Electricity

• What is electric charge?

  Electric charge is a fundamental property of matter that can be positive or negative.
• How is electric current defined?

  Electric current is the flow of electric charge through a conductor.
• In which unit is current measured?

  Current is measured in amperes (A).
• What is the formula for calculating current?

  The formula for current is $I =$$\frac{Q}{t}$.
• If 10 coulombs of charge flow through a wire in 2 seconds, what is the current?

  The current would be 5 amperes.
• What is potential difference in an electrical circuit?

  Potential difference (voltage) is the energy transferred per unit charge between two points in an electrical circuit.
• In which unit is potential difference measured?

  Potential difference is measured in volts (V).
• How is resistance defined in an electrical circuit?

  Resistance is the opposition to the flow of electric current in a circuit.
• In which unit is resistance measured?

  Resistance is measured in ohms (Ω).
• What does Ohm's Law describe?


  Ohm's Law describes the relationship between voltage, current, and resistance.
• What is the mathematical expression of Ohm's Law?

  Ohm's Law is expressed as $V =$$IR$.
• What is the formula for calculating power in electrical circuits?

  The formula for power is $P =$$VI$.
• In which unit is power measured?

  Power is measured in watts (W).
• How does the total resistance in a series circuit compare to that in a parallel circuit?

  The total resistance in a series circuit is the sum of individual resistances, while in a parallel circuit, it is less than the smallest individual resistance.
• What are the 3 key features of UK mains electricity?


  Alternating current, 230 volts , 50 Hz.
• What is the difference between Direct current (DC) and Alternating current (AC). 


  In DC, the charges move continuously in one direction. In AC, charges continuously change direction (p.d. is continuously changed direction)
• In a 3-core cable, what are the features of the live wire?

  Alternates its potential difference from maximum +325V to -325V, 50 times per second.
• In a 3-core cable, what are the features of the neutral wire?

  Potential difference = 0V - completes the circuit between the appliance and the local substation
• In a 3-core cable, what are the features of the earth wire?

  Potential difference = 0V - only carries current in the event of a fault
• Why is the live wire dangerous even when the switch in a mains circuit is open?


  Because it is at a very high p.d. compared to earth, so that if a person touched it they would be electrocuted as current flowed through them to earth
• How is electrical power transferred in the National Grid?

  In a transformer, at a very high potential difference between power stations and consumers, then stepped down to 230V before use.
• Why do Transformers improve effeciency?


  A high potential difference across the transmission cables means a lower current needed to tranfer the same amount of power (P=IV). This means less heat loss and less energy wasted.
• Why is the earth wire a good safety feature?

  It is the path of lowest resistance (shortest) to the Earth so current will flow through it instead of a human in a fault.
• Which wire is the fuse connected to?

  the live wire
• What is DC current?

  Direct current, only flows in one direction (doesn't cross 0V). An example of a DC supplier is batteries or cells- typical cells supply around 1.5V
• What is AC?

  Alternating Current, that changes direction/ potential difference (goes above and below 0V). Example: Sockets (mains) use it, UK mains is at 230V.
• What is static electricity?

  Buildup of electric charge on an insulator.
• What causes the current to flow in the wire?


  The difference in potential difference between the live wire (230V) to the neutral wire (0V).
• When insulators are rubbed together, electrons are transferred from one material to another causing them to become charged.
• Explain why a polyethene rod becomes negatively charged when rubbed with a dry cloth.

  The materials are both insulators ... Friction ... Causes electrons to move from the cloth to the rod ... rod gains electrons so becomes negative while cloth looses electrons, becoming positive.
• What is a forcefield?

  an area around a charged object where another will experience an electrostatic force
• What is an electrostatic force?

  A non-contact force between electrically charged objects such as attract or repel.
• Which was does the arrow point for a positive forcefield?

  away from the centre (repel)
• which way do the arrows point for a negative forcefield?

  towards the centre (attract)
• where is the forcefield strongest?

  in the centre as the lines get closer together.
• Draw a forcefield diagram to show an opposite charge to this one which has a greater charge.

  1 mark for more lines. 1 mark for arrows in the correct direction.
• how do sparks occur?

  Electric charge builds up, object more negatively charged. Potential difference between charged object and earth / earthed object. Strong electric field between them causes electrons in air particles to be removed. Ionised air is conductive so current flows through causing a spark. Object is now discharged.
• what is an ohmic conductor?

  conductor where current is directly proportional to p.d so resistance is constant (at a constant temperature).
• how do you calculate the resistance of IV graphs?

  1/ gradient. That means that a flatter line is a higher resistance. To calculate the resistance always calculate gradient (if it is a curve draw a tangent) rather than using just 2 points.
• Why is the shape of a filament lamp current-p.d graph a soft s shape?

  higher current, temperature increases. The positive ions in the wire vibrate more and collide more often with electrons flowing through it so the resistance increases.