Voltage

• state that the e.m.f. of a source of electrical energy is measured in volts

• show understanding that e.m.f. is defined in terms of energy supplied by a source in driving charge round a complete circuit

• state that the potential difference across a circuit component is measured in volts

• use and describe the use of a voltmeter

The v_____ is a measure of how big a push the e_______ are given as they leave the cell or b_____. The bigger the voltage, the bigger the p___.

The cell, battery or p____ s_____ produces the voltage.

Voltage

lamps

oltagelectrons

atteryush

ower upply

Voltage

lamps

There are two types of voltage e.m.f. and potential difference (p.d.).A cell or power supply increases the amount of energy in a circuit.The energy given to each unit of chargeas it passes arond the circuit, is called the e.m.f.This can be thought of as the electrical pressure pushing the current around a circuit.When the electrical current flows through a component like a bulb, it loses energy.For example in a bulb light energy is given out. The difference in the energy of each unit of charge is known as the potential difference.

Series circuit

Measuring the voltage

• Measure the voltage using a voltmeter.• Connect the voltmeter “across” the component

i.e. one lead to either side.• This is the potential difference across the

component.• If there are no components attached to a power

supply the e.m.f. is equal to the potential difference measured.

Set up the two circuits shown:

Now measure the voltage across each component in your circuits

A.

B.

What do you notice about your readings?

Energy and charge

We can express a relationship between charge, energy and potential difference mathematically using the equation:

Energy = Charge x Voltage

E=QV

Charge measured in Coulombs (C)

Energy measured in Joules (J)

Voltage measured in Volts (V)

Formula triangles

E

Q V

x

Formula triangles help you to rearrange formula, the triangle for E=QV is shown below:

Whatever quantity you are trying to find cover it up and it will leave you with the calculation required.

So if you were trying to find charge, Q…..

…you would cover Q up…

…and you are left with the sum…

Q = E

V

Formula triangles

A charge of 100 C is delivered at a potential difference of 5V. How much energy is delivered?

E = QV

= 100 C x 5 V

= 500 J

In an examination you must always give the units.

If you do not you will lose a mark.

What is 500J in KJ?

0.5 kJ

To do

• P178-179• Answer questions 1&2

Name this circuit symbol.

A. Ammeter

B. Switch

C. Bulb

D. Voltmeter

V

What component is used to measure electrical current?

A. Switch

B. Resistor

C. Ammeter

D. Voltmeter

Which component changes electrical energy into light energy?

A. Bulb

B. Switch

C. Cell

D. Battery

Which component is a store of chemical energy?

A. Bulb

B. Battery

C. Switch

D. Resistor

Which of these materials is not a conductor of electricity?

A. Water

B. Graphite

C. Lead

D. Wood

Progress check

• state that the e.m.f. of a source of electrical energy is measured in volts

• show understanding that e.m.f. is defined in terms of energy supplied by a source in driving charge round a complete circuit

• state that the potential difference across a circuit component is measured in volts

• use and describe the use of a voltmeter