place? How can friction affect this?	By electrostatic forces between the positive nucleus and the negative electrons. The further the electron is from the nucleus the weaker the force. Sometimes some of the outer electrons can be removed by rubbing. The two rubbed objects will become oppositely charged.
Why does lightning occur?	Charge is built up by friction in the clouds. When it becomes large enough to break down the insulation of the air the charge flows between the cloud and the earth. The release of energy appears as a flash of light. This is an example of electrostatics.
What makes a good conductor? Give an example of a good and bad conductor.	If the outer electrons are loosely held and can move around freely then it will be a good conductor, these are known as free electrons. Good: metal. Bad: plastic.
What is an electric current?	The rate of flow of electrons carrying a negative charge.
Why is charge conserved but energy used up in a circuit?	Energy is transferred to components but the electrons have a continuous push from the power supply which keeps them flowing.
What is the purpose of the power supply?	To push the electrons around the circuit, the greater the power the faster they will flow.
What does a voltmeter measured?	The amount of energy transferred at a particular point in a circuit, measured in volts.
Complete the statement: In an electric circuit charge is 1 and energy 2.	1: conserved 2: transfered
What is potential difference? What is one volt in terms of this?	The difference in energy per charge either side of a component. For example charges before a lamp will have more energy than charges on the other side as energy will be transferred to the lamp. The potential difference between two points is one volt if one joule of electrical energy is transferred to another form of energy when one unit of charge passes between the two point.
What is a variable resistor?	A device that allows you to vary the amount of resistance in a circuit by moving a slide or rotating a knob so that more or less resistance wire is connected to the circuit. Resistance is a measure of how much a conductor opposes a current and is measured in ohm. The better the conductor the lower the resistance.
Use an equation to describe the relationship, voltage and current?	resistance= voltage/current
What is Ohm's law?	That the resistance is equal to 1/ the gradient of a graph of voltage against current. This means that the current-voltage relationship across a metallic conductor is directly proportional.
Describe a series circuit. Include information about current, voltage and resistance. What happens if you double the number of cells?	In a series circuit components are connected in one line, there is only one route around the circuit. If one component fails, i.e. if the circuit is broken in one place, the whole circuit will fail. The current is the same everywhere in a series circuit, as there is only one path through the circuit which all the charge has to follow. In a series circuit the supply voltage, or potential difference, is shared between the components. This means in a series circuit we can find the total voltage supplied by finding the sum of the voltages at each component. Since the current is constant the resistance and the voltage depend solely on one another within each component. This means that if the resistance of one component is double that of another the voltage will also be doubled over that component. If the resistance in the whole circuit is increased, the current will be smaller. Increased resistance means the whole battery has to do extra work which means the charges will flow slower. If the number of cells in a circuit is doubled the voltage will be doubled and the resistance will stay the same, so the current also doubles. Much of this can be worked out using the formula Resistance = voltage/current.
What is a series circuit? Include information about the current and voltage.	When components, connected in parallel, are each connected separately to the power supply. This means if one component fails the other components will continue to gain energy. The current is shared between the different branches and the total current is equal to the sum of the currents in the circuit. Each branch is supplied with the full voltage.
What is a thermistor and a LDR?	Thermistor: semiconductor device in which resistance changes with temperature. In the most common thermistors the resistance decreases as the temperature increases. They can be used with a thermostat to control temperature. LDR: light dependent resistor is similar but in a LDR the resistance varies with the amount of light falling on it. The darker it gets the greater the resistance. This can be used to switch on street lighting when it gets darj.
What symbol is used for a thermistor?	Image: 150px-Thermistor.svg (image/png)
What symbol is used for a LDR?	Image: th1.2.4.11.gif (image/gif)
What happens to the resistance of metals as the temperature increases? Why?	As the temperature increases the positive ions in the metal have more energy and move more. The free electrons jiggle about more and so there overall speed decreases. Therefore the resistance increases.
In practice how do LDRs and thermistors work?	The extra heat or light energy provides more free electrons, therefore there is a greater rate of flow. Their resistance therefore decreases as the current increases.
What is a magnetic field?	A space in which a magnetic force acts.
What happens when there is relative movement between a coil of wire and a magnet?	A voltage is induced and a current flows
How can the current and voltage be increased during electromagnetic induction?	By using a stronger magnet; by using more coils; faster relative motion; using an iron core.
How is visualising a magnetic field using magnetic field lines helpful?	It can be used to explain induced voltage, the field lines need to cut through the electrical conductor to induce a voltage. The greater the rate at which the field lines are cut, the bigger the voltage induced.
What is an alternating current and why is it produced?	When the direction of the current changes at regular intervals. The number of times it changes in a second is measured in Hz. A frequency of 1 Hz means it changes direction once a second. It is produced because as the coil spins in a uniform magnetic field different parts of the magnet are subject to different p.d. at different points. When the position is reversed a negative p.d. is produced. This results in the alternating current, the frequency will be the same as the frequency of the rotating coil that induced it.
What is the magnetic effect of a current?	As a current flows through a wire it produces a magnetic field.
What is the motor effect?	When a current flows through a wire in a region where there is another magnetic field the wire experiences a force and moves. The biggest force is felt when the magnetic field and the current are perpendicular. A motor uses the motor effect but is assembled so the force gives continuous rotation. It is like a generator in reverse. The coil is connected to a power supply and a force is produced that makes the coil turn.
Explain why the motor effect works.	The magnetic field around the current-carrying wires and the magnetic field of the permanent magnet combine. The fields reinforce each other above the wire and cancel out below the wire. This makes it turn. This happens to each long side of the coil in the motor. This means one side up and one side is pushed down making it turn.
What is a split ring communicator? How is it used in a motor?	A metal ring split into two halves. The fixed communicator swaps contact with the coil swaps contact with the coil to reverse the current through the coil. This happens each half cycle and ensures the turning effect is always in the same direction and the coil rotates continuously.
What is inside a transformer?	Two copper wire coils: a primary coil- which the current is fed into- and a secondary coil which is connected to the appliance. These have an iron core inside and surrounding the coils.
What is a step up transformer?	When the primary coil is smaller than the secondary coil the voltage supplied increases.
What is a step down transformer?	When the primary coil has more turns than the secondary coil the voltage is reduced.
How does electricity flow from the primary coil to the secondary coil when they are not connected?	The a.c. voltage in the primary coil creates an ever-changing magnetic field around it. The magnetic soft iron core channels the field through the secondary coil, this changing magnetic field cuts through the secondary turns and an a.c. voltage is produced across the ends of the secondary coil.
What is the relationship between voltage in the primary and secondary coils and the number of coils?	(voltage across primary coil/ voltage across secondary coil) = (number of turns in primary coil/ number of turns in secondary coil)
What is d.c.?	Direct current; it flows in only one direction
Why is an alternating current good?	Their generators produce more power than any other forms of electricity production. Another advantage of the generators is that a large amounts of fuels can be used to turn the turbines. Transformers only work with a.c.
Why is electricity transferred at high voltage?	Less energy is lost
What is the equation for electrical power?	Power= voltage x current

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	Created by harry.vinall almost 10 years ago