12.2 - potential difference and power

what delivers the energy in a circuit?

the electrons from the battery

how much energy do the electrons in a circuit take from the battery?

a fixed amount, determined by the potential difference of the battery

when are electrons in a circuit supplied with energy?

when passing through the battery (via the positive terminal)

where do electrons in a circuit enter the battery?

the positive terminal

why do electrons in a circuit enter the battery at the positive terminal?

due to actual current - because electrons are negatively charged, they are attracted to the positive terminal of the battery

how is the energy from the battery shared across the electrons?

each electron is supplied with a fixed amount of energy from the battery that is replenished when the energy is delivered to a component in the circuit

do the electrons in a circuit need to replenish their energy?

if there are no components in the circuit, then no. otherwise they need to replenish it after delivering their energy to a component in the circuit

how much energy do electrons transfer to components in a circuit?

some or all of their energy

explain how a torch bulb connected to a battery in a circuit is lit up

• electrons are delivered a fixed amount of energy from the battery when passing through it

• electrons move around the circuit, delivering all / some of its energy to the torch bulb when passing through it

• electrons are replenished with energy when re-entering the battery via the positive terminal

• repeat

where does the energy come from in a circuit?

the battery, and carried by the electrons (charge carriers)

what does the battery have the potential to do if it’s not part of a complete circuit?

transfer energy from its chemical store

when does a battery transfer energy from its chemical store?

when the battery is not part of a complete circuit

when does the electron do work?

when passing through a component, transferring some / all of it’s energy

how much of the electron’s energy is replenished when passing through the battery?

up to the fixed amount? or by the same amount?

what is the work done by an electron (when passing through a component) equal to?

its lost energy

what is the electron’s lost energy equal to?

it’s work done (when passing through a component)

what is the work done (/ energy transfer) per unit charge (of a charge carrying electron) defined as?

the potential difference across the component

what is the potential difference across the component defined as?

• the work done (/ energy transfer) per unit charge (of a charge carrying electron)

• the difference in electrical energy per coulomb between 2 points (safer to use this)

which definition for potential difference should you use?

the difference in electrical energy per coulomb between 2 points

what is the unit for potential difference?

• volts (V)

• joules per coulomb (Js^-1)

what is the symbol for potential difference?

pd

what is a volt equal to?

1 joule per coulomb

what is 1 joule per coulomb equal to?

a volt

what is the equation linking voltage, work done, and charge?

V = W / Q

• V = potential difference across the component

• W = work done when charge Q flows through the component

• Q = charge flowing through the component

explain what is happening here -

• W = QV

• 37J = 3C x 12V

if the pd across a component in a circuit is 12V, then 3C of charge passing through the component would transfer 36J of energy from the battery to the component (since work done = energy transferred)

what is the symbol for work done?

W

what is the unit for work done?

joules (J)

what is the emf?

the energy transferred from chemical energy to electrical energy per coulomb of charge (i.e., the energy required to move a unit electric charge by an energy source)

what is the symbol for emf?

ε

what is the unit for emf?

volts (V)

what does emf stand for?

electromotive force

what is the equation linking electrical energy, charge, and emf?

E = Qε

what does the equation E = Qε tell you?

the electrical energy produced when charge Q passes through the source, for a source of emf ε

what happens to the electrical energy produced by the source?

• transferred to other parts of the circuit

• some are dissipated in the source due to the source’s internal resistance

why is electrical energy produced by the source dissipated in the source?

due to the source’s internal resistance

where is electrical energy produced by the source transferred to?

other parts of the circuit

what happens when an electric current passes through a component with resistance?

• it has a heating effect

• it has a magnetic effect

when does an electric current have a heating effect?

when it passes through a component with resistance

when does an electric current have a magnetic effect?

when it passes through a component with resistance

what is the magnetic effect of an electric current utilised for?

• electric motors

• loudspeakers

what is useful energy?

electric energy from the source transferred into a useful form related to a component in the circuit

what is wasted / dissipated energy?

electrical energy from the source transferred into a non-useful form, such as heat or sound

how do electric heaters work?

the work done on the device is transferred as thermal energy because charge carriers repeatedly collide with atoms in the device and transfer energy to them, so atoms vibrate more and resistor becomes hotter

what does the work done equal in an electric heater?

work done = electric energy transferred to thermal energy

what do electric heaters get hot?

charge carriers repeatedly collide with atoms in the device, transferring energy to them, making the atoms vibrate more and the resistor becomes hotter

are electric heaters resistors?

yes

are electric motors resistors?

yes

are loudspeakers resistors?

yes

what occurs in resistors only?

electric currents produced by the source have a heating and magnetic effect

what is a resistor?

a component designed to have a certain resistance regardless of the current through it

what is the work done equal in an electric motor (moving at constant speed)?

work done = electric energy transferred to the load and surroundings by the motor

why is the kinetic energy constant in an electric motor (that’s turning at constant speed)?

because the work done on the motor is equal to the energy transferred to the load and surroundings by the motor

what happens to the charge carriers in an electric motor (moving at constant speed)?

they need to be forced through the wires of the spinning motor coil against the opposing force on the electrons, due to the motor’s magnetic field

why do the charge carriers in an electric motor (moving at constant speed) need to be forced through the wires of the spinning motor coil against the opposing force on the electrons?

due to the motor’s magnetic field

what are the charge carriers in an electric motor forced through?

the wires of the spinning motor coil against the opposing force on the electrons, due to the motor’s magnetic field

what are the charge carriers in an electric motor forced against?

the opposing force on the electrons due to the motor’s magnetic field

what is the work done for a loudspeaker transferred as?

kinetic energy transferred as sound energy

what are the charge carriers in a loudspeaker forced through?

the wires of the vibrating loudspeaker coil against the force on them due to the loudspeaker magnet

what are the charge carriers in a loudspeaker forced against?

the force due to the loudspeaker magnet

what is the equation linking charge flow, current and time?

Q = I x Δt

what is the equation for work done by the charge carriers?

W = I x V x Δt

• W = QV

• W = (I x Δt) V

• W = I x V x Δt

what is the equation for electrical power?

P = I x V

• W = I x V x Δt

• ΔE (energy transfer) = W (work done)

• E = I x V x Δt

• P = E / Δt

• P = (I x V x Δt) / Δt

• P = IV

what is the symbol for energy transferred?

ΔE

what is the equation for power, energy, and time?

power = energy / time

what is the unit for power?

watts (W)

what is the symbol for power?

P

1 volt = ?

1 volt = 1 W / ampere