electricity (p2)

Question 1

what is charge?

Answer 1

• a measure of the total current that flowed within a certain period of time.
• charge is represented with a Q, and is measured in coulombs (C)

Question 2

what do cells contain?

Answer 2

cells contain chemical energy that is transferred to electrical energy and is carried by the current. when this energy passes through a component, the electrical energy is transformed into other types of energy.

Question 3

describe what is meant by resistance:

Answer 3

the current flowing through a conductor (e.g. a metal wire) can collide with atoms in the metal, and is transferred into other forms of energy, e.g. thermal. the resistance tells us the voltage required to drive a current through a component.

Question 4

what is the correlation between current and potential difference (voltage)?

Answer 4

the current moving through a resistor is directly proportional to the potential difference. this means that the resistance is constant - it doesn’t change if we increase the current. this is an ohmic conductor.

Question 5

what’s the link between resistance and temperature?

Answer 5

the resistance will only stay constant if the temperature remains constant.

Question 6

describe current:

Answer 6

an electrical current is the flow of electrical charge around the circuit. this moves from the negative end of the cell to the positive end. however, it’s always drawn in the opposite direction, the ‘conventional current’
- the current is the same all around a series circuit

Question 7

what is power?

Answer 7

the rate of electrical energy transfer within a circuit. e.g. if 1J of energy was transferred in 1 second, that would be 1W.

Question 8

what is a parallel circuit?

Answer 8

• contains branches
• some of the current passes through both of the branches; the current in the branches add up to the total current leaving the cell

Question 9

what is alternating current (a.c)?

Answer 9

the direction of the current is constantly flowing back and forth. this occurs when we use an alternating potential difference (fluctuates between pos. and neg.)
- all mains supply in the uk is a.c (50Hz, 230 V)

Question 10

what is direct current (d.c)?

Answer 10

produced by a direct potential difference (either positive or negative the entire time). the charge/current is flowing in the same direction the entire time.
- d.c is found in cells/batteries

Question 11

what are the purposes of different wires in a three core cable?

Answer 11

• each wire is made of copper, but has an outer insulating coating of plastic

> brown: live wire. carries potential difference from supply to appliance. always 230v. connected to a fuse in the plug. (extremely dangerous. fatal if touched.)

> blue: neutral wire. completes circuit with the live wire. has a potential difference of around 0v.

> green and yellow striped wire: earth wire. a safety wire to stop the appliance from becoming live.

Question 12

what would occur if someone touched a live wire?

Answer 12

live wire on its own: even with an open switch, the live wire can still be extremely dangerous, as the section before the switch is still charged at 230V. the Earth is 0v, so touching the live wire means that a current flows through the person and into the Earth, and they’d be electrocuted.

Question 13

what happens if a person touches the live wire, but an earth wire is present?

Answer 13

• metal-cased appliances are dangerous, as if the live wire comes loose and touches the metal case, it could make the casing live (if someone were to touch this live case, they would receive an electric shock).
• the metal case is attached to the Earth wire (which is incredibly low resistance). if the case becomes live, a huge current flows to the Earth. the fuse melts and shuts off the current.
• this prevents anyone from getting an electric shock from touching the case.

Question 14

describe the journey of electricity from the power station to a home:

Answer 14

• the electricity passes from the power station, and through a step-up transformer, increasing the potential difference to several hundred thousand volts.
• less energy is lost in the power cables when the potential difference is very high
• the electricity passes through a step-down transformer, which reduces the potential difference to around 230v.
• it then reaches the home.

Question 15

describe the static electricity that occurs between cloth and a rod:

Answer 15

• cloth is an insulator, and is covered in electrons, which can’t move through the cloth
• a plastic rod is also an insulator, and is covered with electrons which can’t move through
• when the two are rubbed together, electrons move from the plastic onto the cloth, and electrons move onto the cloth, giving it a negative charge, and giving the rod a positive charge.

Question 16

what occurs on a vandegraph, why does it make people’s hairs stand on end?

Answer 16

a moving belt passes over the top plastic roller. over time, the plastic roller transfers electrons to the belt. as the belt moves over the bottom roller, electrons pass off the belt and are transferred onto the Earth. the top roller becomes positively charged, and this builds up on a metal dome. someone who holds onto this dome now has an overall positive charge too. because of this, the hairs have the same charge, and so they repel.

Question 17

describe the different shapes of graphs when investigating the relationship between current and potential difference for bulbs, diodes and resistors:

Answer 17

• filament lamp (s-shaped curve, curved towards the ends): rises possible directly proportionally at the beginning, but as we increase voltage, we increase the current, therefore increasing the resistance, causing the filament to reach a higher temperature, therefore it’s not an ohmic conductor anymore, as the temperature is no longer constant.
• resistor (straight line): straight line through origin. ohmic conductor. as the voltage increases, the current increases, as long as the temperature is constant.
• diode: for all the negative potential difference values, there is 0 current flow in the opposite direction, as the resistance is too high. at approximately 1 volt, the current will then start to rise, and will rise in a directly proportional way - the bigger the voltage, the bigger the current.

Question 18

what is ohm’s law?

Answer 18

a law stating that electric current is directly proportional to potential difference.

Question 19

what is an oscilloscope?

Answer 19

a device that shows how the potential difference changes over time.

Question 20

what does a circuit breaker do?

Answer 20

• electromagnetic device, it’s like a fuse that can be reset.
  > checks if the voltage it gives out and
  receives is the same. if it isn’t it means
  there’s been an earth leakage (the current
  has escaped the circuit), and the circuit
  breaker trips.

Question 21

what does the fuse do?

Answer 21

it has a certain amount of voltage that it can withstand, and if a voltage that’s too high passes through it, the fuse blows, and stops the current flow altogether.

Question 22

what do double insulated devices do?

Answer 22

equipment with an insulated wire and insulated case (usually plastic) doesn’t need an earth wire, as there’s no chance that the user can come into contact with the current flow.