Energy Transfers in Circuits Flashcards

All 21 cards in this deck

When there is a current in a resistor, what happens to the resistor?

There is an energy transfer which heats the resistor.

Why does a resistor heat up when there is a current in it (in terms of particles)?

Electrons collide with the ions in the metal lattice, transferring energy to the ions and making them vibrate more.

What happens to the electrical energy when a current does work against electrical resistance?

It is dissipated as thermal energy in the surroundings.

How does using wires with lower resistance reduce unwanted energy transfer?

Fewer collisions between electrons and lattice ions, so less energy is dissipated as thermal energy in the wires.

Give two ways of making a connecting wire have a lower resistance.

Use a thicker wire (larger cross-sectional area) and a shorter wire; also use a material that is a better conductor, e.g. copper.

Using $P = I^2 \times R$, why do low-resistance cables waste less energy for the same current?

Power dissipated is proportional to resistance, so halving $R$ halves the power wasted as heat.

Give two useful applications of the heating effect of an electric current.

Heating water in a kettle or immersion heater; cooking in an electric oven or toaster (also filament lamps, hair dryers).

Give two disadvantages of the heating effect of an electric current.

Energy is wasted as thermal energy, reducing efficiency; components or wires can overheat and cause a fire.

What is the equation for energy transferred in terms of current, potential difference and time?

$E = I \times V \times t$ (energy in J, current in A, p.d. in V, time in s)

In $E = I \times V \times t$, what unit must the time be in?

Seconds. Minutes or hours must be converted first, e.g. 7 min = 420 s.

How is power defined?

The energy transferred per second.

What is the unit of power, and what is 1 watt equal to?

The watt (W); 1 W = 1 joule per second.

What is the equation linking power, energy transferred and time?

$P = E \div t$ (power in W, energy in J, time in s)

What is the equation for electrical power in terms of current and potential difference?

$P = I \times V$ (power in W, current in A, p.d. in V)

What is the equation for electrical power in terms of current and resistance?

$P = I^2 \times R$ (power in W, current in A, resistance in $\Omega$)

True or false? Doubling the current in a resistor of fixed resistance doubles the power dissipated.

False. $P = I^2R$, so doubling the current multiplies the power by four.

In an electric motor (e.g. in a washing machine), what is the main energy transfer?

Electrical energy is transferred to the kinetic energy store of the moving parts (some is wasted as thermal energy and sound).

In a heating device such as a kettle, what is the main energy transfer?

Electrical energy is transferred to the thermal energy store of the water (and surroundings).

True or false? For the same time of use, a 2000 W kettle transfers twice as much energy as a 1000 W heater.

True. Energy transferred $= P \times t$, so double the power means double the energy in the same time.

What supplies the electrical energy in a battery-powered device compared with a mains appliance?

A battery supplies a d.c. p.d. from its chemical energy store; mains appliances use a.c. mains.

What does the power rating of an appliance tell you?

The most energy it can transfer every second while it is running, i.e. its maximum operating power in watts.