IGCSEPhysicsEdexcelPhysics (Modular)Unit 1Flashcards2. ElectricityElectrical Power & Mains Electricity

Electrical Power & Mains Electricity (Edexcel IGCSE Physics (Modular): Unit 1): Flashcards

Exam code: 4XPH1

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  • Define power.

    Power is the rate of energy transfer, or the amount of energy transferred per second.

  • Define fuse.

    A fuse is a safety device that cuts off the flow of electricity to an appliance if the current becomes too large, due to a fault or a surge.

  • State the equation linking electrical power to current and potential difference.

    P = IV

    Where:

    • P = power in watts (W)

    • I = current in amperes (A)

    • V = potential difference in volts (V)

  • The unit of power is the watt (W), which is the same as a ______ per second.

    The unit of power is the watt (W), which is the same as a joule per second.

  • What equation is used to calculate the current an appliance needs, when selecting a fuse?

    I = \frac{P}{V}

    Where:

    • I = current in amperes (A)

    • P = power in watts (W)

    • V = voltage in volts (V)

  • True or False?

    When selecting a fuse, you should always choose the rating that is closest below the calculated current.

    False.

    You should always choose the next size up from the calculated current. Too low a rating causes the fuse to blow unnecessarily, while too high a rating fails to protect the appliance quickly enough.

  • An appliance draws a current of 3.1 A. Fuses are available rated at 3 A, 5 A and 13 A. Which should be used, and why?

    A 5 A fuse, because it is the next size up from 3.1 A. A 3 A fuse would blow as soon as the appliance was switched on, while a 13 A fuse would allow too much extra current through before it finally blew.

  • Define work done in an electrical circuit.

    Work done is the energy transferred when charge flows through a circuit.

  • The energy transferred by electrical work in a component depends on the current, the potential difference, and the ______ the component is used for.

    The energy transferred by electrical work in a component depends on the current, the potential difference, and the time the component is used for.

  • State the equation linking energy transferred to power and time.

    E = P \times t

    Where:

    • E = energy transferred in joules (J)

    • P = power in watts (W)

    • t = time in seconds (s)

  • Since P = IV, how can the equation E = P \times t be rewritten in terms of current and potential difference?

    E = I \times V \times t

    Where I = current in amperes (A) and V = potential difference in volts (V).

  • Calculate the energy transferred in 1 minute when a current of 0.7 A passes through a potential difference of 4 V.

    E = I \times V \times t

    E = 0.7 \times 4 \times 60

    E = 168 \text{ J}

  • True or False?

    'Work done' and 'energy transferred' mean the same thing in electrical calculations.

    True.

    The two terms are used interchangeably: work done in a circuit is equal to the energy transferred.

  • Why must the time t always be converted into seconds before using the energy transfer equations?

    Because P is in watts (joules per second) and E in joules, so t must be in seconds for the units in the equation to be consistent.

  • Define double insulation.

    Double insulation describes an appliance with two layers of insulation: the insulation around its wires, and a non-metallic case that acts as a second layer. Such appliances do not require an earth wire.

  • Define earthing.

    Earthing is a safety feature in which an additional earth wire provides a low resistance path to the earth if a live wire touches an appliance's metal casing, reducing the risk of electrocution.

  • Name three common electrical safety hazards in the home.

    • Damaged insulation — an exposed live wire risks electrocution

    • Overheating of cables — too much current through too small a wire (or a tightly coiled long wire) can cause a fire or melt the insulation

    • Damp conditions — moisture in contact with live wires can cause a short circuit or electrocution

  • Give two advantages of a circuit breaker over a fuse.

    • It does not melt and break, so it can be reset and used again

    • It works much faster

  • A potential difference as small as ______ can pose a serious hazard to a person.

    A potential difference as small as 50 V can pose a serious hazard to a person.

  • True or False?

    Circuit breakers are sometimes called "fuse boxes", and work in exactly the same way as a fuse.

    False.

    A circuit breaker is an automatic electromagnetic switch, not a melting wire. Unlike a fuse, it does not need replacing and can simply be reset after breaking the circuit.

  • Why are the conducting parts of electrical wires covered with an insulating material such as rubber?

    To prevent a person coming into direct contact with the conducting (metal) core, reducing the risk of electrocution.

  • Define current.

    Current is the rate of flow of charge.

  • Why does the temperature of a resistor (or wire) increase when current flows through it?

    The collisions of the free electrons within the wire cause some of the transferred energy to be dissipated into the surroundings by heating.

  • Give three domestic appliances that use the heating effect of an electric current.

    Any three from: electric heaters, electric ovens, electric hob, toasters, kettles

  • A current passing through a resistor results in the ______ transfer of energy.

    A current passing through a resistor results in the electrical transfer of energy.

  • True or False?

    When current flows through a resistor, all of the electrical energy is transferred usefully, with none dissipated as heat.

    False.

    Some of the energy is always dissipated into the surroundings by heating, due to collisions of the free electrons within the wire.

  • Define direct current (d.c.).

    A steady current, constantly flowing in the same direction in a circuit, from positive to negative.

  • Define alternating current (a.c.).

    A current that continuously changes its direction, going back and forth around a circuit.

  • What are the frequency and typical potential difference of UK mains electricity?

    UK mains electricity has a frequency of 50 Hz and a potential difference of around 230 V.

  • How do the terminals of a d.c. supply differ from those of an a.c. supply?

    A d.c. supply has a fixed positive terminal and a fixed negative terminal. An a.c. supply has two identical terminals that change from positive to negative and back again.

  • The frequency of an alternating current is the number of times the current changes direction ______ each second.

    The frequency of an alternating current is the number of times the current changes direction back and forth each second.

  • True or False?

    Direct current is produced by cells and batteries, whereas alternating current is produced by electrical generators, such as mains electricity.

    True.

    This is a key distinction between the two: cells and batteries supply d.c., while generators (including mains supplies) produce a.c.

  • Even though an a.c. supply's terminals keep swapping polarity, in which direction does the current flow at any instant?

    At any instant, the current still flows from the (currently) positive terminal to the negative terminal — it is the terminals' polarity that swaps, causing the current's direction to change over time.

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