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**
3.6 Potential difference and
energy transfer**

**
Introduction to calculating electrical energy
transferred**

**
Energy transfer per unit of charge = potential difference (p.d.) and
calculations based on
E = QV**

In the previous section we looked at how to calculate the
quantity of charge moving in a circuit, but said nothing about the energy
transferred.

**
Reminders**:

Electrical circuits, terms used, circuit symbols, parallel
circuits and series circuits explained

**Potential difference** (p.d. in
volts, **V**) is the energy transferred per unit charge as electric
charge moves from one point to another in an electric circuit.

It is measured with a voltmeter,
which is always wired in parallel across a circuit component.

**An electrical current transfers
energy**

Just think of all the electrical
appliances you use - all need supplying with energy to work!

**A power supply does work on a
charge and transfers energy to it.**

Work must be done on the charge
to increase its potential energy.

**Electric charge is measured in
coulombs (C)**

Charge and its movement has
already been dealt with in
Part 3.5 (Q = It)

**Charges transfer energy to
components as they pass through by doing work against the resistance of
the component.**

If work is done, then energy is
transferred.

**If electrical charge experience a potential difference,
that charge will flow transferring energy.**

The **energy is supplied** from the energy store of the
**power supply** - battery, mains electricity etc.

**When charge passes through any p.d. fall it releases energy**
(from a higher to a lower potential energy level).

e.g. in a thin wire
**resistance**,
thermal (heat) energy is released.

**The potential difference between two
points is equal to the work done per unit charge.**

**
potential difference (V) = work done (**energy
transferred in** J) ÷ charge (C)**

i.e. 1 volt corresponds to 1 joule
per coulomb or
V = J/C

The bigger the fall in p.d., the greater the energy
transferred, because the charge starts off with a greater potential
energy.

Therefore a power supply with a **bigger** source
**p.d.** (V)
can supply **more energy** to the circuit per unit of electric charge (the
coulomb, C).

**The bigger the p.d., the more energy the same quantity of
electric charge can carry.**

**
Another equation to calculate electrical
energy transfers**

The
quantity of energy carried can be calculated from the equation:

energy transferred = charge x potential
difference.

**
E = QV**, so **Q = E/V**
and **V = E/Q, **

(learn to rearrange, its better than
using the formula triangle)

**
E **
= energy transferred in joules (**J**)

**
Q **
= quantity of
electric charge in coulombs (**C**)

**
V **
= potential difference (**V**)

Noting that: V = E/Q =
energy transferred per unit charge
(J/C)

**Just in passing and some reminders:**

The more energy transferred in a given
time, the greater the power of a device or electrical appliance.

The p.d. **V** tells you how much
energy each unit of electrical charge transfers,

so, **V = E/Q**, (units
J/C), see E = QV calculations below).

The current** I** tells you how much
charge passes a given point in a circuit per unit time (coulombs/second, **
C/s**).

This means both p.d. **V** and current**
I** **affect the rate at which energy is transferred** to an appliance
from the electrical energy store to other energy stores.

AND some **mathematical connections**
based on section 2. **
Q = It** and here in section 3
**E = QV**

From Q = It and E = QV, substituting
gives **E = ItV, **

so** (i)
E = IVt** (I in A,
t in s, V in volts)

Rearranging E = IVt gives IV =
E/t

This connects with the
equations for power

**(ii)** Power = energy
transferred / time taken =
**E/t** (J/s),
and

(**iii**) Power = current
x voltage = P (W) = I (A) x V (V), **
P = IV**

From (ii) and (iii) E/t =
IV, so **E = IVt**, which is equation (i) !!!

**
Calculation q****uestions based on E = QV**

(sometimes involving other electricity equations too)

**Q1** An electric motor of a
model car is powered by a 1.5 V battery.

If 120 C of charge passes through the
motor circuit in the moving car,

(a) how much energy is transferred?

(b) Describe the likely energy store
changes when the car is running.

ANSWERS

**Q2** What quantity of
charge is needed to transfer 500 J of energy if the p.d. of a circuit is
24.0 V?

ANSWERS

**Q3** What potential
difference is required in a circuit to transfer 2000 J of energy with a
charge of 50 coulombs?

ANSWERS

**Q4**
A 12.0 V battery passes a current of 2.0 A through a lamp for 5 minutes.

(a) Calculate how much charge passed
through the lamp.

(b) Calculate how much electrical energy
was transferred by the lamp.

ANSWERS

**Q5**
An appliance has a power of 1.5 kW and works of a 230 V mains supply.

If the appliance is used for 15 minutes,
how much charge has flowed through the circuit?

ANSWERS

INDEX of electricity
section 3 notes on current, voltage, resistance, energy & charge
transfer in circuits including Ohm's Law investigations

**
Keywords, phrases and learning objectives for calculations in electricity
involving charge, time, current, power and energy**

Know how are potential difference and energy
transfer relate

Be able to rearrange and use the following equations E = QV Q = E/V V = E/Q
in calculations e.g. use in problem
solving on energy J transferred given p.d. voltage V and charge in
coulombs C.

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