Topic 5: Electricity and Magnetism

Question 1

Give the law of conservation of charge.

Answer 1

Charge can neither be created or destroyed.
i.e. if two objects are rubbed together and electrons move, both objects will gain equal but opposite charge.

Question 2

Define an electric field.

Answer 2

A region of space where a charged object experiences a force due to its charge.

Question 3

Describe field lines.

Answer 3

1) The closer the field lines, the stronger the field.
2) The direction of the field shows the force that would act on a positive test charge placed at that point.

Question 4

Define a test charge.

Answer 4

One that does not disturb the electric field it is placed in.

Question 5

What direction are the arrows for a positive and negative point charge?

Answer 5

Arrows for a positive point charge go out while arrows for a negative point charge go in.

Question 6

Give Colulomb’s Law.

Answer 6

The force between two point charges is:
- proportional to the product of their charges
- inversely proportional to the square of their separation

Question 7

Define the electric field strength.

Answer 7

The force per unit charge experienced by a small positive test charge placed in the field.

Question 8

What is a uniform electric field created with?

Answer 8

Between two parallel, opposite (but equally) charged plates.

Question 9

Define the potential difference between two points.

Answer 9

The work done (or energy transferred) when one unit of charge moves between two points.

Question 10

Define the electronvolt.

Answer 10

The energy an electron would gain by being accelerated by a potential difference of 1 volt. 1eV = 1.6 x10⁻¹⁹J

Question 11

Define current.

Answer 11

The rate of flow of electric charge. Measured in amperes (A) or Cs⁻¹.

Question 12

Describe conventional current.

Answer 12

The opposite direction to electron flow (i.e. from the positive terminal to the negative terminal).

Question 13

Define drift velocity.

Answer 13

The speed at which electrons move due to the current. I = nAvq

Question 14

What causes drift velocity?

Answer 14

Within a conductor, electrons have high random speeds. An electric field will exert a force on the electrons causing them to accelerate in a direction opposite to the direction of the field. So the net velocity (drift velocity) of the electrons is in the opposite direction to the electric field.

Question 15

Describe a series circuit.

Answer 15

• The current is the same around the circuit.
• The sum of the potential differences across the components adds up to the supply voltage.

Question 16

Describe a parallel circuit.

Answer 16

• The current splits up between the branches.
• The potential difference across each branch is the same.

Question 17

How do we measure current in a circuit?

Answer 17

An ammeter is used. It must be connected in series with the component that is having the current flowing through it measured. An ideal ammeter would have a resistance of zero so it does not affect the current in the circuit which it is connected.

Question 18

How do we measure potential difference in a circuit?

Answer 18

A voltmeter is used. It must be connected in parallel with the component that is having the potential difference across it measured. An ideal voltmeter would have an infinite resistance so it does not draw any current from the circuit to which it is connected.

Question 19

Define resistance.

Answer 19

The ratio of potential difference across the component to current flowing through the component. “A measure of the ease of current flow”.

Question 20

Equations for resistance in series and parallel circuits.

Answer 20

Series:
Rtotal = R1 + R2 + …

Parallel:
1/Rtotal = 1/R1 + 1/R2 + …

Question 21

Define resistivity (p).

Answer 21

Electrical resistivity is a property of a substance that gives a measure of how the substance opposes the flow of electric current. Measured in Ωm.

Question 22

What are the three types of sensors that can be used in potential divider circuits?

Answer 22

1) Light dependent resistors (LDRs)
2) Thermistors
3) Strain gauges

Question 23

Define electrical power.

Answer 23

The amount of energy used or supplied per unit time.

Question 24

Kirchhoff’s Laws

Answer 24

1) At any junction in a circuit, the total current entering the junction equals the total current leaving (conservation of charge).
2) Around any closed path of a circuit, the total of all the potential differences is zero (conservation of energy).

Question 25

Describe primary cells.

Answer 25

Primary cells cannot be recharged, they are an irreversible chemical reaction and once the chemicals have been used up the cell no longer produces electrical energy. Examples include Zn-C and alkaline batteries.

Question 26

Describe secondary cells.

Answer 26

Secondary cells are designed to be recharged. They undergo a reversible chemical reaction. Reversing the current through the cell by attaching it to an external DC power source will reverse the chemical reaction, producing more reactants. Examples include Pb-acid car batteries, Ni-Cd and Lithium ion batteries.

Question 27

What is capacity given in?

Answer 27

Amp-hours (Ah). 1Ah is equal to 3600C.

*The greater the capacity of the cell, the greater current it can deliever for a given period of time.

Question 28

Define EMF.

Answer 28

It is the energy supplied per unit charge by the power supply to the circuit.

*What the battery is trying to put out.

Question 28

What is the shape of the graph of terminal potential difference against time?

Answer 28

Remains at emf for most of the time and then there is a rapid final drop in terminal pd due to an increase in internal resistance.

Question 29

Define terminal potential difference.

Answer 29

The potential difference measured across the terminals of a battery when it is part of a circuit and a current is flowing. It is a measure of the energy available to the external circuit.

*What the circuit gets!

Question 30

How can a bar magnet be drawn?

Answer 30

Lines coming out of North and going into the South.

Question 31

How is the magnetic field drawn going into and out of the page?

Answer 31

Into the page is crosses. Out of the page is dots. Think ‘o’ for out.

Question 32

Describe the rule used to determine moving charges.

Answer 32

RH slap rule for positive charges.
LH slap rule for negative charges.

Thumb is the velocity of the charge/direction. Fingers is the magnetic field and the palm is the force.

Question 33

Define internal resistance.

Answer 33

The resistance of the components inside the cell/battery. It causes an energy loss in the cell.