Sensing

Question 1

What is current?

Answer 1

Current is the rate of flow of charge and can be defined mathematically as: I = Q/t. Current is measured in ampheres (amps, A), which is one coulomb of charge flowing through a point in one second.

Question 2

How do you measure current?

Answer 2

With an ammeter connected in series with the components of a circuit.

Question 3

What is charge?

Answer 3

Charge is simply a measure of how many electrons flow through a particular point and is mathematically defined as Q = It. One electron has a charge of around -1.6 x 10^-19. This means that there are 6.25 x 10¹⁸ electrons in one coulomb of charge.

Question 4

What is potential difference (voltage)?

Answer 4

Potential difference, or voltage, is a measure of the energy converted in a component per unit charge moved and is defined mathematically as V = P/Q. Volts is the unit for potential difference where 1 volt is one joule of energy lost per coulomb charge, this means volts can be alternatively expressed as JC^-1.

Question 5

How do you measure potential difference?

Answer 5

With a voltmeter connected in parallel with the main circuit.

Question 6

What is power?

Answer 6

Power is the rate of transfer of energy and is defined mathematically as P = E/t, or power = total energy transferred/time taken. In electrical circuits, P = VI since voltage is the energy transfered per coulomb and current is the number of coulombs that flow per second.

Question 7

What is resistance?

Answer 7

Resistance is a measure of how bad a component is at letting through a current. It is mathematically defined as R = V/I. Resisitance is measured in ohms (Ω) where 1 Ω is when 1 volt will make a current of 1 amp flow through the component.

Question 8

Why is resistance important?

Answer 8

The resistance equation can be rearranged to form the well known V = IR relationship. This tells us the voltage required to make a specific current flow through the component. This relationship can also be used to define the relationship P = I²R which can be used to work out the power dissipated by a component.

Question 9

What is conductance?

Answer 9

• Conductance is a measure of how good a component is at letting through a current and is mathematically defined as G = 1/R = I/V.
• Conductance is the reciprocal of resistance so is therefore inversely proportional to resistance; so as resistance decreases, conductance increases and vice versa.
• Conductance is measured in Siemens (S).

Question 10

What is Ohm’s law?

Answer 10

Ohm’s law states that for ohmic conductors, resistance is constant for all currents and potential differences given a constant temperature. This means that current is directly proportional to potential difference for ohmic conductors.

Question 11

What does ohm’s law allow us to do?

Answer 11

We are able to work out the resistance of a component with any given current and p.d. since it is constant.

Question 12

What are some examples of non-ohmic behaviour?

Answer 12

1. A light bulb filament is made from metal, which is an ohmic conductor. However, the behaviour of a light bulb filament is non-ohmic (current is not proportional to p.d.) becuase as p.d. increases, resistance also increases due to the filament heating up, so rate of current-increase decreases.
2. A thermisor is designed so that as temperature increases, resistance decreases.

Question 13

How do you work out total resistance for a series circuit?

Answer 13

R_T = R₁ + R₂ + … + R_n-1 + R_n

Question 14

How do you work out total resistance for a parallel circuit?

Answer 14

1/R_T = 1/R₁ + 1/R₂ + … + 1/R_n-1 + 1/R_n

Question 15

How do you work out total conductance for a series circuit?

Answer 15

1/G_T = 1/G₁ + 1/G₂ + … + 1/G_n-1+ 1/G_n

Question 16

How do you work out total conductance for a parallel circuit?

Answer 16

G_T = G₁ + G₂ + … + G_n-1+ G_n

Question 17

What is emf (electromotive force)?

Answer 17

Emf is the total energy produced per coulomb by a cell/power supply without the lost of energy due to the internal resistance of the cell/power source and is mathematically defined as ε = Ir + IR = V_I + V_C.

Question 18

How is emf calculated?

Answer 18

ε = V_I+ V_C can be rearranged so that:

V_C = ε - V_I = ε - Ir.

This is in the form y = mx + c where y = V_C, m = -r, x = I and c = ε. This means that if a graph V_Cagainst I is plotted, the gradient would be the internal resistance and the y-intersect would be the emf. This plot is generated by changing the resistance of a variable resistor in a circuit.

Question 19

What is a potential divider?

Answer 19

When there’s more than one resistor in a circuit, the voltage is shared between the resistors by their ratios. This means that the voltage across any given resistor R_x is given by V_x = V_T x (R_x/R_T).

Question 20

How is a potential divider used?

Answer 20

1. Old volume dials used potential dividers. The knob controlled a variable resistor in series with a fixed resistor (sound producer). Increasing the resistance of the variable resistor decreased voltage across sound producer, meaning it produced a smaller sound.
2. Most sensors depend on a change in resistance and a change in voltage to measure a change in the environment. Having a variable resistor in series with a sensor controls its sensitivity and it voltage to prevent damage.